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evaluated. Simultaneous mapping of normal tissue with 99mTc-sulfur colloid adds sensitivity. Gallium accumulation in an area of absent 99mTc-sulfur colloid uptake in a liver distorted by advanced cirrhosis is highly suggestive of a hepatoma (Fig. 4). In less well defined clinical situations, gallium uptake is nonspecific and may be present in focal infection or inflammation and a wide variety of neoplasms.

Fluorodeoxyglucose

Fluorine-18 labelled fluorodeoxyglucose (18F-FDG) is a glucose analogue taken up by cells through the same transport system as glucose. Following cellular uptake, FDG is phosphorylated. Since FDG-phosphate cannot be metabolized further, it is effectively "trapped" in the cell and can be used as a marker of glucose metabolism. As with gallium, uptake is nonspecific. Because of the high sensitivity of anatomic imaging modalities in detecting liver masses, the greatest utility of FDG imaging is in the detection of extrahepatic sites for tumour staging (see Chapter 15). Availability of positron emitting radioisotopes, including 18F, has been limited to date but is improving.

Figure 3. Metastases from malignant carcinoid. A 46 year old woman with past resection of a small bowel carcinoid presents with carcinoid syndrome. A CT image during CT arterial portography (a) shows multiple hypodense masses (arrows). The corresponding 111In-pentetreotide SPECT transaxial image (b) confirms that these are somatostatin receptor positive metastases (arrows).

Figure 3. Metastases from malignant carcinoid. A 46 year old woman with past resection of a small bowel carcinoid presents with carcinoid syndrome. A CT image during CT arterial portography (a) shows multiple hypodense masses (arrows). The corresponding 111In-pentetreotide SPECT transaxial image (b) confirms that these are somatostatin receptor positive metastases (arrows).

Clinical Role in the Evaluation of the Biliary Tree

Following bolus injection of 99mTc-IDA agents, initial images show blood pool and uniform hepatic activity which, in the absence of hepatocellular disease or cholestasis, clears rapidly. Bile duct activity appears within 10 to 15 minutes of injection.

In healthy fasting patients, the gallbladder is usually visualized within 15 to 30 minutes after injection of the 99mTc-IDA, and almost always within 60 minutes (Fig. 5). Confirmation of the anterior location of the activity on lateral or oblique views is recommended to avoid mistaking duodenal activity for the gallbladder. All excreted bile may enter the gallbladder if the sphincter of Oddi is closed (a not uncommon finding in the fasting patient) or variable portions may drain to the duodenum. The greatest utility of the filling phase is to confirm patency of the cystic duct.

As long as no additional bile enters the gallbladder, externally detected activity is proportional to relative volume and is independent of gallbladder shape or geometry. Contraction of the gallbladder may be stimulated by infusion of a synthetic active octapeptide of cholecystokinin (CCK-8 or sincalide, KinevacĀ®) or endogenously released CCK after a meal. CCK-8 infusion is easier to control and is the preferred method.

The most commonly used single quantitative measurement that reflects emptying is the ejection fraction (EF), which represents the percentage of initial volume or activity evacuated with contraction. It is calculated from the following formula:

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