Control of Scatter Radiation
At the completion of this chapter, the student should be able to do the following:
1 Identify the x-rays that constitute image-forming radiation.
2 Recognize the relationship between scatter radiation and image contrast.
3 List three factors that contribute to scatter radiation.
4 Discuss three devices developed to minimize scatter radiation.
5 Describe beam restriction and its effect on patient radiation dose and image quality.
6 Describe grid construction and its measures of performance.
7 Evaluate the use of various grids in relation to patient dose.
CONTRAST AND contrast resolution are important characteristics of image quality. Contrast arises from the areas of light, dark, and shades of gray on the x-ray image. These variations make up the radiographic image. Contrast resolution is the ability to image adjacent similar tissues. X-Radiation produced by Compton scatter produces noise, reducing image contrast and contrast resolution. It makes the image less visible.
Three factors contribute to increased scatter radiation: increased kVp, increased x-ray field size, and increased patient thickness. Beam-restricting devices are designed to control and minimize scatter radiation by limiting the x-ray field size to only the anatomy of interest. The three principal types of beam-restricting devices are aperture diaphragm, cones or cylinders, and collimators. By removing scattered x-rays from the remnant beam, the grid removes a major source of noise, thus improving radiographic image contrast.
The two principal characteristics of any image are spatial resolution and contrast resolution. Some refer to these together as image detail or visibility of detail. In fact, these qualities are quite distinct and are influenced by different links of the imaging chain.
Spatial resolution is determined by focal-spot size and other factors that contribute to blur. Contrast resolution is determined by scatter radiation and other sources of image noise. Two principal tools are used to control scatter radiation: beam-restricting devices and grids.
Production of Scatter Radiation
Two types of x-rays are responsible for the optical density (OD) and contrast on a radiographic image: those that pass through the patient without interacting and those that are Compton scattered within the patient. X-rays that exit from the patient are remnant x-rays and those that exit and interact with the image receptor are called image-forming x-rays (Figure 11-1).
FIGURE 11-1 Some x-rays interact with the patient and are scattered away from the image receptor (a). Others interact with the patient and are absorbed (b). X-rays that arrive at the image receptor are those transmitted through the patient without interacting (c) and those scattered in the patient (d). X-rays of types c and d are called image-forming x-rays.
Proper collimation of the x-ray beam has the primary effect of reducing patient dose by restricting the volume of irradiated tissue. Proper collimation also improves image contrast. Ideally, only those x-rays that do not interact with the patient should reach the image receptor.
As scatter radiation increases, the radiographic image loses contrast and appears gray and dull. Three primary factors influence the relative intensity of scatter radiation that reaches the image receptor: kVp, field size, and patient thickness.
kVp
As x-ray energy is increased, the absolute number of Compton interactions decreases, but the number of photoelectric interactions decreases much more rapidly. Therefore, the relative number of x-rays that undergo Compton scattering increases.
Table 11-1 shows the percentage of x-rays incident on a 10-cm thickness of soft tissue that will undergo photoelectric absorption and Compton scattering at selected kVp levels. Kilovoltage, which is one of the factors that affect the level of scatter radiation, can be controlled by the radiologic technologist.
TABLE 11-1
Percent Interaction of X-rays by Photoelectric and Compton Processes and Percent Transmission Through 10 cm of Soft Tissue
It would be easy enough to say that all radiographs should be taken at the lowest reasonable kVp because this technique would result in minimum scatter and thus higher image contrast. Unfortunately, it is not that simple.
Figure 11-2 shows the relative contributions of photoelectric effect and Compton scatter to the radiographic image. The increase in photoelectric absorption results in a considerable increase in patient radiation dose.
FIGURE 11-2 The relative contributions of photoelectric effect and Compton scattering to the radiographic image.
Also, fewer x-rays reach the image receptor at low kVp—a phenomenon that is usually compensated for by increasing the mAs. The result is still a higher patient radiation dose.
With large patients, kVp must be high to ensure adequate penetration of the portion of the body that is being radiographed. If, for example, the normal technique factors for an anteroposterior (AP) examination of the abdomen are inadequate, the technologist has the choice of increasing mAs or kVp.
Increasing the mAs usually generates enough x-rays to provide a satisfactory image but may result in an unacceptably high patient radiation dose. On the other hand, a much smaller increase in kVp is usually sufficient to provide enough x-rays, and this can be done at a much lower patient radiation dose. Unfortunately, when kVp is increased, the level of scatter radiation also increases, leading to reduced image contrast.
Collimators and grids are used to reduce the level of scatter radiation. Figure 11-3 shows a series of radiographs of a skull phantom taken at 70, 80, and 90 kVp with the use of appropriate collimation and grids, with the mAs adjusted to produce radiographs of equal OD.
FIGURE 11-3 Each of these skull radiographs is of acceptable quality. The technique factors for each are shown along with the resultant patient exposures. (Courtesy Donald Sommers, Lincoln Land Community College.)
Most radiologists would accept any of these radiographs. Notice that the patient dose at 90 kVp is approximately one third that at 70 kVp. In general, because of this reduction in patient dose, a high-kVp radiographic technique is preferred to a low-kVp technique.
Field Size
Another factor that affects the level of scatter radiation and is controlled by the radiologic technologist is x-ray beam field size. As field size is increased, scatter radiation also increases (Figure 11-4).
