Windows and the Hounsfield Scale

The color scale assigned to tissues of various densities with CT can be varied to accentuate different tissues, a process called “windowing.” The CT color scale is a gray-shade scale, from black to white. Less dense tissues appear blacker, denser tissues appear whiter, and tissues with intermediate densities occupy intermediate gray shades. Because the human eye and brain cannot perceive subtle variations in gray shades, it is usually not useful to distribute the available gray shades uniformly across the full range of densities when displaying the image. Instead, the distribution of the gray shades is varied depending on the tissue of greatest interest. In the case of abdominal structures, the most common window setting (a “soft-tissue” window) uses a fairly widely distributed gray scale, because the abdomen contains materials of highly variable density, from the normal air in the bowel (−1000 HU) to the subcutaneous, peritoneal, and retroperitoneal fat (-50 to -100 HU), to the liquid bile and blood (~+20 HU), to muscles and abdominal solid organs (~+40 to +100 HU), to the bones of the vertebral column, pelvis, and ribs (~+200 to +1000 HU) ( Figures 9-2 and 9-3 ).

The Hounsfield CT density scale.

On the Hounsfield scale, air is assigned a value of −1000 Hounsfield units (HU), water a value of 0 HU, and bone a value of several hundred to +1000 HU. Fat has a value of −50 HU, soft tissue a value of +40 HU. The grayscale color corresponding to these values varies, depending on the window setting selected. A, Body or soft-tissue window. The Hounsfield scale is centered on the tissues of interest, with white and black at the extreme ends of the spectrum. A cursor placed on various tissues displays the Hounsfield density. B, Lung window. By moving the gray scale toward low Hounsfield values, detail of low-density lung tissue is accentuated. Detail of high-density structures is lost. Although the tissue density numeric values are unchanged, the appearance is quite different.

The Hounsfield CT density scale and fat as a contrast agent.

Familiarity with the appearance of various tissue densities on CT is simple but essential.On a noncontrast CT viewed with a soft-tissue window, air appears black, fat appears nearly black, simple fluids are dark gray, and solid tissues are intermediate gray. Bone and calcifications are bright white.Fat can be a useful “contrast agent” on CT. To identify the appearance of fat, start with the patient’s subcutaneous fat. Normal intraperitoneal and retroperitoneal fat have the same tissue density as subcutaneous fat. Notice how intraperitoneal fat separates loops of bowel and retroperitoneal fat separates the kidneys from paraspinal muscles. IVC, inferior vena cava.

The lung window shifts the gray scale to accentuate the appearance of tissues of very low density—air and lung tissue (see Figure 9-2 ). Although this window setting is designed to allow inspection of the lung parenchyma and to allow the differentiation of lung from surrounding pneumothorax, it is extremely useful in inspection of the abdomen, pelvis, and retroperitoneum for pathologic air or gas. On a typical soft-tissue window setting, small areas of abnormal air can be difficult to identify, because air (black on this setting) can appear similar to fat (a dark gray shade, depending on the exact window setting). On a lung window setting, all tissues except air or gas appear quite bright (white or nearly white) by comparison, drawing attention to air, which remains black. Even tiny quantities of free peritoneal air can be readily recognized using this setting. Air in other abnormal locations can also be more readily identified.

Throughout this chapter, unless otherwise specified, the appropriate window to evaluate for pathologic findings is the soft-tissue window. A window is defined by a center value and a width. The center value is the Hounsfield density to which the middle gray shade is assigned. The width is the number of Hounsfield units around the center value to which the brightest and darkest shades in the grayscale are assigned. The precise values used for soft-tissue and other window settings may vary from institution to institution, but common values are shown in Table 9-2 .

Fat and Air as Contrast Agents

We soon discuss extrinsically administered contrast agents, but it is useful first to consider intrinsic contrast provided by normal differences in body tissue density. When we look at a CT image, we cannot distinguish two structures in physical contact with one another that share identical ( or even similar) densities. When a tissue of a different density intervenes, the transition from one tissue to another is clearly demarcated—tissue contrast. Because air or gas is dramatically less dense than other body tissues (−1000 HU, compared with the least dense competing tissue, fat, ~−50 HU), it provides outstanding tissue contrast. As a consequence, air can be recognized without the administration of any exogenous contrast agents. CT is therefore exquisitely sensitive for the detection of abnormal air or gas, as we discuss later in the section on pneumoperitoneum and other conditions in which pathologic gas assists in diagnosis.

A second key intrinsic contrast agent is fat. Again, fat is quite different in density from most other abdominal tissues and can separate other organs of similar density (see Figures 9-2 and 9-3 ). Fat is normally present in subcutaneous tissue, within the peritoneum, and in the retroperitoneum. In these locations, it separates normal organs from one another, allowing us to recognize individual loops of bowel in the abdomen or the demarcation of retroperitoneal organs such as the psoas muscles and kidneys, even without the addition of exogenous contrast agents. In the thinnest patients, a paucity of peritoneal and retroperitoneal fat is a diagnostic disadvantage, potentially increasing the necessity of exogenous contrast agents ( Figure 9-4 ). In patients of average or somewhat above-average body mass index, fat actually provides helpful tissue contrast and may decrease the need for extrinsic contrast. Fat becomes a disadvantage in extremely obese patients, in whom it contributes to excessive image noise ( Figure 9-5 ). Some very obese patients may actually exceed the CT scanner weight, size, and x-ray tube capacity, as described later.

Fat, a useful “contrast agent” for CT.

In very thin patients, the absence of intraperitoneal fat makes it extremely difficult to distinguish structures. Oral and intravenous contrast are particularly helpful in this setting.In this noncontrast CT of a very thin patient, anatomic structures are difficult to distinguish from one another, caused by the paucity of intraperitoneal fat.

Fat stranding, CT without IV contrast.

Fat stranding is an inflammatory change in fat, resulting from extravasation of fluid from tissue lymphatic vessels. When viewed with soft-tissue windows, stranding changes the appearance of fat from its usual near-black to a hazy gray.Compare the subcutaneous fat in this patient, which shows no stranding, to the intraperitoneal fat in the right abdomen, which shows stranding. Fat in the patient’s left abdomen does not show stranding.The overall image quality of this scan is poor because of the patient’s obesity, which results in a worsened signal-to-noise ratio.

Fat Stranding

Fat provides an additional diagnostic benefit, acting as the substrate for inflammatory fat stranding (see Figure 9-5 ). When inflammation occurs, microscopic lymphatic vessels within fat become relatively leaky, and the water content of adipose tissue increases. Because water is denser than normal fat, increasing the water content of adipose tissue increases its density. As you can anticipate from our earlier discussion of density, increasing the density of a tissue increases the brightness or whiteness of the tissue on the CT image. Normal fat appears dark—nearly black on a standard soft-tissue window setting. Therefore inflamed fat appears whiter, assuming a smoky or hazy gray appearance called fat stranding. In the presence of an infectious or inflammatory process such as appendicitis, diverticulitis, cholecystitis, or colitis, the fat around the affected organ often demonstrates stranding, which can be a key diagnostic finding in these scenarios. In patients with little or no peritoneal fat, there is no substrate for stranding, and the diagnosis of these conditions can be difficult. For these reasons, “fat is our friend” in CT diagnosis. (The origin of this phrase is uncertain— the author’s father is the only source known to him ). Because inflammatory fat stranding accompanies many forms of acute abdominal pathology, where there is “smoke” (fat stranding) there is often “fire” (acute pathology).

