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(Radiographics. 2000;20:725-749.)
© RSNA, 2000

SCIENTIFIC EXHIBIT

Tailored Helical CT Evaluation of Acute Abdomen1

(CME available in print version and on RSNA Link)

Bruce A. Urban, MD and Elliot K. Fishman, MD

1 From the Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins Medical Institutions, 600 N Wolfe St, Baltimore, MD 21287. Received March 8, 1999; revision requested May 5 and received June 3; accepted June 8. Address reprint requests to B.A.U. (e-mail: burban@jhmi.edu).


   Abstract
Top
 Abstract
LEARNING OBJECTIVES FOR TEST...
 Introduction
 Helical CT Technique
 Helical CT Findings in...
 Conclusions
 References
 
Helical computed tomography (CT) allows rapid, cost-effective evaluation of patients with acute abdominal pain. Tailoring the examination to the working clinical diagnosis by optimizing constituent factors (eg, timing of acquisition, contrast material used, means and rate of contrast material administration, collimation, pitch) can markedly improve diagnostic accuracy. Rapid (>=3 mL/sec) intravenous injection of contrast material is required for optimal assessment of acute pancreatitis, ischemic bowel, aortic aneurysm, and aortic dissection. Narrow collimation and small reconstruction intervals can help detect calculi in the biliary system and genitourinary tract. Tailored helical CT in patients with acute pyelonephritis usually involves several acquisitions through the kidneys during various phases of renal enhancement. In patients with suspected renal infarction, CT protocol must include an acquisition during the corticomedullary phase. Helical CT with 5-mm collimation through the lower abdomen and pelvis is used to evaluate patients with suspected diverticulitis. Use of both oral and intravenous contrast material can help localize small bowel perforation and characterize related complications. Tailored helical CT for assessment of abdominal hemorrhage consists of initial unenhanced CT followed by optional contrast material–enhanced CT. Clear communication between the radiologist, the patient, and the referring physician is essential for narrowing the differential diagnosis into a working diagnosis prior to helical CT.

Index Terms: Abdomen, acute conditions, 72.25, 74.723, 751.291, 76.285, 762.81, 77.291, 81.2121, 82.21 • Abdomen, CT, **.12112 • Appendicitis, 751.291 • Cholecystitis, 76.285 • Computed tomography (CT), helical technology • Computed tomography (CT), utilization • Gallbladder, calculi, 762.289, 762.81 • Intestines, stenosis or obstruction, 74.723 • Nephritis, 81.2121 • Pancreatitis, 77.291 • Stomach, ulcer, 72.25 • Ureter, calculi, 82.81


   LEARNING OBJECTIVES FOR TEST 4
Top
 Abstract
 LEARNING OBJECTIVES FOR TEST...
Introduction
 Helical CT Technique
 Helical CT Findings in...
 Conclusions
 References
 
After reading this article and taking the test, the reader will be able to:

• Discuss the importance of narrowing the differential diagnosis into a working diagnosis to optimize helical CT technique.

• Recognize the CT appearance of common pathologic conditions that manifest with acute abdominal pain.

• Understand the role of helical CT in the evaluation of acute abdominal pain.


   Introduction
Top
 Abstract
 LEARNING OBJECTIVES FOR TEST...
 Introduction
Helical CT Technique
 Helical CT Findings in...
 Conclusions
 References
 
The term acute abdomen refers to any clinical condition characterized by severe abdominal pain that develops over a period of hours. Rapid, accurate diagnosis is essential if morbidity and mortality are to be significantly decreased. Clinical assessment is often difficult, and laboratory and conventional radiologic findings are often nonspecific. The development of cross-sectional imaging has had a tremendous impact on the diagnosis and treatment of acute abdomen (1–3). In particular, computed tomography (CT) has gained widespread acceptance as a reliable and highly accurate modality in the evaluation of affected patients (1–4). CT is most often indicated in patients with severe abdominal pain who may require surgery or other forms of intervention. It is probably most beneficial in patients who present with confusing or conflicting clinical signs and symptoms. Conventional CT has prospectively demonstrated an accuracy of nearly 95% in acute abdomen (1). The introduction of helical CT technology, with advances in contrast dynamics and high-resolution volumetric data acquisition, has further enhanced the utility of CT in abdominal imaging (5,6). Helical CT is a rapid, cost-effective procedure and provides diagnostic information that can help determine appropriate clinical management (7). It is useful not only in diagnosing the primary abnormality but also in detecting and characterizing the full extent of disease. Helical CT is the imaging modality of choice for patient triage, and many hospitals now have helical CT scanners on-site in the emergency department. Undoubtedly, the need for conventional radiology has diminished due to the increasing utility of helical CT in abdominal imaging (4,8).

In this article, we discuss various protocols and techniques for optimizing the helical CT examination. We also discuss and illustrate helical CT findings in a variety of conditions that can manifest with acute abdominal pain, including those affecting the pancreas, biliary system, spleen, genitourinary tract, gastrointestinal tract, and vascular system.


   Helical CT Technique
Top
 Abstract
 LEARNING OBJECTIVES FOR TEST...
 Introduction
 Helical CT Technique
Helical CT Findings in...
 Conclusions
 References
 
Suggested routine helical CT protocol for patients with nonlocalized signs and symptoms of acute abdominal pain is shown in Table 1. However, it is also important to try to tailor the CT examination to the specific clinical findings and area of interest. Scanning parameters will vary depending on the working clinical diagnosis (Table 2). The remainder of the examination is performed following targeted evaluation. Failure to tailor the examination can greatly reduce the ability to accurately and confidently detect disease. Communication between the radiologist, the patient, and the referring physician is essential for narrowing the differential diagnosis into a working diagnosis prior to scanning.


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  		TABLE 1. Suggested Routine Helical CT Protocol for Nonlocalized Acute Abdominal Pain
 

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  		TABLE 2. Variations in Helical CT Protocol for Acute Abdomen Based on Working Clinical Diagnosis
 
Contrast Material
Intravenously Administered Contrast Material.—Use of intravenous contrast material is recommended in most cases. Exceptions include evaluation of suspected ureteral colic and, in some instances, dedicated evaluation of suspected appendicitis and diverticulitis. Intravenous contrast material opacifies the abdominal vasculature and provides useful information regarding enhancement of the parenchymal organs and intestine (5).

The amount of contrast material administered and the rate of injection will vary depending on the working diagnosis. In general, 120 mL of iodinated contrast material injected at a rate of 2 mL per second is adequate. Images are usually obtained during the portal venous phase of enhancement beginning approximately 70 seconds after initiation of injection.

In dedicated vascular scanning for evaluation of certain conditions such as suspected aortic aneurysm or dissection, initial precontrast images can be helpful in localizing intramural hematoma or impending rupture. CT (especially CT angiography) should be performed after intravenous bolus administration of contrast material; injection of 150 mL of contrast material at a rate of 3–4 mL per second is recommended. Scanning should be initiated during the arterial phase of enhancement beginning 20–30 seconds after initiation of injection (9).