FIGURE 11-4 Collimation of the x-ray beam results in less scatter radiation, reduced dose, and improved contrast resolution.
Figure 11-5 shows two AP views of the lumbar spine obtained on a 35 × 43 cm image receptor. Figure 11-5, A, was taken full field, uncollimated; in Figure 11-5, B, the field size is collimated to the spinal column. Image contrast is noticeably poorer in the full-field radiograph because of the increased scatter radiation that accompanies larger field size.
FIGURE 11-5 The recommended technique for lumbar spine radiography calls for collimation of the beam to the vertebral column. The full-field technique results in reduced image contrast. A, Full-field technique. B, Preferred collimated technique. (Courtesy Mike Enriquez, Merced Community College.)
Compared with a full-field size, radiographic exposure factors may have to be increased for the purpose of maintaining the same OD when the exposure is made with a smaller field size. Reduced scatter radiation results in lower radiographic OD, which must be raised by increasing technique.
Patient Thickness
Imaging thick parts of the body results in more scatter radiation than does imaging thin body parts. Compare a radiograph of the bony structures in an extremity with a radiograph of the bony structures of the chest or pelvis. Even when the two are taken with the same screen-film image receptor, the extremity radiograph will be much sharper because of the reduced amount of scatter radiation (Figure 11-6).
FIGURE 11-6 Extremity radiographs appear sharp because of less tissue and, hence, less scatter radiation. Posteroanterior view of the hand. (Courtesy Rees Stuteville, Oregon Institute of Technology.)
The types of tissue (muscle, fat, bone) and pathology, such as a fluid-filled lung, also play a part in the production of scatter radiation.
Figure 11-7 shows the relative intensity of Compton scattered x-rays as a function of the thickness of soft tissue for a 20- × 25-cm field. Exposure of a 3-cm-thick extremity at 70 kVp produces about 45% scatter radiation. Exposure of a 30-cm-thick abdomen causes nearly 100% of the x-rays to exit the patient as scattered x-rays. With increasing patient thickness, more x-rays undergo multiple scattering, so that the average angle of scatter in the remnant beam is greater.
Normally, patient thickness is not controlled by the radiologic technologist. If you recognize that more x-rays are scattered with increasing patient thickness, you can produce a high-quality radiograph by choosing the proper technique factors and by using devices that reduce scatter radiation to the image receptor, such as a compression paddle (Figure 11-8).
FIGURE 11-8 When tissue is compressed, scatter radiation is reduced, resulting in a lower patient dose and improved contrast resolution.
Compression of anatomy improves spatial resolution and contrast resolution and lowers the patient radiation dose.
Compression devices improve spatial resolution by reducing patient thickness and bringing the object closer to the image receptor. Compression also reduces patient dose and improves contrast resolution. Compression is particularly important during mammography.
Control of Scatter Radiation
Effect of Scatter Radiation on Image Contrast
One of the most important characteristics of image quality is contrast, the visible difference between the light and dark areas of an image. Contrast is the degree of difference in OD between areas of a radiographic image. Contrast resolution is the ability to image and distinguish soft tissues.
Even under the most favorable conditions, most remnant x-rays are scattered. Figure 11-9 illustrates that scattered x-rays are emitted in all directions from the patient.
FIGURE 11-9 When primary x-rays interact with the patient, x-rays are scattered from the patient in all directions.
If you could image a long bone in cross section using only transmitted, unscattered x-rays, the image would be very sharp (Figure 11-10, A). The change in OD from dark to light, corresponding to the bone–soft tissue interface, would be very abrupt; therefore, image contrast would be high.
FIGURE 11-10 Radiographs of a cross section of long bone. A, High contrast would result from the use of only transmitted, unattenuated x-rays. B, No contrast would result from the use of only scattered x-rays. C, Moderate contrast results from the use of both transmitted and scattered x-rays.
On the other hand, if the radiograph were taken with only scatter radiation and no transmitted x-rays reached the image receptor, the image would be dull gray (Figure 11-10, B). The radiographic contrast would be very low.
In the normal situation, however, image-forming x-rays consist of both transmitted and scattered x-rays. If the radiograph were properly exposed, the image in cross-sectional view would appear as in Figure 11-10, C. This image would have moderate contrast. The loss of contrast results from the presence of scattered x-rays.
Two types of devices reduce the amount of scatter radiation that reaches the image receptor, beam restrictors and grids.
Beam Restrictors
Basically, three types of beam-restricting devices are used: the aperture diaphragm, cones or cylinders, and the variable-aperture collimator (Figure 11-11).
Aperture Diaphragm
An aperture is the simplest of all beam-restricting devices. It is basically a lead or lead-lined metal diaphragm that is attached to the x-ray tube head. The opening in the diaphragm usually is designed to cover just less than the size of the image receptor used. Figure 11-12 shows how the x-ray tube, the aperture diaphragm, and the image receptor are related.
FIGURE 11-12 Aperture diaphragm is a fixed lead opening designed for a fixed image receptor size and constant source-to-image receptor distance (SID). SDD, source-to-diaphragm distance.
The most familiar clinical example of aperture diaphragms may be radiographic imaging systems for trauma. The typical trauma system has a fixed source-to-image receptor distance (SID) and is equipped with diaphragms designed to accommodate film sizes of 13 × 18 cm, 20 × 25 cm, and 25 × 30 cm. Radiographic imaging systems for trauma can be positioned to image all parts of the body (Figure 11-13).
