Digestive System

• Esophagram preparation. No preparation procedures are required when the esophagus is imaged, but if an esophagus and stomach examination is being performed, the stomach and duodenum preparation is to be followed.

• Stomach and duodenum preparation. Adequate preparation of the upper gastrointestinal (GI) system eliminates residual stomach debris, which may obscure abnormalities, and prevents excessive fluid from accumulating in the stomach, which could dilute the barium suspension enough to interfere with optimal mucosal coating (see Image 5). The preparation procedure for the upper GI system includes NPO (nothing orally) after midnight or at least 8 hours before the examination, and avoidance of gum and tobacco products before the procedure.

• Small intestine preparation. Optimal preparation of the small intestine for a small bowel study is obtained through a patient preparation procedure that includes a low-residue diet for 1 to 2 days before the examination, NPO after midnight and until the examination, and avoidance of gum and tobacco products before the examination (these are thought to increase salivation and gastric secretions).

• Large intestine preparation. Adequate cleansing of the large intestine for a barium enema is obtained through a patient preparation procedure including a low-residue diet for 2 to 3 days before the examination, followed by a clear liquid diet 1 day before the examination, laxatives the afternoon before the examination, and a suppository or cleansing enema the morning of the examination. Remaining fecal material may obscure the mucosal surfaces and, when barium-coated, may mimic polyps and small tumors; remaining residual fluid causes dilution of the barium suspension, resulting in poor mucosal coating and coating artifacts (see Image 11).

    On arrival in the fluoroscopic department, the patient should be queried to determine whether proper preparation instructions were given and followed. Adequate preparation can be assumed if the patient's last bowel movement lacked solid fecal material.

• Visibility of Digestive System Details. An optimal kilovoltage peak (kVp) technique, as shown in Table 12-1, sufficiently penetrates the barium-coated mucosal surface and provides the contrast needed to distinguish the mucosal patterns. Use a grid to reduce the scatter radiation that reaches the IR, thereby reducing fog, increasing the visibility of the recorded details, and providing a higher contrast image. To obtain optimal density, set a milliampere-seconds (mAs) level manually based on the patient's abdominal thickness or choose the appropriate automatic exposure control (AEC). For double-contrast examinations, using the AEC may be contraindicated because choosing the cell beneath the barium pool may result in overexposure of the area containing the air contrast (see Image 6).

• Pendulous breasts. Pendulous breasts may overlap and prevent clear visualization of the colic flexures unless they are shifted superiorly and laterally. Such movement also prevents excessive radiation exposure to the breasts.

• Exposure times. Short exposure times are needed when imaging the digestive system to control the image blur that may result from peristaltic activity within the system. Peristalsis is the contraction and relaxation movement of the smooth muscles in the walls of the digestive system that mixes food and secretions and moves the materials through the system. Peristaltic activity of the stomach and large or small intestine can be identified on an image by sharp bony cortices and blurry gastric and intestinal gases or barium (see Image 9).

• Body habitus. The body habitus determines the size, shape, and position of the stomach and the abdominal cavity placement of the large intestine. Being familiar with these differences will help the technologist adjust the central ray centering and IR placement for optimal demonstration of the required digestive structures.

    Hypersthenic. The hypersthenic patient's abdomen is broad and deep from anterior to posterior. The stomach is positioned high in the abdomen and lies transversely at the level of the T9 to T12, with the duodenal bulb at the level of T11 to T12. The colic flexures and transverse colon tend to be positioned high in the abdomen (Figure 12-1). Using the sthenic patient as the reference point, this habitus will require a more superior and medial central ray centering and IR placement for AP and PA projections, a more superior and anterior central ray centering and IR placement for the lateral position of the stomach, and two crosswise IRs to include the entire large intestine for barium enema projections.

    Asthenic. The asthenic patient's abdomen is narrow; the stomach is positioned low in the abdomen and runs vertically along the left side of the vertebral column, typically extending from T11 to L5, with the duodenal bulb at the level of L3 to L4. The small and large intestinal structures tend to be positioned low in the abdomen (Figure 12-2). Using the sthenic patient as the reference point, this habitus will require lower and more lateral centering of the central ray for stomach projections.

    Sthenic. The sthenic habitus is the most common. The abdomen is less broad than the hypersthenic habitus, yet not as narrow as the asthenic. The stomach also rests at a position between the hypersthenic and asthenic habitus and typically extends from T10 to L2, with the duodenal bulb at the level of L1 to L2. The small and large intestinal structures tend to be centered in the abdomen (Figure 12-3).

