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Gastrointestinal Imaging |
1 From the Department of Radiology, Hospital of the University of Pennsylvania, 3400 Spruce St, Philadelphia, PA 19104. From the 1998 RSNA scientific assembly. Received October 9, 1998; revision requested December 12; revision received January 29, 1999; accepted April 26. Address reprint requests to W.T.M. (e-mail: miller2@oasis.rad.upenn.edu).
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Abstract |
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MATERIALS AND METHODS: Radiologic reports and clinical charts were reviewed in all patients who underwent US and CT within 48 hours of each other for evaluation of acute right upper quadrant pain. Radiologic findings and clinical outcome were correlated.
RESULTS: CT was the initial imaging study in 57 patients, and CT findings resulted in underdiagnosis or misdiagnosis of acute biliary disease in eight of 11 patients. Follow-up US results were suggestive of the correct diagnosis and provided additional clinical information in seven of these eight patients. US findings resulted in altered clinical treatment in six of 11 patients with acute biliary disease. US was the initial study in 66 patients, and US findings were suggestive of biliary disease or the correct diagnosis in seven of seven patients with acute biliary disease. Follow-up CT did not result in changes in clinical treatment in any patient with acute biliary disease.
CONCLUSION: Initial US is better than initial CT in patients suspected of having acute biliary disease. Follow-up CT provides no additional information regarding the biliary system, and its use should be limited to those patients with a wider differential diagnosis or with confusing clinical symptoms and signs.
Index terms: Bile ducts, calculi, 766.2891 • Bile ducts, CT, 766.12114, 766.12115 • Bile ducts, US, 766.12981 • Computed tomography (CT), comparative studies, 766.12114, 766.12115 • Gallbladder, calculi, 762.2891 • Gallbladder, CT, 762.12114, 762.12115 • Gallbladder, US, 762.12981 • Ultrasound (US), comparative studies, 766.12981
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Introduction |
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Modern cross-sectional biliary imaging uses ultrasonography (US) and computed tomography (CT) in both screening and follow-up roles. Recent use of magnetic resonance (MR) imaging in the evaluation of acute cholecystitis suggests possible applications, but cost-effectiveness remains a clinical hurdle. Endoscopic retrograde cholangiopancreatography (ERCP) and MR cholangiopancreatography provide detailed analysis of the extrahepatic biliary system. Although the role of MR cholangiopancreatography continues to evolve, it provides a noninvasive means for evaluation of the extrahepatic biliary system, is applicable in patients who are not candidates for ERCP, and offers a potential replacement for ERCP in a diagnostic role. US remains the preferred examination for possible acute biliary disease, however, because it is widely available, affords flexibility in imaging applications, can be performed at the bedside, and is safe to use in pregnant and pediatric patients.
The success rate with diagnostic US is greater than 95% (1,2). It has been our experience, however, that many patients often undergo imaging with both US and CT for evaluation of possible acute biliary disease. To our knowledge, there are no reports in which the utility of both CT and US for the evaluation of possible acute biliary disease has been addressed. Clearly, guidelines should be established that suggest a correct imaging protocol. The purpose of this study was to examine the outcomes in patients who underwent both US and CT within 48 hours of each other for evaluation of possible biliary disease.
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MATERIALS AND METHODS |
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Helical CT was performed with a HiSpeed Advantage CT scanner (GE Medical Systems, Milwaukee, Wis). CT was performed through the pancreas and liver with 5-mm collimation and a pitch of 1.5:1.0 followed by CT through the remainder of the abdomen and pelvis with 7-mm collimation and a pitch of 1.3:1.0. Patients were intravenously administered 150 mL of an ionic contrast agent (iothalamate meglumine, Conray 60; Mallinckrodt, St Louis, Mo) at a dose of 282 mg of iodine per milliliter infused at a rate of 2 mL/sec, with a 70 second delay before scanning. Patients in whom ionic contrast material was contraindicated received 100 mL of a nonionic contrast agent (iohexol, Omnipaque 300; Nycomed, Princeton, NJ) at a dose of 300 mg iodine per milliliter. All patients received oral contrast material: Inpatients received 30 mL of diatrizoate meglumine and diatrizoate sodium (367 mg iodine per milliliter; Gastroview; Mallinckrodt) in 1 L of water, and outpatients received 36 oz (1,080 mL) of barium sulfate, (2.1% wt/vol, 2.0% wt/wt; Redi-Cat 2; E-Z-Em, Westbury, NY).