Orally Administered Contrast Material.—Use of oral contrast material is also recommended in most cases. Typically, 750–1,000 mL of water-soluble contrast agent containing 3% iodine is administered. Exceptions include cases in which high-grade small bowel obstruction or ureteral colic is suspected. In some patients with suspected gastric disease or gastrointestinal bleeding, water can be used as an oral contrast agent (10–14). In addition, oral contrast material should not be used for CT angiography because it can interfere with 3D imaging (9). When oral contrast material is used to exclude appendicitis or pelvic disease, it is recommended that imaging be delayed at least 1 hour to allow optimal bowel opacification. Rectal contrast material is not routinely used, although its use has been advocated as an alternative protocol for evaluation of suspected appendicitis or diverticulitis (15,16).

Collimation
Scanning parameters will vary somewhat depending on the type of scanner available. In general, a collimation of 5–7 mm is preferred for routine scanning. Narrow (3-mm) collimation is suggested for CT angiography, evaluation of suspected ureteral colic, and dedicated evaluation of suspected acute pancreatic or biliary conditions (9–12). However, use of narrow collimation can also increase noise and degrade image quality, especially in the pelvis and in obese patients.

Pitch
Most scanners provide high-quality diagnostic images with a pitch of 1.5–1.6. An increased pitch of 2 is necessary in cases requiring narrow collimation and significant patient coverage (eg, evaluation of ureteral colic and vascular disease) (9–12).

Phases of Acquisition
Single acquisitions performed during either the arterial phase (beginning 20–30 seconds following intravenous injection of contrast material) or the portal venous phase (beginning 70–90 seconds after injection) are adequate in most patients. Occasionally, images should be acquired during both phases, especially for dedicated contrast material–enhanced evaluation of the liver or kidneys. Delayed images (acquired beginning 4 minutes after injection) are also helpful in cases of suspected pyelonephritis or in the work-up of suspected pelvic disease, when opacification of the bladder may be desired (17–19).


   Helical CT Findings in Acute Abdomen
Top
 Abstract
 LEARNING OBJECTIVES FOR TEST...
 Introduction
 Helical CT Technique
 Helical CT Findings in...
Conclusions
 References
 
Pancreas
Patients with acute pancreatitis often present with midepigastric pain, nausea, and vomiting. It has been shown that the clinical severity of disease correlates well with CT findings; thus, helical CT can often be used to predict clinical outcome in affected patients (20,21). Tailored evaluation with helical CT requires rapid (3 mL/sec) intravenous bolus administration of contrast material to optimize glandular enhancement as well as narrow collimation for improved resolution (22–24). Dual-phase scanning incorporating portal venous phase images optimizes opacification of the veins surrounding the pancreas but is not necessary for diagnosis in most patients with acute pancreatitis (23,24).

Helical CT findings in acute pancreatitis include glandular enlargement due to interstitial parenchymal edema as well as increased attenuation and stranding in the peripancreatic fat (Fig 1). Glandular enlargement is typically diffuse rather than focal. Focal enlargement may be indistinguishable from cancer (25,26). The pancreatic contour in acute pancreatitis may be irregular, with focal areas of decreased attenuation representing necrosis or edema (27). Acute pancreatitis can progress to a pathologic condition characterized by extensive phlegmon formation along with peripancreatic fluid collections, hemorrhage, peripancreatic abscess, and extraglandular fat necrosis (Fig 2). Pancreatic exudate most often collects in the immediate peripancreatic space, the anterior pararenal space, the lesser sac, and eventually in the peritoneum itself, with dissection pathways that include the transverse mesocolon and the mesenteric root (25,26). Helical CT offers several advantages in the evaluation of acute pancreatitis (23,24). Most important, it can help stage disease involvement and detect complications. Helical CT can demonstrate necrosis or hemorrhage within he pancreas itself as well as extension of the inflammatory process into contiguous organs (28). Thrombosis of the portal or splenic vein with collateral vessel formation is clearly depicted. Helical CT can also help detect an unsuspected, life-threatening pseudoaneurysm. Pancreatic abscesses may develop with superinfection in acute pancreatitis and contribute significantly to mortality in this setting. Gas within a fluid collection is highly suggestive of abscess formation (25,26).



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  		Figure 1.   Glandular enlargement in a 49-year-old woman with acute pancreatitis. Axial CT scan obtained with intravenous contrast material shows a moderate amount of inflammatory fluid (arrows) surrounding a minimally enlarged pancreas (p). The pancreas demonstrates normal attenuation without evidence of necrosis.

 


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  		Figure 2.   Pancreatic abscess in a 38-year-old man with acute pancreatitis. Axial CT scan obtained with rapid bolus administration of intravenous contrast material shows an air-fluid level (A) in the lesser sac anterior to the underlying pancreas (arrow), whose enhancement implies viability.

 
Acute pancreatic fluid collections can evolve into true pseudocysts as the inflammatory reaction walls itself off around the periphery. In contrast to the dynamic fluid collections in acute pancreatitis, pseudocysts have defined capsules and are more often seen in the setting of subacute or chronic pancreatitis (25,26). The fluid in pseudocysts usually has homogeneous low attenuation similar to that of water. Pseudocyst contents that exhibit a heterogeneous pattern of increased attenuation suggest the possibility of superimposed hemorrhage or infection (Fig 3). Helical CT can clearly define the location and extent of pseudocysts and is helpful for surgical planning in affected patients. Pancreatic pseudocysts may invade vascular structures adjacent to the pancreas such as the portal or splenic vein with consequent compression or even obstruction (29). Pseudocysts may also erode into the spleen or into pancreatic or peripancreatic arteries with resultant acute hemorrhage (29,30).



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  		Figure 3.   Hemorrhagic pancreatic pseudocyst in a 44-year-old man who presented with acute abdominal pain. Axial CT scan obtained with intravenous contrast material demonstrates calcifications from chronic pancreatitis in the head of the pancreas. A high-attenuation focus of blood (arrow) is seen within the low-attenuation pseudocyst, a finding that is consistent with hemorrhage.

 
If hemorrhage is clinically suspected, initial unenhanced CT should be included in the examination because visualization of hemorrhagic fluid can be difficult following intravenous injection of contrast material.

Biliary System
Acute Cholecystitis.—Although CT is not typically used to screen patients with presumed cholecystitis, patients with acute abdominal pain often undergo CT as an initial diagnostic work-up. Routine helical CT protocol with intravenous contrast material is used for most patients. The most sensitive helical CT findings in acute cholecystitis are inflammation and significant thickening ( > 3 cm) of the gallbladder wall with increased attenuation in the setting of a distended gallbladder (31). Transient focal areas of increased attenuation in the liver can be seen adjacent to the inflamed gallbladder, findings that probably indicate hepatic arterial hyperemia and early venous drainage (32,33). Other findings include haziness of the pericholecystic fat, pericholecystic fluid, and increased attenuation of the bile (34). A combination of some or all of these findings is highly specific for acute cholecystitis, approaching the sensitivity of ultrasonography (US) (Fig 4).



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  		Figure 4.   Acute cholecystitis in a 40-year-old man. Axial CT scan obtained with intravenous contrast material demonstrates the classic features of acute cholecystitis: distention of the gallbladder (gb), wall thickening with enhancement, and focal pericholecystic fluid and inflammation. Surgery revealed acute cholecystitis with a xanthogranulomatous component.

