4

Abdomen

David M. Kuehn

Chapter Outline

Viewing Abdominal Radiographs

Evaluating the Intestinal Air or Gas Pattern

Too Much Bowel Gas

Too Little Bowel Gas

Gas in the Wrong Places

Gastrointestinal Contrast Studies

Upper Gastrointestinal Series

Antegrade Small Bowel Examination

Enteroclysis

Retrograde Small Bowel Examination

Barium Enema

Study of Gallbladder and Biliary Tract

Urinary Tract Examinations

Excretory Urography and Computed Tomography Urography

Other Urinary Tract Examinations

Abdominal Ultrasound

Computed Tomography and Magnetic Resonance Imaging of the Abdomen

How to Read Abdominal Computed Tomography and Magnetic Resonance Imaging

Angiography

Imaging Features of Gastrointestinal Abnormalities Using Traditional Contrast Radiographs

Imaging Features of Genitourinary Abnormalities

Obstetric and Gynecologic Imaging

Imaging Accessory Digestive Organs

Special Problems in Abdominal Imaging

Trauma

Gastrointestinal Bleeding

Acute Abdomen

Key Points

Careful history and physical examination allow diagnosis of most abdominal complaints. When diagnosis remains uncertain following these procedures, an abdominal radiograph is often the first diagnostic imaging procedure requested. Recall that in women of childbearing age, consideration of possible pregnancy should precede a radiograph.

The anteroposterior (AP) radiograph (often referred from “KUB,” i.e., kidney, ureter, bladder), the most frequently performed abdominal imaging study, is performed with the patient supine (Fig. 4.1A). An upright radiograph (Fig. 4.1B) is useful in searching for free intraperitoneal air and/or intestinal air–fluid levels. If the patient cannot stand, a decubitus radiograph obtained with the patient lying on either the right or, preferably, the left side (Fig. 4.1C) can be substituted.

VIEWING ABDOMINAL RADIOGRAPHS

Step 1 is to position the radiograph correctly on the imaging reader device, with the film R (right side) marker opposite the viewer’s left side and the patient’s head toward the top of the film. On the AP upright radiograph, there should be a sign indicating an upright view, usually an arrow near the R or L marker, pointing toward the patient’s head. Similarly, decubitus radiographs should be clearly labeled as such and should note which side is up or down.

Step 2 is to glance at the entire radiograph in a relaxed manner to allow an obvious abnormality to jump out at you. When you do discover an abnormality, do not terminate your subsequent search.

Step 3 is to evaluate the radiograph systematically. Any system or checklist will suffice. Table 4.1 will work, until you develop your own. First, locate the water density liver and spleen silhouettes. One clue to locating liver and spleen edges is the presence of bowel gas in the right and left upper abdominal quadrants. Such bowel gas permits an indirect estimate of the location of the hepatic and splenic borders, because the gas is located at the lower edges of the liver and spleen. With a little experience, you will recognize a normal-sized liver. When the liver shadow extends to the iliac crest, it is usually enlarged. Also with more experience, you will readily detect an enlarged spleen (splenomegaly; Fig. 4.2).

In the normal radiograph, psoas muscle margins are usually visible. A nonvisible psoas margin should alert you to a possible abnormality adjacent to that structure. As your eyes drift toward the renal shadows, evaluate their size, shape, and position. The renal silhouettes are visible because they are water density structures (gray) surrounded by variable amounts of retroperitoneal fat (black). You should attempt to locate the upper and lower renal poles as well as their medial and lateral borders. If the renal long axis is not parallel with the psoas muscle margin, you should consider a mass or other water density abnormality in the kidney or the retroperitoneum. Always look for calcifications (white) in the abdomen, especially in the region of the kidneys, ureters, urinary bladder, and the gallbladder (discussed later).

The term Aunt Minnie, coined by the late Dr. Ben Felson, refers to the unmistakable and unforgettable appearance of your Aunt Minnie, or Uncle Al, or any other family character. A radiologic Aunt Minnie describes an image appearance so classic that, once you see it, you never forget it. The following abdominal radiographic Aunt Minnies (Figs. 4.3–4.9) are commonly encountered. File them away in your visual-cerebral computer, and your ability to recognize them will make you a star in the eyes of your colleagues, teachers, and patients.