FIGURE 11-13 Typical trauma radiographic imaging system used for imaging the skull, spine, and extremities. Such units are flexible and adaptable for examination of many body parts. (Courtesy Fischer Imaging.)
X-ray imaging systems dedicated specifically to chest radiography can be supplied with fixed-aperture diaphragms. Such aperture diaphragms for chest radiography are designed to expose all of a 35- × 43-cm image receptor except for a 1-cm border.
Cones and Cylinders
Radiographic extension cones and cylinders are considered modifications of the aperture diaphragm. Figure 11-14 presents a diagram of a typical extension cone and cylinder. In both, an extended metal structure restricts the useful beam to the required size. The position and size of the distal end act as an aperture and determine field size.
FIGURE 11-14 Radiographic cones and cylinders produce restricted useful x-ray beams of circular shape.
In contrast to the beam produced by an aperture diaphragm, the useful beam produced by an extension cone or cylinder is usually circular. Both of these beam restrictors are routinely called cones even though the most commonly used type is actually a cylinder.
One difficulty with using cones is alignment. If the x-ray source, cone, and image receptor are not aligned on the same axis, one side of the radiograph may not be exposed because the edge of the cone may interfere with the x-ray beam. Such interference is called cone cutting.
At one time, cones were used extensively in radiographic imaging. Today, they are reserved primarily for examinations of selected areas. Figure 11-15 shows how a cone improves image contrast when used in examination of the frontal sinuses.
Variable Aperture Collimator
The light-localizing variable-aperture collimator is the most commonly used beam-restricting device in radiography. The photograph in Figure 11-16 shows an example of a modern automatic variable-aperture collimator. Figure 11-17 identifies the principal parts of such a collimator.
Not all x-rays are emitted precisely from the focal spot of the x-ray tube. Some x-rays are produced when projectile electrons stray and interact at positions on the anode other than the focal spot. Such radiation, which is called off-focus radiation, increases image blur.
To control off-focus radiation, a first-stage entrance-shuttering device that has multiple collimator blades protrudes from the top of the collimator into the x-ray tube housing.
The leaves of the second-stage collimator shutter are usually made of lead that is at least 3 mm thick. They work in pairs and are independently controlled, thereby allowing for both rectangular and square fields.
Light localization in a typical variable-aperture collimator is accomplished with a small lamp and mirror. The mirror must be far enough on the x-ray tube side of the collimator leaves to project a sufficiently sharp light pattern through the collimator leaves when the lamp is on.
The collimator lamp and the mirror must be adjusted so that the projected light field coincides with the x-ray beam. If the light field and the x-ray beam do not coincide, the lamp or the mirror must be adjusted. Such coincidence checking is a necessary evaluation of any quality control program. Misalignment of the light field and x-ray beam can result in collimator cutoff of anatomical structures.
Today, nearly all light-localizing collimators manufactured in the United States for fixed radiographic equipment are automatic. They are called positive-beam–limiting (PBL) devices. Positive beam limitation was mandated by the U. S. Food and Drug Administration in 1974. That regulation was removed in 1994, but PBL prevails.
When a film-loaded cassette is inserted into the Bucky tray and is clamped into place, sensing devices in the tray identify the size and alignment of the cassette. A signal transmitted to the collimator housing actuates the synchronous motors that drive the collimator leaves to a precalibrated position, so the x-ray beam is restricted to the image receptor in use.
Even with PBL, when appropriate, the radiologic technologist should manually collimate more tightly to reduce patient dose and improve image quality.
Depending on the tube potential, additional collimator filtration may be necessary to produce high-quality radiographs with minimum patient exposure. Some collimator housings are designed to allow easy changing of the added filtration. Filtration stations of 0, 1, 2, and 3 mm Al are the most common.
Total FiltrationEven in the zero position, however, the added filtration to the x-ray tube is not zero because collimator structures intercept the beam. In addition to the inherent filtration of the tube, the exit port, usually plastic, and the reflecting mirror provide filtration. The added filtration of the collimator assembly is equivalent to approximately 1 mm Al.
Radiographic Grids
Scattered x-rays that reach the image receptor are part of the image-forming process; indeed, the x-rays that are scattered forward do contribute to the image. An extremely effective device for reducing the level of scatter radiation that reaches the image receptor is the radiographic grid, a carefully fabricated section of radiopaque material (grid strip) alternating with radiolucent material (interspace material). The grid is positioned between the patient and the image receptor.
This technique for reducing the amount of scatter radiation that reaches the image receptor was first demonstrated in 1913 by Gustave Bucky. Over the years, Bucky’s grid has been improved by more precise manufacturing, but the basic principle has not changed.
The grid is designed to transmit only x-rays whose direction is on a straight line from the x-ray tube target to the image receptor. Scatter radiation is absorbed in the grid material. Figure 11-18 is a schematic representation of how a grid “cleans up” scatter radiation.
FIGURE 11-18 The only x-rays transmitted through a grid are those that travel in the direction of the interspace. X-rays scattered obliquely through the interspace are absorbed.
X-rays that exit the patient and strike the radiopaque grid strips are absorbed and do not reach the image receptor. For instance, a typical grid may have grid strips 50 µm wide that are separated by interspace material 350 µm wide. Consequently, even 12.5% of x-rays transmitted through the patient are absorbed.
| Question: | A grid is constructed with 50-µm strips and a 350-µm interspace. What percentage of x-rays incident on the grid will be absorbed by its entrance surface? |
| Answer: |
Primary beam x-rays incident on the interspace material are transmitted to the image receptor. Scattered x-rays incident on the interspace material may or may not be absorbed, depending on their angle of incidence and the physical characteristics of the grid.