 
Helical CT can also demonstrate complications of acute cholecystitis. Air within the gallbladder lumen or wall indicates the presence of emphysematous cholecystitis, which in turn implies underlying gangrenous changes (Fig 5). Care must be taken not to mistake air introduced at recent endoscopic retrograde cholangiopancreatography or prior surgery for acute emphysematous cholecystitis. Infectious cholangitis can complicate cholecystitis, manifesting as hepatic abscesses (Fig 6) (35). Perforation is a relatively late finding in patients with cholecystitis and often necessitates surgical intervention. CT is reasonably sensitive in diagnosing and localizing gallbladder perforation (Fig 7) (36). Some patients may present with acute cholecystitis and have fairly unremarkable CT findings. Correlation of CT findings with those at other modalities, especially US and nuclear magnetic resonance imaging, is vital if the diagnosis of cholecystitis remains in doubt. This is particularly important for early diagnosis leading to rapid intervention in the intensive care unit patient or the severely ill patient in whom the gallbladder may be distended at baseline evaluation (37).



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  		Figure 5.   Emphysematous cholecystitis in a 66-year-old man. Axial CT scan obtained with intravenous contrast material shows air filling the distended gallbladder lumen (gb) and wall (arrow). Pathologic analysis revealed extensive necrosis with adherence to the adjacent liver.

 


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  		Figure 6.   Infectious cholangitis in an 82-year-old woman with acute cholecystitis. Axial CT scan obtained with intravenous contrast material demonstrates multiple hepatic abscesses (arrows), which were a complication of cholecystitis in this case.

 


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  		Figure 7.   Gallbladder perforation in a 59-year-old woman with acute cholecystitis. Axial CT scan obtained after bolus injection of intravenous contrast material demonstrates a liver abscess with heterogeneous attenuation (A) adjacent to the distended gallbladder (gb). Continuity is demonstrated between the gallbladder and the abscess, indicating the site of rupture (arrow). Aspiration yielded frank pus.

 
Choledocholithiasis.—Patients with choledocholithiasis often present with recurrent episodes of right upper quadrant pain, fever, and jaundice. Tailored helical CT protocol includes narrow collimation and small (2–3-mm) reconstruction intervals to optimize detection of cholelithiasis and choledocholithiasis. In older studies, conventional CT demonstrated sensitivities as high as 90% for the detection of common bile duct stones (38). In a study by Neitlich et al (39), helical CT demonstrated a sensitivity of 88%, a specificity of 97%, and an accuracy of 94% in the diagnosis of common bile duct stones. The most reliable CT finding is a high-attenuation nidus within the duct (Figs 8, 9) (38,39). Helical CT with narrow collimation and small reconstruction intervals is particularly helpful in detecting small stones and the subtle, alternating low- and high- attenuation rings seen with mixed cholesterol-calcium stones, which often result in distal common bile duct obstruction. In the absence of a pancreatic head mass or a detectable cause of obstruction, the differential diagnosis of a distal common bile duct obstruction should include stone, biliary stricture, and small ampullary mass.



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  		Figure 8a.   Biliary obstruction from an impacted common bile duct stone in an 81-year-old woman with choledocholithiasis. (a) On an axial CT scan obtained with intravenous contrast material, the common bile duct (cbd) is moderately dilated and tortuous. (b) CT scan obtained inferior to a shows an impacted high-attenuation nidus from a gallstone at the ampulla (arrow).

 


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  		Figure 8b.   Biliary obstruction from an impacted common bile duct stone in an 81-year-old woman with choledocholithiasis. (a) On an axial CT scan obtained with intravenous contrast material, the common bile duct (cbd) is moderately dilated and tortuous. (b) CT scan obtained inferior to a shows an impacted high-attenuation nidus from a gallstone at the ampulla (arrow).

 


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  		Figure 9.   Gallstones in a 75-year-old man with choledocholithiasis and pancreatitis. Axial CT scan obtained with intravenous contrast material shows very subtle gallstones within the gallbladder lumen (white arrow) and common bile duct (black arrow). Stranding is seen in the peripancreatic fat.

 
Spleen
Patients with splenic infarction typically present with left upper quadrant pain, although in some cases the disease may be clinically silent. Common causes of splenic infarction include bacterial endocarditis, portal hypertension, and underlying splenomegaly (29). Pancreatitis can also extend into the splenic hilum and result in infarction (29,30). Tailored helical CT protocol requires intravenous bolus administration of contrast material. At helical CT, focal infarcts appear as wedge-shaped areas of decreased attenuation that extend to the surface of the spleen (Fig 10) (41–43). Most infarcts are easily appreciated against the normal inhomogeneous pattern of early splenic enhancement. Global infarction can manifest as diffuse areas of decreased attenuation in the spleen and can mimic splenic abscess or tumor (Fig 11). In some cases of global infarction, the splenic periphery remains enhanced due to perfusion from capsular vessels. Helical CT performed during peak contrast material enhancement allows reliable depiction of the splenic artery and vein (41).



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  		Figure 10.   Splenic infarction in a 41-year-old woman. Axial CT scan obtained with intravenous contrast material demonstrates multiple infarcts in an enlarged spleen (arrows). Most splenic abnormalities, including infarcts, can be differentiated from normal inhomogeneous splenic enhancement.

 


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  		Figure 11.   Global splenic infarction in a 32-year-old man with human immunodeficiency virus (HIV) infection who presented with severe left upper quadrant pain. Axial CT scan obtained with intravenous contrast material demonstrates an enlarged, infarcted spleen (S) resulting from portal vein thrombosis. The spleen appears similar to a large abscess.

 
Genitourinary Tract
Acute Pyelonephritis.—Acute pyelonephritis is a common clinical diagnosis in patients who present with fever, chills, and flank tenderness. Infections typically result from ascending retrograde spread through the collecting ducts into the renal parenchyma. Patients are referred for CT evaluation of acute pyelonephritis when symptoms are poorly localized or complications are suspected (44–46). Tailored helical CT in patients with acute pyelonephritis usually consists of several acquisitions through the kidneys during various phases of renal enhancement (47). Typically, images are acquired during the corticomedullary phase beginning 30 seconds after initiation of injection and during either the nephrographic phase (70–90 seconds after injection) or the excretory phase (5 minutes after injection).

Helical CT findings in acute pyelonephritis consist of focal areas of striated or wedge-shaped hypoperfusion, resulting in a characteristic "patchy" nephrogram (Fig 12) (48–50). Striations result from stasis of contrast material within edematous tubules that demonstrates increasing attenuation over time. The infected kidney is usually enlarged, and there is often stranding in the perinephric fat (Fig 13). Poorly enhanced areas of focal pyelonephritis can mimic a renal mass at conventional CT. Helical CT is probably more specific in differentiating infection from tumor (47). It is also helpful in detecting subtle cases of acute pyelonephritis; clues to the diagnosis include loss of normal, sharp corticomedullary differentiation and delayed appearance of the cortical nephrogram, abnormalities that are visualized only during early dynamic contrast enhancement. Delayed views of the infected kidney may demonstrate a nephrogram with increased attenuation (17).



