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Biliary Imaging
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Biliary Imaging | ||
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Related narrative: Bile Leak, Common Duct Stones, Laparoscopic Ultrasound Multiple imaging modalities are available and new ones are evolving for diagnostic visualization the biliary tree. Accurate diagnostic information allows accurate planning for managing benign and malignant obstructive conditions. Ultrasound is the preferred initial screening test for biliary obstruction because it is cheap, quick, non-invasive, relatively accurate and readily available. There is a wide range of reported accuracy for ultrasound determining the level of obstruction (23-95%) and the cause of obstruction (38-81%). For choledocholithiasis, the sensitivity (false negatives) ranges from 20-80%. The specificity is high at 90%. Ultrasound provides information on ductal dilatation and is becoming the preferable modality for diagnosing acute cholecystitis (gallbladder wall thickening, pericholecystic fluid). Intraoperative ultrasound differentiates biliary from vascular structures, locates bile ducts obscured by inflammation and helps clarify unclear anatomy and anomalous ducts. Radionuclide imaging is based on dynamic transit of a radiopharmaceutical (99m Tc-labelled N-substituted iminodiacetic acid/HIDA) through the hepatocytes to the biliary collecting system. After 2 hours of fasting, the patient is given an intravenous dose of 3-10 mCi of 99mTcHIDA intravenously. The patient is placed supine with the scintillation camera anterior and one-minute sequential images are recorded for one hour followed by additional images for up to 24 hours. The first 10 minutes visualizes the size, shape and position of a normally functioning liver. The bile ducts should visualize within 20 minutes. Most unobstructed gallbladders visualize within 60 minutes and the isotope should appear in the duodenum in the same time period if the common duct is open. Presence of isotope in the liver and not in the gallbladder is indicative of acute cholecystitis since the outlet of the gallbladder is invariably obstructed in this condition. False positive HIDA scans may occur if the patient is not fasting, or has been fasting for a long period, if opiates have been administered (causing sphincter of Oddi spasm and failure of isotope to pass into the intestine), and in patients with a history of ethanol abuse. The primary utility of HIDA scan at the present time is as an adjunct to other imaging modalities when dynamic assessment of bile flow is needed, such as in post-operative bile leak (see bile leak folio and discussion). Other radionuclide imaging includes gallium-67 for assessment of hepatocellular carcinoma (HCC) in cirrhosis, and Tc99m labeled RBC's for assessing blood flow and volume of hepatic hemangiomas. CT scanning has a relatively limited role in diagnosing biliary ductal disease and primarily provides structural information on the presence of hepatic lesions. It also detects signs of cholecystitis, however, as mentioned above, US is the first line diagnostic modality for this condition. Latest generation spiral and multidetector scanners with enhanced data processing have shortened scan times and increased sensitivity. Contrast-enhanced, multiplanar and multiphasic imaging have further increased the utility of this modality. CT provides information on volume and location of post-operative biloma for percutaneous drainage and symptomatic relief when indicated. Magnetic resonance cholangiopancreatography (MRCP) is a non-invasive biliary ductal imaging modality that has evolved to equal the accuracy of direct invasive techniques of percutaneous transhepatic cholangiography (PTC) and endoscopic retrograde cholangiopancreatography (ERCP) and is replacing those techniques in clinical use. MRCP relies on the intrinsic electromagnetic relaxation times of tissues to generate images. Fluids such as bile and pancreatic juice have long T1 and T2 relaxation times, with a dark, low signal T1 image and a bright, high signal T2 image. MRCP uses heavily T2 weighted sequences to create an isolated fluid-only imaged cholangiogram and pancreatogram. Intraluminal filling defects such as stones appear as a dark void in the bright fluid image. There is a 4-6 hour fast to allow gallbladder filling and gastric emptying. Initial limitations of fine spatial resolution are resolving with the use of faster pulse sequences and respiratory-triggered imaging. Sensitivity has been reported from 57-91% and specificity from 98-100%. The accuracy is influenced by technical factors (fast spin echo and breath-holding pulse sequences, increased coil strength and surface coil application) that are rapidly improving in the speed of image acquisition and therefore image definition. MRCP has the advantage over PTC and ERCP of visualizing ducts both proximal and distal to an obstruction, but cannot differentiate a transected duct from an obstructed duct. Its use avoids the 3% morbidity (sepsis, bleeding, bile leak) and small percent mortality of ERCP. ERCP remains the first choice for therapeutic biliary intervention, but is being replaced by MRCP for diagnostic purposes. MRCP is contraindicated in patients with pacemakers, neurostimulators and any ferromagnetic implants such as aneurysm clips. References: Grainger & Allison's Diagnostic Radiology: a Textbook of Medical Imaging, 4th ed, Churchill Livingston, 2001, pp 1287-1294. Townsend: Sabiston textbook of surgery, 16th ed, WB Saunders, 2001, hepatobiliary complications, pp 219, 220. Reddy SI, Liver imaging, a hepatologist's perspective, Clinics in Liver Disease, vol 6, #1, Feb 2002. Ros PR & Koenraad JM, Hepatic imaging, an overview, Clinics in Liver Disease, vol 6, #1, Feb 2002. MacEneaney P et al, Update on magnetic resonance cholangiopancreatography, Gastroenterology clinics, Vol 31, #3, September 2002. Coakley FV, Qayyum A, Magnetic resonance cholangiopancreatography, Gastrointestinal Endoscopy, vol 55, #7, June 2002.
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