Free Fluid

Free fluid deserves special discussion, partly because the familiarity of many emergency physicians with ultrasound might lead to confusion when interpreting CT scan. On ultrasound images, simple fluid appears black. On an abdominal CT image viewed on a typical soft-tissue window, simple fluid appears intermediate gray—a shade usually indicating a solid organ on ultrasound images. On this same CT window setting, air appears black and fat appears dark gray, or nearly black. If you are an emergency physician with extensive ultrasound experience, until you have become accustomed to viewing abdominal CT, you may mistakenly see fat as “fluid” on CT images and see fluid as “solid tissue.”

Free fluid on CT can have several diagnostic meanings, depending on the scenario. Because CT differentiates tissues based on density and x-ray attenuation, CT cannot perfectly distinguish types of simple fluids (new advances in dual-energy CT, discussed later in this chapter, may help to alleviate this shortcoming). For example, blood, bile, purulent fluid, and ascites may differ in density by only a few Hounsfield units, depending on factors such as the hematocrit of the blood. As discussed in Chapter 10 , Chapter 12 , in the setting of trauma, free fluid may be blood from solid-organ injury, fluid from bowel injury, or urine or bile resulting from injury to the relevant organ systems. In the setting of nontraumatic abdominal pain, fluid may be blood, ascites, purulent fluid, bile, or urine—clinical history, as well as other CT abnormalities, may help to narrow the differential diagnosis. Free fluid can be seen in infectious and inflammatory conditions, as well as mesenteric ischemia and bowel perforation. Although nonspecific, it is a potentially important sign of pathology and should be sought carefully. We discuss free fluid in more detail when we cover individual forms of pathology.

Intravenous Contrast Timing

Contrast injected into a peripheral vein should not allow immediate visualization of arterial structures or end organs. Depending on the vascular bed, organ, or pathology of interest, the timing of CT image acquisition relative to the moment of contrast injection is adjusted, allowing sufficient time for the injected contrast to traverse the right heart and lungs, enter the left heart and aorta, fill arteries, and reach target organs. For some disease processes, the timing of contrast is relatively unimportant. These include fixed structural abnormalities or inflammatory processes, for which the degree of enhancement may be less crucial. For other disease processes, contrast timing is critical. For example, for evaluation of mesenteric ischemia, contrast is desired first in the mesenteric arterial system and then in the portal venous system, because either may be affected. CT images are typically acquired first during the arterial phase, and then (after a delay of 60-90 seconds) additional images are acquired during a portal venous phase. Some neoplastic processes have characteristic enhancement properties when early, middle, and late phases of enhancement are compared on subsequent CT images, although these are generally not emergency department time-dependent diagnoses. Modern CT scanners use software to assist contrast timing, a process described in more detail in Chapter 7 . As you can see, the timing of CT image acquisition relative to IV contrast administration can have diagnostic importance, so communication of the differential diagnosis to the radiologist is necessary to maximize diagnostic accuracy. A “generic” CT scan of the abdomen typically uses a delay of approximately 25 seconds, resulting in arterial-phase enhancement of organs. Some scanners use automated triggering software, which initiates the CT scan when the enhancement of the aorta or other target organs reaches a specific Hounsfield unit threshold, rather than using a fixed delay. This can provide better enhancement, because the rapidity of enhancement depends on factors such as the caliber and the location of the catheter used to infuse contrast, which vary from patient to patient.

Enteral Contrast (Oral and Rectal)

Enteral contrast has several potential functions. Contrast can be administered into the intestine orally, by nasogastric or orogastric tube, or rectally (as described in more detail later). Enteral contrast agents can be “positive,” “neutral,” or “negative” contrast agents. The most common form of enteral contrast for emergency applications is positive contrast—meaning contrast that is radiographically dense and appears white on typical soft-tissue CT windows. Positive contrast agents include iodinated contrast agents such as Gastrografin (meglumine diatrizoate) and barium sulfate contrast agents, which can be used in patients with allergy to iodinated contrast agents. These have high density, positive on the Hounsfield scale. Neutral contrast agents are similar in density to water (~0 Hounsfield units) and include (not surprisingly) water, and Volumen, a low concentration barium suspension (0.1% barium). Although water is an excellent contrast agent, it is rapidly absorbed from the intestine and therefore is of little use in the jejunum and ileum. Negative contrast agents are actually lower in density than water and include gases such as carbon dioxide, used for applications such as CT colonography, that are not typically employed in the emergency department. We discuss the theoretical benefits of each type of contrast later.

All enteral contrast agents share two common properties: they distend the lumen of small and large bowel, and they distinguish bowel from “nonbowel” abdominal structures that may otherwise appear similar. The normal small bowel is a tubular structure, usually filled with some combination of fluid (near water density, 0 Hounsfield units, or moderately dark gray on soft-tissue CT window) and air (−1000 Hounsfield units or black on all window settings). Depending on the orientation of the bowel and the image plane, the small bowel may appear circular, elliptic, or tubular in cross section. Pathologic processes adjacent to the bowel can sometimes be difficult to distinguish from the normal bowel in the absence of positive enteral contrast, because they may have a similar appearance. For example, an abscess cavity adjacent to the bowel could have a similar appearance, with a circular cross section filled with dark gray fluid (pus) and air (black). Therefore one potentially important role of positive oral contrast agents is to identify the bowel, allowing structures that fail to fill with oral contrast to be identified as nonbowel structures.

A second potential role of positive enteral contrast is to identify a point of obstruction of the bowel. Conceptually, if the bowel is obstructed at a single point, orally administered contrast (or contrast given by nasogastric or orogastric tube) should fill the bowel to the point of obstruction but pass no farther; therefore oral contrast might be valuable in identifying not only the presence of an obstruction but also the exact location of the obstruction. In reality, oral contrast appears to be unnecessary in most cases of obstruction, as described in more detail later in this chapter in the section titled “Oral Contrast Agents and Computed Tomography of Suspected Small-Bowel Obstruction.” The reason is simple: the bowel almost always contains some intrinsic neutral contrast composed of a slurry of swallowed saliva, partially digested food, and GI secretions. If the bowel is obstructed, it will appear distended, with this neutral contrast proximal to the point of obstruction. Distal to the point of obstruction, intestinal peristalsis will move this “contrast” farther along and the bowel will not appear distended.

A third potential role of positive enteral contrast is to reveal intestinal perforation. If enteral contrast is administered and then seen to collect in the peritoneal cavity, outside of the apparent lumen of the bowel, perforation can be diagnosed. As is the case for bowel obstruction, this theoretical advantage appears to be less useful in reality. Most intestinal perforations are likely fairly small, limiting the likelihood that enteral contrast would actually leak out of the bowel lumen. In addition, if a point of perforation is located not in a dependent loop of bowel but rather at a higher point (with the patient in the typical supine position for CT), enteral contrast may fail to leak out of bowel. Studies in the setting of blunt trauma show oral contrast to have limited or no utility in the diagnosis of bowel injury, as described in detail in Chapter 10 . Nonetheless, large perforations of bowel, or fistulas connecting the bowel lumen to other structures or cavities, might be revealed by positive enteral contrast.