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  		Figure 12.   Acute bilateral pyelonephritis in a 34-year-old woman. Axial CT scan obtained with intravenous contrast material demonstrates patchy, striated nephrograms.

 


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  		Figure 13.   Acute pyelonephritis in a 69-year-old man. Axial nephrographic-phase CT scan obtained with intravenous contrast material reveals patchy enhancement of the left kidney. The kidney is minimally enlarged with perinephric stranding, secondary findings that also suggest infection.

 
Renal Infarction.—Patients with renal infarction typically present with acute flank pain and may have hematuria. Renal infarcts are typically the result of embolism (usually in a diseased heart), aortic dissection, trauma, or thrombosis (51,52). Tailored helical CT must include an acquisition during the corticomedullary phase of enhancement (beginning 30 seconds after initiation of injection) when renal arterial and venous opacification is greatest. Helical CT findings include one or more focal parenchymal defects that involve both the cortex and medulla and extend to the capsular surface of the kidney. Segmental infarcts of the anterior or posterior renal arteries demonstrate a characteristic appearance at CT (Fig 14) (53). The larger ventral branch supplies the anterior lateral portion of the kidney, whereas the smaller dorsal branch supplies the posterior medial aspect. With global infarction, the entire kidney is nonperfused (Fig 15). Characteristic rim enhancement representing collateral capsular perfusion may be seen surrounding the nonfunctioning, infarcted kidney (51).



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  		Figure 14.   Abdominal aortic aneurysm and renal infarction in a 67-year-old man. On an axial arterial-phase CT scan obtained with intravenous contrast material, the lateral aspect of the left kidney demonstrates the characteristic appearance of renal infarction (arrow).

 


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  		Figure 15.   Global infarction in a 67-year-old man with aortic dissection. The patient presented with acute left flank pain. Axial CT scan obtained with intravenous contrast material reveals a dissection flap occluding the left renal artery (arrow). Flow to the left kidney is obstructed.

 
Ureteral Stones.—Patients with acute pain from ureteral calculi present with hematuria and flank pain radiating to the groin. Helical CT provides a rapid and accurate test for the presence of ureteral calculi (10–12). A significant advantage of using helical CT is the ability to document the presence of stones without intravenous injection of contrast material because the vast majority of calculi are radiopaque at CT (54). In addition, lack of breathing misregistration with helical CT ensures continuous coverage of the entire collecting system and improves detection of small stones. These advantages make helical CT the screening modality of choice at most institutions (12). Helical CT has proved as sensitive as intravenous pyelography in detecting ureteral obstruction and more sensitive in identifying calculi as the source of obstruction (11). In clinical studies, helical CT obviated intravenous pyelography in 90% of cases (55). In 292 patients with 100 proved ureteral stones, Smith et al (10) found helical CT to be 97% sensitive, 96% specific, and 97% accurate in the depiction of ureteral calculi.

Tailored helical CT in patients with suspected ureteral calculi requires a continuous breath-hold acquisition from the top of the kidneys to the bladder base. Narrow (3-mm) collimation and small reconstruction intervals (also 3 mm) are essential for optimal detection of small calculi (10–12). Identification of hydronephrosis, hydroureter, and obstructing calculi is usually straightforward (Fig 16). Characteristic locations of obstructing calculi include the ureteropelvic junction and the ureterovesical junction. Prone scans may be needed to differentiate a ureterovesical junction stone from a recently passed stone. Secondary signs of perinephric stranding and edema provide supporting evidence for acute obstruction (55, 56). Focal periureteral stranding can also help localize subtle calculi (Fig 17). In addition, helical CT can be used to document stone size and predict clinical outcome (12,57). Rarely, phleboliths can mimic renal calculi and present a diagnostic dilemma. Furthermore, HIV–positive patients undergoing indinavir therapy can present with nonopaque stones (58). In such cases, intravenous contrast material may be required to localize the ureter. However, we have found that unenhanced CT provides rapid and accurate diagnosis in most patients.



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  		Figure 16a.   Obstructing ureteral calculus in a 51-year-old woman. (a) Axial unenhanced CT scan demonstrates right hydronephrosis. (b) CT scan obtained inferior to a clearly depicts an obstructing calculus in the midureter (arrow).

 


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  		Figure 16b.   Obstructing ureteral calculus in a 51-year-old woman. (a) Axial unenhanced CT scan demonstrates right hydronephrosis. (b) CT scan obtained inferior to a clearly depicts an obstructing calculus in the midureter (arrow).

 


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  		Figure 17.   Ureteral calculus in a 24-year-old man. Axial unenhanced CT scan shows focal stranding around the middle of the right ureter, a finding that helps localize a subtle calculus (arrow).

 
Pelvic Inflammatory Disease.—Patients with pelvic inflammatory disease typically present with vaginal discharge, pelvic pain, fever, and an elevated white cell count. US is the initial modality of choice in most patients. Occasionally, patients are referred for CT when signs and symptoms are poorly localized or complications such as tubo-ovarian abscess are suspected. Helical CT with intravenous contrast material is required to define the adnexal anatomy and potential disease (18,19). Occasionally, delayed images obtained following bladder opacification (5 minutes after contrast material injection) are needed to help differentiate a cystic mass or collection from the bladder (18,19). Typical helical CT findings in patients with pelvic inflammatory disease include unilateral or bilateral adnexal masses, hydrosalpinx, and pelvic ascites (59). Tubo-ovarian abscesses manifest as complex masses with septations and thick, irregular walls (59,60). Helical CT demonstrates characteristic peripheral enhancement of the fallopian tubes and abscess capsule (Fig 18) (18,19). Tubo-ovarian abscesses can be difficult to differentiate from ovarian neoplasms or from abscesses secondary to other causes such as inflammatory bowel disease or appendicitis. In addition, ovarian cysts and a small amount of ascites are normal in premenopausal patients, and correlation with clinical history and US findings remains essential.



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  		Figure 18.   Tubo-ovarian abscess and pyosalpinx in a 41-year-old woman. On a CT scan obtained with intravenous contrast material, the left fallopian tube is dilated and filled with fluid. Subtle tubular enhancement is also seen (arrows).

 
Gastrointestinal Tract
Appendicitis.—Acute appendicitis is one of the most common causes of acute abdominal pain. Patients present with right lower quadrant pain, nausea, and vomiting and often have an elevated white cell count. CT has long been recognized as having a high diagnostic accuracy in patients with acute appendicitis (61,62). Initial studies have shown conventional CT with both oral and intravenous contrast material to be highly diagnostic for appendicitis (61). More recently, other investigators have shown that tailored helical CT evaluation with 5-mm collimation with or without oral or intravenous contrast material can be just as accurate in the diagnosis of appendicitis, having a reported accuracy of 94%–98% (15,63). Unenhanced helical CT is helpful in that it eliminates the risk of an adverse reaction to intravenous contrast material as well as the time required for oral or rectal administration of contrast material. Obviously, if intravenous or oral contrast material is not used, this technique has limited value in patients with minimal intraabdominal fat. Unenhanced helical CT with oral or rectal contrast material alone has been implemented in the community hospital setting in a study of 100 patients and has demonstrated excellent diagnostic accuracy (95%) (64).