If the angle of a scattered x-ray is great enough to cause it to intersect a lead grid strip, it will be absorbed. If the angle is slight, the scattered x-ray will be transmitted similarly to a primary x-ray. Laboratory measurements show that high-quality grids can attenuate 80% to 90% of the scatter radiation. Such a grid is said to exhibit good “cleanup.”
| Question: | When viewed from the top, a particular grid shows a series of lead strips 40 µm wide separated by interspaces 300 µm wide. How much of the radiation incident on this grid should be absorbed? |
| Answer: | If 300 + 40 represents the total surface area and 40 represents the surface area of absorbing material, then the percentage absorption is as follows: |
Grid Ratio
A grid has of three important dimensions: the thickness of the grid strip (T), the width of the interspace material (D), and the height of the grid (h). The grid ratio is the height of the grid divided by the interspace width (Figure 11-19).
FIGURE 11-19 Grid ratio is defined as the height of the grid strip (h) divided by the thickness of the interspace material (D). T, width of the grid strip.
High-ratio grids are more effective in reducing scatter radiation than are low-ratio grids. This is because the angle of scatter allowed by high-ratio grids is less than that permitted by low-ratio grids (Figure 11-20).
FIGURE 11-20 High-ratio grids are more effective than low-ratio grids because the angle of deviation is smaller.
In general, grid ratios range from 5 : 1 to 16 : 1; higher-ratio grids are used most often in high-kVp radiography. An 8 : 1 to 10 : 1 grid is frequently used with general-purpose x-ray imaging systems. Whereas a 5 : 1 grid reduces approximately 85% of the scatter radiation, a 16 : 1 grid may reduce as much as 97%.
Grid Frequency
The number of grid strips per centimeter is called the grid frequency. Grids with high frequency show less distinct grid lines on a radiographic image than grids with low frequency.
If grid strip width is held constant, the higher the frequency of a grid, the thinner its interspace must be and the higher the grid ratio.
The use of high-frequency grids requires high radiographic technique and results in a higher patient radiation dose.
As grid frequency increases, relatively more grid strip is available to absorb x-rays; therefore, the patient radiation dose is high because a higher radiographic technique is required. The disadvantage of the increased patient radiation dose associated with high-frequency grids can be overcome by reducing the width of the grid strips, but this effectively reduces the grid ratio and therefore the absorption of scatter radiation.
Most grids have frequencies in the range of 25 to 45 lines per centimeter. Grid frequency can be calculated if the widths of the grid strip and of the interspace are known. Grid frequency is computed by dividing the thickness of one line pair (T + D), expressed in µm, into 1 cm:
| Question: | What is the grid frequency of a grid that has a grid strip width of 30 µm and an interspace width of 300 µm? |
| Answer: | If one line pair = 300 µm + 30 µm = 330 µm, how many line pairs are in 10,000 µm (10,000 µm = 1 cm)? |
Specially designed grids are used for mammography. Usually, a 4 : 1 or a 5 : 1 ratio grid is used. These low-ratio grids have grid frequencies of approximately 80 lines/cm.
Interspace Material
The purpose of the interspace material is to maintain a precise separation between the delicate lead strips of the grid. The interspace material of most grids consists of aluminum or plastic fiber; reports are conflicting as to which is better.
Aluminum has a higher atomic number than plastic and therefore may provide some selective filtration of scattered x-rays not absorbed in the grid strip. Aluminum also has the advantage of producing less visible grid lines on the radiograph.
On the other hand, use of aluminum as interspace material increases the absorption of primary x-rays in the interspace, especially at low kVp. The result is higher mAs and a higher patient dose. Above 100 kVp, this property is unimportant, but at low kVp, the patient dose may be increased by approximately 20%. For this reason, fiber interspace grids usually are preferred to aluminum interspace grids.
Still, aluminum has two additional advantages over fiber. It is nonhygroscopic, that is, it does not absorb moisture as plastic fiber does. Fiber interspace grids can become warped if they absorb moisture. Also, aluminum interspace grids of high quality are easier to manufacture because aluminum is easier to form and roll into sheets of precise thickness.
Grid Strip
Theoretically, the grid strip should be infinitely thin and should have high absorption properties. These strips may be formed from several possible materials. Lead is most widely used because it is easy to shape and is relatively inexpensive. Its high atomic number and high mass density make lead the material of choice in the manufacture of grids. Tungsten, platinum, gold, and uranium all have been tried, but none has the overall desirable characteristics of lead.
Grid Performance
Perhaps the largest single factor responsible for poor radiographic image quality is scatter radiation. By removing scattered x-rays from the remnant beam, the radiographic grid removes the source of reduced contrast.
Contrast Improvement Factor
The characteristics of grid construction previously described, especially the grid ratio, usually are specified when a grid is identified. Grid ratio, however, does not reveal the ability of the grid to improve image contrast. This property of the grid is specified by the contrast improvement factor (k). A contrast improvement factor of 1 indicates no improvement.