Now, evaluate the bowel gas pattern (see the next section). Last but not least, look at the bones systematically, beginning with the visible ribs and spine (Fig. 4.10). Study the pedicles of the lower dorsal and lumbar spine, proceeding from head to foot. They resemble automobile headlights on an AP radiograph. A missing pedicle indicates a destructive process, such as metastatic disease. Evaluate all visible bones, including the pelves, hips, and femurs, for their overall density and any abnormalities.

Use a similar search system for the AP upright abdominal radiograph while being especially alert for free air beneath the diaphragms. Free intraperitoneal air is usually visualized only on an upright radiograph, because only this position allows free air to rise to the subdiaphragmatic regions.

EVALUATING THE INTESTINAL AIR OR GAS PATTERN

Intestinal gas (black) provides a natural contrast medium that can be useful for detecting abdominal disease. When evaluating the intestinal gas pattern, ask yourself several important questions. Is the bowel gas pattern normal? Remember that there is normally some air or gas in the stomach, small intestine, colon, and rectum. With experience, you will begin to recognize abnormal amounts of air in the gastrointestinal (GI) tract. This is similar to recognizing a normal heart on a chest radiograph. If the gas pattern is not normal, ask more questions. Is there too much or too little air? Is the air in the wrong place?

Too Much Bowel Gas

Here, where differential diagnosis includes adynamic ileus and bowel obstruction, we need a systematic approach to arrive at the correct diagnosis. In adynamic ileus (also referred to as paralytic ileus or just ileus), there is too much bowel gas in the entire GI tract, including the small and large intestines (Fig. 4.11). Adynamic ileus may arise from intra-abdominal cases or as a reflex phenomenon from disease elsewhere. The multiple causes are listed in Table 4.2. If you identify comparable amounts of gas in the small and large intestines and in the rectum, this generally indicates adynamic ileus. Air in the rectum may be a key differential point.

In intestinal obstruction, another reason for too much bowel gas, there is usually air-filled, dilated intestine proximal to the point of obstruction and little or no air distal to the obstruction (Fig. 4.12). In both ileus and obstruction, often the dilated small and large bowels containing too much air will have air–fluid levels noted on upright and decubitus radiographs.

If a diagnosis of obstruction versus adynamic ileus is not readily apparent, it is necessary to obtain additional studies to arrive at the correct diagnosis. These include barium studies, computed tomography (CT) (Fig. 4.13), and ultrasound (US). Note the relative ease of identifying small versus large bowel using CT.

If you diagnose intestinal obstruction, you next need to determine the location of the obstruction. Is the obstruction in the small or large bowel? In small bowel obstruction, there are loops of dilated small bowel proximal to the obstruction site and little or no gas in the colon or the rectum. In large bowel obstruction, there is dilated colon proximal to the obstruction site but little or no air distally and minimal air in the rectum.

Sometimes it is difficult to differentiate dilated small bowel from large bowel. One way is to identify the valvulae conniventes and colon haustra. Valvulae conniventes are regularly spaced, thin mucosal folds that extend across the entire small bowel lumen (see Fig. 4.12). On the other hand, the colon can usually be identified by the somewhat irregularly spaced transverse bands, called colon septa or haustral folds, that do not extend completely across the colon lumen (see Fig. 4.11).

Sigmoid volvulus is a dramatic clinical event that occurs predominantly in elderly patients with a long history of constipation. The chronic constipation results in a redundant sigmoid mesentery that has the potential to twist on itself like a garden hose. If twisting occurs, there is complete or partial obstruction, and an abdominal radiograph shows a dramatically dilated sigmoid colon. Barium enema is confirmatory with complete obstruction to the retrograde flow of barium at the site of the twist (Fig. 4.14). The obstruction can often be relieved by gently passing a sigmoidoscope past the point of the obstruction or twist.

Too Little Bowel Gas

When the abdominal radiographs show a paucity or absence of bowel gas, the differential diagnosis listed in Table 4.3 should be entertained.

Gas in the Wrong Places

There are several situations in which air is found outside of the intestinal lumen (Table 4.4). Free air in the peritoneal cavity results from any process that perforates the intestinal tract. AP supine and upright abdominal radiographs should be performed if there is clinical suspicion of gut perforation. The upright position allows free intraperitoneal air to rise to the subdiaphragmatic regions of the abdomen (Fig. 4.15). If the upright view is not possible owing to the patient’s condition, a decubitus radiograph will suffice. On a decubitus radiograph, the air rises to the nondependent portion of the peritoneal cavity (Fig. 4.16). Either technique has the potential to identify as little as 2 cc of free intraperitoneal air, as long as the patient is in the upright or decubitus position approximately 5 minutes prior to the radiograph.