A dilated, fluid-filled appendix is the most specific helical CT finding in acute appendicitis (15,63). Calcified appendicoliths and periappendiceal inflammation are helpful secondary findings. Enhancement of the appendiceal wall is often seen following intravenous bolus administration of contrast material and is another specific sign of inflammation (Fig 19). However, in patients with adequate intraperitoneal fat, diagnosis can be made without oral or intravenous contrast material because the focal nature of the periappendiceal stranding is obvious. Helical CT can also demonstrate complications of appendicitis, including perforation, small bowel obstruction, and mesenteric venous thrombosis (Fig 20). Many other conditions can lead to inflammation and abscess formation in the right lower quadrant and mimic findings of acute appendicitis at both clinical examination and radiography (65–67). These include Crohn disease and cecal diverticulitis, among others (Fig 21). Essentially all inflammatory processes of the gastrointestinal tract, including inflammatory bowel disease and infectious enteritis and colitis, can manifest with pain and produce inflammatory stranding in the mesenteric fat. One must be certain that the appendix is the cause of inflammation before making the diagnosis of acute appendicitis.



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  		Figure 19a.   Acute appendicitis. Axial CT scans obtained with intravenous contrast material in a 27-year-old woman (a) and a 62-year-old man (b) show a minimally distended appendix with an enhancing wall (arrow). Stranding is seen in the periappendiceal fat. These findings are pathognomonic for acute appendicitis.

 


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  		Figure 19b.   Acute appendicitis. Axial CT scans obtained with intravenous contrast material in a 27-year-old woman (a) and a 62-year-old man (b) show a minimally distended appendix with an enhancing wall (arrow). Stranding is seen in the periappendiceal fat. These findings are pathognomonic for acute appendicitis.

 


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  		Figure 20a.   Perforating appendicitis in a 44-year-old man. (a) Axial CT scan obtained with intravenous contrast material demonstrates subtle findings of a minimally enhancing, tortuously dilated appendix (arrowheads). This finding was initially misinterpreted as the normal terminal ileum. (b) CT scan obtained inferior to a demonstrates minimal fluid in the pelvis (arrow). B  = bladder. In this case, use of oral contrast material would likely have aided in making the correct diagnosis. Although reported sensitivities for detecting acute appendicitis are similar with any combination of oral, intravenous, and rectal contrast material (or with none of the three), we prefer using a combination of the first two whenever possible.

 


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  		Figure 20b.   Perforating appendicitis in a 44-year-old man. (a) Axial CT scan obtained with intravenous contrast material demonstrates subtle findings of a minimally enhancing, tortuously dilated appendix (arrowheads). This finding was initially misinterpreted as the normal terminal ileum. (b) CT scan obtained inferior to a demonstrates minimal fluid in the pelvis (arrow). B  = bladder. In this case, use of oral contrast material would likely have aided in making the correct diagnosis. Although reported sensitivities for detecting acute appendicitis are similar with any combination of oral, intravenous, and rectal contrast material (or with none of the three), we prefer using a combination of the first two whenever possible.

 


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  		Figure 21.   Crohn disease mimicking acute appendicitis in an 18-year-old woman. Axial CT scan obtained with intravenous contrast material shows minimal thickening of the distal terminal ileum (white arrow). Focal abscesses in the right lower quadrant (black arrows) mimic perforating appendicitis.

 
Diverticulitis.—Diverticular disease of the colon is common, affecting up to 65% of patients with acute abdominal pain by age 65 years and typically involving the sigmoid colon. Up to 25% of these patients will develop diverticulitis (68). Patients often present with fever, left lower quadrant pain, and an elevated white cell count. CT is sensitive in proved cases of diverticulitis and has replaced barium enema examination in the initial evaluation of patients with suspected diverticulitis. CT findings in acute diverticulitis include colonic wall thickening in the presence of diverticula, pericolonic fat stranding, phlegmon, air bubbles, abscess, and dependent free fluid (Fig 22) (69,70). CT is up to 93% sensitive and approaches 100% specificity and accuracy in the diagnosis or exclusion of diverticulitis (16,71). CT is also much more sensitive than barium enema examination in determining the presence and nature of pericolonic complications (72). In addition, CT is excellent for detecting other causes of left lower quadrant pain that may mimic diverticulitis and can suggest alternative diagnoses in 78% of cases when other causes are present (16).



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  		Figure 22.   Acute diverticulitis in a 62-year-old man. Axial CT scan obtained with intravenous contrast material demonstrates focal thickening and pericolonic stranding (arrow), both of which are classic features of acute diverticulitis.

 
Tailored helical CT evaluation of patients with suspected diverticulitis includes 5-mm-collimation scanning through the lower abdomen and pelvis. Although helical CT with rectal contrast material alone is highly accurate for diagnosis (16), intravenous bolus administration of contrast material is helpful in the detection and characterization of pericolonic inflammation and is therefore recommended in most patients. Helical CT with intravenous contrast material accentuates the characteristic rim enhancement of pericolonic abscesses and helps differentiate neighboring bowel loops from surrounding inflammatory changes (Fig 23). Bowel lumen opacification is helpful for depiction of wall thickening and is best achieved if rectal rather than oral contrast material is used. Most authors suggest the administration of 400–800 mL of 3% iodinated contrast material (16). In practice, however, the diagnosis can be established in most patients without use of rectal contrast material, which can be reserved for the occasional patient with equivocal findings.



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  		Figure 23.   Diverticular abscess in a 49-year-old man. Axial CT scan obtained with intravenous contrast material shows a small abscess adjacent to an abnormally thickened sigmoid colon (arrow). The rim enhancement of the fluid collection implies infection.

 
Peptic Ulcer Disease.—Patients with peptic ulcer disease present with epigastric pain, nausea, and vomiting. At times, symptoms are nonlocalized or indistinguishable from those in acute pancreatitis or cholecystitis, and CT is often performed in such cases. Alternatively, patients with known ulcer disease may undergo CT for assessment of severe complications of ulcer penetration, including pancreatitis, perforation, and abscess formation.

Tailored helical CT of the stomach and duodenum is best performed after adequate distention following the ingestion of either water or positive oral contrast agent. Intravenous bolus administration of contrast material is also necessary if water is used (13,14). Peptic ulcer disease often manifests at CT as focal thickening (Fig 24). Unfortunately, gastric and duodenal thickening are often nonspecific, and correlation with the patient's symptoms is poor (73). In addition, many patients have chronic thickening, often due to infection with Helicobacter pylori, which can produce substantial thickening in the body and antrum of the stomach (74). Helical CT is probably most helpful and specific in patients with complications from penetrating ulcers (Fig 25) (75), who will demonstrate inflammatory changes in the adjacent soft tissues and organs, including the pancreas, liver, and lesser omentum. These changes are easily identified at helical CT (75,76). CT is also valuable in the detection of clinically unsuspected perforation, although determining the precise site of perforation is often difficult (76).



