Image Analysis Guidelines

Patient and Facility Identification

The correct patient's name and age or birth date, and the facility's name, should be permanently photoflashed onto the identification (ID) plate or displayed on the digital display monitor. This information should be typed and legible. Evaluate all the images within a routine series to ensure that these have been correctly imprinted or displayed on the image. Never assume that an image has been correctly photoflashed or assigned to the correct patient. Always double-check. Flash cards can be easily switched or forgotten.

If the wrong information has been photoflashed onto the ID plate when using a screen-film system, a corrected information sticker should be placed over the incorrect patient information before the images are sent to be interpreted.

Time and Date

The examination time and date must be accurate to distinguish the images in a timed series and to match images with their accompanying requisition and report.

Identification Plate Placement Guidelines

Box 1-2 provides guidelines to help prevent the ID plate from superimposing the anatomy of interest when using screen-film radiography. In digital radiography, the ID plate placement is not as sensitive as it is for a screen-film radiograph because after the image is displayed, the blocker location can be moved to a location away from the anatomy of interest.

Using the Collimator Guide for Marker Placement

When collimating less than the size of the IR used, it can be difficult to determine exactly where to place the marker on the IR so that it will remain within the collimated field and not obscure anatomic areas of interest. The best way of accomplishing this is first to collimate the desired amount and then use the collimator guide (Figure 1-20) to determine how far from the IR's midline to place the marker. Although different models of x-ray equipment have different collimator guides, the information displayed by all is similar. Each guide explains the IR coverage for the source–image receptor distance (SID) and amount of longitudinal and transverse collimation being used. If a 14- × 17-inch (35- × 43-cm) IR is placed in the Bucky tray at a set SID, and the collimator guide indicates that the operator has collimated to an 8- × 17-inch (20- × 43-cm) field size, the marker should be placed 3.5 to 4 inches (10 cm) from the IR's longitudinal midline to be included in the collimated field (Figure 1-21). If the field was also longitudinally collimated, the marker would also have to be positioned within this dimension. In the preceding example, if the collimator guide indicates that the longitudinal field is collimated to a 15-inch (38-cm) field size, the marker would have to be placed 7.5 inches (19 cm) from the IR's transverse midline (see Figure 1-21).

Determining Mismarking Using the Identification Plate

If both sides of the body are demonstrated on the same radiograph, as with an anteroposterior (AP) projection of the pelvis or abdomen, evaluate it to ensure that an R marker is placed to the right of the vertebral column or an L marker is placed to the left of the vertebral column. When the screen-film system is used, this can be accomplished by using the patient ID plate because it is permanently built into the cassette and is always in the same place. Begin by displaying the image on the view box in the same manner that the IR was placed in the Bucky diaphragm. (For posteroanterior [PA] projections, display the image as if the patient's back were facing you. This is not the proper way to display such an image but is a method for determining marker accuracy.) The ID plate is in the lower left corner or the upper right corner on a lengthwise image and in the upper right or lower left corner on a crosswise image. Then, position yourself as the patient was positioned with respect to the IR. If the patient was in an AP projection, turn your back to the image; if the patient was in a PA projection, face the image. The marker on the image should correspond to your right or left side—an R marker on your right side or an L marker on your left side.

When a marker on an image is only faintly visible, circle it and rewrite the information it displays next to it. Do not write the information over it (Figure 1-22).

Mismarked Images

If the R or L marker does not appear on the image or the image has been mismarked, it is best to repeat the image. Do not guess or rely on what you may believe to be a sure sign. The heart shadow, which is normally located on the left side of the thorax, may be shifted toward the right because of a disease process or because the patient has situs inversus (total or partial reversal of the body organs).

Size Selection and Placement of Image Receptor

The IR used for a procedure should be just large enough to include the region being examined. Deciding whether to place the long axis of the IR crosswise (CW), lengthwise (LW), or diagonally in respect to the long axis of the part being examined is a simple matter of positioning it so that all the required anatomy can easily be demonstrated on the IR size chosen. This is mostly dictated by the body habitus, part length, and IR system used. As a general rule, the long axis of the part is aligned with the long axis of the IR.

Body Habitus

There are four types of body habitus to consider when positioning a patient for a PA-AP chest or AP abdomen projection—hypersthenic, sthenic, hyposthenic, and asthenic. The hypersthenic patient has a wide, short thorax and a broad peritoneal cavity, with a high diaphragm (Figure 1-23). This body habitus requires the IR to be placed CW for PA-AP chest projections to include the entire lung field and requires two CW IRs to be used for an AP abdomen projection to demonstrate the entire peritoneal cavity. The asthenic patient has a long, narrow thoracic cavity and narrow peritoneal cavity, with a lower diaphragm (Figure 1-24). The sthenic and hyposthenic types of body habitus have thoracic and peritoneal cavities, with lengths and widths that are between those of the hypersthenic and asthenic body habitus (Figures 1-25 and 1-26). The sthenic, hyposthenic, and asthenic types of body habitus require the IR to be placed LW for the PA-AP chest and AP abdomen projections to include the entire lung field and peritoneal cavity, respectively.

Long Bones

When imaging long bones, such as the forearm, humerus, lower leg, or femur, which require one or both joints to be included on the image, choose an IR that is long enough to allow 1 to 2 inches (2.5 to 5 cm) of the IR to extend beyond each joint space. This is needed to prevent the off-centered joint(s) from being projected off the IR because they will move in the direction in which the diverged x-ray beams that are used to record them on the image are moving (Figure 1-27).

Images of the humerus and lower leg may be placed diagonally on the IR to have enough length that both joints can be included on a single image when a screen-film IR system is used (Figure 1-28). This is not advisable when using a cassette-based computed radiography (CR) system because an exposure field that is not parallel with the edges of the imaging plate may result in poor exposure field recognition and histogram analysis errors. For CR systems, when the structure is too long for the part to fit on one cassette, two separate images should be obtained, with each including a joint and portion of the midshaft and with overlapping of at least 2 inches (5 cm) between the images.

Multiple Projections on Same Image Receptor

How the IR is placed (LW or CW) often dictates how many images can be placed on the IR. For example, a 10- × 12-inch (24- × 30-cm) IR placed CW can accommodate three images of the wrist, but the same-sized IR placed LW has space available for only two images. When an extremity is imaged and more than one projection of the structure is exposed on the same IR, the images should be evenly spaced on the IR and similar anatomic structures should be located at the same end of the IR. For example, when the AP and lateral projections of the forearm are placed on the same 14- × 17-inch (35- × 43-cm) IR, the AP is positioned on half of the IR and the lateral on the other, with a defined collimated border around each, and the elbows in the two images are to be demonstrated at the same end of the IR (Figure 1-29). Failure to keep this alignment makes displaying and viewing of the image difficult (Figure 1-30).

Collimation

Proper collimation is accomplished when the beam of radiation is narrow enough to include only the areas of interests. Good collimation practices result in the following: (1) decrease the radiation dosage by limiting the amount of patient tissue exposed; (2) improve the visibility of recorded details by reducing the amount of scatter radiation that reaches the IR; and (3) reduces histogram analysis errors when using digital radiography. As a general rule, each image should demonstrate a small collimated border around the entire image of interest. The only time that this rule does not apply is when the entire IR must be used to prevent clipping of needed anatomy, as in chest and abdominal imaging. This collimated border not only demonstrates good collimation practices but also can be used to determine the exact location of central ray placement. Make an imaginary X on the image by diagonally connecting the corners of the collimated border (Figure 1-31). The center of the X indicates the central ray placement for the image.

Accurate placement of the central ray and alignment of the long axis of the part with the collimator's longitudinal light line are two positioning practices that will aid in obtaining tight collimation. When collimating, do not allow the collimator's light field to mislead you into believing that you have collimated more tightly than what has actually been done. When the collimator's central ray indicator is positioned on the patient's torso and the collimator is set to a predetermined width and length, the light field demonstrated on the patient's torso does not represent the true width and length of the field set on the collimator. This is because x-rays (and the collimator light, if the patient was not in the way) continue to diverge as they move through the torso to the IR, increasing the field size as they do so (Figure 1-32). The thicker the part being imaged, the smaller the collimator's light field that appears on the patient's skin surface. On a very thick patient, it is often difficult to collimate the needed amount when the light field appears so small but, on these patients, tight collimation demonstrates the largest improvement in the visibility of the recorded details.

Learn to use the collimator guide (see Figure 1-20) to determine the actual IR coverage. For example, when an AP lumbar vertebral projection is taken, the transversely collimated field should be reduced to an 8-inch (20-cm) field size. Because greater soft tissue thickness has nothing to do with an increase in the size of the skeletal structure, the transverse field should still be reduced when a thick patient part is being imaged. Accurately center the patient by using the centering light field and then set the transverse collimation length to 8 inches by using the collimator guide. Be confident that the IR coverage will be sufficient, even though the light field appears small.

