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travel through a fluid-filled structure, no attenuation occurs. Thus, when those sound waves hit the back of the gallbladder, they will be so strong that they will obscure an accurate picture of the wall thickness. For gallbladder wall thickness, greater than 3 mm is abnormal [ 3 , 4 ]. Table 7.1 Differential for thickened gallbladder wall [3,4] Postprandial Renal failure Ascites Hepatitis Hypoalbuminemia HIV/AIDS Adenomyomatosis Multiple myeloma Cholecystitis Congestive heart failure 156 Diagnostic ultrasound Gallbladder ultrasound
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A common bile duct that is dilated is evidence of obstruction. This is the second measurement required for a complete evaluation. The common bile duct is typically less than 6 mm in transverse diameter [ 5 ]. The CBD diameter should be measured from inner wall to inner wall. The diameter can increase with age, and some authors have recommended that the diameter should be less than one-tenth of the patient’s age. In patients who have had a cholecystectomy, the CBD may normally range up to 1 cm [ 5 ]. The range of CBD diameters and their implications is shown in Table 7.2 . Biliary obstruction, regardless of the etiology, will be demonstrated by a dilated biliary tree. Dilatation of the extrahepatic ducts implies CBD obstruc- tion. This can eventually lead to intrahepatic duct dilatation. (Note that dilatation of the intrahepatic ducts alone suggests obstruction within the common hepatic duct or more proximal). Figure 7.8 Posterior acoustic enhancement (area around asterisk) distal to the anechoic gallbladder, in two views. For more detailed explanation of this phenomenon, see Chapter 1 . Table 7.2 Common bile duct diameters [5] CBD Implication 2–5 mm normal range 6–8 mm clinical correlation required 6 mm 11% normal subjects 7 mm 4% normal subjects > 8 mm abnormal Diagnostic ultrasound 157 Gallbladder ultrasound
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Scanning tips Trouble with gallbladder scanning Rib shadow in the way? ± Try angling the probe obliquely to sneak in between the ribs. ± Have the patient take a deep breath to lower the diaphragm and bring the gallbladder lower in the abdomen below the ribs. Can’t see the gallbladder at all? ± Try having the patient roll onto his or her left side to bring the gall- bladder more anterior in the peritoneal cavity, or if the patient is sitting up have him or her lean forward. ± One unorthodox view is to have the patient get on his or her hands and knees and scan the abdomen this way so gravity works in your favor to pull the gallbladder toward the anterior abdominal wall. ± It is always easier to see the gallbladder if the patient has been NPO, because this causes the gallbladder to dilate. If feasible, you can wait for an hour to see if the dilating gallbladder will be easier to find. ± If any liver parenchyma is visible, try to trace the portal vein tributaries back to the main portal vein as described in the text. The gallbladder will reliably be located near the portal vein at this point. This technique is analogous to tracing retinal vessels back toward the optic disc in the eye exam.
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