• From full inspiration to expiration the diaphragm position moves from an inferior to a superior position. This movement also changes the pressure placed on the abdominal structures. On full expiration the right side of the diaphragm dome is at the same transverse level as the eighth thoracic vertebrae, whereas on inspiration it may be found at the same transverse level as the ninth or tenth posterior rib. Exposing upper GI and small and large intestine projections on full expiration allows increased abdominal space for the structures to be demonstrated without segment overlapping and foreshortening (see Image 7).

• Contrast. The goal of the esophagram is to demonstrate the workings and appearance of the pharynx and esophagus. This is accomplished through the fluoroscopic procedure and overhead projections that are obtained in an esophagram. Barium is used to demonstrate the pharynx and esophagus during this examination. A 30% to 50% weight or volume barium suspension is ingested continuously during the exposure, or two to three spoonfuls with toothpaste consistency; thick barium is ingested before exposing the esophagus, filling it with barium. The patient may also be asked to swallow cotton balls soaked in thin barium, barium-filled gelatin capsules, barium tablets, or marshmallows when a radiolucent foreign body or stricture is suspected. Adequate filling of the esophagus has occurred when the entire column is filled with barium. Only aspects of the esophagus that are filled with barium will be adequately demonstrated (see Images 1 and 3).

    For the overhead projections, place a cup full of barium in the patient's hand and place the straw in the patient's mouth so that the patient can ingest barium during the exposure, or ask the patient to swallow two spoonfuls of thick barium and then give a third spoonful that is swallowed immediately before the exposure.

• A PA oblique esophagram projection (right anterior oblique [RAO]) is obtained by placing the patient prone on the imaging table and then rotating the torso toward the right side until the midcoronal plane is at a 35- to 40-degree angle with the imaging table (Figure 12-5). The patient's left elbow and knee may be flexed and used to support the body rotation.

• Inaccurate patient rotation. If less than the desired 35 to 40 degrees of obliquity is obtained on a PA oblique esophagram projection, the vertebrae will be superimposed over the esophagus and the right sternal clavicular end (see Image 2). If the patient is rotated more than 40 degrees, more than 0.5 inch (1.25 cm) of the right sternal clavicular end will be demonstrated to the left of the vertebrae.

• To place the midesophagus in the center of the exposure field, center a perpendicular central ray approximately 3 inches (7.5 cm) to the left of the spinous processes and 2 to 3 inches (5 to 7.5 cm) inferior to the jugular notch. Center the IR to the central ray.

• Open the longitudinally collimated field the full 17-inch (43-cm) IR length. Transverse collimation should be to a 6-inch (15-cm) field size.

• A 14- × 17-inch (35- × 43-cm) IR placed lengthwise should be adequate to include all the required anatomic structures.

Posteroanterior Oblique Esophagram Projection (RAO Position) Analysis

• To obtain a lateral esophagram projection, place the patient on the imaging table in a lateral recumbent position. Whether the patient is lying on the right or left side is not significant (Figure 12-7). Flex the patient's knees and hips for support, and position a pillow or sponge between the knees. The pillow or sponge should be thick enough to prevent the side of the pelvis situated farther from the IR from rotating anteriorly but not so thick as to cause posterior rotation. To avoid vertebral rotation, align the shoulders, posterior ribs, and posterior pelvis perpendicular to the imaging table and IR by resting an extended flat palm against each, respectively, and then adjusting patient rotation until the hand is positioned perpendicular to the IR.

• Detecting thorax rotation. Rotation can be detected on a lateral esophagram projection by evaluating superimposition of the right and left posterior surfaces of the vertebral bodies and superimposition of the posterior ribs. Because the two sides of the thorax and vertebrae are mirror images, it is very difficult to determine from a rotated lateral esophagram projection which side of the patient was rotated anteriorly and which posteriorly. If the patient was only slightly rotated, one way of determining which way the patient was rotated is to evaluate the amount of posterior rib superimposition. If the patient's elevated side was rotated posteriorly, the posterior ribs demonstrate more than 0.5 inch (1.25 cm) of space between them (see Chapter 8, Image 40). If the patient's elevated side was rotated anteriorly, the posterior ribs are superimposed on slight rotation (see Chapter 8, Image 41) and demonstrate greater separation as rotation of the patient increases.

• Humeral and shoulder positioning. Placing the humeri anteriorly at a 90-degree angle with the torso or separating the shoulders by positioning the arm and shoulder closer to the imaging table slightly forward and the arm and shoulder farther away from the imaging table back, while maintaining a lateral thorax (Figure 12-8), prevents the shoulders and humeri from being superimposed over the esophagus.

• Center a perpendicular central ray to the midcoronal plane at a level 2 to 3 inches (5 to 7.5 cm) inferior to the jugular notch to center the midesophagus at the center of the exposure field. Center the IR to the central ray.

• Open the longitudinally collimated field the full 17-inch (43-cm) IR length. Transverse collimation should be to a 6-inch (15-cm) field size.