Another example of air in the wrong place is pneumatosis intestinalis (Fig. 4.17). Causes of this are listed in Table 4.5. Gas-filled abscesses can be found in any location, including the abdomen (Fig. 4.18).

GASTROINTESTINAL CONTRAST STUDIES

For inspection of the mucosal surface of the esophagus, stomach, and duodenum, endoscopy is often preferred. To evaluate the gut lumen and wall, traditional radiologic GI contrast studies are accurate, safe, and less expensive than the endoscopic studies and enjoy excellent patient acceptance. These studies consist of fluoroscopy and radiographs obtained following introduction of barium sulfate (metallic density or white) and/or air (black) into the GI tract.

Upper Gastrointestinal Series

For an upper GI series, the patient swallows liquid barium, often combined with gas-producing crystals, under fluoroscopy to visualize the esophagus, stomach, and small intestine (Fig. 4.19). When both barium and air are used, the process is referred to as a double-contrast study. When barium is used alone, it is a single-contrast study. Preparation for an upper GI series consists simply of nothing by mouth (non per os [NPO]) for 8 to 12 hours prior to the study. When perforation of the upper GI tract is suspected, water-soluble contrast media is used.

Antegrade Small Bowel Examination

The usual small bowel examination is performed after an upper GI series by having the patient drink additional barium. Serial radiographs of the abdomen are performed at 15- to 30-minute intervals thereafter to evaluate the small bowel as barium passes through (Fig. 4.20). Fluoroscopy is commonly used as a supplement to study the terminal ileum when barium begins to enter the colon or to further investigate abnormalities seen on the serial radiographs.

Enteroclysis

Enteroclysis is a focused examination of the small intestine, wherein air and barium sulfate are introduced directly into the small intestine via a nasointestinal tube. Under fluoroscopy, the tip of the tube is placed just beyond the duodenal–jejunal junction and contrast is injected (Fig. 4.21). Advantages of this procedure are that the small bowel can be distended and the stomach and duodenum do not obstruct visualization. The main disadvantages are the discomfort associated with a nasal tube and the radiation exposure.

Retrograde Small Bowel Examination

On occasion, especially when disease of the terminal ileum is suspected and previous examinations are nondiagnostic, barium can be refluxed from a filled colon into the ileum. Although the procedure is useful, there is considerable patient discomfort alleviated slightly by antispasmodic agents.

Barium Enema

Introduction of barium sulfate and/or air into the colon via a rectal tube is called a lower GI series or barium enema. For this study, it is important to have a clean colon; this is best accomplished with laxatives and large amounts of orally ingested fluids. Barium and often air are administered via a rectal tube under fluoroscopic observation. When both air and barium are used, it is called a double-contrast study (Fig. 4.22), whereas barium alone is a single-contrast study. A properly performed barium enema has minimal associated discomfort. The double-contrast study is preferred to evaluate intraluminal and mucosal diseases, such as small ulcers and polyps. Again, if colon perforation is suspected, a water-soluble contrast medium is used.

Colonoscopy, an expensive alternative to colon barium studies, can directly visualize the mucosa. However, it requires conscious sedation because of patient discomfort. Virtual colonoscopy is an examination of the entire colon using multidetector CT and a dedicated software program so that the colon is displayed throughout its length with stacked images created to form a three-dimensional (3D) picture of the colon at each level. The examination is usually performed after administration of an agent that tags fecal material, which can then be subtracted from the viewed images. Before the examination begins, the colon is insufflated with air so that the images resemble the interior view of the colon as would be seen by endoscopy. Virtual colonoscopy has the ability to discover almost all colon cancers (Fig. 4.23) as well as larger polyps (Fig. 4.24) (which are premalignant). The examination takes but a few minutes and does not require the sedation and analgesics required for optical colon endoscopy. As experience with the technique has increased, it seems more accurate than barium enema techniques and perhaps as good as optical colonoscopy. Its disadvantages are the use of radiation and probably reduced detectability of flat mucosal lesions. It is much less expensive than optical colonoscopy. At the time of this writing, indication for its use as a substitute for screening optical colonoscopy is imprecise, but considerable improvement of the technique is anticipated.