Box 1-4 lists guidelines to follow when collimating and evaluating collimation accuracy on images.

Rotating Collimator Head

On some x-ray equipment, the collimator head can be rotated without rotating the entire tube column. This capability allows the technologist to increase collimation on anatomic structures such as the clavicle, which is not aligned directly with the longitudinal or transverse axis of the light field. Rotating just the collimator head does not affect the alignment of the beam with the grid; this alignment is affected only when the tube column is rotated and is demonstrated on the image by visualization of grid lines artifacts and grid cutoff. Rotation of the collimator head should be avoided when using cassette-based digital radiography because it may affect the exposure field recognition process.

Overcollimation

Evaluate all images to determine whether the required anatomic structures have been included. Poor centering or overcollimation can result in the clipping of required anatomy (Figure 1-36).

Clipping of required anatomy can also result from overcollimation on a structure that is not placed in direct contact with the IR, such as for a lateral third or fourth finger or lateral hand projection. Clipping occurs because the divergence of the x-ray beam has not been taken into consideration during collimation. To prevent clipping, view the shadow of the object projected onto the IR by the collimator light (Figure 1-37). It will be magnified. This magnification is similar to the magnification that the x-ray beam undergoes when the image is created. Allow the collimated field to remain open enough to include the shadow of the object, ensuring that the object will be shown in its entirety on the image.

Positioning Routines and Understanding the Reason for the Procedure

Most positioning routines require AP-PA and lateral projections to be taken to demonstrate superimposed anatomic structures, localize lesions or foreign bodies, and determine alignment of fractures. When joints are of interest, oblique projections are also added to this routine to visualize obscured areas better. In addition to these, special projections may be requested for more precise demonstration of specific anatomic structures and pathologic conditions.

To appreciate the importance of the anatomic relationships on an image, one must understand the clinical reason for what the procedure is to demonstrate for the reviewer. This is particularly important when obtaining special projections that are not commonly performed and require specific and accurate anatomic alignment to be useful. For example, an optimally positioned tangential (supraspinatus outlet) shoulder projection (Figure 1-38) should demonstrate the supraspinatus outlet (opening formed between acromion and humeral head) and the posterior aspects of the acromion and acromioclavicular (AC) joint in profile. The technologist produces these anatomic relationships when the patient's midcoronal plane is positioned vertically and can be ensured that the proper positioning was obtained when the superior scapular angle is positioned at the level of the coracoid tip on the image. From this optimal image, the radiologist can evaluate the supraspinatus outlet for narrowing caused by variations in the shape (spur) or slope of the acromion or AC joint, which has been found to be the primary cause of shoulder impingements and rotator cuff tears. If instead of being vertical, the patient's upper midcoronal plane was tilted toward the IR, the resulting image would demonstrate the superior scapular angle positioned above the coracoid tip, preventing clear visualization of the acromion and AC joint deformities, because their posterior surfaces would no longer be in profile and would narrow or close the supraspinatus outlet (Figure 1-39). Because the reviewer would be unable to diagnose outlet narrowing that results from variations in the shape or slope of the acromion or AC joint, this image would not be of diagnostic value.

Correlating the Anatomic Relationships and Positioning Criteria

For each projection in the procedural analysis sections of this book, there is a list of analytic criteria to use when evaluating the anatomic relationships that should be seen on an optimal image of that projection, an explanation that correlates it with the specific positioning procedure, and a description of related positioning errors. This information is needed to reposition the patient properly if a poorly positioned image is obtained because only the aspect of the positioning procedure that was inaccurate should be changed when repeating the image. For example, a PA chest projection that is demonstrated without foreshortening visualizes the manubrium superimposed by the fourth thoracic vertebra, with approximately 1 inch (2.5 cm) of the apical lung field visible above the clavicles. This analysis criterion is demonstrated on the image when the patient's midcoronal plane is positioned vertically. If a PA chest projection demonstrates all the required analysis criteria, with the exception of the manubrium and fourth thoracic vertebral alignment, the technologist who understands the correlation between the analysis criteria and positioning procedure would know to adjust only the positioning of the patient's midcoronal plane before repeating the image.

Identifying Anatomic Structures

An optimal image should appear as much like the real object as possible, but because of unavoidable distortion that results from the shape, thickness, and position of the object and beam, part, and IR alignment, this is not always feasible, resulting in some anatomic structures appearing different than the real object. Using skeletal bones positioned in the same manner as the projection will greatly aid in identification of the anatomic structures on an image. Closely compare the visualization of the anatomic structures on the skeletal scapular bone photograph and x-ray image shown in Figure 1-40. Note that the superior scapular angle and lateral borders of this surface on the skeletal image are well demonstrated, obscuring the coracoid, but on the x-ray image the superior scapular angle is seen as a thin cortical line, its lateral borders are not demonstrated, and the coracoid can be clearly visualized. Also, note that the superior surface of the spine is visualized on the skeletal bone image between the lateral and medial scapular spine borders, but is not seen on the x-ray image.

When identifying anatomic structures, one must consider how anatomy may appear different from the real object. The following concepts, when understood and applied to how the procedure was obtained, can help with identification of the anatomic structures on the image.

Distinguishing Between Structures of Similar Shape and Size

The most difficult structures to identify are those that are identical in shape and size, such as the femoral condyles or talar domes. For these structures, three methods may be used to distinguish the structures from one another.

Determining the Degree of Patient Obliquity

To align the anatomic structures correctly, it is necessary to demonstrate precise patient positioning and central ray alignment. How accurately the patient is placed in a true AP-PA, lateral, or oblique projection, whether the structure is properly flexed or extended, and how accurately the central ray is directed and centered in relation to the structure determines how properly the anatomy is aligned. Because few technologists carry protractors, there must be a method for determining whether the patient is in a true AP-PA or lateral projection, or specific degree of obliquity. For every projection described, an imaginary line (e.g., for the midsagittal or midcoronal plane, a line connecting the humeral or femoral epicondyles) is given that can be used to align the patient with the IR or imaging table. When the patient is in an AP-PA projection, the reference line should be aligned parallel (0-degree angle) with the IR (Figure 1-50, A) and, when the patient is in a lateral projection, the reference line should be aligned perpendicular (90-degree angle) to the IR (see Figure 1-50, B). For a 45-degree AP-PA oblique projection, place the reference line halfway between the AP-PA projection and the lateral position (see Figure 1-50, C). For a 68-degree AP-PA oblique projection, place the reference line halfway between the 45- and 90-degree angles (see Figure 1-50, D). For a 23-degree AP-PA oblique projection, place the reference line halfway between the 0- and 45-degree angles (see Figure 1-50, E). Even though these five angles are not the only angles used when a patient is positioned for images, they are easy to locate and can be used to estimate almost any other angle. For example, if a 60-degree AP-PA oblique projection is required, rotate the patient until the reference line is positioned at an angle slightly less than the 68-degree mark. I have used the torso to demonstrate this obliquity principle, but it can also be used for extremities. When an AP-PA oblique projection is required, always use the reference line to determine the amount of obliquity. Do not assume that a sponge will give you the correct angle. A 45-degree sponge may actually turn the patient more than 45 degrees if it is placed too far under the patient or if the patient's posterior or anterior soft tissue is thick.

Determining the Degree of Extremity Flexion

For many examinations, a precise degree of structure flexion or extension is required to adequately demonstrate the desired information. Technologists need to estimate the degree to which an extremity is flexed or extended when positioning the patient and when evaluating images. When an extremity is in full extension, the degree of flexion is 0 (Figure 1-51, A), and when the two adjoining bones are aligned perpendicular to each other, the degree of flexion is 90 degrees (see Figure 1-51, B). As described in the preceding discussion, the angle found halfway between full extension and 90 degrees is 45 degrees (see Figure 1-51, C). The angle found halfway between the 45- and 90-degree angles is 68 degrees (see Figure 1-51, D), and the angle found halfway between full extension and a 45-degree angle is 23 degrees (see Figure 1-51, E). Because most flexible extremities flex beyond 90 degrees, the 113- and 135-degree angles (see Figure 1-51, F) should also be known.

Demonstrating Joint Spaces and Fracture Lines

For an open joint space or fracture line to be demonstrated, the central ray or diverged rays recording the area must be aligned parallel with the joint or fracture line of interest (Figures 1-52 and 1-53). Failure to accomplish this alignment will result in closed joint or poor fracture visualization because the surrounding structures are projected into the space or over the fracture line (Figures 1-54 and 1-55).

Focal Spot Size

A detail that is smaller than the focal spot used to produce the image will not be visible. This is why a small focal spot is recommended when fine detail demonstration is important, such as images of the extremities. Compare the trabecular patterns on the ankle images in Figures 1-57 and 1-58. Figure 1-57 was taken using a large focal spot and Figure 1-58 was taken using a small focal spot. Note how the use of a small focal spot increases the visibility of the bony trabeculae.