Distension of bowel with enteral contrast can be important for determining the thickness of the bowel wall. Bowel wall thickening can suggest inflammation (as in infectious colitis or inflammatory bowel disease), ischemia (as in ischemic colitis or “shock bowel”), or a mass lesion (as in a sessile colonic polyp or small-bowel lymphoma). Without enteral contrast agents, the bowel may be relatively nondistended or even fully collapsed. In these cases, the thickness of the bowel wall can be difficult to estimate, because the wall may be folded upon itself or simply thickened because of lack of distension. Enteral contrast agents distend the bowel, helping to resolve this issue. Positive enteral contrast agents can obscure some details of the bowel wall, because they are extremely dense relative to the bowel and can create image artifacts. Negative or neutral contrast agents may be more useful in depicting the bowel wall structure and the mucosal surface, but these agents are relatively rarely used today in emergency medicine.

Positive enteral contrast agents can obscure diagnostically important abnormalities of bowel wall enhancement with IV contrast. Bowel wall enhancement usually occurs to a limited extent in normally perfused bowel. Increased bowel wall enhancement indicates increased blood flow, which can be seen in infectious and inflammatory states, including appendicitis. Decreased enhancement can indicate hypoperfusion, for example, in a closed-loop bowel obstruction or embolic mesenteric ischemia. When positive enteral contrast is present immediately adjacent to the bowel wall, the degree of bowel wall enhancement with IV contrast can be obscured. As we discuss later in this chapter, positive enteral contrast can be relatively contraindicated for this reason when bowel wall enhancement is predicted to play an important role in the patient’s diagnosis.

Positive enteral contrast agents also obscure extravasation of injected contrast agents into the bowel lumen, because both are quite dense and appear white on an soft-tissue CT window. Some new CT protocols for diagnosis of acute GI bleeding therefore do not use enteral contrast agents. We discuss these in more detail later in this chapter.

Finally, enteral contrast agents can assist in recognition of mass lesions within the bowel—though these are not usually emergency department diagnoses. When a contrast agent is used to distend the bowel and fill its lumen, mass lesions such as polyps are visible as “filling defects” within the bowel lumen. Accurate diagnosis of bowel mass lesions such as polyps requires a cleansing bowel prep to remove stool, which could otherwise have a similar appearance. Some protocols for CT colonography take an additional step, with the patient ingesting a contrast agent to “tag” stool with positive contrast, allowing any residual stool to be recognized. Usually such protocols use carbon dioxide gas as the negative enteral contrast agent to distend the bowel. Gases including atmospheric air and carbon dioxide appear black on CT, providing outstanding contrast against soft tissues. Carbon dioxide is often used, because it is rapidly reabsorbed from the gut lumen and exhaled, limiting discomfort from intestinal distension.

Rectally administered enteral contrast is rarely used for CT in the emergency department, but it has some putative benefits. The same potential advantages of enteral contrast agents described earlier apply to rectal contrast. Rectal administration of contrast can allow rapid opacification of the distal bowel without waiting for the transit of orally administered contrast, which can take hours, depending on factors such as the speed of intestinal peristalsis and the presence of ileus. Some protocols use rectal contrast for detection of appendicitis or diverticulitis for this reason. In cases of penetrating torso trauma, rectal contrast has been advocated by some radiologists to assist in detection of colonic perforation, because leaking contrast into the pelvis, peritoneum, or retroperitoneum might thus be seen. Rectal contrast could be useful in detection of perirectal or sigmoid abscesses by differentiating the bowel lumen from an adjacent fluid- and gas-filled abscess. Whether rectal contrast is necessary in these cases is discussed in more detail for individual conditions later in this chapter. Rectal contrast is usually administered as a retention enema of 400-800 mL of Gastrografin (3% meglumine diatrizoate–saline solution) through a rectal tube. The contrast is infused under gravity pressure only from a height of 3 feet over 2-4 minutes.

Urinary Contrast Agents

Urinary contrast agents can be instilled into the bladder through a catheter for detection of bladder rupture or mass lesions, as discussed in Chapter 12 .

Contrast Injection Rate and Venous Catheter Size

The rate of contrast injection is important in vascular applications of abdominal CT. For CT angiography (CTA), rates of 4 to 5 mL per second are desirable to achieve sufficient opacification of vascular structures and thus allow recognition of filling defects, intimal flaps, and stenotic regions. Standard angiocatheters of a 14- to 20-gauge size are capable of achieving flow rates of 5 mL per second with power injectors using a pressure limit of 325 psi. Catheters that are 22 gauge and smaller do not reach the desired flow rate before exceeding the safe pressure limit and are therefore not adequate for CTA. Flow rates through catheters are inversely proportional to the catheter length and directly proportional to the radius raised to the fourth power, so a short, large-bore catheter is desirable for CTA. For nonangiographic applications, contrast flow rates are less important, and smaller catheters may be sufficient.

Reactions to Iodinated Vascular Contrast

Contrast reactions are sometimes called contrast allergies, although the specific mechanism of these reactions is thought to be anaphylactoid, not mediated through true allergy. Nonetheless, from a practical clinical standpoint, they have effects much like those of true allergies and are treated similarly.

Contrast Nephropathy

Contrast nephropathy is a serious potential consequence of exposure to iodinated vascular contrast. It is a leading cause of in-hospital renal failure, occurring in 150,000 cases per year in the United States. Contrast nephropathy is defined by a 25% increase in baseline creatinine or an absolute increase of 0.5 mg/dL within 48 hours after the administration of iodinated contrast. Less than 1% of patients experiencing contrast nephropathy require dialysis. Contrast nephropathy is discussed in detail in Chapter 7 .

Dialysis Patients and Iodinated Intravenous Contrast

Iodinated IV contrast agents are safe in patients receiving long-term hemodialysis for renal failure. Because these patients already have severe and irreversible renal dysfunction, contrast agents do not pose any additional risk for contrast nephropathy. In addition, the volume of contrast given is usually no more than 120-150 mL and does not generally pose a threat of vascular volume overload and pulmonary edema. Patients should be dialyzed as usual but do not require emergency dialysis following iodinated contrast administration. This should not be confused with the situation for gadolinium contrast agents used for MRI. As discussed in Chapter 15 these agents pose a small but potentially life-threatening risk in patients with end-stage renal disease, including dialysis patients. Gadolinium should be avoided in these patients whenever possible, and dialysis should be performed as soon as possible after any administration of gadolinium.

Dual-Energy Computed Tomography

A new CT technique called dual-energy or spectral CT may eliminate some of these issues of obscuration of native tissues by exogenous contrast agents of similar density. In dual-energy CT, two x-ray tubes are used to scan the patient simultaneously, using two distinct x-ray voltages and x-ray tube currents. The combination of absorption spectra of a material at different energy levels of incident radiation gives the material a unique signature, allowing differentiation of two materials of similar density that may appear identical with conventional CT. This process could allow digital subtraction of iodinated contrast from an image, revealing calcifications having a different absorption spectrum. Bone structures can also be digitally removed, improving CTA. Although dual-energy scanners are commercially available, research into their applications is limited and they have not yet entered wide clinical use.