• A 14- × 17-inch (35- × 43-cm) IR placed lengthwise should be adequate to include all the required anatomic structures.

Lateral Esophagram Projection Analysis

• To obtain a PA esophagus projection, place the patient prone on the imaging table. Position the shoulders and anterior superior iliac spines (ASISs) at equal distances from the imaging table to prevent rotation, and draw the patient's arms away from the abdominal area to prevent them from being superimposed over the abdominal region (Figure 12-10). Special attention should be given to female patients who have had one breast removed. The side of the patient on which the breast was removed may need to be placed at a greater OID than the opposite side to prevent rotation.

• Detecting rotation. Rotation is readily detected on a PA esophagus projection by evaluating the position of the esophagus with respect to the vertebrae and the distances from the vertebral column to the sternal clavicular ends. On a nonrotated esophagus projection, the vertebrae and esophagus are superimposed and the distances from the vertebrae to the sternal clavicular ends are equal on both sides. On a rotated PA projection, the side of the vertebrae toward which the esophagus is rotated and the sternal clavicular end that demonstrates less vertebral column superimposition represents the side of the chest positioned farther from the IR (see Image 4).

• Center a perpendicular central ray to the midsagittal plane at a level 2 to 3 inches (5 to 7.5 cm) inferior to the jugular notch for the AP projection and at the level of T5 to T6 (2 to 3 inches superior to the inferior scapular angle) for the PA projection to center the midesophagus at the center of the exposure field. Center the IR to the central ray.

• Open the longitudinally collimated field the full 17-inch (43-cm) IR length. Transverse collimation should be to a 6-inch (15-cm) field size.

• A 14- × 17-inch (35- × 43-cm) IR placed lengthwise should be adequate to include all the required anatomic structures.

Posteroanterior Esophagram Projection Analysis

• The single-contrast PA stomach and duodenum projection demonstrates barium-filled organs with normally present gas. The primary goal of a single-contrast upper GI study is to demonstrate abnormalities of the stomach and lumen contour. A 30% to 50% weight or volume barium suspension is typically used for this study.

• The goal of a double-contrast study is to visualize abnormalities in the mucosal details and contour and lumen of the stomach and duodenum.

    Negative (radiolucent) contrast is most commonly obtained by having the patient swallow effervescent granules, powder, or tablets that rapidly release 300 to 400 mL of carbon dioxide on contact with the fluid in the stomach. The carbon dioxide causes gastric distention and smoothing of the rugae.

    Positive (radiopaque) contrast is obtained by having the patient drink a high-density (up to 250% weight or volume) barium suspension. The barium provides the thin coating that covers the mucosal surface. To obtain adequate mucosal coating of the area, the barium is washed over the gastric surface by having the patient turn 360 degrees and then positioning the patient so that the barium pool will be placed away from the area of interest. Because the barium will slowly flow toward the lowest level after coating, the patient should be rotated between projections or the sequence of projections should be taken to optimize coating of the area of interest to maintain an optimal mucosal covering. Table 12-2 indicates where the barium will pool and which aspect of the upper GI tract is best demonstrated in the most commonly obtained stomach and duodenum projections.

    Failure to obtain good mucosal coating may result in missed or simulated lesions. The quality of the mucosal coating depends on the properties of the barium suspension, the volume of barium and gas, the frequency of washing, and the amount of fluid or secretions and viscosity of mucus in the stomach. Although proper double-contrast filling is primarily the fluoroscopist's responsibility, the technologist's scope of practice does play a part in some of the causes of poor coating, such as using the wrong type of barium, improperly preparing the barium suspension, or performing poor lower intestine preparation. A thorough mixing of the barium suspension is required before the patient ingests the material.

• Hypersthenic habitus. The patient has been rotated 70 degrees, as identified by the demonstration of the left lumbar zygapophyseal joints in the posterior third of the vertebral bodies. The duodenal bulb and descending duodenum are in profile, and the long axis of the stomach demonstrates foreshortening with a closed lesser curvature (see Figure 12-11).

• Sthenic habitus. The patient has been rotated approximately 45 degrees, as identified by the demonstration of the left lumbar zygapophyseal joints at the midline of the vertebral bodies. The duodenal bulb and descending duodenum are in profile, and the long axis of the stomach is partially foreshortened, with a partially closed lesser curvature (see Figure 12-12).

• Asthenic habitus. The patient has been rotated approximately 40 degrees, as identified by the demonstration of the left lumbar zygapophyseal joints in the anterior third of the vertebral bodies. The duodenal bulb and descending duodenum are in profile, the long axis of the stomach is demonstrated without foreshortening, and the lesser curvature is open (see Figure 12-13).