Using a small focal spot may not be feasible when imaging structures that require a high milliampere-seconds (mAs) setting because milliamperage (mA) settings of 300 mA or below can only be used when the small focal spot is chosen and the need for a long exposure time to obtain the density required may result in patient motion. Weigh the expected exposure time and the possibility of motion against the advantages gained by choosing a small focal spot. If the patient's thickness measurement is large or the patient's ability to hold still is not reliable, a large focal spot and high milliamperage is the better choice.

Distances

By using the shortest possible OID and the longest feasible SID, size distortion can be kept to a minimum. This is especially important, because size distortion reduces an object's recognizability and blurs the recorded details.

In nonroutine clinical situations, the technologist may be unable to get the part as close to the IR as possible. For example, if a patient lying flat on the imaging table were unable to straighten his or her knee and had to keep it bent, the knee would be up off the table and have an increased OID. The technologist can compensate for this by increasing the SID above the standard used. It is often not feasible to increase the SID the needed amount to offset the magnification completely because the ratio between the OID and SID must remain the same for equal magnification to result. For example, an image taken at a 1-inch OID and 40-inch SID would demonstrate the same magnification as one taken at a 4-inch OID and 160-inch SID because both have a 1:40 ratio. When the SID is increased to offset magnification, it is also necessary to increase the mAs using the square law to maintain density.

Image Receptor Systems

Image receptors are chosen for their ability to demonstrate fine recorded details or for their speed. For maximum recorded detail sharpness, choose a single-emulsion film with a low-speed single-screen IR. This single-emulsion system (referred to as a slow detail system) eliminates the unsharpness caused by the parallax and crossover effects seen when a thick or large crystal screen emulsion is used to increase speed. Figure 1-59 demonstrates a hand image exposed using a slow detail screen and a fast screen system. Compare the trabecular patterns on these hand images. As a general rule, the higher the relative speed of the intensifying screen used, the lower will be the sharpness of detail demonstrated. Use a low-speed system for most extremity examinations. This system is for tabletop work only and should not be used when a grid is used or when a thick body part is being imaged. A medium-speed system should be used when the detail capability provides little benefit. For example, hands and feet can have hairline fractures that may be visualized only on a detail screen; because a fracture of the femoral bone is very obvious, however, we do not need to see small details to identify it. Use a high-speed system for examinations such as those performed for scoliosis, in which the bony cortical outlines are all that need to be demonstrated for measurements to be taken. Such a system should not be used when information about the structure itself is desired.

Motion

The term motion unsharpness refers to lack of detail sharpness in an image that is most often caused by patient movement during the exposure. This movement can be voluntary or involuntary. Voluntary motion refers to the patient's breathing or otherwise moving during the exposure. It can be controlled by explaining to the patient the importance of holding still, making the patient as comfortable as possible on the imaging table, using the shortest possible exposure time, and using positioning devices. Voluntary motion can be identified on an image by blurred bony cortical outlines (Figure 1-61). Involuntary motion is movement that the patient cannot control. Its effects will appear the same as those of voluntary motion in most situations, with the exception of within the abdomen. In the abdomen, peristaltic activity of the stomach and small or large intestine can be identified on an image by sharp bony cortices and blurry gastric and intestinal gases (Figure 1-62). The only means of decreasing the blur caused by involuntary motion is to use the shortest possible exposure time, which in some cases is not good enough. At times, normal voluntary motions such as breathing or shaking can become involuntary motions. For example, an unconscious patient is unable to control breathing and a patient with severe trauma may be unable to control shivering.

Double-Exposed Image

A double-exposed image may also appear blurry and can easily be mistaken for an image affected by patient motion (Figure 1-63). When evaluating a blurry image, look at the cortical outlines of bony structures that are lying longitudinally and transversely. Is there only one cortical outline representing each bony structure, or are there two? Is one outline lying slightly above or to the side of the other? If one outline is demonstrated, the patient moved during the exposure, but if two are demonstrated, the image was exposed twice, and the patient was in a slightly different position for the second exposure.

Effective Communication

Taking the time to explain the procedure to the patient and giving clear, concise instructions during the procedure will help the patient understand the importance of holding still and maintaining the proper position, reducing the need for repeat radiographic exposures and additional radiation dose.

Immobilization Devices

If the patient moves during a procedure, the resulting image will be blurred. Such images have little or no diagnostic value and need to be repeated with additional exposure to the patient. Using appropriate immobilization devices can eliminate or minimize patient motion, which is especially important when imaging children, who may have a limited ability to understand and cooperate.

Source-Skin Distance

Mobile radiography units do not have the SID lock that department equipment is required to have to prevent exposures from being taken at an unsafe SID. When operating mobile radiography units, the technologist must maintain a source-skin distance (SSD) of at least 12 inches (30 cm) to prevent an unacceptable entrance skin dose. The entrance skin dose represents the absorbed dose to the most superficial layers of skin. As the distance between the source of radiation and the person increases, radiation exposure decreases. The amount of exposure decrease can be calculated using the inverse square law.

Compensating Filters

Compensating filters are constructed of aluminum or lead-acrylic and are used to create more homogeneous density across objects that vary in thickness. The filter absorbs x-ray photons before they reach the IR. The thicker part of the filter absorbs more photons than the thinner part. When a compensating filter is used, a technique is set that will adequately expose the thickest part being examined. If the filter has been accurately positioned, it will absorb the excessive radiation directed toward the thinnest structures, resulting in uniform image density throughout the image. Compensating filters that are positioned between the focal spot and the patient will reduce radiation dose to structures positioned beneath the filter.

Grids

Grids are used to improve radiographic contrast by eliminating some of the scatter radiation before it exposes the IR and should be used when imaging anatomic parts that are 4 inches (10 cm) or larger and using tube potentials above a 60-kilovoltage peak (kVp). When a grid is added, the mAs must be increased to compensate for the density loss that occurs as scatter is reduced. This increase will cause a direct increase in patient dose. To keep patient exposure as low as possible, grids should be used only when appropriate and the grid ratio should be kept to the lowest possible value that will provide sufficient contrast improvement as based on the kVp range used (see later, Table 1-1).

Screen-Film Image Receptor Speed

Relative screen speed is a major factor in reducing the radiation dose to the patient. By changing to a faster screen that requires less radiation to produce the image, the technologist can significantly reduce the patient's radiation dose. Unfortunately the reduction in dose gained from switching to a higher speed screen must be weighed against the sharpness of detail loss that occurs.

Possible Pregnancy

When imaging a female of childbearing age, it is essential that the technologist question the patient regarding the possibility of pregnancy. In some departments this is required of all females older than 11 years. Teenage girls may not admit to being pregnant until they reach the radiology department. If there is hesitancy rather than denial, additional questioning should occur, with follow-up questions such as, “Are you sexually active? If so, are you taking precautions?” If the patient is to have a procedure that requires significant pelvic exposure and there is some doubt as to her pregnancy status, it is recommended that a pregnancy test be performed.

Avoiding unnecessary radiation exposure or limiting it during the embryonic stage of development is essential because it is in this stage that the embryonic cells are dividing and differentiating and they are extremely radiosensitive and easily damaged by ionizing radiation.

Gonadal Shielding

Proper gonadal shielding practices have been proven to reduce radiation exposure of the female and male gonads. Gonadal shielding is recommended in the following situations:

Professional technologists must always strive to improve skills and develop better ways to ensure good patient care while obtaining optimal images. All images should be evaluated for the accuracy of gonadal shielding.

Gonadal Shielding in the AP Projection for Female Patients

Shielding the gonads of the female patient for an AP projection of the pelvis, hip, or lumbar vertebrae requires more precise positioning of the shield to prevent the obscuring of pertinent information. The first step in understanding how to shield a woman properly is to know which organs should be shielded and their location. These are the ovaries, uterine (fallopian) tubes, and uterus. The uterus is found at the patient's midline, superior to the bladder. It is approximately 3 inches (7.5 cm) in length; its inferior aspect begins at the level of the symphysis pubis and it extends anterosuperiorly. The uterine tubes are bilateral, beginning at the superolateral angles of the uterus and extending to the lateral sides of the pelvis. Tucked between the lateral side of the pelvis and the uterus and inferior to the uterine tubes are the ovaries. The exact level at which the uterus, uterine tubes, and ovaries are found varies from patient to patient. Figures 1-64 and 1-65 show images from two different hysterosalpingograms. Note the variation in the location of the uterus, uterine tubes, and ovaries in these two patients. Because the location of these organs within the inlet pelvis cannot be determined with certainty, the entire inlet pelvis should be shielded to ensure that all the reproductive organs' have been protected.

To shield the female gonads properly, use a flat contact shield made from at least 1 mm of lead and cut to the shape of the inlet pelvis (Figure 1-66). Oddly shaped and male (triangular) shields do not effectively protect the female patient (Figure 1-67). The dimensions of the shield used should be varied according to the amount of magnification that the shield will demonstrate, which is determined by the OID and SID and by the size of the patient's pelvis, which increases from infancy to adulthood. Each department should have different-sized contact or shadow shields for variations in female pelvic sizes for infants, toddlers, adolescents, and young adults.