Most grids have contrast improvement factors of between 1.5 and 2.5. In other words, the image contrast is approximately doubled when grids are used. Mathematically, the contrast improvement factor, k, is expressed as follows:
| Question: | An aluminum step wedge is placed on a tissue phantom that is 20 cm thick and a radiograph is made. Without a grid, analysis of the radiograph shows an average gradient (a measure of contrast) of 1.1. With a 12 : 1 grid, radiographic contrast is 2.8. What is the contrast improvement factor of this grid? |
| Answer: |
The contrast improvement factor usually is measured at 100 kVp, but it should be realized that k is a complex function of the x-ray emission spectrum, patient thickness, and the tissue irradiated.
Bucky Factor
Although the use of a grid improves contrast, a penalty is paid in the form of patient radiation dose. The quantity of image-forming x-rays transmitted through a grid is much less than that of image-forming x-rays incident on the grid. Therefore, when a grid is used, the radiographic technique must be increased to produce the same image receptor signal. The amount of this increase is given by the Bucky factor (B), also called the grid factor.
The Bucky factor is named for Gustave Bucky, the inventor of the grid. It is an attempt to measure the penetration of primary and scatter radiation through the grid. Table 11-2 gives representative values of the Bucky factor for several popular grids.
Two generalizations can be made from the data presented in Table 11-2:
1. The higher the grid ratio, the higher is the Bucky factor. The penetration of primary radiation through a grid is fairly independent of grid ratio. Penetration of scatter radiation through a grid becomes less likely with increasing grid ratio; therefore, the Bucky factor increases.
2. The Bucky factor increases with increasing kVp. At high voltage, more scatter radiation is produced. This scatter radiation has a more difficult time penetrating the grid; thus, the Bucky factor increases.
As the Bucky factor increases, radiographic technique and the patient radiation dose increase proportionately.
Whereas the contrast improvement factor measures improvement in image quality when grids are used, the Bucky factor measures how much of an increase in technique will be required compared with nongrid exposure. The Bucky factor also indicates how large an increase in patient radiation dose will accompany the use of a particular grid.
Grid Types
The simplest type of grid is the parallel grid, which is diagrammed in cross section in Figure 11-21. In the parallel grid, all lead grid strips are parallel. This type of grid is the easiest to manufacture, but it has some properties that are clinically undesirable, namely grid cutoff, the undesirable absorption of primary x-rays by the grid.
FIGURE 11-21 A parallel grid is constructed with parallel grid strips. At a short source-to-image receptor distance (SID), some grid cutoff may occur.
The attenuation of primary x-rays becomes greater as the x-rays approach the edge of the image receptor. The lead strips in a 35- × 43-cm grid are 43 cm long. Across the 35-cm dimension, the signal intensity reaches a maximum along the center line of the image receptor and decreases toward the sides.
Grid cutoff can be partial or complete. The term is derived from the fact that the primary x-rays are “cut off” from reaching the image receptor. Grid cutoff can occur with any type of grid if the grid is improperly positioned, but it is most common with parallel grids.
This characteristic of parallel grids is most pronounced when the grid is used at a short SID or with a large-area image receptor. Figure 11-22 shows the geometric relationship for attenuation of primary x-rays by a parallel grid. The distance from the central ray at which complete cutoff will occur is determined by the following:
FIGURE 11-22 With a parallel grid, optical density (OD) decreases toward the edge of the image receptor. The distance to grid cutoff is the source-to-image receptor distance (SID) divided by the grid ratio.
For instance, in theory, a 10 : 1 grid used at 100 cm SID should absorb all primary x-rays farther than 10 cm from the central ray. When this grid is used with a 35- × 43-cm image receptor, OD should be apparent only over a 20- × 43-cm area of the image receptor.
The radiographs in Figure 11-23 were taken with a 6 : 1 parallel grid at 76 and 61 cm SID (A and B, respectively). They show increasing degrees of grid cutoff with decreasing SID.
FIGURE 11-23 A, Radiograph taken with a 6 : 1 parallel grid at a source-to-image receptor distance (SID) of 76 cm. B, Radiograph taken with 6 : 1 parallel grid at an SID of 61 cm. Optical density decreases from the center to the edge of the image and to complete cutoff. (Courtesy Dawn Stark, Mississippi State University.)
| Question: | A 16 : 1 parallel grid is positioned for chest radiography at 180 cm SID. What is the distance from the central axis to complete grid cutoff? Will the image satisfactorily cover a 35- × 43-cm image receptor? |
| Answer: |
No! Grid cutoff will occur on the lateral 6.2 cm (17.5−11.3) of the image receptor.
Crossed Grid
Parallel grids clean up scatter radiation in only one direction along the axis of the grid. Crossed grids are designed to overcome this deficiency. Crossed grids have lead grid strips that run parallel to the long and short axes of the grid (Figure 11-24). They are usually fabricated by sandwiching two parallel grids together with their grid strips perpendicular to one another.
FIGURE 11-24 Crossed grids are fabricated by sandwiching two parallel grids together so their grid strips are perpendicular.
They are not too difficult to manufacture and therefore are not excessively expensive. However, they have found restricted application in clinical radiology. (It is interesting to note that Bucky’s original grid was crossed.)
Crossed grids are much more efficient than parallel grids in cleaning up scatter radiation. In fact, a crossed grid has a higher contrast improvement factor than a parallel grid of twice the grid ratio. A 6 : 1 crossed grid will clean up more scatter radiation than a 12 : 1 parallel grid.