• To place the pylorus in the center of the exposure field, center a perpendicular central ray halfway between the vertebrae and lateral rib border of the elevated side at a level 1 to 2 inches (2.5 to 5 cm) superior to the inferior rib margin for the sthenic habitus. Center the central ray at a level 2 inches (5 cm) superior to the sthenic patient centering for the hypersthenic habitus, and at a level 2 inches inferior for the asthenic habitus. Center the IR to the central ray.

• Open the longitudinally collimated field the full 14-inch (35-cm) IR length. Transverse collimation should be to the vertebrae and lateral rib border.

• An 11- × 14-inch (28- ×35-cm) IR placed lengthwise should be adequate to include all the required anatomic structures.

• Hypersthenic habitus. The stomach is aligned almost horizontally, with the duodenal bulb at the level of T11 to T12. The lesser and greater curvatures are demonstrated almost on end, with the greater curvature being more anteriorly situated and the esophagogastric junction almost on end (see Figure 12-15).

• Sthenic habitus. The stomach is aligned almost vertically, with the duodenal bulb at the level of L1 to L2. The stomach is somewhat J-shaped and its long axis is partially foreshortened. The lesser and greater curvatures, esophagogastric junction, pylorus, and duodenal bulb are in partial profile (see Figure 12-16).

• Asthenic habitus. The stomach is aligned vertically, with the duodenal bulb at the level of L3 to L4. The stomach is J-shaped and its long axis is demonstrated without foreshortening. The lesser and greater curvatures, esophagogastric junction, pylorus, and duodenal bulb are in profile (see Figure 12-17).

• To position the pylorus in the center of the collimated field for the sthenic habitus, center a perpendicular central ray halfway between the vertebrae and left lateral rib border at a point approximately 1 to 2 inches (2.5 to 5 cm) superior to the lower rib margin. For the hypersthenic habitus, direct the central ray just to the left of the vertebrae at a level 2 inches (5 cm) superior to the sthenic habitus centering point. For the asthenic habitus, direct the central ray 2 inches inferior to the sthenic habitus centering point. Center the IR to the central ray.

• Open the longitudinally collimated field the full 14-inch (35-cm) IR length. Transverse collimation should be to the vertebrae and lateral rib border.

• An 11- × 14-inch (28- × 35-cm) IR placed lengthwise should be adequate to include all the required anatomic structures.

Posteroanterior Stomach and Duodenum Projection Analysis

• Hypersthenic habitus. The duodenal bulb and descending duodenum are in profile, and the long axis of the stomach demonstrates foreshortening with a closed lesser curvature (see Figure 12-19).

• Sthenic habitus. The duodenal bulb and descending duodenum are in profile, and the long axis of the stomach is partially foreshortened with a partially closed lesser curvature (see Figure 12-20).

• Asthenic habitus. The duodenal bulb and descending duodenum are in profile, the long axis of the stomach is demonstrated without foreshortening, and the lesser curvature is open (see Figure 12-21).

• To place the pylorus in the center of the exposure field, center a perpendicular central ray halfway between the midcoronal plane and anterior abdomen at the level of the inferior rib margin for the sthenic habitus. For the hypersthenic habitus, direct the central ray at a level 2 inches (5 cm) superior to the sthenic habitus centering point. For the asthenic habitus, direct the central ray 2 inches inferior to the sthenic habitus centering point. Center the IR to the central ray.

• Open the longitudinally collimated field the full 14-inch (35-cm) IR length. Transverse collimation should be to the vertebrae and anterior abdomen border.

• An 11- × 14-inch (28- × 35-cm) IR placed lengthwise should be adequate to include all the required anatomic structures.

Lateral Stomach and Duodenum Projection (Right Lateral Position) Analysis

• Hypersthenic habitus. The patient has been rotated 60 degrees, as identified by the demonstration of the left lumbar zygapophyseal joints in the posterior third of the vertebral bodies. The duodenal bulb and descending duodenum are in profile, and the pylorus is superimposed over the vertebrae (see Figure 12-23).

• Sthenic habitus. The patient has been rotated 45 degrees, as identified by the demonstration of the left lumbar zygapophyseal joints at the midline of the vertebral bodies. The duodenal bulb and descending duodenum are in profile, and the vertebrae are demonstrated with little if any pyloric superimposition (see Figure 12-24).

• Asthenic habitus. The patient has been rotated 30 degrees, as identified by the demonstration of the left lumbar zygapophyseal joints in the anterior third of the vertebral bodies. The duodenal bulb and descending duodenum are in profile, and the vertebrae are demonstrated with little if any pyloric superimposition (see Figure 12-25).