To position the shield on the patient, place the narrower end of the shield just superior to the symphysis pubis and allow the wider end of the shield to lie superiorly over the reproductive organs. Side-to-side centering can be evaluated by placing an index finger just medial to each anterior superior iliac spine (ASIS). The sides of the shield should be placed at equal distances from the index fingers. When imaging children, do not palpate the pubic symphysis because they are taught that no one should touch their “private parts.” Instead use the greater trochanters to position the shield because they are at the level of the superior border of the pubic symphysis. It may be wise to tape the shield to the patient. Patient motion such as breathing may cause the shield to shift to one side, inferiorly, or superiorly.

Gonadal Shielding in the AP Projection for Male Patients

The reproductive organs that are to be shielded on the male are the testes, which are found within the scrotal pouch. The testes are located along the midsagittal plane inferior to the symphysis pubis. Shielding the testes of a male patient for an AP projection of the pelvis or hip requires more specific placement of the lead shield to avoid obscuring areas of interest. For these examinations, a flat contact shield made from vinyl and 1 mm of lead should be cut out in the shape of a right triangle (one angle should be 90 degrees). Round the 90-degree corner on this triangle. Place the shield on the adult patient with the rounded corner beginning approximately 1 to 1.5 inches (2.5 to 4 cm) inferior to the palpable superior symphysis pubis. When accurately positioned, the shield frames the inferior outlines of the symphysis pubis and inferior ramus and extends inferiorly until the entire scrotum is covered (Figure 1-68). Each department should have different-sized male contact shields for the variations in male pelvic sizes for infants, youths, adolescents, and young adults.

Gonadal Shielding in the Lateral Projection for Male and Female Patients

When male and female patients are imaged in the lateral projection, use gonadal shielding whenever (1) the gonads are within the primary radiation field and (2) shielding will not cover pertinent information. In the lateral projection, male and female patients can be similarly shielded with a large flat contact shield or the straight edge of a lead apron. Begin by palpating the patient's coccyx and elevated ASIS. Next, draw an imaginary line connecting the coccyx with a point 1 inch posterior to the ASIS, and position the longitudinal edge of a large flat contact shield or half-lead apron anteriorly against this imaginary line (Figure 1-69). This shielding method can be safely used on patients being imaged for lateral vertebral, sacral, or coccygeal projections without fear of obscuring areas of interest (Figure 1-70).

Shielding of Radiosensitive Cells Not Within Primary Beam

Shielding of radiosensitive cells should be done whenever they lie within 2 inches (5 cm) of the primary beam. Radiosensitive cells are the eyes, thyroid, breasts, and gonads. To protect these areas, place a flat contact shield constructed of vinyl and 1 mm of lead or the straight edge of a lead apron over the area to be protected. Because the atomic number of lead is so high, radiation used in the diagnostic range will be readily absorbed in the shield.

Collimation

Tight collimation reduces the radiation exposure of anatomic structures that are not required on the image. For example, its use on chest images will reduce exposure of the patient's thyroid; on a cervical vertebral image, it will reduce exposure of the eyes; on a thoracic vertebrae image, it will reduce exposure of the breasts; and, on a hip image, it will reduce exposure of the gonads.

Exposure Factors to Minimize Patient Exposure

Selection of appropriate technical exposure factors for a procedure should focus on producing an image of diagnostic quality with minimal patient dose. This is accomplished by selecting the highest practical kilovoltage and the lowest mAs that will produce an image with sufficient information. Also, when the patient has difficulty holding still or halting respiration, the shortest possible exposure time should be used by selecting a high-milliampere station.

Automatic Exposure Control Backup Timer

The backup timer is a safety device that prevents overexposure to the patient when the automatic exposure control (AEC) is not properly functioning or the control panel is not set correctly. When using the AEC, set the AEC backup time at 150% to 200% of the expected manual exposure time. Once the backup time is reached, the exposure will automatically terminate.

Anatomic Artifacts

These are anatomic structures of the patient or x-ray personnel that are demonstrated on the image but should not be there (Figure 1-71). Note in the figure how the patient's other hand was used to help maintain the position. This is not an acceptable practice. Many sponges and other positioning tools are available to aid in positioning and immobilizing the patient. Whenever the hands of the patient, x-ray personnel, or others must be within the radiation field, they must be properly attired with lead gloves.

Personnel and Family Members in Room During Exposure

Appropriate immobilization devices should be used and all personnel and family members should leave the room before the x-ray exposure is made. If the patient cannot be effectively immobilized or left alone in the room during the exposure, lead protection attire such as aprons, thyroid shields, glasses, and gloves should be worn by the personnel during any x-ray exposure. Anyone remaining in the room should also stand out of the path of the radiation source and as far from it as possible.

• Did you read the technique chart correctly and set the mAs and kVp as stated on the technique chart?

• Was the patient measured correctly?

• Was the patient properly positioned in respect to the activated ionization chamber(s)?

• Was the SID set correctly?

• Was the OID at a minimum?

• Was a grid used, if required, and was it positioned correctly with the central ray?

• Did you use the correct receptor system, and was the correct IR placed in that system?

• Did you use maximum collimation?

• Was a compensating filter used when applicable?

• Did you position the part to use the anode heel effect to your advantage?

Milliampere-Seconds

Density is directly related to the quantity of radiation that reaches the IR and mAs is the controlling factor for quantity. With adequate penetration, an increase in mAs will cause an increase in radiographic density and a decrease in mAs will cause a decrease in radiographic density. If an image is exposed using excessive mAs, it demonstrates density that is so dark that some or all of the bony and soft tissue structures of interest are not well visualized. A dark image produced using excessive mAs can be distinguished from one that was produced using very high kVp by evaluating the contrast on the image.

An overexposed image obtained using excessive mAs will demonstrate acceptable contrast as long as the kVp is optimal. Even though the overall image will be dark, the cortical outlines of the bone should remain high in contrast. Figure 1-72 demonstrates an accurately exposed and overexposed AP pelvic projection.

An underexposed image obtained using insufficient mAs will demonstrate density that is so light that some or all of the anatomic structures cannot be evaluated. Evaluating the visualization of the bony trabecular patterns is valuable when determining acceptability of such an image because usually on light images, unless underexposure is extreme, the demonstration of the soft tissue structures is better. Because underexposed and underpenetrated images demonstrate low density, it is necessary to learn to identify each technical error. One way to distinguish whether an image has been underexposed rather than underpenetrated is to study the bony cortical outlines of the structures of interest. On an image that has been underexposed, the cortical outlines are visible, even though their density is light, whereas an underpenetrated image will not demonstrate areas of the cortical outlines that were not penetrated. Figure 1-73 shows accurately exposed and underexposed AP pelvic projections.

When image density is inadequate, mAs is the controlling factor of choice. How much adjustment in mAs should be made to obtain an optimal image is directly proportional to the amount of increase or decrease in density that is needed. Typically, three adjustments are made when making density changes:

Kilovoltage

The penetrating ability of the primary x-ray beam is controlled by the kilovoltage peak that is used. Radiographic density is affected by a change in kVp because it alters the quality of photons, which changes the ratio of penetrated to absorbed photons. The higher the kVp, the greater is the number of photons that penetrate the patient and reach the IR, resulting in greater image density; the lower the kVp, the greater is the number of photons absorbed in the patient, decreasing the numbers reaching the IR and image density. Because no amount of mAs will ever compensate for insufficient kVp, an optimum kVp level based on the tissue's composition and thickness is required for each body part. Optimum kVp is defined as the kVp that will provide adequate body part penetration and sufficient gray scale.

An image that has been adequately penetrated demonstrates the cortical outlines of the thinnest and thickest bony structures of interest. If these structures have not been penetrated (not enough kVp used), the image demonstrates too little density but, in comparison with an underexposed image, the cortical outlines of the thickest parts of the structure are not visible. Compare the bottom images in Figures 1-76 and 1-73. Note that if a transparency were laid over the images and an outline of the bony structures drawn on the transparency, with lines made only where the cortical outlines of the bone were clearly visible, many of the hip structures around the acetabulum would not be drawn. If the cortical outlines of the structure of interest can be seen even though the image density is light, it can be concluded that the mAs needs adjusting; if the cortical outlines of the structure cannot be seen and the image density is light, a kVp adjustment is required.

An underpenetrated image may also demonstrate inadequate density. The amount of density adjustment needed must also be considered when deciding how much kVp adjustment is to be made:

Patient Condition

Additive and destructive patient conditions that result in change to the normal bony structures, soft tissues, or air or fluid content of the patient may require technical changes as compensation before exposing the patient. Additive diseases cause tissues to increase in mass density or thickness, resulting in them being more radiopaque, whereas destructive diseases cause tissues to break down, resulting in them being more radiolucent. Table 1-3 lists common additive and destructive diseases that may require technique adjustments and provides a starting point for adjusting technical factors for the condition.