Controversies Around Oral Contrast

Oral contrast use for abdominal CT is a topic of significant controversy. Garra et al. noted that the median interval required to consume 2 L of oral contrast was more than 100 minutes, with no significant difference between patients randomized to receive a prophylactic antiemetic and those receiving placebo. Consequently, oral contrast use can substantially increase the length of stay of an emergency department patient. For that individual, oral contrast use may lead to a delay in diagnosis, which could be clinically important for conditions such as appendicitis or, more critically, for vascular pathology such as abdominal aortic aneurysm or mesenteric ischemia. In addition to causing potential harm for the patient receiving contrast, the delay associated with oral contrast use may delay the care of other patients waiting to be seen, a critical issue in crowded modern U.S. emergency departments. The risks for these delays must be weighed against the potential benefit of increased diagnostic accuracy that might result from oral contrast use. Numerous studies have attempted to determine the effect of oral contrast on diagnostic accuracy of CT for a range of conditions. We summarize these studies here, exploring them in more detail in individual sections on specific pathology.

Multiple studies suggest that CT without oral contrast is highly sensitive and specific for important emergency department diagnoses, including appendicitis, diverticulitis, pancreatitis, small-bowel obstruction, pneumoperitoneum, mesenteric ischemia, abdominal aortic aneurysm and dissection, and blunt abdominal injuries.

Radiation Exposure From Abdominal and Pelvic Computed Tomography

CT provides a relatively large ionizing radiation exposure, leading to growing controversy about its wide and repeated use, particularly in young patients. Radiation risks for CT are discussed later in this chapter in the sections titled “ Appendicitis Imaging in the Pediatric Patient ” and “ Imaging the Pregnant Patient With Possible Appendicitis. ” Radiation risks are also discussed in Chapter 7 , Chapter 8 .

Focused Abdominal Computed Tomography Interpretation

Although emergency medicine is already overflowing with abbreviations, we introduce one more here to emphasize a similarity to the focused assessment with sonography in trauma (FAST) ultrasound examination familiar to most emergency physicians. When performing a FAST examination (discussed in Chapter 10 ), the emergency physician looks at each of four anatomic regions in turn, answering a single yes-or-no question for each:

• 1.
  

  Right upper quadrant—Is free fluid present in the hepatorenal space?

  
  
• 2.
  

  Left upper quadrant—Is free fluid present in the splenorenal or subdiaphragmatic space?

  
  
• 3.
  

  Suprapubic region—Is free fluid present in the pelvis?

  
  
• 4.
  

  Subxiphoid region—Is fluid present in the pericardial sac?

The examination is thus brief, and the questions are clear and clinically-oriented. Although other structures may be seen in the field of view, the operator is trained to focus on a single question, avoiding distraction. The same approach can be used to interpret abdominal CT, which can otherwise baffle novices because of the number of structures and forms of pathology that can be seen. We call our focused interpretation FACT, because it follows the same principles as FAST.

Let’s establish some simple rules for our discussion. First, we assume that you can only view axial images, because some centers do not yet routinely provide images in other planes. Second, we assume a familiarity with window settings and CT tissue appearances (discussed earlier), as well as with basic anatomy. Third, we assume that you are viewing the images on a digital computer workstation or picture archiving and communication system (PACS). The process is the same for reviewing printed images, although the window settings and other image characteristics cannot be modified. The axial images are conventionally displayed with the right side of the patient’s body on the left side of the viewing monitor and the anterior abdomen facing the top of the screen. The viewpoint is that of an observer standing at the foot of the patient’s bed, looking toward the head.

When interpreting abdominal CT using the FACT approach, you should focus your attention on one organ or structure at a time, with a single or limited number of questions for each organ. Although other structures may be visible in each axial image, you should intentionally ignore other structures to avoid distraction. Our approach requires you to make multiple passes through the stack of axial CT images, but you’ll find that with practice you become extremely fast at this approach.

Our approach requires you to look through the abdominal CT images several times using three window settings: soft-tissue, lung, and bone. Most organs require inspection using the soft-tissue window, with a few exceptions deserving mention here. Free air (pneumoperitoneum) is often more readily recognized on lung window settings, because this setting leaves air or gas black while rendering all other tissues quite bright or white. On a standard soft-tissue window, fat is fairly black, and small quantities of free air may be difficult to recognize against this background. A lung window is also necessary when reviewing the cephalad axial slices for pneumothorax or pulmonary parenchymal opacity. A pneumothorax looks black on this setting, whereas the lung parenchyma is readily visible. On a soft-tissue window, both normal lung parenchyma and a pneumothorax appear black and can be indistinguishable. Perform your inspection of the chest and abdomen for free air and lung pathology using the lung window, and then switch to the soft-tissue window to complete your evaluation of other structures and pathology. A final window setting that may be more selectively used is the bone window. As the name suggests, a bone window allows detailed evaluation of bone for fracture or lytic lesions. The same images viewed with a soft-tissue window can obscure fine structural abnormalities of bone, because the bone appears bright white with “bloom artifact” obscuring cortical fractures or other abnormalities. An unexpected use of a bone window includes evaluation for renal colic. Although ureteral calcifications are visible on a soft-tissue window, you may prefer to search for stones using bone window settings, which tend to render the abdomen in three primary shades: white for calcified structures, dark gray for soft tissues and fluids, and black for air. As a consequence, this window setting is also sometimes used to evaluate for free air.

To illustrate the FACT approach to interpretation, let’s start with the example of evaluating the liver for traumatic injury, a topic examined in detail in Chapter 10 . We won’t belabor the findings of liver trauma here, because they are extensively reviewed in that chapter.

Begin your search by locating the liver, which is visible high in the abdomen, in a subdiaphragmatic position. The first several axial slices typically include chest structures above the diaphragm, but ignore these at the moment, because you are focused on a single question: Does the liver show signs of traumatic hemorrhage or contusion? The axial slices spanning the liver also show the spleen, aorta, stomach, pancreas, kidneys, and spine, but ignore these for now, concentrating on the liver. You will return to each of the other organs during your assessment. On each slice containing the liver, inspect the liver parenchyma and enhancement. Once you have passed through the caudadmost liver slice and are satisfied that you have answered your focused question, stop your evaluation and move to the next organ or disease process on your checklist. For example, in the case of nontraumatic abdominal pain, the aorta is a key organ to assess for abnormalities, particularly aortic aneurysm rupture or dissection. Because the aorta begins in the chest and continues through the pelvis after bifurcating into the iliac arteries, this inspection requires review of the entire stack of CT slices. However, the questions to be answered remain simple:

• 1.
  

  Is an aortic aneurysm (defined by an aortic diameter greater than 3 cm) present?

  
  
• 2.
  

  Is a dissection flap visible within the contrast-filled aorta?

  
  
• 3.
  

  Is blood or contrast visible outside of the confines of the aorta?

Again, intentionally ignore other chest, abdominal, and pelvic structures during your inspection of the aorta, which should take only seconds. The aorta normally appears as a rounded circular structure in cross section, with a fixed location, just anterior to the thoracic and lumbar spine and slightly left of the midline. Measure the aortic diameter on a single axial CT slice to develop a baseline understanding of its size, and then scroll through the digital stack of images from cephalad to caudad. Don’t evaluate major aortic branches at this stage, though as you become more adept you may choose to incorporate their evaluation here.