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  		Figure 24.   Duodenitis in a 72-year-old man. The patient presented with postoperative nausea, vomiting, and abdominal pain. Axial CT scan obtained with intravenous contrast material shows moderate duodenal thickening (arrowheads).

 


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  		Figure 25.   Perforated duodenal ulcer in a 62-year-old man. Axial CT scan obtained with intravenous contrast material demonstrates stranding in the right anterior pararenal space (arrow), a common location for the traversal of fluid in patients with perforated ulcer. Surgery confirmed the presence of a large perforated ulcer.

 
Small Bowel Obstruction.—Small bowel obstruction is a common cause of acute abdominal pain. Patients often present with nausea, vomiting, and abdominal tenderness. Small bowel obstruction has a variety of causes, the most common of which are adhesions (64%–79% of cases), hernia (15%–25%), and tumor (10%–15%) (77). The utility of CT in the evaluation of small bowel obstruction depends somewhat on the degree and suspected cause of obstruction. At most institutions, CT has supplanted the small bowel follow-through study as the initial examination of choice because it can help confirm the need for or obviate surgery. CT has proved to be very useful in cases of high-grade small bowel obstruction, with a sensitivity of 90%–96%, a specificity of 96%, and an accuracy of 95% (78,79). In low-grade obstruction, however, the accuracy of CT is only about 50%, and follow-up CT or barium examination may be indicated. In all cases of small bowel obstruction (including low-grade obstruction), sensitivity drops to about 60%, with a specificity of 80% and an accuracy of 66% (80). For this reason, a small bowel follow-through study is often performed following CT if clinical concerns for low-grade obstruction persist.

Tailored helical CT evaluation for suspected high-grade small bowel obstruction is best performed without oral contrast material. Patients with high-grade small bowel obstruction already have large amounts of fluid in the bowel that acts as a natural contrast agent, especially when combined with intravenous bolus injection of contrast material, allowing opacification of the normal bowel wall as well as bowel masses. Intravenous contrast material also allows accurate assessment of the extent of bowel wall thickening. However, in cases of low-grade obstruction or vague pain, use of oral contrast material is indicated because it improves accuracy in the detection of inflammation and abscesses and can optimize identification of a transition zone by increasing the load factor on the bowel lumen.

The essential helical CT finding in small bowel obstruction is a definable transition from dilated to decompressed small bowel. Careful inspection of the transition point and luminal contents of the bowel will often reveal the underlying cause of obstruction (78,79). Hernias are the most common positive finding and are usually seen in the inguinal region or abdominal wall (Fig 26) (81,82). Abdominal wall hernias account for the great majority of external hernias (82,83). A congenital or acquired weakness or defect in the muscular layers of the abdominal wall produces the potential hernia site. Other types of abdominal wall hernias include incisional hernias, parastomal hernias, and spigelian hernias (78,82,83). Indirect inguinal hernias are also very common and are caused by acquired weakness and dilatation of the internal inguinal ring, which results from a defect in the transverse fascia (83). Less frequently, an obstructing mass or tumor implant may be identified (Fig 27). Intussusception may also result in obstruction and is often associated with an underlying mass that serves as a lead point (84,85). In the absence of an obstructing mass or hernia at CT, small bowel obstruction is most likely due to adhesions, especially in patients with a history of surgery (Fig 28). A finding of adhesions as the cause of small bowel obstruction effectively excludes an obstructing mass or hernia in the majority of patients.



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  		Figure 26a.   Small bowel obstruction from inguinal hernia in a 55-year-old man. (a) Axial CT scan obtained with intravenous contrast material shows moderately dilated small bowel loops with minimal mural thickening and enhancement. (b) CT scan obtained inferior to a shows a transition point at the site of an incarcerated right inguinal hernia (arrow).

 


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  		Figure 26b.   Small bowel obstruction from inguinal hernia in a 55-year-old man. (a) Axial CT scan obtained with intravenous contrast material shows moderately dilated small bowel loops with minimal mural thickening and enhancement. (b) CT scan obtained inferior to a shows a transition point at the site of an incarcerated right inguinal hernia (arrow).

 


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  		Figure 27.   Small bowel obstruction from metastatic gastric cancer in a 68-year-old man. Axial CT scan obtained with bolus administration of intravenous contrast material demonstrates a markedly distended small bowel with air-fluid levels. An enhancing serosal implant is seen involving a bowel loop in the midabdomen (arrow).

 


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  		Figure 28.   Small bowel obstruction from adhesions in a 61-year-old man who had undergone renal transplantation. Axial CT scan obtained with intravenous contrast material shows a moderately dilated proximal small bowel. Collapsed small bowel loops are seen in the right lower quadrant (arrowheads). No discrete mass is seen in the region of transition in the lower abdomen, and there is no evidence of hernia or intussusception.

 
Helical CT can also be useful in differentiating simple from strangulated small bowel obstruction (86,87). Strangulated small bowel obstruction involves mechanical obstruction proximal to the involved loop and closed-loop obstruction with venous congestion of the involved loop (88). Initially, venous outflow is occluded in the involved loop, resulting in distention and engorgement of vessels. Bowel hemorrhage ensues, resulting in transudation of fluid into the peritoneal cavity. Findings that suggest strangulation include poorly enhanced or unenhanced bowel wall and the serrated beak sign, both of which are 100% specific for strangulation (86). The serrated beak sign is caused by twisting of bowel with regional engorgement of the mesenteric vasculature, mesenteric edema, and bowel wall thickening. Closed-loop obstructions may have a peculiar C- or U-shaped configuration, and fluid in the mesenteric leaves may radiate out from the point of strangulation (Fig 29). An unusual course and diffuse engorgement of the mesenteric vasculature with mesenteric haziness are highly specific for strangulation (86).



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  		Figure 29.   Closed-loop small bowel obstruction from adhesions in a 44-year-old woman. Axial CT scan obtained with intravenous contrast material shows distended small bowel loops in the left midabdomen with collapsed loops in the right lower quadrant. Strangulated obstruction is suggested by the bowel wall thickening with mesenteric edema radiating toward the point of strangulation (arrow). Surgery revealed extensive transmural ischemia with mucosal and submucosal hemorrhage.

 
Ischemic Bowel.—Patients with bowel ischemia can present with symptoms ranging from relatively minor discomfort to acute abdominal pain, which makes clinical diagnosis difficult (89). Predominant causes of intestinal ischemia include vascular occlusion or thrombosis, whether from arterial or venous disease, and hypoperfusion. Usually, a combination of these factors is seen, and the degree to which a given factor predominates determines patient outcome. Ischemia can also result from secondary vascular compromise in patients with bowel obstruction, including closed loop obstruction, hernia, and intussusception. CT plays a key role in identifying early changes of ischemia and in determining the underlying cause of ischemic bowel (90–92). However, CT findings in patients with ischemic bowel, even when newer helical CT techniques are used, can be nonspecific (93). In patients with obstruction, helical CT has only an 80% positive predictive value for ischemia, and up to 20% of patients will have negative findings at laparotomy for ischemia (94). However, helical CT has a 95% negative predictive value for ischemia and may help indicate when a more conservative initial approach is warranted (94).