• To position the pylorus in the center of the exposure field, center a perpendicular central ray halfway between the vertebrae and left abdominal margin at a level midway between the xiphoid process and inferior rib margin for the sthenic habitus. Center the central ray at a level 2 inches (5 cm) superior to the sthenic habitus central ray centering for the hypersthenic habitus and 2 inches inferior for the asthenic habitus. Center the IR to the central ray.

• Open the longitudinally collimated field the full 14-inch (35-cm) IR length. Transverse collimation should be to the vertebrae and lateral rib border.

• An 11- × 14-inch (28- × 35-cm) IR placed lengthwise should be adequate to include all the required anatomic structures.

Posteroanterior Oblique Stomach and Duodenum Projection (Left Posterior Oblique Position) Analysis

• Hypersthenic habitus. The stomach is aligned almost horizontally, with the duodenal bulb at the level of T11 to T12. The lesser and greater curvatures are demonstrated almost on end, with the greater curvature being more anteriorly situated, and the esophagogastric junction is almost on end (see Figure 12-27).

• Sthenic habitus. The stomach is aligned almost vertically, with the duodenal bulb at the level of L1 to L2. The stomach is somewhat J-shaped, and its long axis is partially foreshortened. The lesser and greater curvatures, esophagogastric junction, pylorus, and duodenal bulb are in partial profile (see Figure 12-28).

• Asthenic habitus. The stomach is aligned vertically, with the duodenal bulb at the level of L3 to L4. The stomach is J-shaped, its long axis is demonstrated without foreshortening, and the lesser and greater curvatures, esophagogastric junction, pylorus, and duodenal bulb are demonstrated in profile (see Figure 12-29).

• To position the pylorus in the center of the exposure field, center a perpendicular central ray halfway between the vertebrae and left abdominal margin at a level midway between the xiphoid process and inferior rib margin sthenic habitus. Center the central ray just medial to the left side of the vertebrae at a level 2 inches (5 cm) superior to the sthenic patient centering for the hypersthenic habitus and at a level 2 inches inferior for the asthenic habitus. Center the IR to the central ray.

• Open the longitudinally collimated field the full 14-inch (35-cm) IR length. Transverse collimation should be to the lateral rib border.

• An 11- × 14-inch (28- × 35-cm) IR placed lengthwise should be adequate to include all the required anatomic structures.

SMALL INTESTINE: POSTEROANTERIOR PROJECTION

See Figures 12-31, 12-32, 12-33 and Box 12-10.

A marker indicating the amount of time that has elapsed since the patient ingested the contrast medium is included within the collimated field and is not superimposed over anatomic structures of interest.

The abdomen demonstrates a PA projection. The spinous processes are aligned with the midline of the vertebral bodies, the distances from the pedicles to the spinous processes are the same on both sides, and the iliac alae are symmetrical.

The small intestine is centered within the exposure field. The stomach and proximal aspects of the small intestine are included within the collimated field on projections taken early in the series, and the small intestine and cecum are included on projections taken later in the series.

• Good double-contrast lower intestinal filling. Good gaseous distention is demonstrated when the bowel lumina are distended, eliminating the mucosal folds and allowing all parts of the barium-coated mucosal lining of the colon and any small intraluminal lesions to be visualized. Good lower intestinal barium coating has been obtained when the surface positioned farther from the IR on recumbent projections or superiorly on erect projections, also called the nondependent surface, demonstrates a thin layer of barium coating on the mucosal surface, and when the surface positioned closer to the IR on recumbent projections or inferiorly on erect projections, also called the dependent (decubitus) surface, demonstrates a thin layer of barium coating of the highest structures, with barium pooled in the lower crevices. The barium pools are used to wash away residual fecal material from the dependent surface, coat the mucosal surface, and fill any depressed lesions as the patient is rotated. Ideally, barium should fill one third of the large intestine diameter; overfilling or underfilling may result in obscured lesions or inadequate intestinal washing, respectively. See Table 12-3 to determine where barium pooling will occur on a lower intestine image. Pooling will occur in the anterior surface on prone projections, posterior surface on supine projections and, inferiorly, between the mucosal folds, on erect projections.

• Poor double contrast. Poor gaseous distension results in pockets of large barium pools, compacted intestinal segments with tight mucosal folds. Poor mucosal coating is demonstrated by thin, irregular, or interrupted barium coating or excessive barium pooling. Poor coating may cause lesions to be easily missed. Although proper double-contrast filling is primarily the fluoroscopist's responsibility, the technologist's scope of practice may play a part in some of the causes for poor coating, such as using the wrong type of barium, improperly preparing the barium suspension, or performing poor lower intestinal preparation.

• To obtain a PA large intestine projection, place the patient prone on the imaging table. To obtain an AP large intestine projection, place the patient supine on the imaging table. Position the shoulders and ASISs at equal distances from the imaging table to prevent rotation, and draw the patient's arms away from the abdominal area to prevent them from being superimposed over the abdominal region (Figure 12-37).

