Multidetector Computed Tomography

Recent improvements in MDCT technology allow thinner collimation and faster scanning that provides multiphasic, high-resolution images of the abdomen and pelvis. These hardware developments, coupled with advances in three-dimensional imaging software and the availability of cheaper data storage capacity, have provided new opportunities for imaging a variety of pathologic processes. Isotropic imaging now is possible, providing two-dimensional multiplanar reformations, CT urography, CT colonography, CT enterography, and three-dimensional imaging formats, with minimal artifacts, and all derived from a single data acquisition. The patient can usually be scanned in less than 15 seconds, allowing for acquisition of thin-section images during multiple phases of the intravenous contrast bolus. This multiphasic evaluation permits high-resolution imaging of not only solid organs and hollow viscera but also the abdominal and pelvic vasculature. The data from these various series then can be reconstructed at thinner intervals and transferred to a workstation, in which multiplanar reformatted images, maximal-intensity projection images, and three-dimensional volume-rendered images can all be created.

MDCT is ideally suited for imaging the postoperative patient. Opacification of the gastrointestinal (GI) tract with positive contrast material is of critical importance for differentiation among bowel loops, intraperitoneal fluid collections, enlarged lymph nodes, and abscesses. For this purpose, at least 500 mL of dilute (2%) barium sulfate or iodinated compounds should be administered orally or through a nasogastric tube at least 1 hour before the examination. Water-soluble contrast media are preferable if there is any potential risk for intraperitoneal leakage from the GI lumen. Furthermore, water-soluble media tend to stimulate GI peristalsis and more readily opacify the distal small bowel and colon. Intravenous administration of iodinated contrast media is equally important in the detection of abscess and various vascular and parenchymal lesions in the postoperative abdomen. Other modifications, such as rectal contrast enema or scanning the patient in the decubitus or prone position, also are options when attempting to resolve a particular diagnostic dilemma.

The CT examination of the postoperative abdomen should extend from the dome of the diaphragm to the pelvic floor. This approach is warranted because many postoperative complications, particularly abscess and fluid collections, are often located at a site remote from the surgical field because of extension along preexisting anatomic pathways within the abdominal cavity.

Magnetic Resonance Imaging

Significant advances in MRI hardware and software, combined with routine use of phased-array coils and parallel imaging, permit fast and high spatial resolution imaging of the abdomen and pelvis. Short breath-hold scans can produce superb multiphase images without significant peristaltic or respiratory motion artifacts. MRI generally is employed as a secondary imaging test when better characterization of a mass discovered on MDCT or ultrasound is needed.

MRCP is a pivotal tool in the evaluation of pancreaticobiliary disease. This MRI technique exploits the fluid present within the bile ducts, pancreatic duct, and gallbladder, causing the bile and pancreatic juice to have a high signal intensity from which quality endoscopic retrograde cholangiopancreatography (ERCP) images of the ductal system can be created. MRCP can be obtained without the associated complications of ERCP while offering comparable sensitivity, specificity, and accuracy. Thus, bile duct stones and biliary obstruction often are best evaluated with MRCP. When combined with conventional MRI and magnetic resonance angiography, MRCP offers superb preoperative staging of hepatic, biliary tract, and pancreatic primary malignancies.

Ultrasound

Ultrasound is the favored initial imaging test for the evaluation of gallstones, biliary dilation, and cholecystitis and also has a primary role in pediatrics, obstetrics, and gynecology. Ultrasound has several inherent advantages over CT, including lower cost, an ability to perform real-time imaging, portability, a lack of ionizing radiation, and an absence of contraindications. Recent advances in gray-scale and color-flow Doppler techniques, as well as tissue harmonics, have enhanced the ability of ultrasound to identify and characterize abdominal and pelvic masses. In addition, endosonography and transrectal ultrasound are important in the T and N staging of patients with early esophageal, gastric, and rectal cancers.