Tailored evaluation with helical CT requires rapid (>=3 mL/sec) intravenous administration of contrast material for optimal vascular opacification. Helical CT is very helpful in defining the patency of the major intestinal vessels, particularly the superior mesenteric artery and superior mesenteric vein (Fig 30). Intravenous contrast material is also helpful in characterizing bowel wall thickening, which can manifest as a low-attenuation ring of edema (Fig 31) (94). However, a finding of bowel wall edema is nonspecific and can be seen with inflammatory or infectious causes as well (Fig 32) so that clinical correlation is required. Water can be used as an alternative to positive oral contrast agent in patients with presumed ischemia, especially in the setting of obstruction. In these patients, intravenous contrast material is essential for depiction of the thickened, edematous bowel wall, which can easily be appreciated against the obstructed, fluid-filled intestine.



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  		Figure 30a.   Diffuse bowel ischemia in a 49-year-old woman. The patient presented with recurrent pancreatic cancer and had a history of Whipple disease. (a) Axial CT scan obtained with intravenous contrast material reveals a tumor surrounding the pancreas and mesenteric vessels (white arrowheads). The superior mesenteric artery is patent (straight arrow), but the superior mesenteric vein is thrombosed (curved arrow). Infarcts are seen in the left kidney (black arrowheads). (b) On a CT scan obtained inferior to a, minimal bowel wall thickening is seen in both the small and large intestine (arrows). Surgery revealed diffuse ischemia.

 


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  		Figure 30b.   Diffuse bowel ischemia in a 49-year-old woman. The patient presented with recurrent pancreatic cancer and had a history of Whipple disease. (a) Axial CT scan obtained with intravenous contrast material reveals a tumor surrounding the pancreas and mesenteric vessels (white arrowheads). The superior mesenteric artery is patent (straight arrow), but the superior mesenteric vein is thrombosed (curved arrow). Infarcts are seen in the left kidney (black arrowheads). (b) On a CT scan obtained inferior to a, minimal bowel wall thickening is seen in both the small and large intestine (arrows). Surgery revealed diffuse ischemia.

 


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  		Figure 31.   Intussusception and secondary bowel ischemia in an 80-year-old woman. Axial CT scan obtained with intravenous contrast material shows a colocolic intussusception (black arrow). There is no evidence for a lead point. Low-attenuation bowel wall ischemia is also seen (white arrow) and was confirmed at surgery.

 


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  		Figure 32.   Ischemic colitis in a 79-year-old man. Axial CT scan obtained with intravenous contrast material reveals a focal low-attenuation area representing thickening in the transverse colon (arrowheads). Focal stranding is seen in the pericolonic fat.

 
Air within the bowel wall (pneumatosis intestinalis), mesentery, and portal venous system are late CT signs of bowel ischemia and usually portend a grave prognosis (Figs 33, 34). A multitude of entities can manifest with air in the bowel wall, including conditions that disrupt the bowel mucosa (eg, bowel obstruction, inflammatory bowel disease), entities that increase bowel permeability (eg, graft-versus-host disease, acquired immunodeficiency syndrome, steroid therapy), and pulmonary disease (eg, chronic obstructive pulmonary disease) (95,96). CT findings must always be correlated with clinical history and symptoms in patients with pneumatosis intestinalis (96).



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  		Figure 33.   Ischemic small bowel and pneumatosis intestinalis in a 41-year-old woman. The patient presented with acute thrombosis of multiple vessels including the superior mesenteric artery. Unenhanced CT scan reveals air within the bowel wall (arrow). Pneumatosis is often best appreciated with a lung window setting.

 


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  		Figure 34.   Bowel necrosis and mesenteric air in a 53-year-old woman. Unenhanced CT scan demonstrates air within the mesenteric vessels (arrowheads). Surgery revealed transmural necrosis of the distal ileum, cecum, and sigmoid colon.

 
Gastrointestinal Perforation.—Patients with bowel perforation usually present with obvious indications of peritonitis, although symptoms can be masked in immunosuppressed patients or in patients undergoing steroid therapy. Spontaneous bowel perforation usually occurs with underlying disruption of the mural integrity of the intestine. Perforation generally indicates a catastrophic complication that can result from a multitude of potential causes, including severe intestinal inflammation, peptic ulcer disease, diverticulitis, infarction, trauma, and closed-loop obstruction (97). Perforation can also complicate neoplasms, and the search for an underlying mass is warranted in the appropriate clinical setting. Iatrogenic perforation is occasionally seen following endoscopic procedures, especially endoscopic biopsy or sphincterotomy (Fig 35) (98).



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  		Figure 35.   Perforating gastric cancer in a 61-year-old man who had undergone endoscopy. Axial CT scan obtained with intravenous contrast material demonstrates extensive extraluminal air, predominantly in the retroperitoneum. The exact site of perforation can be difficult to determine if there is a large amount of air, especially in patients with iatrogenic perforation.

 
Helical CT is ideally suited for rapid evaluation of the abdomen in patients with acute pain from suspected perforation. CT is often indicated when free air is seen but the perforation site is unclear at conventional radiography. Because CT is more sensitive than conventional radiography in the detection of subtle pneumoperitoneum, it is also indicated when free air is strongly suspected despite normal abdominal radiographic findings (99,100). Unenhanced CT is often performed to expedite evaluation. If possible, oral and intravenous contrast material should be used to help localize the perforation and characterize complications including peritonitis and abscess formation. In supine CT, the anterior peritoneal surface of the liver is the most nondependent portion of the body and is often the location where extraluminal air can first be detected. However, detection of the actual site of perforation is often challenging because the location of the free air does not necessarily correlate with the site of perforation (76,101). Extravasation of oral contrast material is the most accurate localizer but is absent in many patients (Fig 36). If present, focal fluid, air, or inflammatory changes near the site of perforation are often helpful for localization.



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  		Figure 36.   Perforated benign gastric ulcer in an elderly man. Axial CT scan obtained with intravenous contrast material demonstrates free air anterior to the liver (black arrow) as well as focal extraluminal oral contrast material (white arrow), which helped localize the perforation in the stomach.

 
Vascular System
Aortic Aneurysm Rupture.—With the advent of helical CT, quality multiplanar and 3D vascular images can now readily be produced, and at many institutions helical CT angiography has essentially replaced conventional angiography in the evaluation of aortic aneurysms (9,102). Tailored evaluation requires rapid (>=3 mL/sec) intravenous bolus administration of contrast material for optimal vascular opacification. Narrow (3-mm) collimation is also preferred for optimal vascular opacification and high-quality 3D images. Oral contrast material is not administered because it can interfere with CT angiographic reconstruction. Helical CT can be used to accurately measure the diameter of the aneurysm and allows direct visualization of mural thrombus and the outer aneurysm wall. In addition, the origin and length of the aneurysm and its relationship to the renal and iliac arteries can readily be assessed (102,103).