This process is repeated for each of the major abdominal, pelvic, and retroperitoneal organs, as well as visible structures of the chest. A systematic approach requires that all structures be evaluated in this manner ( Table 9-3 ). However, emergency physicians may elect to use a limited CT interpretation approach.

Limited Interpretation of Computed Tomography

Although a systematic approach minimizes the likelihood of missing pathology, in some cases the differential diagnosis is appropriately quite narrow and a more narrowly focused interpretation approach may be reasonable. A 23-year-old male with nontraumatic right lower quadrant tenderness likely has appendicitis, terminal ileitis, a ureteral stone, hernia, or testicular pathology. Abdominal aortic aneurysm and splenic rupture may be far from the differential, and it may be appropriate for the emergency physician to answer only a subset of the imaging questions for this patient, just as a focused history and physical examination may be appropriate. The radiologist bears a heavier burden, with responsibility for identifying even incidental findings of potential significance on the images. As an emergency physician, if you perform a limited initial interpretation of the CT, be sure that some system is in place for a more comprehensive interpretation by a radiologist and for any findings to be communicated to you and your patient.

X-ray Findings of Gastric Outlet Obstruction

The x-ray findings of gastric outlet obstruction include a grossly distended stomach, with a paucity of gas in the remainder of the abdomen if sufficient time has elapsed for peristalsis to empty the bowel distal to the obstruction (see Figure 9-12 ). The stomach may displace the transverse colon inferiorly. If obstruction is caused by ulcer, free air may be seen.

Upper Gastrointestinal Series (Fluoroscopic Examination)

A traditional means of assessing for gastric outlet obstruction is with contrast swallow study. In this case, contrast fails to move beyond the stomach. Irregularities in the mucosal surface, such as craters from ulcer, can be seen. The thickness of the stomach wall cannot be measured directly with this technique, because only the mucosal surface is visible.

Computed Tomography Findings of Gastric Outlet Obstruction

On CT, the stomach may appear grossly distended, and thickening of the pyloric region may be seen with gastric outlet obstruction. Gastric varices can be seen with IV contrast–enhanced CT. In rare cases of gastric volvulus, the stomach wall may fail to enhance normally and beaking at the point of volvulus may be seen, as described later for large- and small-bowel volvulus. If oral contrast is administered, it will not move beyond the stomach. Stomach anatomy can be difficult to visualize with CT, although thin collimation (slice thickness) and coronal plane reformations can assist. No large studies have examined CT for gastric outlet obstruction, so the description of the appearance is based on case reports.

Infantile Pyloric Stenosis

Pyloric stenosis causes gastric outlet obstruction in infants, with 95% of cases diagnosed between 3 and 12 weeks of age. A family history and male gender are risk factors, although sporadic nonfamilial cases and cases in female patients occur. The typical history is of projectile emesis immediately after feeding. In patients without the classic palpable olive on physical examination, the diagnosis was made historically using upper GI contrast studies, but today ultrasound is the predominant imaging modality. X-ray is nonspecific but usually shows gross gastric distension and often a paucity of distal bowel gas (see Figure 9-12 ). Ultrasound is highly sensitive and specific for pyloric stenosis and does not expose the patient to ionizing radiation, an important consideration in pediatric patients. Ultrasound findings of pyloric stenosis (see Figure 9-13 ) include a pylorus 14 mm or greater in length and 10 mm or greater in diameter. The use of a pyloric thickness 5.5 mm or greater does not improve diagnostic accuracy, although some authors suggested a thickness of 4 mm as a diagnostic criterion.

Pyloric stenosis, ultrasound.

This 3-week-old male presented with 2 days of projectile, nonbilious emesis. His ultrasound shows typical findings of pyloric stenosis. In the long-axis views (A through C), a thickened and elongated pylorus is seen. The stomach proximal to the pylorus is distended with fluid and particulate matter. In the short-axis (transverse) view (D), a thickened pylorus is seen. The patient underwent pyloromyotomy and recovered uneventfully. Compare with the x-ray in Figure 9-12 .Figure 9-13,cont’d

Hernanz-Schulman et al. studied 152 infants with suspected pyloric stenosis and found the sensitivity and specificity of ultrasound to be 100% for patients requiring surgery. Van der Schouw et al. also found that ultrasound improved the diagnosis of pyloric stenosis in infants without a palpable abdominal olive suspected of pyloric stenosis. An ultrasound-measured pyloric length of 14 mm or greater and a diameter of 10 mm or greater, along with the age at which vomiting began (<5 weeks), base excess, and a bicarbonate level of 28 mmol/L or greater, yielded high sensitivity and specificity.

X-ray of Small-Bowel Obstruction

X-ray diagnosis of small-bowel obstruction relies on two primary findings: dilated loops of small bowel and air–fluid levels (see Figures 9-14 and 9-15 ). Dilated loops of small bowel are visible on supine, upright, or lateral decubitus images. The small bowel is normally less than 3 cm in diameter, so a diameter exceeding this is considered pathologic. The small bowel is characterized by internal folds called plicae circularis (or valvulae conniventes), which completely traverse the diameter of the bowel. These distinguish the small bowel from the large bowel, which is marked by haustra, segmentations that do not completely traverse the bowel diameter. In a dilated loop of small bowel, the adjacent plicae circularis can give the tubular bowel the appearance of a “stack of coins.”

Air–fluid levels are not visible on a supine x-ray, because air and fluid layer in the plane of the x-ray detector and do not form a silhouette against each other. Air–fluid levels are visible on upright or decubitus abdominal x-rays, because fluid settles into dependent loops of small bowel. Normal small bowel contains both air and fluid, so scattered air–fluid levels can be a normal finding or a finding in ileus. However, in the presence of a mechanical obstruction, multiple air–fluid levels occur. As loops proximal to the obstruction fill with fluid, displacing air, air–fluid levels may be replaced by tiny pockets of air, called the “string of beads” or “string of pearls” sign on the upright x-ray. On a supine x-ray, these air pockets may appear elongated and slitlike, caused by air trapped along plicae circularis—sometimes called the “stretch” sign.

Air is present in the small bowel primarily from swallowed air; gas within the large bowel is partly caused by gas production by anaerobic bacteria. Fluid is normally present in the small bowel because of swallowed fluids, including saliva and ingested materials, and gut secretions. In the presence of a mechanical obstruction, peristalsis distal to the point of obstruction may ultimately push air and fluid out of the distal bowel. Thus a classic appearance for small-bowel obstruction includes air–fluid levels, dilated loops of small bowel, and an absence of bowel gas in the distal bowel and rectum. However, mechanical bowel obstructions can occur along a spectrum, both in terms of severity (partial or complete) and in terms of duration of obstruction before x-rays are obtained, affecting the resulting images. Understandably, an x-ray obtained moments after the onset of even a complete obstruction may not yet show dilated bowel proximal to the obstruction or an absence of gas distal to the obstruction. A partial obstruction, which allows some bowel contents to traverse the point of obstruction, may never develop a classic x-ray appearance.