Combined Positron Emission Tomography and Computed Tomography

Most malignancies exhibit increased metabolic activity that results in increased use of glucose. PET imaging exploits the fact that a glucose analog, [18]F-fluorodeoxyglucose (FDG), shows increased intracellular accumulation in malignant tissue. PET-CT is a fixed combination of PET and CT scanners in a combined imaging system. The nearly simultaneous data acquisitions lead to minimization of spatial and temporal mismatches between modalities by eliminating the need to move the patient during the examination. The result is a fused image that provides biologic and anatomic information. Imaging generated from the combination of anatomic and metabolic data often provides more sensitive and specific information concerning the extent of malignancy than anatomic imaging alone. PET-CT is an increasingly popular method for the staging of a number of malignancies (including lung, breast, colorectal, and esophageal cancer), as evident from cancer treatment guidelines, such as those available from the National Comprehensive Cancer Network.

Preoperative Assessment of Gallstones

Ultrasound is the gold standard for the noninvasive diagnosis of cholelithiasis, with an accuracy of more than 96%. A sonographic diagnosis of cholelithiasis must fulfill three major criteria: (1) an echogenic focus, (2) acoustic shadowing from the focus, and (3) gravitational dependence of the focus. Confidence in the diagnosis is increased if a 5-mm or larger defect meets all three criteria. Stones smaller than 2 to 3 mm may be difficult to visualize. Small stones, however, are usually multiple, which assists in their detection.

The CT appearance of gallstones is variable, depending on their composition, the pattern of calcification, and the presence of lamellation, fissuring, or gas. Stones with high cholesterol content are difficult to visualize because they are isodense with the surrounding bile. Well-calcified stones are readily detected on CT. Stones that are denser than bile may be seen because of a rim or nidus of calcification. The CT attenuation of gallstones correlates more closely with the cholesterol content of the stones than with the calcium content. On CT, gallstones can be simulated by the enhancing mucosa of a contracted gallbladder wall or neck, which often folds upon itself.

On MRI, most gallstones produce little or no signal because of the restricted motion of water and cholesterol molecules in the crystalline lattice of the stone. To optimize gallbladder and ductal stone visualization on MRI, T2-weighted imaging sequences are used that produce bright bile. MRI is superior to CT in detecting small calculi because of the inherent contrast between low-signal-intensity stones and high-signal-intensity bile.

Choledocholithiasis

Choledocholithiasis is found in 7% to 20% of patients undergoing cholecystectomy and in 2% to 4% of patients after cholecystectomy. These stones usually are asymptomatic unless they obstruct the common bile duct (CBD). Small calculi may intermittently cause colicky pain as they obstruct the ampulla of Vater, but they generally pass into the duodenum. Larger stones (between 5 and 10 mm) are difficult to pass and can result in intermittent long-term symptoms and sequelae, such as cholangitis and sepsis.

The detection of bile duct stones is easiest in the setting of biliary dilation. Unfortunately, biliary dilation is present in only 66% to 75% of patients with bile duct stones, so their detection may be difficult. Sonographically, a bile duct stone may appear as an echogenic focus within the fluid-filled duct that may cast an acoustic shadow. A ductal stone also may appear as an echogenic curved line so that only the anterior margin is visualized, with homogeneous echogenicity throughout the stone, or without acoustic shadowing. Adjacent duodenal and colonic gas can make it difficult to image the distal common bile duct. Because of these factors, sonography has a sensitivity of only 18% to 45% in the detection of choledocholithiasis.

As with gallstones, the CT appearance of CBD stones is variable, depending on their composition and pattern of calcification. High-attenuation stones can be seen easily within the duct lumen even in the absence of biliary dilation. Only 20% of CBD stones have a homogeneous high density. Other findings include a rim of high attenuation (which may be difficult to detect when impacted against the duct wall), soft tissue attenuation, and homogeneous near-water attenuation. CBD stones rarely may have sufficient cholesterol to appear lower in attenuation than surrounding bile. MDCT with coronal reformatted images has a sensitivity of 76% to 80% in the detection of bile duct stones.