Gray-scale US of the right upper quadrant was performed with an HDI UltraMark 9 unit or an HDI 3000 unit (ATL, Bothell, Wash) by using a 2.0–4.0-MHz multifrequency curved-array transducer or with a Logiq 700 unit (GE Medical Systems) by using a 3.0-MHz curved-array transducer. Measurements were performed with electronic calipers. Color flow and duplex Doppler imaging techniques were not used during evaluation of the gallbladder. US was performed either by a US technologist or by a radiologist. Images obtained by a technologist were immediately reviewed by an attending radiologist, who also performed a directed examination immediately after the technologist's examination.
US and CT studies were interpreted independently of each other by attending radiologists in either the US section or in the abdominal CT section. Studies obtained on the same day were interpreted blindly, without knowledge of the other study findings. When studies were obtained on separate days, however, radiologists had access to the report of the initial study and referred to the report at his or her discretion.
A radiologic diagnosis was rendered for each patient and for each CT and US examination (Table 1). A normal study was recorded if no abnormalities were noted or if isolated gallbladder distention was demonstrated. The diagnosis of nonacute biliary disease was recorded if gallbladder sludge or gallstones with or without sludge were noted. The diagnosis of acute disease was recorded if isolated pericholecystic fluid or biliary sludge and/or gallstones with gallbladder distention, gallbladder wall thickening, and/or pericholecystic fluid were noted (3,4). The US Murphy sign was not considered, because it was not routinely recorded in the radiologic report. An indeterminate diagnosis was recorded when isolated gallbladder wall thickening was noted, because this is a nonspecific finding and often is seen in a partially collapsed or physiologically contracted gallbladder or in other disease states such as chronic cholecystitis, hepatitis, and metabolic disturbances causing hypoalbuminemia and ascites (4,5). A diagnosis of choledocholithiasis was recorded if stones were seen in the common duct. A diagnosis of possible choledocholithiasis was recorded when ductal dilatation was noted without other findings.
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RESULTS |
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Initial US with CT Follow-up
US was the initial imaging examination in 66 patients. Of these, 55 patients had nonbiliary disease, four had acute cholecystitis or acute exacerbation of chronic cholecystitis, three had choledocholithiasis, and four had other biliary disease (sclerosing cholangitis in one, graft-versus-host disease in one, cholangiocarcinoma in one, and stricture of the distal portion of the common bile duct secondary to pancreatitis in one). In 117 patients suspected of having acute biliary disease (exclusive of other biliary disease), US sensitivity, specificity, positive predictive value, and negative predictive value for acute biliary disease (exclusive of choledocholithiasis) were 83%, 95%, 75%, and 97%, respectively (Table 2).
In the four patients with acute cholecystitis, initial US demonstrated acute disease in two patients and nonacute disease in two patients. No patient had normal findings at initial US. In the two patients with acute disease at US, follow-up CT demonstrated acute disease in one and was indeterminate in the other. The two patients with nonacute biliary disease at US had the same findings at follow-up CT. Thus, CT did not change the clinical treatment in any of these four patients with acute cholecystitis (Table 4).
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DISCUSSION |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Scintigraphy undoubtedly plays an important role in the evaluation of acute biliary disease; however, scintigraphy was deliberately excluded from our analysis because we elected to evaluate the utility of obtaining two studies that are used to evaluate anatomy rather than to compare them with physiologic imaging.
Our findings suggest that (a) US is more sensitive than CT for help in identifying acute biliary disease, (b) follow-up CT is not necessary in cases of acute biliary disease that have been positively diagnosed at US, (c) CT should be reserved for patients with wider differential diagnostic considerations or who present with unusual symptoms or signs, and (d) patients with choledocholithiasis suspected on the basis of US results should be referred directly for ERCP.
In patients suspected of having acute biliary disease, US proved to be more useful as the screening examination because it had greater sensitivity and positive and negative predictive values than did CT, with both modalities demonstrating equal specificity. In the study population, 11 of 123 patients had acute cholecystitis or acute exacerbation of chronic cholecystitis, which was correctly diagnosed at US in nine (82%) patients and at CT in four (36%). In addition, two patients had gallstone pancreatitis and one had symptomatic cholelithiasis. In all three of these patients, US demonstrated gallstones that were overlooked at CT (which demonstrated a normal gallbladder). If these three patients are included, a total of 14 patients had acute biliary disease, which was detected at US in 12 (86%) and at CT in four (29%).