This advantage of the crossed grid increases as the operating kVp is increased. A crossed grid identified as having a grid ratio of 6 : 1 is constructed with two 6 : 1 parallel grids.
Three serious disadvantages are associated with the use of crossed grids. First, positioning the grid is critical; the central ray of the x-ray beam must coincide with the center of the grid. Second, tilt-table techniques are possible only if the x-ray tube and the table are properly aligned. Finally, the exposure technique required is substantial and results in higher patient radiation dose.
Focused Grid
The focused grid is designed to minimize grid cutoff. The lead grid strips of a focused grid lie on the imaginary radial lines of a circle centered at the focal spot, so they coincide with the divergence of the x-ray beam. The x-ray tube target should be placed at the center of this imaginary circle when a focused grid is used (Figure 11-25).
FIGURE 11-25 A focused grid is fabricated so that grid strips are parallel to the primary x-ray path across the entire image receptor.
Focused grids are more difficult to manufacture than parallel grids. They are characterized by all of the properties of parallel grids except that when properly positioned, they exhibit no grid cutoff. Radiologic technologists must take care when positioning focused grids because of their geometric limitations.
Every focused grid is marked with its intended focal distance and the side of the grid that should face the x-ray tube. If radiographs are taken at distances other than those intended, grid cutoff occurs.
Moving Grid
An obvious and annoying shortcoming of the grids previously discussed is that they can produce grid lines on the image. Grid lines are the images made when primary x-rays are absorbed within the grid strips. Even though the grid strips are very small, their image is still observable.
The presence of grid lines can be demonstrated simply by radiographing a grid. Usually, high-frequency grids present less obvious grid lines compared with low-frequency grids. This is not always the case, however, because the visibility of grid lines is directly related to the width of the grid strips.
A major improvement in grid development occurred in 1920. Hollis E. Potter hit on a very simple idea: Move the grid while the x-ray exposure is being made. The grid lines disappear at little cost of increased radiographic technique. A device that does this is called a moving grid or a Potter-Bucky diaphragm (“Bucky” for short).
Focused grids usually are moving grids. They are placed in a holding mechanism that begins moving just before x-ray exposure and continues moving after the exposure ends. Two basic types of moving grid mechanisms are in use today: reciprocating and oscillating.
Reciprocating Grid
A reciprocating grid is a moving grid that is motor-driven back and forth several times during x-ray exposure. The total distance of drive is approximately 2 cm.
Oscillating Grid
An oscillating grid is positioned within a frame with a 2- to 3-cm tolerance on all sides between the frame and the grid. Delicate, springlike devices located in the four corners hold the grid centered within the frame. A powerful electromagnet pulls the grid to one side and releases it at the beginning of the exposure. Thereafter, the grid oscillates in a circular fashion around the grid frame, coming to rest after 20 to 30 seconds.
Disadvantages of Moving Grids
Moving grids require a bulky mechanism that is subject to failure. The distance between the patient and the image receptor is increased with moving grids because of this mechanism; this extra distance may create an unwanted increase in magnification and image blur. Moving grids can introduce motion into the cassette-holding device, which can result in additional image blur.
Fortunately, the advantages of moving grids far outweigh the disadvantages. The types of motion blur discussed are for descriptive purposes only. The motion blur generated by moving grids that are functioning properly is undetectable. Moving grids are usually the technique of choice and therefore are used widely.
Grid Problems
Most grids in diagnostic imaging are of the moving type. They are permanently mounted in the moving mechanism just below the tabletop or just behind the vertical chest board.
To be effective, of course, the grid must move from side to side. If the grid is installed incorrectly and moves in the same direction as the grid strips, grid lines will appear on the radiograph (Figure 11-26).
The most frequent error in the use of grids is improper positioning. For the grid to function correctly, it must be precisely positioned relative to the x-ray tube target and to the central ray of the x-ray beam. Four situations characteristic of focused grids must be avoided (Table 11-3). Only the off-level grid is a problem with parallel and crossed grids.
TABLE 11-3
| Type of Grid Misalignment | Result |
| Off level | Grid cutoff across image; underexposed, light image |
| Off center | Grid cutoff across image; underexposed, light image |
| Off focus | Grid cutoff toward edge of image |
| Upside down | Severe grid cutoff toward edge of image |
| Off center, off focus | Grid cutoff on one side of image |
Off-Level Grid
A properly functioning grid must lie in a plane perpendicular to the central ray of the x-ray beam (Figure 11-27). The central ray x-ray beam is the x-ray that travels along the center of the useful x-ray beam.
FIGURE 11-27 If a grid is off level so that the central axis is not perpendicular to the grid, partial cutoff occurs over the entire image receptor.
Despite its name, an off-level grid in fact is usually produced with an improperly positioned x-ray tube and not an improperly positioned grid. However, this can occur when the grid tilts during horizontal beam radiography or during mobile radiography when the image receptor sinks into the patient’s bed.
If the central ray is incident on the grid at an angle, then all incident x-rays will be angled, and grid cutoff will occur across the entire radiographic image, resulting in lower OD or intensity at the digital image receptor.
Off-Center Grid
A grid can be perpendicular to the central ray of the x-ray beam and still produce grid cutoff if it is shifted laterally. This is a problem with focused grids, as shown in Figure 11-28, in which an off-center grid is shown with a properly positioned grid.
FIGURE 11-28 When a focused grid is positioned off center, partial grid cutoff occurs over the entire image receptor.