Automatic Exposure Control

The AEC allows the mAs to be automatically determined by controlling the exposure time, but it is the technologist's responsibility to set an optimum kVp and optimal mA manually. Optimum mA refers to using a high enough mA at a given focal spot size to minimize motion, but not so high that the exposure times are shorter than the AEC's minimum response time. The minimum response time is the time that it takes for the circuit to detect and react to the radiation received; this is determined by the AEC manufacturer.

Guidelines for Using Automatic Exposure Control and Evaluating Images Produced

See earlier discussions of kVp, penetration, and contrast scale to determine the appropriate adjustment in kVp to be made when penetration and contrast scale are inadequate. If less than optimum kVp is used for the anatomy of interest, this anatomy will be underpenetrated and underexposed, although the radiographic density of any anatomic structures that were penetrated may be adequate as long as the backup time is long enough to accommodate the increased exposure time needed to compensate. An underexposed light image will result if the kVp is so low that inadequate penetration occurred in all anatomic structures beneath the activated chamber(s). No amount of mAs will compensate for inadequate penetration.

Exposures taken with an exposure time that is less than the minimum response time will result in overexposed, dark images. This is because the AEC circuit does not have enough time to detect and react to the radiation received to shut the exposure off in the time needed to produce the ideal image. The mA station should be decreased until exposure times are sufficient to produce the desired density.

The backup timer is the maximum time that the x-ray exposure will be allowed to continue. Once the backup time is met the exposure will automatically terminate. If the set backup time is too short the exposure will prematurely stop before adequate exposure has reached the ionization chamber(s), resulting in an underexposed, light image. If the set backup time is too long and the AEC is not functioning properly or the control panel is not correctly set, the exposure will continue much longer than needed to produce adequate density, resulting in an overexposed, dark image and excessive radiation dose to the patient.

Recommendations for ionization chamber selection can be found at the beginning of each procedural analysis chapter of the book. Failure to properly activate the correct ionization chamber(s) and center the anatomic structures of greatest interest beneath them will result in images that are over- or underexposed. An overexposed image results when the ionization chamber chosen is located beneath a structure that has a higher atomic number or is thicker or denser than the structure of interest. For example, when an AP abdomen projection is taken, the outside ionization chambers should be chosen and situated within the soft tissue, away from the lumbar vertebrae, to yield the desired abdominal soft tissue density. If the chamber situated under the lumbar vertebrae is used instead, the capacitor (device that stores energy) requires a longer exposure time to reach its maximum filling level and terminate the exposure. This occurs because of the high atomic number of bone and the higher number of photons that bone absorbs compared with soft tissue. The result will be an image with adequate bone density but overexposed soft tissue (Figure 1-78).

An underexposed image results, however, when the ionization chamber chosen is located beneath a structure that has a lower atomic number or is thinner or less dense than the structure of interest. When an AP lumbar vertebral projection is taken, the center ionization chamber is chosen and centered directly beneath the lumbar vertebrae. If one or both of the outside chambers are used or the center ionization chamber is off center, the image is underexposed, because soft tissue, which has a lower atomic number than bone, is above the activated chamber (Figure 1-79).

Each activated ionization chamber measures the average amount of radiation striking the area it covers. The part of the chamber not covered with tissue will collect radiation so quickly that it will charge the capacitor to its maximum level, terminating the exposure before proper density has been reached, resulting in an underexposed light image (Figure 1-80).

For example, an AP thoracic vertebrae projection that has inadequate side-to-side collimation will demonstrate too much scatter through the lungs, hitting the AEC before the vertebrae can be adequately exposed.

For example, it is best not to use the AEC on an AP atlas and axis (open-mouthed) projection of the dens. With this examination, the upper incisors, occipital cranial base, and mandible add thickness to the areas superior and inferior to the dens and atlantoaxial joint. This added thickness causes the area of interest to be overexposed, because more time is needed for the capacitor to reach its maximum level as photons are absorbed in the thicker areas (Figure 1-81).

Radiopaque materials, such as metal, lead sheets, or sandbags, have a much higher atomic number than that of the bony and soft tissue structures of the body. When a radiopaque material is situated within the activated chamber(s), the AEC will attempt to expose the radiopaque structure adequately, resulting in the anatomic structures being overexposed and a dark image (Figure 1-82).

If a higher speed screen-film IR system is used with an AEC that is calibrated for a lower screen-film system, an overexposed dark image will result; if a lower speed screen-film IR system is used with an AEC that is calibrated for a higher screen-film system, an underexposed light image will result. The AEC will not adjust for changes in receptor system speeds without the thyristor being recalibrated for the system.

The density controls change the preset thyratron sensitivity so that the exposure time will be increased or decreased, adjusting radiographic density by the density control setting amount. Typical density control settings change the exposure level by increments of 25%, with the +1 and +2 buttons increasing the exposure and the −1 and −2 buttons decreasing the exposure. The 1 buttons will result in a 25% density change and the 2 buttons in a 50% density change. Some facilities have the AEC density controls set to obtain a 100% density increase and a 50% density decrease.

Correcting Poor Automatic Exposure Control Images

Tables 1-1 and 1-2 list reasons why an AEC image may have inadequate density. When an unacceptable AEC image is produced, the technologist needs to consider each potential cause to determine the correct adjustment to make before repeating the image. Many imaging units include an mAs readout display, on which the amount of mAs used for the image is shown after the exposure. In situations in which it is not advisable to repeat an unacceptable image using the AEC, the technologist can revert to a manual technique by using this readout to adjust the mAs to the value needed.

Source–Image Receptor Distance

Increasing the SID will decrease radiographic density. Decreasing the SID will increase density by the inverse square law, because the area through which the x-rays are distributed is spread out or condensed, respectively, with distance changes. It takes only a 20% change in SID for a visible density change to occur. When the technologist uses a manual technique and varies the SID from the standard, the mAs should be adjusted by using the direct square law ([new mAs]/[old mAs] = [new distance squared]/[old distance squared]), preventing an over- or underexposed image. Failure to make this adjustment may require the image to be repeated. If this occurs, leave the SID at the same setting and adjust the mAs the needed amount (see Tables 1-1 and 1-2).

Object–Image Receptor Distance

Although it is standard to maintain the lowest possible OID, there are situations in which increasing the OID is unavoidable. Increasing the OID may result in a noticeable density loss because of the reduction in the amount of scatter radiation detected by the IR when a portion of the scattered x-rays generated in the patient are scattered away from the IR. The amount of density loss will depend on the degree of OID increase and the amount of scatter that would typically reach the IR for such a procedure, which is determined by the kVp selected, field size, and patient thickness. As tube potentials are raised above 60 kVp, scatter radiation is directed in a more increasingly forward direction, so the image will demonstrate significant density loss as the OID is increased and the scatter is diverged away from the IR. With tube potentials below 60 kVp, there is a decrease in the number of scatter photons that are scattered in a forward direction, so an increase in OID will not result in a significant enough change in the amount of scatter reaching the IR or image density to be noticeable. A larger field size and body part thickness will affect the amount of scatter produced, with more production resulting in increased reduction of scatter reaching the IR as the OID increases. When the OID is increased, causing significant scatter radiation to be diverted from the IR, the mAs should be increased by about 10% for every centimeter of OID to compensate for the loss in density that such a change will cause.

Grids

When a grid is added or the technologist changes from one grid ratio to another, radiographic density will be inadequate unless the mAs is adjusted (see Table 1-2) to compensate for the resulting change in scatter radiation cleanup. When changing to a higher grid ratio, an increase in mAs is needed or insufficient radiographic density will result; when changing to a lower grid ratio, a decrease in mAs is needed or excessive radiographic density will be demonstrated.

Inadequate radiographic density will also be seen when the grid and central ray are misaligned, causing a decrease in the number of transmitted photons that reach the IR. The density decrease caused by grid cutoff can be distinguished from other density problems by the appearance of grid lines (small white lines) on the image where the cutoff is demonstrated. (See later, “Equipment-Related Artifacts,” for examples of grid cutoff.)

Air-Gap Technique

An alternative to using a grid is to use the air-gap technique. To use the air-gap technique, move the IR 10 to 15 cm from the patient. The long OID will cause scatter radiation that would typically expose the IR at a short OID to scatter away from the IR. Because much of the scatter radiation is not being directed toward the IR, density is decreased and contrast is enhanced. When the air-gap technique is used, increase the mAs by about 10% for every centimeter of air gap. It is usually equivalent to using an 8:1 grid.