STUDY OF GALLBLADDER AND BILIARY TRACT

In years past, the oral cholecystogram was performed to visualize the gallbladder following the oral ingestion of special iodinated compounds that are excreted into the biliary system and subsequently concentrated in the gallbladder. The study is seldom performed now because of the greater accuracy of US. With US, one can examine the liver and biliary tract as well as the gallbladder. CT and magnetic resonance imaging (MRI) are needed in certain situations to complement US.

In endoscopic retrograde cholangiopancreatography (ERCP), the endoscopist passes a fiberoptic scope under fluoroscopic control antegrade through the esophagus, stomach, and duodenum and retrogrades into the common bile duct. The pancreatic ducts can also be cannulated. Contrast media can be injected into any of these structures and appropriate radiographs obtained (Fig. 4.25). ERCP is usually performed when less-invasive studies (CT, US, MRI, or contrast studies) are indeterminate or nondiagnostic or as part of a therapeutic endoscopic procedure.

URINARY TRACT EXAMINATIONS

The first methodology to examine the urinary tract directly (about 1,900 present) was to inject radiopaque material directly into the bladder or other urinary structures (retrograde cystography or pyelography) at the time of cystoscopy. It was later discovered that intravenous contrast material that is excreted by the kidneys could be given with relative safety; excretory urography (EU) was developed shortly thereafter. Other names given for EU are intravenous urogram and intravenous pyelogram. Multislice CT has now evolved as the new standard, CT urography (CTU). A multifaceted radiologic approach to genitourinary (GU) problems is now possible, with supplemental US and MRI examination.

EU with traditional radiographs, although still a useful technique, is performed much less frequently in the investigation of GU disease than is CTU, principally because the renal parenchyma, pelvicalyceal system, ureter, and bladder can be more accurately visualized with multislice techniques. US remains a valuable technique to complement radiographic investigation.

Excretory Urography and Computed Tomography Urography

Both EU and CTU involve the administration of an intravenous contrast medium that is excreted by the kidneys. In CTU the contrast medium is delivered in bolus fashion to maximize renal parenchymal visualization. In EU one recognizes the urinary structures as seen through overlying bowel gas, soft tissue, and so on. In CTU, multidetector CT is utilized, permitting visualization of urinary structures without overlying structures. A further advantage is the ability to reconstruct images in any plane—axial, coronal, or sagittal. A final advantage of CTU is a reconstruction technique so that the urinary tract is viewed in 3D with all other structures subtracted.

Disadvantages of CTU include a higher patient radiation dose (about double) and additional cost. Three-dimensional reconstruction requires image manipulation by a specially trained technologist. Thus, EU remains an accepted technique for most children, for many follow-up studies, and at sites without multidetector CT. Both types of examinations are featured in this chapter.

EU and CTU do not require special patient preparation, merely abstaining from food and liquids for several hours before contrast administration.

The timing of EU and CTU radiographs can be varied, depending upon the patient’s clinical problems. Nevertheless, both techniques usually require examination during the nephrographic phase (for visualization of the renal cortex), followed by image(s) of the pelvicalyceal system and bladder, which are opacified later. Delayed films can be obtained for hours, or even days, in situations such as ureteral obstruction or renal failure.

Viewing an Excretory Urogram or Computed Tomography Urogram

An EU study begins with a preliminary or scout radiograph that includes the entire abdomen. You can evaluate this preliminary radiograph using the same system as described previously. The radiographs obtained immediately following intravenous contrast media demonstrate the nephrogram phase wherein the contrast media is located in the renal capillaries, glomeruli, and proximal convoluted tubules (Fig. 4.26A). Compare the nephrograms for symmetry, as size discrepancy is suspicious for unilateral renal disease.

Next, evaluate the later postcontrast injection radiographs, at which times the contrast media is normally present in the calyces, renal pelves, portions of the ureters, and urinary bladder (Fig. 4.26B). Normal calyces are sharp in outline with various numbers and geometry. Oblique, prone, and abdominal compression radiographs are often obtained to better display portions of the urinary tract.

The CTU is evaluated for the same factors as the EU, albeit in different fashion. A computed abdominal radiograph is performed, followed by axial scans of the abdomen before, immediately after, and at a later time following the bolus of contrast material (Fig. 4.27). In special situations (e.g., study of renal donor), immediate postcontrast scans can be obtained to visualize renal arteries and later the veins. As in EU, pay attention to size and symmetry of the kidneys, pelvicalyceal systems, and bladder.

Other Urinary Tract Examinations

Direct injection of contrast material into the bladder or ureter (retrograde pyelogram) is of value when a detailed view of a portion of the ureter or pelvicalyceal system is necessary. It is often an adjunct to endoscopy.