In the case of ileus, when small bowel is diffusely “stunned” or inactive, no focal point of obstruction is present. The bowel may appear dilated, and air–fluid levels may be seen, but air and gas are usually distributed throughout the bowel, including distally through the large bowel to the rectum. Classically, this appearance differs from that of a complete mechanical bowel obstruction, in which gas and fluid are evacuated from bowel distal to the point of obstruction by peristalsis. However, in reality, an ileus may appear identical to an early complete mechanical bowel obstruction or a partial bowel obstruction, so the presence of gas in the distal bowel cannot distinguish these or rule out complete obstruction. A number of other bowel gas patterns have been described, including a “nonspecific bowel gas pattern,” but unfortunately, bowel obstruction may be present in patients with normal, nonspecific, ileus, or obstruction x-ray patterns. Air in the rectum should not be attributed to digital rectal examination, which does not introduce significant amounts of air.

X-ray is relatively poor for distinguishing ileus from mechanical obstruction. Air–fluid levels occurring at different heights within the same loop of bowel (called differential or dynamic air–fluid levels) have been described as more typical of mechanical obstruction than ileus, with an increasing likelihood of obstruction with increasing height differential. Harlow et al. found that the presence of differential loops is only 52% sensitive and 71% specific for mechanical obstruction—inadequate for clinical utility. With height differentials greater than 20 mm, the specificity is 94%, with less than 25% sensitivity.

This insensitivity of x-ray strongly limits its utility in the evaluation of small-bowel obstruction. Because a normal or nondiagnostic x-ray can occur in the presence of bowel obstruction (see Figure 9-18 and the related CT in Figure 9-19 ), CT (discussed later) is often performed following a normal x-ray. Moreover, because CT is commonly used to identify the point of obstruction, identify a cause such as hernia (see Figures 9-20 and 9-21 ), confirm the degree of obstruction, and identify ischemic bowel related to obstruction, an abnormal x-ray is commonly followed by CT today. Consequently, x-rays have little impact on patient management, other than introducing diagnostic delay and additional cost. Rarely, x-rays may be useful in the sickest patients (unsuitable for CT), when CT is unavailable, or in patients with such a low pretest probability of bowel obstruction that the physician is willing to accept an insensitive test result without further evaluation. X-rays can be performed portably, allowing other resuscitation measures to be continued in unstable or very ill patients. Early identification of free air or bowel obstruction with x-ray may facilitate care such as nasogastric tube placement or surgical consultation in these sickest patients.

Incarcerated inguinal hernia, CT with IV and oral contrast.

This 46-year-old male presented with right inguinal pain and swelling. An attempt to reduce an apparent right inguinal hernia was unsuccessful. This axial slice shows a loop of small bowel entering a right inguinal hernia. Figure 9-21 shows a coned-in view focused on the region of the hernia. Whereas the radiologist identified this as an indirect inguinal hernia, the operative report noted a direct hernia through the inguinal ring.

Incarcerated inguinal hernia, CT with IV and oral contrast, soft-tissue window.

Same patient as Figure 9-20 . This series of axial slices shows a loop of small bowel entering a right inguinal hernia. A through L progress from cephalad to caudad. The minimal stranding and absence of bowel wall thickening are reassuring, making frank bowel infarction less likely.

Computed Tomography Findings of Complete Versus Incomplete Small-Bowel Obstruction

Using CT without oral contrast, high-grade obstruction is suggested by a 50% difference in the caliber of proximal dilated bowel and collapsed bowel distal to the transition point. Using CT with oral contrast, complete obstruction is diagnosed if no enteral contrast passes the point of obstruction at delayed scans 3 to 24 hours after the initial CT. Partial obstruction is diagnosed if some contrast material passes the point of obstruction.

Computed Tomography Findings of Bowel Ischemia Associated With Bowel Obstruction

On CT scan, a closed-loop obstruction can be seen, indicating a high risk for bowel ischemia (see Figure 9-19 ). In such an obstruction, characterized by a grossly distended U-shaped loop of bowel with an abrupt transition to collapsed distal bowel loops, compression of the blood supply can lead to gangrene without rapid surgical intervention. Other findings concerning for ischemia from strangulating obstruction are listed in Table 9-6 .

Although detection of ischemia associated with small-bowel obstruction is a prime indication for CT, the sensitivity and specificity have wide reported ranges in the medical literature. The ACR appropriateness criteria call CT an “excellent means of detecting complications of bowel obstruction such as ischemia and strangulation.”

Frager et al. found CT to be 100% sensitive but only 61% specific. Balthazar et al. reported the sensitivity of CT as 83%, with 93% specificity. Ha et al. found the overall sensitivity of CT to be 85%, although individual findings such as poor bowel wall enhancement (sensitivity = 34%, specificity = 100%) or a serrated beak (sensitivity = 32%, specificity = 100%) were less sensitive but highly specific. Zalcman et al. reported an overall sensitivity of CT of 96% and specificity of 93% for ischemia in the setting of small-bowel obstruction. Mallo et al. conducted a systematic review from 1966 to 2004 and found 11 studies of CT for ischemia associated with small-bowel obstruction, with 743 patients. For ischemia, the aggregate sensitivity of CT was 83% (range = 63%-100%) and specificity was 92% (range = 61%-100%). Sheedy et al. evaluated the prospective performance of CT for detection of small-bowel obstruction-associated ischemia in 60 patients and found poor sensitivity (14.8%) with high specificity (94.1%). Decreased segmental bowel wall enhancement with IV contrast was the most specific finding (100% specificity).

Jang et al. compared simple visual assessment of bowel wall enhancement for evidence of ischemia with a quantitative measure (maximal attenuation of a region of interest) in which the Hounsfield unit enhancement of bowel wall was measured. Visual inspection had a sensitivity of 91.7% and specificity of 66.7%. In comparison, quantitative measurements provided better sensitivity and specificity. A maximum attenuation of less than 85 Hounsfield units was 100% sensitive but only 45% specific for ischemia. Adopting a lower threshold improved specificity at the expense of sensitivity. If the maximum bowel wall attenuation was less than 45 Hounsfield units, the sensitivity was only 25% but specificity was 100%. The authors also assessed a technique in which the Hounsfield density of the same region on a noncontrast CT (performed immediately before CT with IV contrast) was subtracted from the Hounsfield attenuation on IV-contrasted CT. This provided the best combination of sensitivity and specificity but would require that patients undergo two CT scans, increasing the radiation exposure.

Oral Contrast Agents and Computed Tomography of Suspected Small-Bowel Obstruction

Just as small-bowel obstruction can be diagnosed with x-ray using no enteral contrast, CT without enteral contrast can demonstrate small-bowel obstruction. In the case of obstruction, the small bowel is typically already distended with ingested saliva, partially digested food, and intestinal secretions—providing intrinsic enteral contrast. Unlike high-density enteral contrast agents (e.g., barium sulfate or Gastrografin, also called diatrizoate meglumine), which appear white on CT using soft-tissue window settings, native fluid within the bowel is relatively low in density, near water density. Low-density fluid appears dark gray on soft-tissue window settings and is readily differentiated from air and fat. High-density contrast agents are designated “positive” contrast agents, whereas low-density contrast such as native bowel contents are called “neutral” contrast agents. Multiple studies demonstrate that positive enteral contrast agents are not generally needed for the diagnosis of small-bowel obstruction. In its published appropriateness criteria, the ACR states that the use of positive enteral contrast may actually disguise bowel wall changes indicative of ischemia, which we discuss later.