Screen-Film Receptor System Speed

Intensifying screens convert x-ray energy into light energy through fluorescence. The more light that is emitted for a given x-ray exposure, the greater is the relative speed value of the intensifying screen. Rare earth receptor systems are the most commonly used today and are defined with the descriptive names of extremity, medium speed, regular speed, and high speed, with relative speeds of 80, 300, 400 and 800, respectively. When changing from one intensifying screen speed to another, a new mAs must be calculated because each relative speed value produces a different density or the image will be too dark or too light. Each screen has a conversion factor that is calculated by forming a fraction with 100 in the numerator and the relative speed value of the screen in the denominator. Then divide the numerator by the denominator; the decimal produced is the conversion factor (e.g., a rare earth screen at a relative speed value of 400 would have a conversion factor of 100/400 = 0.25). Tables 1-1 and 1-2 list the rare earth receptors conversion factors and the formula for calculating the new mAs when changing from one screen speed to another.

Collimation

An increase or decrease in the area exposed on the patient, as determined by collimation, changes the amount of scatter radiation produced and hence the amount of scatter reaching the IR and the overall radiographic density. The amount of density change will depend on the field size and the amount of scatter that would typically reach the IR for such a procedure, which is determined by the kVp selected and patient thickness (see earlier). The mAs needs to be changed to compensate for density changes when the collimation field size is changed and the part produces significant scatter radiation. If the field size is changed from a size that will cover a 14- × 17-inch IR to a size that will cover a 10- × 12-inch IR, the mAs should be increased by about 35%. If the change is to an 8- × 10-inch IR, the mAs should be increased by about 50%. Failure to make a mAs adjustment may result in an image with inadequate density.

Compensating Filter

With some examinations, when an exposure is set that will adequately demonstrate one structure of interest, other structures of interest are overexposed. This occurs because of the differences in thickness among the structures being imaged. AP projections of the shoulder, feet, and thoracic vertebrae are three examples that demonstrate this problem. To offset this thickness difference and obtain homogeneous density, place a compensating filter over or under the thinnest structures (Figure 1-83). A compensating filter absorbs x-ray photons before they reach the IR. The thicker part of the filter absorbs more photons than the thinner part. Set a technique that will adequately expose the thickest part being examined. If the filter has been accurately positioned, it will absorb the excessive radiation directed toward the thinnest structures, resulting in uniform image density throughout the image. Figures 1-84 and 1-85 show AP foot projections. The image in Figure 1-84 was taken without using a compensating filter, whereas the image in Figure 1-85 was taken with a compensating filter positioned over the distal metatarsals and phalangeal regions. Note the increased visibility of anatomic structures covered by the compensating filter.

If the compensating filter is inaccurately positioned, there will be a density variation defining where the filter was and was not placed over the structures (Figure 1-86). Having too much of the filter positioned over or under a structure that does not need it will result in too many of the photons being absorbed and too little radiographic density in the area.

Anode Heel Effect

Another exposure factor that should be considered when positioning long bones and the vertebral column, when a long 17-inch (43-cm) field length is used to accommodate the structure, is the anode heel effect. When this field length is used, a noticeable density variation occurs across the entire field size that is significant enough between the ends of the field that when they are compared, it can be seen. This density variation is a result of the greater photon absorption that occurs at the thicker “heel” portion of the anode compared with the thinner “toe” portion when a long field is used. Consequently, radiographic density at the anode end of the tube is lower because fewer photons emerge from that end of the tube than at the cathode end.

Using this knowledge to our advantage can help produce images of long bones and vertebral column that demonstrate uniform density at both ends. Position the thinner side of the structure at the anode end of the tube and the thicker side of the structure at the cathode end. Set an exposure (mAs) that will adequately demonstrate the midpoint of the structure (where the central ray is centered). Because the anode will absorb some of the photons aimed at the anode end of the IR and the thinnest structure, but not as many of the photons aimed at the cathode end and the thickest structure, a more uniform density across that part will be demonstrated (Figure 1-87). Table 1-4 provides guidelines for positioning structures to take advantage of the anode heel effect. Because the density variation between the ends of the IR is only approximately 30%, the anode heel effect will not adequately adjust for large thickness differences but will help improve images of the structures listed in Table 1-4.

Most imaging rooms are designed so that the patient's head is positioned on the technologist's left side (when facing the imaging table), placing the anode end of the x-ray tube at the head end of the patient. The placement of the anode end of the tube may vary in reference to the patient as the tube is moved into the horizontal position. To identify the anode and cathode ends of the x-ray tube, locate the + and − symbols attached to the tube housing where the electrical supply enters. The + symbol is used to identify the anode end of the tube and the − symbol indicates the cathode end (Figure 1-88).

Controlling Factor (Kilovoltage)

The kVp is the factor that determines the energy of the x-ray photons produced and consequently the differential absorption that occurs in the patient's tissue as the photons traverse the body. High kVp produces high-energy photons that penetrate through body tissues, decreasing differential absorption and producing low-contrast (long-scale) images (Figure 1-94). Low kVp produces low-energy photons that are easily absorbed in the patient's tissues, increasing differential absorption and producing high-contrast (short-scale) images (Figure 1-95).

To obtain the desired level of radiographic contrast and gray scale, the technologist must evaluate the composition of the body part to be imaged and select a kVp level that will produce the appropriate differential absorption. Recommendations for the optimal kVp level to use for each body part can be found at the beginning of each procedural analysis chapter in this book. Optimal kVp is the kVp that will provide adequate body part penetration and sufficient image contrast.

Adjusting Contrast With kVp Using the 15% Rule

Even when optimum kVp levels are used, there are different situations and patient conditions that occur in which this kVp level does not provide the best penetration or contrast. In these cases the kVp must be adjusted from the routinely used optimal level (see Table 1-3). In situations in which the density on the image is adequate but the contrast level is not sufficient to visualize all the anatomic structures adequately, the contrast level can be adjusted by varying the kVp and the density can be maintained by counteracting the kVp adjustment with a comparable mAs adjustment.

In both these situations the image density has remained the same as in the original image, because the density adjustment made with kVp is offset by an equal mAs adjustment. The amount of kVp adjustment depends on how dramatic a contrast change is desired. As a general rule, you should stay relatively close to the optimal kVp level for the structure being imaged to prevent insufficient penetration or excessive image fogging.

Scatter Radiation

Radiographic contrast is also affected by the amount of scatter radiation that reaches the IR, which is determined by the kVp, field size, and patient thickness. Scatter radiation degrades the radiographic contrast by putting a blanket of density, also referred to as fog, over the image. The once individual and distinct gray shades of the image become blended with each other when fog is added to an image, resulting in a very gray low-contrast image. As the amount of scatter radiation reaching the IR increases, the greater is the decrease in radiographic contrast and decrease in detail visibility. Technologists can control the amount of scatter that reaches the IR and improve radiographic contrast by reducing the amount of tissue irradiated, decreasing the field size, and using a grid. The higher the grid ratio, the greater will be the scatter cleanup and the higher the radiographic contrast.

Flat contact shields made of lead can also be used to control the amount of scatter radiation that reaches the IR by eliminating scatter produced in the table from being scattered toward the area of interest. When the anatomic structures being examined demonstrate an excessive amount of scatter fogging along the outside of the collimated borders (e.g., the lateral lumbar vertebrae), place a large, flat contact shield or the straight edge of a lead apron along the appropriate border. This greatly improves the visibility of the recorded details. Compare the lateral lumbar vertebral projections in Figure 1-96 and Figure 1-97. Figure 1-96 was taken with a lead contact shield placed against the posterior edge of the collimator's light field, but a contact shield was not used for Figure 1-97. Note the improvement in visualization of the lumbar spinous processes using a contact shield.

Anatomic Artifacts

An anatomic artifact is any anatomic structure that is within the image that could have been removed. These include those that are superimposed over an area of interest, as well as those that are not superimposed over an area of interest but are still located within the collimated field and could easily have been excluded. A common anatomic artifact is the patient's own hand or arm. Figure 1-98 shows an AP abdomen projection obtained in the supine position in which the patient's hands are superimposed over the upper abdominal region. More than likely the patient was not positioned in this manner when the technologist left the room. After the technologist positioned the patient and before the exposure was taken, however, the patient found a more comfortable position. This example stresses the importance of explaining examinations to patients so that they understand how important it is for them to remain in the position in which the technologist placed them. This also shows the importance of rechecking each patient's position before the exposure is taken if much time has elapsed between positioning the patient and exposing the image. It is also not uncommon for a patient who is experiencing hip or lower back pain from lying on the imaging table to place a hand beneath an affected hip. This will result in superimposition of the hip and hand on the image. Remember, the patient does not know that repositioning because of discomfort is not acceptable. Figure 1-99 shows an AP chest projection produced with a mobile x-ray machine, which was taken with the patient's arms positioned tightly against the sides. Because humeri are not evaluated on a chest image, there is no reason for them to be included, and they could easily have been shifted out of the imaging field.