• Detecting abdominal rotation. Rotation is effectively detected on a PA or AP lower intestine projection by comparing the distances from the pedicles to the spinous processes on both sides, the symmetry of the iliac ala, and the superimposition of the colic flexures.

    PA projection. The side demonstrating the greater distance from the pedicles to the spinous processes, wider iliac ala, and colic flexure with greater ascending and descending limb superimposition is the side positioned farther from the IR (see Image 12).

    AP projection. The side demonstrating the greater distance from the pedicle to the spinous processes, wider iliac ala, and colic flexure with the greater ascending and descending limb superimposition is the side positioned closer to the IR.

    The beam divergence causes very different-appearing iliac alae on an image that is obtained in a supine versus a prone position. Figure 12-38 demonstrates the iliac alae of a supine and prone abdomen; note that the iliac alae in the image obtained with the patient supine are wider than with the patient prone. This information can be used to distinguish whether the image was taken with the patient in a supine or prone position. The narrow iliac ala of a prone projection should not be mistaken for narrowness caused by rotation.

• To position the fourth lumbar vertebra in the center of the exposure field, center a perpendicular central ray with the patient's midsagittal plane at the level of the iliac crest. Center the IR to the central ray. The longitudinal collimated field should remain fully open. Transversely collimate to within 0.5 inch (0.6 cm) of the patient's lateral skin line.

• IR size and direction. A 14-× 17-inch (35- × 43-cm) lengthwise IR should be adequate to include all the required anatomic structures on sthenic and asthenic patients as long as the transverse abdominal measurement is less than 14 inches (35 cm).

    Use two 14- × 17-inch (35- × 43-cm) crosswise IRs on hypersthenic patients and on other patients who have a transverse abdominal measurement of 14 inches (35 cm) or more to include all the necessary anatomic structures (Figure 12-39; see Image 13). Take the first projection with the central ray centered to the midsagittal plane at a level halfway between the symphysis pubis and ASIS. Position the bottom of the second IR so that it includes 2 to 3 inches (5 to 7.5 cm) of the same transverse section of the peritoneal cavity imaged on the first projection to ensure that no middle peritoneal information has been excluded. The top of the IR should extend to the patient's xiphoid (which is at the level of the tenth thoracic vertebra) to make sure that the left colic (splenic) flexure is included.

Posteroanterior or Anteroposterior Large Intestine Projection Analysis

• A grid and lead sheet are placed on the imaging table at the edge of the posteriorly collimated field to reduce the amount of scatter radiation that reaches the IR, providing higher contrast and better visibility of recorded details.

• To obtain a lateral sacral projection, place the patient on the imaging table in a lateral recumbent position. Whether the patient is lying on the right or left side is not significant, although the left side positioning is easier for the technologist.

    Flex the patient's knees and hips for support, and position a pillow or sponge between the knees. The pillow or sponge should be thick enough to prevent the side of the pelvis situated farther from the IR from rotating anteriorly, without being so thick as to cause this side to rotate posteriorly (Figure 12-41).

    To avoid rectal and vertebral rotation, align the shoulders, posterior ribs, and posterior pelvis perpendicular to the imaging table and IR. This is accomplished by resting your extended flat palm against each structure individually and adjusting the patient's rotation until your hand is positioned perpendicular to the imaging table.

• Detecting rotation. Rotation can be detected on a lateral rectum projection by evaluating the degree of femoral head superimposition. On a nonrotated lateral rectal projection, the femoral heads are directly superimposed. On rotation, the femoral heads will move away from each other. When rotation has occurred, evaluate the placement of the femoral heads to determine the way in which the patient was rotated. The femoral head that demonstrates the greater magnification is the one situated farther from the IR (see Images 14 and 15).

• To place the rectosigmoid region in the center of the exposure field, center a perpendicular central ray to the midcoronal plane (between the ASIS and posterior sacrum) at the level of the ASIS. Center the IR to the central ray.

• Open the longitudinal and transverse collimation to the full IR field size.

• A 10- × 12-inch (24- × 30-cm) IR placed lengthwise should be adequate to include all the required anatomic structures.

Lateral Large Intestine (Rectum) Projection Analysis

• Place the marker superiorly, away from anatomic structures of interest, and within the collimated field.