Intravenous Contrast Agents and Computed Tomography of Suspected Small-Bowel Obstruction

Small-bowel obstruction can be diagnosed without IV contrast, but the use of IV contrast can assist in diagnosis of bowel ischemia. Detection of ischemia is an important indication for abdominal CT in clinically suspected or x-ray-confirmed small-bowel obstruction, so IV contrast should be administered when possible. IV contrast may assist in other conditions with similar clinical presentations to small-bowel obstruction, as described in other sections of this chapter and chapters of this book.

Atri et al. compared IV-enhanced CT with nonenhanced CT (no oral, IV, or rectal contrast). Unenhanced CT had a sensitivity of 88.1% and specificity of 77% for small bowel obstruction, not significantly different from the sensitivity and specificity of CT with IV contrast (87.6% and 75%, respectively).

What Is the Transition Point, and What Is Its Significance?

With CT, the point of a mechanical obstruction can be identified and often the cause can be determined. Proximal to this point, the bowel appears dilated beyond 3 cm. Distal to the point of obstruction, bowel loops are decompressed (<3 cm). The point of obstruction, at which dilated bowel loops give way to undilated bowel, is called the transition point or transition zone. Identification of a transition point helps to confirm a mechanical obstruction rather than ileus. In addition, the cause of obstruction may be recognized as an abscess, mass, internal or external hernia (see Figures 9-20 and 9-21 ), volvulus, intussusception (see Figures 9-23 and 9-24 ), ingested foreign body, or inflamed bowel segment, aiding surgical planning. Adhesions may be difficult to identify directly on CT but are a common cause of small-bowel obstruction in patients with CT evidence of small-bowel obstruction without a visualized cause. Use of coronal plane images, in addition to axial images, may assist in detection of the transition point. A recent study questions the clinical significance of the radiographic transition zone. Colon et al. found no difference in the rate of immediate operative therapy or failure of nonoperative therapy in patients with and without transition zones. Transition zones correlated with operative findings in 63% of cases.

Pediatric midgut malrotation and suspected volvulus.

This 10-day-old infant presented with bilious emesis. An upper gastrointestinal contrast study was performed and showed no contrast progressing beyond the second segment of the duodenum, suspicious for malrotation with midgut volvulus. Operatively, the patient was found to have malrotation without volvulus.

Adult intussusception, CT with IV and oral contrast, soft-tissue window.

This 35-year-old male with multiple sclerosis presented with abdominal pain and vomiting for 4 days. CT scan shows an intussusception—pathology more commonly seen in children. The patient ultimately underwent laparotomy and resection of the intussusception. Pathology showed the intussusceptum to be a large, inverted diverticulum, likely a Meckel’s diverticulum given the position in the terminal ileum, though no gastric mucosa was found.These axial CT images show a segment of small bowel with a thickened bowel wall. In some images, the bowel is seen in transverse (short-axis) cross-section, while in others it is oriented in long-axis section.

The short-axis views (A, B) suggest that there is bowel within bowel, confirmed on the long-axis views (C, D) A, C, Axial CT slices. B, D, Close-ups from A and C, respectively.

Adult intussusception, CT.

Same patient as Figure 9-23 . These coronal reconstructions beautifully display intussusception. The proximal bowel is filled with contrast, right up to the point when the bowel suddenly invaginates into the distal bowel segment. A, C, E, Coronal reconstructions. B, D, F, Close-ups from A, C, E, respectively.

Ultrasound for Small-Bowel Obstruction

Ultrasound is rarely used in the United States but often used internationally for the diagnosis of small-bowel obstruction. Advantages of ultrasound are portability, lack of any ionizing radiation exposure, and elimination of injected iodinated contrast, with its attendant risks.

Ultrasound findings of small-bowel obstruction are similar to those with other imaging modalities. Dilated loops of bowel greater than 3 cm diameter are seen proximal to a point of mechanical obstruction, with collapsed loops distally. Fluid-filled loops of bowel are readily seen with ultrasound, although air continues to pose a barrier to visualization, scattering the ultrasound beam. Associated ascites are readily recognized. The thickness of the bowel wall can also be assessed. Because ultrasound allows dynamic assessment of bowel motility, peristalsis can be used as an additional criterion. In the presence of obstruction, increased bowel motility with a to-and-fro or whirling motion of intestinal contents may be seen. Conversely, in ileus, an absence of intestinal peristalsis is seen. Hyperechoic ultrasound contrast agents can be administered intravenously to assess bowel wall perfusion. After administration of these agents, lesser echogenicity is seen in ischemic bowel wall compared with normally perfused bowel.

Ultrasound has been compared retrospectively and prospectively with x-ray for the diagnosis of small-bowel obstruction. Ko et al. found ultrasound to be correct in 89% of 54 cases, compared with 71% of cases for x-ray. Sonography also more accurately predicted the degree and cause of obstruction. Ogata et al. prospectively compared ultrasound and x-ray in 50 patients with suspected bowel obstruction, 22 ultimately diagnosed with small-bowel obstruction and 2 with large-bowel obstruction. Ultrasound was 88% sensitive and 96% specific, whereas x-ray was 96% sensitive and 65% specific. This study is limited by selection biases and a small sample size with wide confidence intervals (CIs). Czechowski compared x-ray and ultrasound in 96 patients and found ultrasound to be 91% sensitive in patients with strangulation, compared with 30% for x-ray. In cases of simple obstruction, ultrasound was 89% sensitive, compared with 78% for x-ray. However, the small number of cases (9 simple obstructions and 10 strangulation obstructions) results in wide CIs. As described earlier, Suri et al. prospectively compared x-ray, CT, and ultrasound, finding ultrasound to be 83% sensitive and 100% specific, but with significant selection bias and a small patient population.

Hata et al. studied contrast-enhanced ultrasound for assessment of small-bowel obstruction–associated intestinal ischemia in 51 patients with dilated bowel on x-ray. Ultimately, 15 patients were found to have bowel ischemia from strangulation, 5 from superior mesenteric artery (SMA) thromboembolism, and 31 patients had simple obstruction. Diminished or absent color signals from the bowel wall had a sensitivity of 85% for bowel ischemia (95% CI = 62%-97%) and specificity of 100% (95% CI = 91%-100%). Larger studies are needed to confirm these findings. The rate of SMA thrombosis in this series was extraordinarily high (10%) compared with the rate in the general population, suggesting selection bias in this study population.

Midgut Volvulus With Resulting Bowel Obstruction and Ischemia in Adults

Midgut volvulus can result in intestinal obstruction and acute bowel ischemia. The condition is not restricted to pediatric patients but represents a particular concern in preverbal infants who cannot communicate symptoms, sometimes leading to catastrophic diagnostic delay. In adults, midgut volvulus can occur as a result of undiagnosed midgut malrotation. It is a rare cause of mesenteric ischemia in young adults. X-ray findings are nonspecific but can include a “bird’s beak” appearance because of rotation of the bowel around the point of volvulus. CT demonstrates this same appearance, with whirling of the mesentery. Other CT findings of intestinal ischemia are often present, as described earlier.