Most patients who are screened for suspected acute biliary disease will have negative imaging results; therefore, specificity should be considered when evaluating the utility of a screening examination. Indeed, in 123 patients in the study group, 99 (80%) had no identifiable disease or had nonbiliary disease as the source of their symptoms. There were seven false-positive and six indeterminate CT scans. Similarly, there were five false-positive and eight indeterminate US studies. CT and US had almost equal specificity (93% and 95%, respectively) for acute biliary disease. However, our results demonstrate that technically sufficient US of the right upper quadrant has higher positive and negative predictive values for the diagnosis of acute biliary disease: The positive predictive value was 75% for US and 50% for CT, and the negative predictive value was 97% for US and 89% for CT.
These results suggest that US alone is sufficient for screening of acute biliary disease, and a negative US study virtually excludes acute biliary disease. Furthermore, CT is not necessary after US when ruling in or excluding acute biliary disease, since CT has lower sensitivity and positive and negative predictive values in comparison with those of US. In addition, in our series, clinical treatment was never changed when CT was performed as the follow-up examination, whereas the results of initial CT evaluation of suspected acute biliary disease were often inadequate, and follow-up US resulted in altered clinical treatment in 64% of patients.
Our findings reflect the high sensitivity, specificity, and accuracy of US for demonstration of cholelithiasis that has been reported in the literature (1–3,6,7). The sensitivity of Doppler US is similar to that of gray-scale US but affords greater specificity in the diagnosis of acute cholecystitis (8). In our study, the specificity of US was 95% for acute biliary disease, without the use of color flow or Doppler analysis.
The use of the US Murphy sign was not included in this investigation because it was not routinely recorded on the radiologic reports. We do not believe this to be a limitation, however. We believe that the Murphy sign probably increases the sensitivity of US for acute cholecystitis; therefore, because it was not routinely included in the radiologic reports we reviewed, our results are probably an underestimate of the sensitivity of US for acute cholecystitis. In the current investigation and without reference to the Murphy sign, however, US results correctly influenced the clinical outcome in patients initially examined with CT, and subsequent imaging with CT after initial US did not alter treatment.
The lower sensitivity of CT for cholelithiasis is well known, although CT nearly always demonstrates the gallbladder in fasting patients (9). Unlike US, the depiction of gallstone size at CT is largely dependent on the size and composition of stones. Calcified gallstones are easily seen as high-attenuating foci in the gallbladder, and pure cholesterol gallstones are seen as low-attenuating defects in surrounding bile. However, many gallstones are composed of a mixture of calcium, bile pigments, and cholesterol and are isoattenuating with the surrounding bile; thus, such stones may not be detected on CT images regardless of their size (9).
To our knowledge, the sensitivity and specificity of CT for acute cholecystitis have not been evaluated in large prospective studies. However, acute cholecystitis has been diagnosed with CT (10), and the CT findings most commonly associated with acute cholecystitis are gallbladder wall thickening and presence of pericholecystic stranding, gallbladder distention, pericholecystic fluid, subserosal edema, high-attenuating bile, and sloughed membranes (11). Some patients with acute cholecystitis will not have the classic signs and symptoms at the time of presentation, and CT is, therefore, of increased value in patients with a wider differential diagnosis or with confusing clinical signs and symptoms.
Nonetheless, CT findings suggestive of acute cholecystitis should be interpreted cautiously given the low positive predictive value we observed for acute cholecystitis and should prompt the performance of follow-up US. In addition, although we observed a high negative predictive value for CT, follow-up US may be warranted in the setting of high clinical suspicion, to more accurately exclude acute cholecystitis. CT, however, is of greater utility when gallbladder and/or biliary carcinoma are considered and for evaluation of complications of cholecystitis, because the US evaluation may be limited.
In some patients presenting with acute right upper quadrant pain, choledocholithiasis will be diagnosed. In four (3%) of 123 patients in the current study, choledocholithiasis (three patients) or probable choledocholithiasis (one patient) was diagnosed. In the three patients with confirmed choledocholithiasis, US and CT demonstrated ductal dilatation, but neither demonstrated choledocholithiasis. In all three patients, definitive diagnoses were established with results from ERCP performed within 24 hours of the second imaging study. The case of probable choledocholithiasis was diagnosed as acute biliary disease on both initial CT and follow-up US images; neither study demonstrated ductal dilatation. Subsequent ERCP (performed soon after US) demonstrated an edematous ampulla of Vater, which was suggestive of the recent passage of stones; choledocholithiasis and ductal dilatation were not identified at ERCP.