• Using a wedge-compensating filter. When an AP-PA (lateral decubitus) large intestine projection is obtained in a patient with excessive abdominal soft tissue, the soft tissue often drops toward the imaging table or cart. This movement results in a smaller AP measurement at the elevated side than at the side closer to the imaging table or cart. To compensate for this thickness difference, a wedge-compensating filter may be used. Attach the wedge-compensating filter to the x-ray collimator head with the thick end positioned toward the patient's “up” side (thinnest part of abdomen) and the thin end toward the patient's “down” side (thickest part of abdomen). Then set a technique that will accurately expose the middle section of the abdomen. When the filter has been accurately positioned, image density is uniform throughout the abdominal structures. Positioning the filter too close to or too far away from the thickest part of the abdomen results in an overexposed or underexposed area on the image, respectively, and a line of density difference. If the compensating filter is inaccurately positioned, a density variation line will appear, defining where the filter was and was not placed over the structures (see Image 16).

• AP or PA large intestine projections (lateral decubitus position) are obtained by placing the patient in left and right lateral recumbent positions on the imaging table or cart with the back or abdomen resting against a grid cassette or the upright IR holder. To avoid rotation, align the shoulders, the posterior ribs, and the posterior pelvis perpendicular to the imaging table or cart (Figure 12-44). Accomplish this alignment by resting an extended flat hand against each, respectively, and then adjusting the patient's rotation until the hand is positioned perpendicular to the imaging table or cart. Flex the patient's knees to support the patient's lateral position, although do not bring them to a 90-degree angle with the torso or they may be superimposed over the lateral aspect of the distal rectum (see Image 17). It is most common for a patient to rotate the elevated thorax and iliac ala anteriorly. A pillow or other support placed between the patient's flexed knees may help eliminate this forward rotation.

• Detecting abdominal rotation. Rotation is effectively detected on a PA or AP lower intestine projection (lateral decubitus position) by comparing the distance from the pedicles to the spinous processes on each side, the symmetry of the iliac ala, and the superimposition of the colic flexures.

    PA decubitus projection. The side demonstrating the greater distance from the pedicles to the spinous processes, wider iliac ala, and colic flexure with greater ascending and descending limb superimposition is the side positioned farther from the IR (see Image 18).

    AP decubitus projection. The side demonstrating the greater distance from the pedicle to the spinous processes, wider iliac ala, and colic flexure with greater ascending and descending limb superimposition is the side positioned closer to the IR (see Image 19).

• Elevating the patient on a radiolucent sponge or hard surface such as a cardiac board positions the patient's abdomen above the IR's cassette border, preventing part of the abdomen from being clipped and preventing the abdomen from sinking into the table or cart pad. When the patient's body is allowed to sink into the cart pad, artifact lines are superimposed over the lateral abdominal field of the side that is down (see Image 17).

• To position the fourth lumbar vertebra in the center of the exposure field, center a perpendicular central ray with the patient's midsagittal plane at the level of the iliac crest. Center the IR to the central ray.

• The longitudinal collimated field should remain fully open. Transversely collimate to within 0.5 inch (1.25 cm) of the patient's lateral skin line.

• IR size and direction. A 14- × 17-inch (35- × 43-cm) lengthwise IR should be adequate to include all the required anatomic structures on sthenic and asthenic patients, as long as the transverse abdominal measurement is less than 14 inches (35 cm).

• Use two 14- × 17-inch (35- × 43-cm) crosswise IRs on hypersthenic patients and on other patients who have a transverse abdominal measurement of 14 inches (35 cm) or more to include all the necessary anatomic structures. Take the first projection with the central ray centered to the midsagittal plane at a level halfway between the symphysis pubis and ASIS. Position the bottom of the second IR so that it includes 2 to 3 inches (5 to 7.5 cm) of the same transverse section of the peritoneal cavity imaged on the first projection to ensure that no middle peritoneal information has been excluded. The top of the IR should extend to the patient's xiphoid (which is at the level of the tenth thoracic vertebra) to make sure that the left colic (splenic) flexure is included.

Anteroposterior or Posteroanterior Large Intestine Projection (Lateral Decubitus Position) Analysis

• A PA oblique large intestine projection (RAO position) is obtained by positioning the patient prone on the imaging table and then rotating the torso toward the right side until the midcoronal plane is at a 35- to 45-degree angle with the imaging table. In the PA projection, the descending limb of the right colic (hepatic) flexure is superimposed over the ascending limb and the rectum is superimposed over the distal sigmoid. Rotating the patient toward the right side moves the ascending right colic limb from beneath the descending limb and the distal sigmoid from beneath the rectum (transversely), allowing better visualization of these structures. The left elbow and knee are partially flexed and are used to support the patient and maintain accurate obliquity (Figure 12-46).

• Detecting inadequate rotation on a PA oblique projection. Insufficient rotation of the colon is demonstrated on the PA oblique projection when the ascending and descending limbs of the right colic flexure are superimposed and the rectum is superimposed over the distal sigmoid (see Image 20).

• To place the midabdomen in the center of the exposure field, center a perpendicular central ray approximately 1 to 2 inches (2.5 to 5 cm) to the left of the midsagittal plane at the level of the iliac crest. Center the IR to the central ray.