Rupture is a catastrophic complication of abdominal aortic aneurysm, with mortality rates approaching 95%. Up to 40% of patients die within 1 hour of the onset of symptoms (104). Some patients survive long enough to receive medical attention and undergo imaging evaluation for acute abdominal pain. CT is the modality of choice for evaluation of suspected aortic aneurysm rupture. Pertinent helical CT findings include retroperitoneal hematoma and extravasation of intravenous contrast material (Fig 37). Helical CT can often help localize the exact site of bleeding (Fig 38). Hemorrhage is most often seen in the retroperitoneum involving the perirenal spaces (105). The presence of an area of increased attenuation surrounding the aorta does not always indicate rupture; patients with perianeurysmal fibrosis can demonstrate a thick, often calcified mantle of chronic inflammatory tissue. Care must be taken not to mistake perianeurysmal fibrosis for hemorrhage; both may demonstrate dramatic enhancement following bolus infusion of contrast material.



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  		Figure 37.   Ruptured abdominal aortic aneurysm in a 69-year-old man. Axial CT scan obtained with intravenous contrast material demonstrates massive extravasation of contrast material. Intraoperative repair was unsuccessful, and the patient died.

 


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  		Figure 38.   Ruptured abdominal aortic aneurysm in an elderly man. Axial CT scan obtained with intravenous contrast material demonstrates acute extravasation of contrast material from an aneurysm leak (black arrow). Note the interruption of intimal calcifications at the site of rupture (white arrow).

 
At times, patients present with chronic abdominal pain from impending rupture of enlarging abdominal aortic aneurysms. Aneurysm size is the most accurate positive predictive factor in determining which patients are at risk for aneurysm rupture. Studies have shown that in approximately 30%–40% of patients with aneurysms larger than 5 cm in diameter, the aneurysm will rupture within 5 years of detection. An increase of 1 cm or more in the diameter of an abdominal aortic aneurysm over the past 6 months is also a worrisome sign for impending rupture (106). Other indications for impending rupture include the draped aorta sign, the high-attenuation crescent sign, and focal discontinuity of a calcified rim. The draped aorta sign occurs when an area in the posterior wall of the aortic aneurysm is undefinable and is in proximity to the spine. These findings are highly indicative of aortic wall deficiency and contained leak (107). The high-attenuation crescent sign is the result of hemorrhage within the mural thrombus or wall of the aneurysm and may represent the first stage of rupture (108).

Aortic Dissection.—Aortic dissection is defined as the presence of a hematoma in the middle to outer third of the aortic wall with subsequent proximal and distal propagation. The vast majority of dissections lead to a tear in the weakened aortic intima and break into the aortic lumen (109). There are a multitude of predisposing factors for dissection, the most common of which is hypertension. Patients with aortic dissection typically present with sharp, tearing chest pain. Dissection proximal to the aortic root can lead to involvement of the coronary arteries, resulting in arrhythmia, cardiac tamponade, infarction, or even death. Less frequently, aortic dissection can manifest with acute abdominal pain. This is most commonly seen when origins of the renal arteries, celiac axis, or superior mesenteric artery are involved by the dissection flap, resulting in symptoms of ischemia or infarction in the involved vascular distribution.

Helical CT is now considered the screening modality of choice for aortic dissection (110). The accuracy of helical CT in this setting is extremely high, with a sensitivity and specificity of nearly 100% (110). Tailored helical CT requires rapid (>=3 mL/sec) injection of contrast material for optimal vascular opacification. Narrow collimation is also optimal for vascular imaging, especially 3D imaging. Findings of a contrast material–filled double channel with an intervening intimal flap are diagnostic (Fig 39) (110). If one lumen is thrombosed, it may be difficult to differentiate dissection from an aneurysm with mural thrombus. Associated findings that favor the diagnosis of dissection include dilatation of the aorta with compression of the true lumen, irregular contour of the contrast material–filled portion of the aorta, and differential flow to the kidneys from the dissecting intimal flap secondary to involvement of the renal artery origins.



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  		Figure 39.   Aortic dissection in a 54-year-old man. Axial CT scan obtained with intravenous contrast material clearly depicts an intimal flap caused by aortic dissection (arrowhead).

 
Hemorrhage.—Patients can present with pain due to acute abdominal hemorrhage in various locations, including the gastrointestinal tract, retroperitoneum, and abdominal musculature. A falling hematocrit and hypotension are indicators of significant bleeding. Tailored CT evaluation for abdominal hemorrhage consists of initial unenhanced CT, which is useful in detecting the hyperattenuating hematoma, followed by optional contrast-enhanced helical CT with intravenous bolus injection of contrast material (111). The rate of injection should approach 3–4 mL per second for optimal vascular opacification. This helical CT technique has the potential to localize sites of bleeding, which appear as a blush of intravenous contrast material from the involved vessel (Figs 40, 41) (111,112). Spontaneous bowel hematomas are often seen in the setting of underlying coagulopathy (Fig 42) (113). The rectus sheath represents one of the most common sites of spontaneous musculoskeletal hemorrhage, which can mimic an obstructing or incarcerated abdominal wall hernia (114,115). Another relatively common site of spontaneous muscular hemorrhage is the psoas muscle (Fig 43). Bleeding can also result from an underlying abdominal tumor, especially renal cell carcinoma (Fig 44) (116).



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  		Figure 40.   Bleeding Meckel diverticulum in a 25-year-old man. Axial CT scan obtained with intravenous contrast material demonstrates bleeding from ectopic gastric mucosa within the diverticulum (arrow).

 


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  		Figure 41.   Peritoneal hemorrhage in a 33-year-old man who had undergone paracentesis. Axial CT scan obtained with intravenous contrast material demonstrates hemorrhagic ascites throughout the abdomen. A bleeding vessel is identified near the site of recent needle insertion (arrow).

 


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  		Figure 42.   Small bowel hematoma in a 68-year-old man who was undergoing anticoagulation therapy. The patient presented 7 days after undergoing heart surgery. Unenhanced CT scan demonstrates a high-attenuation hematoma in the proximal small bowel (arrows).

 


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  		Figure 43.   Hematoma in a 38-year-old man with hemophilia who presented with acute abdominal pain. Axial CT scan obtained with intravenous contrast material demonstrates bilateral hematomas with heterogeneous attenuation causing minimal enlargement of the psoas muscles (arrows).

 


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  		Figure 44.   Renal cell carcinoma in a 74-year-old man. Axial CT scan obtained after rapid intravenous administration of contrast material demonstrates hemorrhage in the right kidney (arrow). Surgery revealed hemorrhagic cystic degeneration from renal cell carcinoma.

 

   Conclusions
Top
 Abstract
 LEARNING OBJECTIVES FOR TEST...
 Introduction
 Helical CT Technique
 Helical CT Findings in...
 Conclusions
References
 
Helical CT is a rapid and efficient means of evaluating patients with acute abdominal pain. Attention to proper technique and protocol is essential for optimizing the CT examination and maximizing diagnostic accuracy.


   Footnotes
 
** indicates multiple body systems Back

Abbreviations: HIV  = human immunodeficiency virus, 3D  = three-dimensional


   References
Top
 Abstract
 LEARNING OBJECTIVES FOR TEST...
 Introduction
 Helical CT Technique
 Helical CT Findings in...
 Conclusions
 References
 

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