Choledocholithiasis is often considered when ductal dilatation is identified. US is highly sensitive (99%) (12) and accurate (93%) (13) for demonstration of ductal dilatation but is slightly less reliable with regard to the location (60%–92%) and cause (39%–71%) of biliary obstruction (13,14). This is due in part to difficulty in identifying the distal common bile duct (most commonly because of overlying bowel gas or body habitus) in as many as 8% of patients (14). CT, in comparison, is better able to accurately demonstrate the location (97%) and cause (94%) of obstruction (13,15,16).
US has limited sensitivity (55%–85%) for the diagnosis of choledocholithiasis (14, 17,18), but a high specificity (89%–91%) (19,20) and positive predictive value (92%) (21) have been reported. The limited sensitivity of US for demonstration of choledocholithiasis is related to several factors, including gallstones located in nondilated ducts (19–21) or in the distal common duct, lack of surrounding bile (21), and gallstones that do not produce acoustic shadowing (22).
Conventional CT for choledocholithiasis has a sensitivity of 87%–90% (23,24), and, in a recent study (25), nonenhanced helical CT demonstrated greater sensitivity, specificity, and accuracy for the diagnosis of choledocholithiasis. CT is particularly useful for evaluation of the distal common duct and ampulla of Vater, areas that are poorly depicted at US, and CT usually does not require dilatation of the distal duct for identification of stones (24), particularly when helical techniques are used (25). Stones composed of calcium bilirubinate are more easily diagnosed than are stones primarily composed of cholesterol, although only a minority of the former are composed of pure calcium bilirubinate (23). Approximately 10%–20% of stones are composed of pure cholesterol, are low in density, and are difficult to detect with CT because they are isoattenuating with bile (26). These stones account for many of the false-negative CT scans, and concurrent biliary dilatation usually is necessary to visualize stones composed of cholesterol and pigment (23,26). The CT findings suggestive of obstruction caused by choledocholithiasis in contrast to those suggestive of malignant processes have been previously described (15,16,23).
Aside from technical failures, US and CT are equally accurate in demonstrating biliary obstruction (13), and although US may be limited in terms of the determination of a definite etiology, its use as a screening examination is justified, since these patients may then be referred for ERCP to determine more definitive treatment of suspected choledocholithiasis.
The current use of MR cholangiopancreatography as a noninvasive imaging technique for evaluation of the extrahepatic biliary system continues to evolve. Although the role of MR cholangiopancreatography will become more clearly defined, it already is in practice for several indications, including in patients with contraindications to ERCP, in cases of failed previous ERCP, in the presence of biliary-enteric anastomosis, and when ERCP incompletely depicts the ducts. As Barish and Soto (27) described, the eventual role of MR cholangiopancreatography may be to reduce or eliminate the need for diagnostic ERCP, limiting ERCP to those cases in which a therapeutic procedure would be performed simultaneously. At our institution, however, ERCP has not been supplanted by MR cholangiopancreatography and still is performed for diagnostic imaging of the biliary system.
In conclusion, our results suggest that the use of US is warranted as the initial imaging study for evaluation in patients suspected of having acute gallbladder disease. The conditions most likely responsible for such complaints are acute cholecystitis, gallstone pancreatitis, or choledocholithiasis. US is highly sensitive and accurate for the diagnoses of acute cholecystitis and cholelithiasis. Our observations do not support the utility of CT as the initial study for evaluation of suspected acute gallbladder disease or even as a follow-up study in patients with a US diagnosis of acute biliary disease. Rather, CT may be more appropriately used for evaluation in patients with a wider differential diagnosis, a known history of chronic biliary disease, or with confusing clinical symptoms and signs.
CT and US are equally able to demonstrate acute ductal dilatation; in our series, however, both modalities were equally unable to demonstrate choledocholithiasis, which was diagnosed in three of four patients who underwent ERCP. If ductal dilatation without an apparent cause is identified on initial CT or US images and if choledocholithiasis is suspected, our findings do not support performance of additional noninvasive imaging. Rather, patients with ductal dilatation and suspected choledocholithiasis should be referred directly for ERCP to help determine diagnosis and treatment.
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Footnotes |
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Author contributions: Guarantors of integrity of entire study, R.T.H., W.T.M.; study concepts, W.T.M.; study design, R.T.H., W.T.M.; definition of intellectual content, R.T.H., W.T.M.; literature research, R.T.H.; clinical studies, R.T.H.; data acquisition and analysis, R.T.H.; statistical analysis, R.T.H.; manuscript preparation, R.T.H.; manuscript editing and review, R.T.H., W.T.M.
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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