On MRI, CBD stones appear as foci of low signal intensity surrounded by bright bile on T2-weighted sequences. Stones as small as 2 mm can be detected by this technique, which has a sensitivity of 81% to 100% and a specificity of 85% to 99%. MRCP is superior to CT and ultrasound in selecting which patient may benefit from preoperative ERCP. MRCP has been recommended in the patient with gallstones and a moderate to high suspicion of CBD stones, based on clinical, sonographic, and laboratory data. MRCP can rule out CBD stones in up to 48% of patients with a high preoperative probability of CBD stones.

Acute Cholecystitis

The patient with suspected acute cholecystitis should undergo imaging because 60% to 85% of patients with an unconfirmed diagnosis have other causes of right upper quadrant pain, including peptic ulcer disease, pancreatitis, hepatitis, appendicitis, hepatic congestion from right-sided heart failure, perihepatitis from pelvic inflammatory disease (Fitz-Hugh–Curtis syndrome), right lower lobe pneumonia, right-sided pyelonephritis, nephroureterolithiasis, and so forth. Furthermore, imaging can diagnose severe complications that require immediate surgery, such as emphysematous cholecystitis or perforation.

The patient with suspected acute cholecystitis should have an ultrasound as the initial imaging procedure. If the diagnosis is in doubt after the ultrasound, then hepatobiliary scintigraphy or a CT can be performed. MDCT often is performed initially in many cases because the diagnosis is unclear. CT also may be quite helpful in suspected emphysematous cholecystitis or gallbladder perforation. MRI usually is employed to exclude obstructing and nonobstructing biliary tract stones.

The sonographic findings of acute uncomplicated cholecystitis include gallstones that may be impacted in the cystic duct or gallbladder neck, mural thickening (>3 mm), a three-layered appearance of the gallbladder wall, hazy delineation of the gallbladder, localized pain with maximal tenderness elicited over the gallbladder (sonographic Murphy sign), pericholecystic fluid, and gallbladder distention. Gallstones and the sonographic Murphy sign are the most specific indicators of acute cholecystitis, with a positive predictive value of 92%. The sonographic Murphy sign may be difficult to elicit in obtunded patients and in those who have received pain medication. Of note, Murphy sign may be absent in patients with gangrenous cholecystitis.

The CT findings of acute cholecystitis include gallstones, mural thickening of the gallbladder, mural edema, pericholecystic fluid and inflammation, and transient increased enhancement of the liver parenchyma adjacent to the gallbladder due to hyperemia. As stated before, CT is less sensitive (75%) than ultrasound in the depiction of gallstones. Stones with significant calcification or the presence of gas in a noncalcified stone (the Mercedes Benz sign) are best seen with CT. The CT findings of acute cholecystitis have been divided into major and minor criteria; the former include calculi, mural thickening of the gallbladder, pericholecystic fluid, and subserosal edema, whereas the latter include gallbladder distention and sludge. The overall sensitivity, specificity, and accuracy of CT for the diagnosis of acute cholecystitis are 92%, 99%, and 94%.

MRI rivals ultrasound and CT in the depiction of acute cholecystitis. On postgadolinium T1-weighted images, the findings of acute cholecystitis include increased mural enhancement, mural thickening, and transient increased enhancement of the adjacent liver parenchyma. Findings found on T2-weighted images include gallstones, intramural abscess (appearing as a hyperintense foci in the gallbladder wall), and a thickened gallbladder wall.

Radionuclide cholescintigraphy with technetium-99m–labeled iminodiacetic acid analogs (also known as HIDA scan) was first introduced in the late 1970s. In this examination, hepatic parenchymal uptake of the tracer is observed within 1 minute, with peak activity occurring at 10 to 15 minutes. The bile ducts are usually visualized within 10 minutes, and the gallbladder should fill with isotope within 1 hour, if the cystic duct is patent. If the gallbladder is not identified, then delayed imaging (up to 4 hours after injection) should be performed. Prompt biliary excretion of the isotope without visualization of the gallbladder is the hallmark of acute cholecystitis. False-positive results may occur in patients with abnormal bile flow because of hepatic parenchymal disease or secondary to a prolonged fast with a distended, sludge-filled gallbladder. Delayed gallbladder filling also can be seen in the setting of chronic cholecystitis.