• The longitudinal collimated field should remain fully open. Transversely collimate to within 0.5 inch (1.25 cm) of the patient's lateral skin line.

• IR size and direction. A 14- × 17-inch (35- × 43-cm) IR placed lengthwise should be adequate to include all the required anatomic structures.

Posteroanterior Oblique Large Intestine Projection (Right Anterior Oblique Position) Analysis

• A PA oblique large intestine projection (left anterior oblique [LAO]) is obtained by positioning the patient prone on the imaging table and then rotating the torso toward the left side until the midcoronal plane is at a 35- to 45-degree angle with the imaging table. In the PA projection, the descending limb of the left colic (splenic) flexure is superimposed over the ascending limb, and the rectum is superimposed over the distal sigmoid. Rotating the patient toward the left side moves the descending left colic limb from beneath the ascending limb, allowing better visualization of these structures. The right elbow and knee are partially flexed and are used to support the patient and maintain accurate obliquity (Figure 12-48).

• Detecting inadequate rotation on a PA oblique projection. Insufficient rotation of the colon is demonstrated on the PA oblique projection when the ascending and descending limbs of the left colic flexure are superimposed.

• To place the midabdomen in the center of the exposure field, center a perpendicular central ray approximately 1 to 2 inches (2.5 to 5 cm) to the right of the midsagittal plane at the level 1 to 2 inches (2.5 to 5 cm) superior to the iliac crest. Center the IR to the central ray.

• The longitudinal collimated field should remain fully open. Transversely collimate to within 0.5 inch (1.25 cm) of the patient's lateral skin line.

• IR size and direction. A 14- × 17-inch (35- × 43-cm) lengthwise IR placed lengthwise should be adequate to include all the required anatomic structures.

• A PA axial large intestine projection is obtained by positioning the patient prone on the imaging table with the legs extended. Position the shoulders and ASISs at equal distances from the imaging table to prevent rotation (Figure 12-50).

• Detecting rotation on a PA axial projection. Rotation is effectively detected on a PA axial large intestine projection by evaluating the symmetry of the iliac alae and obturator foramen. If the patient was rotated away from the prone position, the iliac ala positioned farther from the IR will increase in width, the iliac ala positioned closer to the IR will narrow, the obturator foramina positioned farther from the IR will narrow, and the obturator foramina positioned closer to the IR will widen.

• A PA axial oblique large intestine projection (RAO position) is obtained by positioning the patient prone on the imaging table and then rotating the torso toward the right side until the midcoronal plane is at a 35- to 45-degree angle with the imaging table. In the PA projection, the rectum is superimposed over the distal sigmoid, obscuring the rectosigmoid junction. Rotating the patient toward the right side moves the sigmoid from beneath the rectum (transversely), allowing better demonstration of this area. The left elbow and knee are partially flexed and are used to support the patient and maintain accurate obliquity (Figure 12-51).

• Detecting inadequate rotation on a PA axial oblique projection. Insufficient pelvic rotation of the rectosigmoid and pelvic area is demonstrated on the PA axial oblique projection when the rectum is superimposed over the sigmoid colon, the right SI joint is too medial to the ASIS, and the left obturator foramen is narrowed. Too much pelvic rotation is demonstrated when the right SI joint is obscured and the left obturator foramen is closed (see Images 21 and 22).

• The PA axial oblique (RAO position) and PA axial projections of the large intestine are obtained to demonstrate the rectosigmoid area with less superimposition. To move the posteriorly situated rectum inferiorly and off the distal sigmoid, a 30- to 40-degree caudal angulation is used, decreasing rectosigmoid superimposition and better demonstrating the area. This angulation also elongates the pelvic structures.

• Inadequate angulation. When the rectosigmoid segment demonstrates inferosuperior overlap, the central ray angulation used was inadequate. If the inferior aspect of the left acetabulum is demonstrated superior to the distal rectum, the central ray was insufficient (see Image 21). If the inferior aspect of the left acetabulum is demonstrated inferior to the distal rectum, the central ray angulation was too great (see Image 22).

• PA axial projection. To place the rectosigmoid area at the center of the exposure field, center the central ray to exit at the level of the ASIS and to the midsagittal plane.

• PA axial oblique projection. To place the rectosigmoid area in the center of the field, center the central ray to the exit at the ASIS and 2 inches (5 cm) to the left of the lumbar spinous processes. Center the IR to the central ray.

• An 11- × 14-inch (28- × 35-cm) or 14- × 17-inch (35- × 43-cm) IR placed lengthwise should be adequate to include all the required anatomic structures.

Posteroanterior Axial Oblique Large Intestine Projection (Right Anterior Oblique Position) Analysis