Double-Exposure Artifacts

A double-exposed image occurs when two exposures are taken on the same IR without processing having been done between them. The images exposed on the IR can be totally different and easy to identify, such as AP and lateral lumbar vertebrae projections (Figure 1-100), or they may be the same image, with almost identical overlap. Double-exposures of images of the same procedure typically appear blurry and can easily be mistaken for patient motion (Figure 1-101). When evaluating a blurry image, look at the cortical outlines of bony structures that are lying longitudinally and transversely:

If one outline is demonstrated, the patient moved during the exposure, but if two are demonstrated, the image was exposed twice and the patient was in a slightly different position for the second exposure. Another indication of a double-exposed image in conventional radiography is its density (Figure 1-102). When the screen-film system is used, a double-exposed image will result in high image density because the film was exposed to radiation twice.

External Artifacts

An external artifact is found outside the patient's body, such as a patient's possession that remained in a pocket or a hospital possession (e.g., needle cap, ice bag) that was lying on top of or beneath the patient. Common external artifacts include earrings, rings, necklaces, bra hooks, dental structures, hairpins, heart monitoring lines, and gown snaps (Figure 1-103). Two external artifacts that are not as common but that do occasionally appear are caused by pillows (Figure 1-104) and by the imprinted designs on shirts and pants (Figure 1-105). Most of these artifacts can easily be avoided with proper patient preparation and positioning. Being aware of as many objects as possible that can create artifacts on the image is the best way of preventing them.

Internal Artifacts

Internal artifacts are found within the patient. They cannot be removed and must be accepted. Examples of commonly seen internal artifacts are the prosthesis (Figures 1-105 and 1-106), pacemaker (Figure 3-16), central venous catheter (Figures 3-12 and 3-13), pleural drainage tube (Figures 1-107 and 3-11), and endotracheal tube (Figure 3-9).

If an artifact that is normally not found within the body is identified on an image, it is the technologist's duty to discretely search and interview the patient or to consult the ordering physician to determine whether the artifact can be located outside the patient's body. If it is not found, it may have been introduced into the patient through one of the body orifices. Your search and interview discoveries should be recorded on the patient's requisition. Figure 1-108 shows a pelvic image of a patient who had swallowed several batteries.

Equipment-Related Artifacts

Equipment-related artifacts are caused by improper use of the imaging equipment.

Grid Alignment Artifacts

Grid alignment artifacts are grid lines that result from the use of stationary grids and the improper use of all grid types. They occur because the grid's lead strips absorb primary radiation and are visible as small white lines on the image. Grid line artifacts caused by improper grid alignment are more noticeable with higher ratio grids and can be avoided by choosing a moving grid whenever possible and by properly aligning the central ray and grid.

Causes of grid cut-off include the following:

When conventional screen-film radiography is used, an image taken with a grid alignment artifact demonstrates a loss in density where the grid lines are shown. The degree of density loss will be greater on the side toward which the central ray is angled and will increase with increased severity of misalignment. Density loss will also be greater when higher grid ratios are used.

Film Handling and Processing Artifacts

Improper film handling and processing can cause artifacts such as film creases, static (Figure 1-115), fog, stains, scratches (Figure 1-116), and hesitation marks (Figure 1-117). They can be avoided by following a good quality control program for the darkroom, film storage, and processor.

Locating and Repeating for Artifacts

Whenever an external or unidentifiable internal artifact is demonstrated on an image, discretely examine and interview the patient to determine the artifact's location. To pinpoint where to look for the artifact, study the image to locate a palpable anatomic structure situated close to the artifact. This is the area where the search should begin.

If an artifact that can be eliminated obscures any portion of the area of interest, the image needs to be repeated. A gown snap superimposed on an area of the lungs on a chest image can easily obscure a small lesion. A ring can easily obscure a hairline finger fracture.

If the artifact is located outside the field of interest, the image does not need to be repeated, although the patient should be discretely examined and interviewed to determine whether the artifact is located externally or internally.

Completing the Requisition Form

These are the sections of the requisition form that should be completed by the technologist: (1) the number and sizes of films that were used during conventional screen-film radiography; (2) the number of images obtained during digital imaging; (3) the mAs, kVp, and distance used; (4) the room number where the images were taken; (5) the technologist's name; (6) the date and time of the procedure; and (7) any additional patient history obtained from the patient-technologist interview (Figure 1-118).

Recording the number and sizes of films or images obtained on the requisition form tells the reviewer how many images are to be evaluated. The name(s) of the technologist(s) involved in the examination is valuable information if a question arises about the image or patient or if the patient is found to have a contagious disease and measures need to be taken to protect the technologist(s). Recording the same date and time of the procedure on the requisition form as are shown on the image, as well as double-checking that the name of the patient is correct, provides a means of verifying that the requisition form goes with a certain set of images.

The patient's history should be completed by the ordering physician before the requisition form arrives in the imaging facility; any information that the technologist has learned from interviewing and observing the patient that might assist the reviewer in the diagnosis should be added, however. This area of the form may also be used to note any situation necessitating departure from the routine examination procedure. For example, if a hand image was taken with the patient's ring still on the finger because the ring could not be removed, the technologist should record this fact in the patient history or notes section of the requisition form so that the reviewer understands why the ring appears on the image.

Repeat-Reject Analysis

The facility's repeat or reject analysis card, an example of which is shown in Figure 1-119, should be filled out to indicate any positioning or technical errors that occurred during the procedure. This information provides your facility with a means of distinguishing areas in which patient service can be improved through in-service personnel training or equipment repair.

Defining Image Acceptability

When an image meets all the necessary requirements, it is considered an optimal image and it should not be repeated. When an image is not optimal but may be acceptable, the question arises as to whether it is poor enough to repeat or whether the information needed can be obtained without exposing the patient to further radiation. Factors that should be considered when making this decision include the following:

Each facility has its own standards that will determine whether an image should be repeated. If standards are low, improving imaging skills can raise them, thereby increasing the accuracy of diagnosis. The age and condition of the patient, as well as the conditions under which the patient was imaged, are most important in the decision to repeat an image. Sometimes a less than optimal image must be accepted because repeating the image is impossible, as in a surgery case; at other times, the patient cannot or will not cooperate. Whenever an examination is accepted that does not meet optimal standards, record on the requisition form any information about the patient's condition or situation that resulted in acceptance of this examination. A less than optimal image may also be passed when the indication for the examination is clearly fulfilled by the images obtained. For example, a lower leg examination is taken without the required knee joint when the patient history states that the patient has had a distal fibular fracture and the indication for the examination is to evaluate the healing of the distal fibular fracture. In this case, it is obvious that the patient's knee is not being evaluated. As long as the distal fibula is included in its entirety on the original image, the image should not be repeated.

It is important that all unacceptable images and those less than optimal images that have been accepted are studied carefully to determine whether the situation(s) that caused them could be eliminated on future examinations. When an image is repeated, the overall radiation dose to the patient increases and the cost of patient care rises because reimaging requires more technologist time, supplies, and equipment use.

1. Based on the requisition or request form, determine the projections that will be needed and the order in which they will be completed. First, obtain the projections that will provide information about the most life-threatening condition (cross-table lateral cervical vertebrae projection if a cervical fracture is questioned or when obtaining an AP chest projection for a patient having difficulty breathing). Speed is of the essence in many mobile and trauma situations, because the patient can be quite ill. Having a thought-out plan of action before starting allows the technologist to work in an organized and speedy manner. As a general rule, after the initial projections associated with life-threatening conditions have been exposed and checked by the radiologist or physician, the remaining projections are exposed in an order that will require the least amount of central ray adjustment. All AP projections are exposed, and then the central ray is moved horizontally for the lateral projections.

    It is important that projections be obtained that are at 90-degree angles from each other (AP-PA and lateral) when fractures and foreign bodies are suspected to determine the degree of bone displacement (Figure 1-120) and depth location.

2. Gather and organize the supplies (e.g., IRs, IR holders, positioning aides, disposable gloves, radiation protection supplies) that will be needed, and determine the starting technical factors (kVp, mAs, AEC) for the needed images. Cover the positioning aids and IRs to protect them from contamination.

3. Determine the degree of patient mobility, alertness, and ability to follow requests. Can the patient be placed in a seated position or be rotated to one side? Can the arm or leg fully extend or flex? When the patient is asked to breathe deeply, can he or she follow the request? Can the patient control movement?

4. Assess the site of interest for physical signs of injury (swelling, bruising, deformity, pain). Understanding the degree of injury will help the technologist prevent further injury during the positioning process.

5. Determine whether positioning devices (e.g., slings, backboards, and casts) and artifacts (e.g., heart leads, clothing, jewelry) may be removed and, if not, whether they will obscure the area of interest on the ordered projections. If positioning devices or artifacts will obscure the area, consult with the radiologist or ordering physician about possible alternatives (e.g., taking a slight oblique instead of a true projection).