Normal Imaging Findings after Uncomplicated Laparoscopic Cholecystectomy

Occasionally, a patient who has undergone laparoscopic cholecystectomy may present postoperatively with pain, fever, leukocytosis, and jaundice. Such a patient requires additional imaging that may reveal certain and unexpected findings. Normal radiographic findings in this setting are described later.

Stone Retention in the Remnant Gallbladder or Cystic Duct

Uncommonly, laparoscopic cholecystectomy may be performed with incomplete excision of the gallbladder secondary to a difficult surgical dissection. This type of case may result in a retained stone within the remnant gallbladder. Because of the limitations in exploring the cystic duct pedicle during a difficult laparoscopic cholecystectomy, a small stone in a particularly long cystic duct remnant may remain. MRCP is superior to both sonography and MDCT in the detection of remnant gallbladder or cystic duct stones.

Common Bile Duct Stones

Retained stones can cause partial or complete biliary obstruction after laparoscopic cholecystectomy. Although intraoperative cholangiography is the gold standard for detecting choledocholithiasis during cholecystectomy, MRCP is valuable in the preoperative and postoperative detection of CBD stones (Fig. 41-1). Small gallstones may migrate into the CBD in the patient with a patulous cystic duct during cephalad retraction of the gallbladder. ERCP is useful in diagnosing and treating these cases.

FIGURE 41-1 Retained common bile duct stones shown on magnetic resonance cholangiopancreatography. Multiple low-signal-intensity stones (short arrows) are highlighted by high-signal-intensity bile in this patient after laparoscopic cholecystectomy. Long arrow indicates cystic duct remnant.

Bile Leakage

Bile leakage is one of the most common complications of laparoscopic cholecystectomy. Most leaks occur from the cystic duct stump. Other causes include laceration, transection, or thermal injury of the CBD or an unrecognized anomalous duct. CT and ultrasound are the initial imaging studies performed in the postoperative patient with suspected bile leak or biliary sepsis. Although both studies are useful in detecting intra-abdominal fluid collections, neither can differentiate between lymphocele, hematoma, seroma, or biloma (Fig. 41-2). Cross-sectional imaging studies can show a fluid collection in the gallbladder fossa, but such imaging cannot determine whether there is active bile leakage. Hepatobiliary scintigraphy (Fig. 41-3A), ERCP, or percutaneous transhepatic cholangiography (PTC) can show an active bile leak and the site of leakage. MRI employing gadoxetate disodium (Eovist), a gadolinium-based agent that is excreted in the bile, also is effective in showing bile leakage (Fig. 41-3B).

FIGURE 41-2 Postoperative biloma on computed tomography. Several low-density fluid collections (arrows) surround the left lobe of the liver 2 weeks after laparoscopic cholecystectomy. Note surgical clips in the gallbladder fossa.

FIGURE 41-3 Bile leak complicating laparoscopic cholecystectomy. A, Two-hour delayed image from a hepatobiliary scintigram demonstrating extrabiliary and extraintestinal isotope throughout the peritoneal cavity, but primarily in the Morison pouch (arrow). B, Magnetic resonance image with Eovist, showing high-signal-intensity contrast material surrounding the liver (straight arrow), in the gallbladder fossa (curved arrow), and within the common hepatic duct (arrowhead).

Acute Biliary Obstruction and Bile Duct Injury

A bile duct injury typically manifests with biliary dilation (Fig. 41-4) or bile leakage. The site of injury is described according to the Bismuth classification. A Bismuth type 1 injury indicates a remaining (noninjured) common hepatic duct longer than 2 cm; a type 2 injury has a remaining common hepatic duct shorter than 2 cm; a type 3 injury is located at the confluence of the right and left hepatic ducts; a type 4 injury extends into the right and left hepatic ducts. Major ductal injury from laparoscopic cholecystectomy generally is high in the biliary system, and multiple ducts often are involved. Mechanisms of CBD injury include malpositioning of surgical clips, thermal injury from sealing and cauterizing instruments, sharp incision, and en bloc excision.