The center of a focused grid must be positioned directly under the x-ray tube target, so the central ray of the x-ray beam passes through the centermost interspace of the grid. Any lateral shift results in grid cutoff across the entire radiograph, producing lower OD. This error in positioning is called lateral decentering.
As with an off-level grid, an off-center grid is more a result of positioning the x-ray tube than the grid. In practice, it means that the radiologic technologist must carefully line up the center of the light-localized field with the center of the image receptor.
Off-Focus Grid
A major problem with using a focused grid arises when radiographs are taken at SIDs unspecified for that grid. Figure 11-29 illustrates what happens when a focused grid is not used at the proper focal distance. The farther the grid is from the specified focal distance, the more severe will be the grid cutoff. Grid cutoff is not uniform across the image receptor but instead is more severe at the edges.
FIGURE 11-29 If a focused grid is not positioned at the specified focal distance, grid cutoff occurs and the optical density (OD) decreases with distance from the central ray.
This condition is not usually a problem if all chest radiographs are taken at 180 cm SID and all table radiographs at 100 cm SID. Positioning the grid at the proper focal distance is more important with high-ratio grids; greater positioning latitude is possible with low-ratio grids.
Upside-Down Grid
The explanation for an upside-down grid is obvious. It need occur only once, and it will be noticed immediately. A radiographic image taken with an upside-down focused grid shows severe grid cutoff on either side of the central ray (Figure 11-30).
FIGURE 11-30 A focused grid positioned upside down should be detected on the first radiograph. Complete grid cutoff occurs except in the region of the central ray.
Combined Off-Center, Off-Focus Grid
Perhaps the most common improper grid position occurs if the grid is both off center and off focus. Without proper attention, this can occur easily during mobile radiography. It is an easily recognized grid-positioning artifact because the result is uneven exposure. The resultant radiograph appears dark on one side and light on the other.
Grid Selection
Modern grids are sufficiently well manufactured that many radiologists do not find the grid lines of stationary grids objectionable, especially for mobile radiography and horizontal views of an upright patient.
Moving grid mechanisms, however, rarely fail, and image degradation rarely occurs. Therefore, in most situations, it is appropriate to design radiographic imaging around moving grids. When moving grids are used, parallel grids can be used, but focused grids are more common.
Focused grids are in general far superior to parallel grids, but their use requires care and attention. When focused grids are used, the indicators on the x-ray apparatus must be in good adjustment and properly calibrated. The SID indicator, the source-to-tabletop distance (STD) indicator, and the light-localizing collimator all must be properly adjusted.
Selection of a grid with the proper ratio depends on an understanding of three interrelated factors: kVp, degree of scatter radiation reduction, and patient radiation dose. When a high kVp is used, high-ratio grids should be used as well. Of course, the choice of grid is also influenced by the size and shape of the anatomy that is being radiographed.
As grid ratio increases, scatter radiation attenuation also increases. Figure 11-31 shows the approximate percentage of scatter radiation and primary radiation transmitted as a function of grid ratio. Note that the difference between grid ratios of 12 : 1 and 16 : 1 is small.
FIGURE 11-31 As the grid ratio increases, transmission of scatter radiation decreases faster than transmission of primary radiation. Therefore, cleanup of scatter radiation increases.
The difference in patient dose is large, however; therefore, 16 : 1 grids are not often used. Many general-purpose x-ray examination facilities find that an 8 : 1 grid represents a good compromise between the desired levels of scatter radiation reduction and patient radiation dose.
In general, grid ratios up to 8 : 1 are satisfactory at tube potentials below 90 kVp. Grid ratios above 8 : 1 are used when kVp exceeds 90 kVp.
The use of one grid also reduces the likelihood of grid cutoff because improper grid positioning can easily accompany frequent changes of grids. In facilities where high-kVp technique for dedicated chest radiography is used, 16 : 1 grids can be installed.
Patient Dose
One major disadvantage that accompanies the use of radiographic grids is increased patient radiation dose. For any examination, use of a grid may result in several times more radiation to the patient than is provided when a grid is not used. The use of a moving grid instead of a stationary grid with similar physical characteristics requires approximately 15% more patient radiation dose. Table 11-4 is a summary of approximate patient doses for various grid techniques with a 400-speed image receptor.
TABLE 11-4
Approximate Entrance Skin Radiation Dose for Examination of the Adult Pelvis with a 400-Speed Image Receptor
Low-ratio grids are used during mammography. All dedicated mammographic imaging systems are equipped with a 4 : 1 or a 5 : 1 ratio moving grid. Even at the low kVp used for mammography, considerable scatter radiation occurs.
The use of such grids greatly improves image contrast, with no loss of spatial resolution. The only disadvantage is the increase in patient dose, which can be as much as twice that without a grid. However, with dedicated equipment and grid, patient dose still is very low.
In general, compared with the use of low-kVp and low-ratio grids, the use of high-kVp and high-ratio grids results in lower patient radiation dose and equal image quality.
One additional disadvantage of the use of radiographic grids is the increased radiographic technique required. When a grid is used, technique factors must be increased over what they were for nongrid examinations: The mAs or the kVp must be increased. Table 11-5 presents approximate changes in technique factors required by standard grids. Usually, the mAs rather than the kVp is increased. One exception to this is chest radiography, in which increased exposure time can result in motion blur.