6. Set an optimal kVp and mAs or AEC for the anatomic structure and projection being imaged. Technical adjustments may also be needed as a result of the increase in absorption that may occur because of the positioning device or artifact. Either increase (+) or decrease (–) the kVp or mAs from the routine amount for the patient thickness measurement, as indicated in Table 1-5.

7. Obtain the requested images. The technologist should use the routine positioning guidelines such as for patient positioning, central ray centering, IR size, and collimation when obtaining mobile and trauma projections. For patients who are unable to follow the routine positioning requirements, adaptations to this setup can be made. Never force the patient into a position. Instead, adjust the central ray and IR. As long as the central ray, part, and IR form the same alignments, identical projections will result. The word part with regard to alignment refers to the specific plane, imaginary line, or anatomic structure used to position the patient with the central ray and IR in routine positioning.

Lateral Projections

Routine lateral foot projections require that the foot's lateral surface be aligned parallel to the IR and the central ray be aligned perpendicular to the part and the IR. In this situation the lateral foot surface is the part, because this is what is used to position the foot in relation to the central ray and IR. If the lateral foot projection is taken on the imaging table, the patient will externally rotate his or her leg until the lateral foot surface is parallel to the IR, and the central ray will be aligned perpendicular to the IR and part (Figure 1-123).

If the lateral foot projection is taken in a standing position, the IR will be positioned vertically and the central ray horizontally. Even though the setup appears different, the central ray, part, and IR alignments are the same as in the previous setup. The lateral foot surface is positioned parallel to the IR, and the central ray is perpendicular to the IR and lateral foot surface. Often, when a cross-table image is created, the projection taken is opposite. For a routine tabletop lateral foot projection, a mediolateral projection is performed, whereas a lateromedial projection is used for cross-table images. To obtain identical images for both pathways, the technologist must maintain the same central ray, part, and IR alignment. This means that the lateral surface of the foot must still be positioned parallel to the IR for a lateromedial projection, even if this surface is not placed directly adjacent to the IR. For a lateral foot projection, this will require the medial aspect of the heel to be positioned slightly away from the IR (Figure 1-124).

If a patient arrives in the radiology department in a wheelchair and is unable to move to the imaging table for the lateral foot projection easily, the projection can be obtained with the patient remaining in the wheelchair. First, align the lateral foot surface with the IR and then align the central ray perpendicular to the IR and lateral foot surface. Again, because the relationships among the central ray, IR, and part are the same as in the two previous setups, the resulting image will be identical (Figure 1-125).

Oblique Projections

For trauma oblique images, begin by aligning the central ray with the plane, line, or anatomic structure that is used for an AP projection of the part being imaged. Next, adjust the central ray in the direction needed to set up the correct alignment between the central ray and structure. Because the degree of angulation in which patients are rotated for oblique projections is always referenced from the AP-PA projection, the amount of angle adjustment would be the same as the required degree of obliquity. For a routine internal AP oblique elbow projection, the central ray is aligned at a 45-degree angle with an imaginary line connecting the humeral epicondyles (the medial epicondyle is placed farther from the tube than the lateral). To obtain the same image in a patient who is unable to rotate his or her arm, the technologist first positions the central ray perpendicular to the line connecting the epicondyles and then adjusts the angle 45 degrees medially, positioning the medial epicondyle farther from the x-ray tube than the lateral epicondyle. The IR would then be angled so that it is perpendicular to the central ray (Figure 1-126).

When an open joint space or a particular anatomic relationship is required, it is the central ray alignment with the part that accomplishes the required results. Although IR alignment with the central ray and part is important to prevent distortion, it does not have an effect on the anatomic relationships that are demonstrated. After the central ray and part are accurately aligned, the IR should be positioned as close to perpendicular to the central ray as possible. If the central ray is not positioned perpendicular to the IR, the resulting image will demonstrate elongation in the direction toward which the central ray was angled, but the anatomic alignment of the structures should be demonstrated as required for the projection. The more acute the central ray and IR angle, the greater will be the elongation. In this situation, the IR will need to be offset from side to side or cephalocaudally in the direction toward which the central ray is angled more than what would occur if the IR were positioned perpendicular to the CR. Because of this offset, careful attention should be given to centering the central ray to the center of the IR.

The law of isometry indicates that the central ray should be set at half of the angle formed between the object and IR to minimize shape distortion. If the patient's knee cannot be fully extended for an AP femur projection, causing the femur to be at a 30-degree angle with the IR, the central ray should be angled 15 degrees. The direction of the angle should be such that it brings the central ray closer to perpendicular with the IR (Figure 1-127).

Technical Considerations

Pediatric imaging requires lower technical values (kVp and mAs) when compared with those for adults. Box 1-7 lists guidelines to follow when selecting technical values for pediatrics patients.

Clothing Artifacts

Because lower kVp is used in pediatrics, clothing artifacts may be problematic in neonates and smaller children (Figure 1-131). The kVp used may not be high enough to burn out creases or folds, particularly in unlaundered material or flame-resistant clothing. It is best to image children without upper clothing or with a tee shirt when modesty is an issue. Skinfolds of neonates may also cause artifacts when they overlie the chest.

1. Obese patients often feel unwelcome in medical settings, where they encounter negative attitudes, discriminatory behavior, and a challenging physical environment. The emotional needs of these patients must be considered when they are imaged. Avoid making remarks about their size, being mindful of terms used such as “big” when referring to special equipment needs or requests for “lots of help” when transferring the patient.

2. Patient weight and body diameter are factors that should be evaluated before transporting the patient to the department or performing the examination. Use this information to determine whether the patient's weight exceeds any of the equipment weight limits, including waiting room chairs or support structures, or his or her diameter exceeds the wheelchair or cart dimensions.

3. Avoid injury to the patient and personnel by making certain that enough people are available to assist if the patient requires moving before or during the procedure. Use moving devices, such as table sliders and lifts, whenever possible.

4. Determine how the positioning procedure (IR size, CW-LW position of IR) will need to be adjusted from the routine to accommodate the increased structure size. For example, to include the entire abdomen on a morbidly obese patient may require a separate IR for each of the four abdominal quadrants, instead of one for the top and bottom.

5. Obese patients have inherently low subject contrast because their muscles have lost strength and have become the consistency of fat, so technical values must be set to enhance subject contrast while producing the lowest possible patient dose to produce an image with sufficient image contrast.

Technical Considerations

Thicker patients attenuate more of the primary x-ray photons than thin patients, requiring the technologist to increase mAs and/or kVp to compensate for the density loss that would result if a change were not made. Thicker patients also demonstrate a higher scatter–to–primary photon ratio (SPR) reaching the IR, causing a loss in image contrast. For example, a typical abdominal image taken on a patient measuring 20 cm will demonstrate a SPR of 3:1, meaning that 75% of the photons striking the IR are scattered photons that carry little or no useful information. For larger patients, the ratio in the abdomen can approach 5:1 or 6:1 (83% to 86%).

When determining how to adjust the technique for a thicker patient, the technologist must consider the effect that the change will have on patient dose and image contrast. As long as the kVp is set to allow penetration through the part, increasing the mAs will generate enough x-rays to provide more photons. As a general rule, for every 4 cm of added tissue thickness, the mAs should be doubled to maintain density. This technique adjustment will have a significant increase on patient dose because the increase in dose is directly proportional to the mAs increase. It will also demonstrate lower image contrast because the SPR will increase with increased thickness.

Another technique adjustment option is to increase the kVp. This will increase the penetration ability of the photons, resulting in more of them reaching the IR. As a general rule, for every centimeter of added tissue thickness, the kVp should be adjusted by 2 kV. With this option, the patient dosage will increase, but not directly, as with mAs, so the amount of dosage increase is significantly less. Image contrast will be also lowered, because an increase in kVp decreases subject contrast and causes scatter radiation to be directed at a forward angle toward the IR, which is difficult for the grid to remove.

For best results when adjusting technique for a thick patient, the kVp should be set as high as possible (to reduce radiation dose), but should not exceed a kVp value that will provide sufficient subject contrast. After the kVp value maximum has been attained, additional adjustments should be made with mAs.

Scatter Radiation Control

One of the biggest obstacles when imaging the obese patient is controlling scatter radiation enough to provide an image that has sufficient image contrast. This is accomplished by using very aggressive, tight collimation, using a high-ratio grid, or using an air-gap technique.

Focal Spot Size

When using a small focal spot, the milliamperage is typically limited to 300 mA or less. Using such a small focal spot may not be feasible when imaging an obese patient because it would require a long exposure time to achieve the needed radiographic density and motion may result.

Automatic Exposure Control

Select a high mA to avoid long exposure times and the potential motion it causes. Also, adjust the backup timer to 150% to 200% of the expected manual exposure time.

When the soft tissue thickness prevents palpation of bony structures, use signs such as depressions or dimples in the soft tissue that suggest where the bony structures are located. Observe closely how the patient is positioned for each image so if a repeat is needed, you can adjust the amount needed from the original positioning.