FIGURE 41-4 Bile duct stricture complicating laparoscopic cholecystectomy. A, Endoscopic retrograde cholangiopancreatography image demonstrating a stricture of the right hepatic duct (arrow). There is dilation of the ductal system proximal to this stricture. B, Magnetic resonance cholangiopancreatography image demonstrating a stricture (arrow) at the confluence of the right and left hepatic ducts.

Late Biliary Obstruction with Stricture

Stricture of the CBD occurring late after laparoscopic cholecystectomy has become a recognized, although uncommon, complication of this procedure. This type of stricture presumably results from mild to moderate thermal injury to the bile duct. Rarely, surgical clips can induce fibrosis or inflammatory changes around the extrahepatic ducts that may lead to a stricture. MRCP is the most effective noninvasive means of diagnosing this type of biliary stricture.

Hemorrhage

Hemorrhage after laparoscopic cholecystectomy (Fig. 41-5) may occur from the cystic artery stump or the right hepatic artery. Bleeding may be secondary to thermal or mechanical injury, as with bile duct injury, or from dislodged hemostatic clips. Bleeding also can occur at a trocar insertion site, from the omentum or abdominal wall. MDCT is the most effective method for detecting bleeding because it will show the hyperdense hemorrhage and may show a focus of active bleeding or a pseudoaneurysm on contrast-enhanced scans. Transcatheter arterial embolization or surgical hemostasis may be considered, particularly if extravasation of the intravenous contrast agent is seen.

FIGURE 41-5 Hemorrhage following laparoscopic cholecystectomy as seen on multidetector computed tomography. A, Axial slice. B, Coronal reformatted. C, Sagittal reformatted. Arrows indicate a large hematoma originating in the gallbladder fossa.

Abscess with Retention of Peritoneal Gallstones or Spilled Clips

Gallstone and bile spillage develops in up to one third of patients during laparoscopic cholecystectomy. Dropped stones can lodge in virtually any crevice of the abdominal cavity. The combination of pneumoperitoneum and peritoneal irrigation may disperse the calculi throughout the peritoneal cavity. Stones have been reported in the right pleural space and have eroded into to colon. Abscess formation occurs in about 0.6% of patients in whom bile spillage alone occurs and in 0.6% to 2.9% of cases in whom both bile and stone spillage occurs. An abscess also can be associated with dropped surgical clips. Most abscesses associated with unretrieved peritoneal gallstones or spilled clips are seen in the perihepatic space (Fig. 41-6), but they may occur anywhere in the peritoneal cavity. Visualization of the spilled stone or clip in the abscess is the key to the radiologic diagnosis of this entity. Sonographically, a typical abscess appears as an echogenic focus with posterior acoustic shadowing. On MDCT, a central or eccentric calcified stone or metallic nidus may be identified.

FIGURE 41-6 Spilled stone during laparoscopic cholecystectomy leading to abscess. Computed tomography scan shows a calcified stone (short arrow) within a low-density, thick-walled abscess (long arrows) along the posterolateral aspect of the right lobe of the liver.

Trocar Site Hernia

An umbilical hernia at the Hasson cannula site is the most common type of trocar hernia, which generally occurs within a few days of surgery and often presents with small bowel obstruction. It can take several months, however, before symptoms of a trocar hernia develop. If the hernia causes small bowel obstruction (Fig. 41-7) and strangulation, operative intervention is needed. A trocar hernia also may be complicated by panniculitis.

FIGURE 41-7 Trocar site hernia following laparoscopic cholecystectomy that resulted in a small bowel obstruction. Computed tomography scan demonstrates a small knuckle of proximal ileum (short arrow) incarcerated in the defect. Note the small bowel feces sign (long arrows), with mottled density material within the dilated ileum just proximal to the obstruction.