Vesicoureteral reflux, a condition in which bladder urine refluxes in retrograde fashion into the ureters, is a common phenomenon in children but infrequent in adults. It can be associated with urinary tract infection. With the voiding cystourethrogram, contrast medium is introduced via a urethral catheter into the bladder. Subsequent fluoroscopy and filming allow one to identify and quantitate vesicoureteral reflux if present (Fig. 4.28). At the completion of the study, the patient voids, with the voiding sequence recorded in some imaging form. This allows detection of urethral abnormalities, which can produce bladder obstruction and secondary vesicoureteral reflux. Cystography and retrograde urethrography are examinations usually performed to detect urinary extravasation in trauma cases.

ABDOMINAL ULTRASOUND

US, being a different modality from x-rays, shows abdominal organs in a different fashion. There are roughly three patterns of reflected US.

1. No reflection of the sound wave. Almost all of the sound passes through the area. This is termed sonolucent and is traditionally viewed as black on images. Fluid, such as in ascites or abdominal cysts, is sonolucent.

2. Reflection and transmission of some sound. Solid organs, such as the kidney or liver, are examples. US waves are reflected, particularly at boundaries of organs of differing echogenicity, such as the boundary between the liver and the kidney.

3. Reflection of all sound. Bone, other calcifications, and air in the gut are examples. One can make use of this by noting such shadowing and the absence of echoes distal to a lesion to help diagnose gallstones and like abnormalities.

There are two major problems in learning to read US images.

1. It requires one to think differently. You are looking at differences in transmission and reflection of US rather than transmitted x-rays.

2. Orientation of the image. This is the chief stumbling block. One may consider the US image as representing a roughly pie-shaped wedge of tissues, less than 1 cm thick, below the US transducer.

Even experienced radiologists and clinicians have considerable difficulty figuring out the nature of the US image if they did not perform the study. Orientation must be provided by the person who performed the scan. In most situations, there is a relatively fixed method of performing abdominal US. In general, one evaluates each area of interest in at least two dimensions, typically axial (transverse) and longitudinal (sagittal). For technical reasons, the direction of the scan beam shows the anatomy best if it is perpendicular to the organ of interest. As few abdominal organs are 100% oriented anterior–posterior or medial–lateral, the scanned images are, to varying degrees, oblique.

Probably the best method to be introduced to US is to attend an imaging session with a knowledgeable mentor who discusses the anatomy as it is being scanned. Combined with this, learn the usual routines for US scanning in your institution and try to figure out how each image was performed. Conventionally, images are labeled as to the method by which they are done, for example, kidney—transverse.

There are many abdominal applications of US related to its widespread availability and cost (it is about half the cost of CT and about one-third that of MRI). Ultrasonography is valuable in the workup of diseases involving the liver, biliary tract, kidneys, abdominal aorta, and abdominal masses. It is particularly useful in defining fluid versus solid (e.g., cyst vs. solid mass) as well as in imaging fluid-filled structures, such as the gallbladder, urinary bladder, and renal pelvis. The various abdominal organs and pathologic processes have their own characteristic echo patterns, as shown in Figure 4.29.

Obstetric and gynecologic US is particularly important because of the absence of significant biologic risk to the fetus or maternal genital structures. In obstetric US, the fetus is surrounded by amniotic fluid, making visualization easier (Fig. 4.30). In addition, one can use real-time US images to evaluate the beating heart. For gynecologic examinations, both transabdominal (Fig. 4.31) and transvaginal techniques are used. Transvaginal imaging has the technical advantage of eliminating echoes from the abdominal wall from the area of interest, allowing better definition of genital organs (Fig. 4.32).

Diagnostic US of the prostate has been disappointing, as it is relatively insensitive to identifying abnormalities of this organ. In the scrotum, US is superb. It localizes the site of disease (e.g., testis vs. epididymis) and often allows specific diagnosis of the abnormality present (Fig. 4.33). Correct diagnosis of epididymitis versus testicular torsion versus orchitis is possible, separating those who need surgery from those who require only medical treatment. Hydrocele and varicocele are easily identified with US. Identification of testicular tumors is good, although identifying tumor type is less reliable.

Related posts:

1 Radiography, Computed Tomography, Magnetic Resonance Imaging, and Ultrasonography: Principles and Indications 12 Interventional Radiology 7 Brain 10 Nuclear Imaging 3 Chest 6 Musculoskeletal System

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