TABLE 11-5
Approximate Change in Radiographic Technique for Standard Grids
| Grid Ratio | mAs Increase | kVp Increase |
| No grid | 1 × | 0 |
| 5 : 1 | 2 × | + 8–10 |
| 8 : 1 | 4 × | + 13–15 |
| 12 : 1 | 5 × | + 20–25 |
| 16 : 1 | 6 × | + 30–40 |
Table 11-6 summarizes the clinical factors that should be considered in the selection of various types of grids.
Air-Gap Technique
A clever technique that may be used as an alternative to the use of radiographic grids is the air-gap technique. This is another method of reducing scatter radiation, thereby enhancing image contrast.
When the air-gap technique is used, the image receptor is moved 10 to 15 cm from the patient (Figure 11-32). A portion of the scattered x-rays generated in the patient would be scattered away from the image receptor and not be detected. Because fewer scattered x-rays interact with the image receptor, the contrast is enhanced.
FIGURE 11-32 When the air-gap technique is used, the image receptor is positioned 10 to 15 cm from the patient. A large fraction of scattered x-rays does not interact with the image receptor.
Usually, when an air-gap technique is used, the mAs is increased approximately 10% for every centimeter of air gap. The technique factors usually are about the same as those for an 8 : 1 grid. Therefore, the patient dose is higher than that associated with the nongrid technique and is approximately equivalent to that of an intermediate grid technique.
The air-gap technique has found application particularly in the areas of chest radiography and cerebral angiography. The magnification that accompanies these techniques is usually acceptable.
In chest radiography, however, some radiologic technologists increase the SID from 180 to 300 cm. This results in very little magnification and a sharper image. Of course, the technique factors must be increased, but the patient dose is not increased (Figure 11-33).
FIGURE 11-33 Increasing the source-to-image receptor distance (SID) to 300 cm from 180 cm improves spatial resolution with no increase in patient dose.
The air-gap technique is not normally as effective with high-kVp radiography, in which the direction of the scattered x-rays is more forward. At tube potentials below approximately 90 kVp, the scattered x-rays are directed more to the side; therefore, they have a higher probability of being scattered away from the image receptor. Nevertheless, at some centers, 120 to 140 kVp air-gap chest radiography is used with good results.
Summary
Two types of image-forming x-rays exit the patient: (1) x-rays that pass through tissue without interacting and (2) x-rays that are scattered in tissue by the Compton interaction and therefore contribute only noise to the image. The three factors that contribute to increased scatter radiation and ultimately to image noise are increasing kVp, increasing x-ray field size, and increasing anatomical thickness.
Although increased kVp increases scatter radiation, the trade-off is reduced patient radiation dose. Beam-restricting devices can be used to control and minimize the increase in scatter. Such devices include the aperture diaphragm, cones or cylinders, and the variable-aperture collimator. The variable-aperture collimator is the most commonly used beam-restricting device in radiographic imaging.
Contrast is one of the most important characteristics of the radiographic image. Scatter radiation, the result of Compton interaction, is the primary factor that reduces image contrast. Grids reduce the amount of scatter that reaches the image receptor.
The two main components of grid construction are the interspace material (aluminum or plastic fiber) and the grid material (lead strips). The principal characteristic of a grid is grid ratio, that is, the height of the grid strip divided by the interspace width. Different grids are selected for use in particular situations. At less than 90 kVp, grid ratios of 8 : 1 and lower are used. At 90 kVp and above, grid ratios greater than 8 : 1 are used.
In all cases, the use of a grid increases patient dose. Table 11-5 summarizes the changes in grid ratio and changes in mAs or kVp that are required. Problems can arise with the use of grids, including off-level, off-center, and upside-down grid errors.
An alternative to use of a grid is the air-gap technique, in which the image receptor is moved 10 to 15 cm from the patient. The air gap allows much of the scatter radiation to miss the image receptor.
1. Define or otherwise identify the following:
2. Why should a radiograph of the lumbar vertebrae be well collimated?
3. With particular references to materials used and dimensions, discuss the construction of a grid.
4. An acceptable IVP can be obtained with technique factors of (1) 74 kVp, 120 mAs, or (2) 82 kVp, 80 mAs. Discuss possible reasons for selecting one technique over the other.
5. Does the radiograph of a long bone in a wet cast result in more or less scatter than that of a long bone in a dry cast?
6. A focused grid has the following characteristics: 100 cm focal distance, 40 µm grid strips, 350 µm interspace, and 2.8 mm height. What is the grid ratio?
7. What happens to image contrast and patient dose as more filtration is added to the x-ray beam?
8. Why does tissue compression improve image contrast?
9. At 80-kVp, approximately what percentage of the x-ray beam is scattered through Compton interaction?
10. Name the devices used to reduce the production of scatter radiation.
11. Compression of tissue is particularly important during what examination?
12. List two reasons for restricting the x-ray beam.
13. Compared with contact radiography, why does air-gap technique increase the patient dose?
14. What is the reason why an unexposed border is shown on the edge of the radiograph?
15. Why does lowering kVp increase the patient dose?
16. What is viewed in the light field of a variable-aperture light-localizing collimator?
17. Explain how grid cutoff can occur.
18. Does a light-localizing collimator add filtration to the x-ray beam?
19. If the light field and the radiation field of a light-localizing collimator do not coincide, what needs to be adjusted?
20. When should the x-ray field exceed the size of the image receptor?
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