Midgut Volvulus in Infants

Midgut volvulus resulting from malrotation is a much feared pediatric emergency, occurring in an estimated 1 in 500 live births. Because 60% of cases present within the first month of life and another 20%-30% present before 12 months, diagnosis is difficult; the patients cannot communicate symptoms. Another 10% of cases present throughout life, including adults of any age. Mortality is around 15%, but short gut syndrome from extensive bowel infarction is a highly morbid consequence. The clinical presentation in neonates is bilious emesis.

Computed Tomography Findings of Intussusception With Resulting Bowel Ischemia

Whereas intussusception in children (discussed in the next section) requires reduction with enema or surgery, in adults, intussusceptions are a rare and potentially incidental finding on CT scans. Sundaram et al. reviewed 95,223 CT scans performed over 7 years and found 136 intussusceptions in 118 patients (0.13% of CT scans). In this study, 88.2% of intussusceptions were enteroenteric, 3.7% were enterocolic, and 4.4% were colocolic or other locations. The authors found 5.9% of patients required surgery, whereas 94.1% were nonsurgical. The discovery that 63% of surgical lesions involved the colon suggested that colonic intussusceptions is more likely surgical—though the small number of surgical cases in this series makes this finding uncertain. CT criteria were not specific for identifying enteroenteric lesions requiring surgery. Many nonsurgical enteroenteric intussusceptions were greater than 3.5 cm in length and thicker than 3 cm, suggesting that length and thickness had poor predictive value. A case of adult intussusception imaged with CT is shown in Figures 9-23 and 9-24 .

Pediatric Intussusception

Intussusception is the leading cause of bowel obstruction and potential infarction in children 3 months to 6 years, with a peak incidence between 3 and 12 months. Most cases are ileocolic, with the terminal ileum carried through the ileocecal valve into the colon. Although a pathologic leadpoint is theorized to account for intussusception, 90%-95% of cases have no identifiable cause. The condition is fatal if not rapidly treated, although most patients recover if treated within 24 hours of onset. Intense colicky pain, vomiting, and currant jelly stools are classic but occur in only about one in five cases. Lethargy can occur. Rapid imaging is key to diagnosis and treatment (see Figures 9-25 through 9-31 , which demonstrate imaging findings with multiple modalities).

Pediatric intussusception, x-ray.

Intussusception can be diagnosed by ultrasound in children, and this has become the primary screening modality in many institutions. X-ray is most often nondiagnostic, although findings consistent with obstruction may be seen.This 17-month-old male with proven intussusception had a nondiagnostic x-ray. Compare this with Figures 9-26 and 9-27 , showing ultrasound and air-contrast enema in the same patient.

Pediatric intussusception, ultrasound.

Same patient as the Figure 9-25 . This 17-month-old male presented with 3 days of bilious emesis. A right lower quadrant tubular mass consistent with ileocolic intussusception is seen. The typical alternating hyperechoic and hypoechoic layers are seen.

Intussusception can be diagnosed by ultrasound in children, and this has become the primary screening modality in many institutions. Ultrasound findings of intussusception include a concentric ring sign of bowel telescoped within bowel. Characteristically, there are alternating areas of increased and decreased echogenicity. A, B, Short-axis (transverse) views.

Pediatric intussusception, air-contrast enema.

Same patient as Figures 9-25 and 9-26 . The patient underwent air-contrast enema. To reduce the risk for bowel perforation, a manometer is used to monitor the pressure of insufflated air, and pressures are kept below 120 mm Hg. In an ileocolic intussusception, a successful reduction is marked by air refluxing through the entire colon and into small bowel. In this patient, the reduction was successful, as seen in B. A, B, Fluoroscopic air-contrast enema images.

Pediatric intussusception, ultrasound.

This 3-month-old presented with bilious vomiting and intermittent lethargy. Ultrasound shows intussusception, with the typical appearance of alternating hyperechoic and hypoechoic concentric layers. This mass extended into the right upper quadrant.

Pediatric intussusception, air-contrast enema.

Same patient as Figure 9-28 . After ultrasound showed intussusception, air-contrast enema was performed. The intussusceptum is seen as a soft-tissue density near the hepatic flexure, silhouetted by air. Although air did push the intussusceptum down to the level of the cecum, air did not reflux into the small bowel and the intussusceptum remained visible in the cecum. The patient underwent operative reduction of the intussusception.

Pediatric intussusception, ultrasound.

A, B, Transverse (short-axis) images. This 2-year-old presented with abdominal pain and vomiting. This ultrasound shows the typical mass of intussusception, with alternating hyperechoic and hypoechoic layers. The ultrasound also showed large lymph nodes (B), in keeping with the theory of an inflammatory leadpoint resulting in intussusception. After ultrasound confirmed intussusception, air-contrast enema was performed but was unsuccessful at reduction. The patient underwent laparotomy, and the intussusception was reduced. Large mesenteric lymph nodes were biopsied and subsequently grew adenovirus in culture.

Pediatric intussusception, air-contrast enema.

Same patient as Figure 9-30 . A, B, Fluoroscopic air-contrast enema images show the intussusceptum (a dark soft-tissue mass, silhouetted by air) first at the level of the hepatic flexure and then in the cecum. But after repeated efforts, the intussusceptum could not be reduced further, necessitating laparotomy.

X-rays can demonstrate a soft-tissue mass at the site of intussusception, a pattern of bowel obstruction, or perforation—but are often normal or nonspecific. Sargent et al. found that x-rays were equivocal in 53% of suspected cases of pediatric intussusception, positive in 21%, and negative in 26%. Smith et al. reported a sensitivity of 80.5% and specificity of 58% for x-rays interpreted by full-time pediatric emergency physicians. Morrison et al. found that when interpreted by emergency physicians, x-ray had a sensitivity of 48% and specificity of 21%. Overall, x-ray appears too insensitive and nonspecific to confirm or exclude intussusception ( see Figure 9-25 ).

Ultrasound has become the mainstay of diagnosis, because it is noninvasive and highly sensitive and specific. Hryhorczuk and Strouse retrospectively reviewed 814 pediatric patients evaluated for intussusception with ultrasound and found ultrasound was 97.9% sensitive and 97.8% specific. Ultrasound is a desirability modality in children because of lack of radiation exposure. Findings include an appearance of alternating hyper echoic and hypoechoic layers (see Figures 9-26, 9-28, and 9-30 ).

CT is not usually required for the diagnosis of intussusception in children. However, CT may be obtained to assess for other conditions, such as appendicitis or mass, only to demonstrate intussusception. CT demonstrates a “target” sign of alternating hyperattenuating and hypoattenuating bowel layers. Findings of bowel obstruction and ischemia (described earlier) may also be present, depending on the duration of the intussusception.

Air-contrast enema, once the mainstay of diagnosis, is usually used only therapeutically today after ultrasound diagnosis (see Figures 9-27, 9-29, and 9-31 ). Air is instilled into the rectum under controlled pressure to reduce the intussusception and is successful in about 85% of cases. Water or barium contrast enema reduction is not commonly performed today because of the safety and efficacy of air-contrast enema. Unsuccessful reduction requires surgical therapy, as does perforation from the enema, which rarely occurs (in about 1.6% of cases). When reduction with air-contrast enema is successful, recurrence takes place in about 7.5% of cases within 36 hours. However, recurrence is not associated with adverse outcomes, and routine admission after successful reduction may not be required.