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Gastric Function Imaging: Technique and Applications
Technical Aspects
Radionuclide gastric emptying studies (scintigraphy) remain the most widely used method for evaluation of gastric function.
Radiopharmaceuticals
Gastric emptying scintigraphy is most commonly performed with technetium-99m ( 99m Tc) sulfur colloid dispersed in a solid and/or liquid bolus.
To be a gastric function tracer, a radioactive marker must meet certain criteria. The criteria for a good liquid-phase marker includes the ability to equilibrate rapidly and be nonabsorbable. 99m Tc sulfur colloid in water meets these criteria. The solid-phase radioactive marker for evaluation of solid gastric emptying requires the ability to bind tightly to the solid food particle. The reason is that liquids empty faster than solids, thereby producing an erroneously shortened solid emptying time. The most well-accepted in-vitro methods for radioactive labeling involves frying eggs with 99m Tc sulfur colloid, resulting in binding to the egg albumin and administering as an egg sandwich.
In dual (solid/liquid) phase studies, indium-111–labeled diethylenetriaminepentaacetic acid ( 111 In-DTPA) is the liquid marker and 99m Tc sulfur colloid is the solid marker.
Technique
Gastric emptying scintigraphy requires the patient to be fasting for 8 to 12 hours. Medications that affect gastric motility should be stopped, if possible. These include calcium channel blockers, anticholinergics, antidepressants, narcotics, gastric acid suppressants, and aluminum-containing antacids. Alcohol consumption and use of tobacco products should be stopped for a minimum of 24 hours.
On the morning of the study, the radiolabeled meal is prepared ( Table 23-1 ). 99m Tc sulfur colloid (1 millicurie) is added to solidifying scrambling eggs and mixed until solidified and then placed between two pieces of toasted bread. Once prepared, the 99m Tc sulfur colloid radiolabeled egg should be consumed within 5 to 10 minutes. Promptly after ingestion, a continuous data acquisition with a frame rate of 30 to 60 seconds per image is performed for 90 minutes (64 × 64 pixels) with the patient positioned in the supine position. Additional imaging at 3 and 4 hours can be performed to identify patients with delayed emptying.
TABLE 23-1
Adult Dosimetry for Gastric Function
| Phase | Adult Dosimetry for Gastric Scintigraphy |
|---|---|
| Liquid | 0.5-1 mCi 99m Tc sulfur colloid |
| Solid | 0.5-1 mCi 99m Tc sulfur colloid ovalbumin 0.5-1 mCi 99m Tc sulfur colloid chicken liver |
A region of interest is drawn over the stomach, and the percent of gastric emptying is determined. The radioactive counts increase as the food travels from the fundus, a posterior structure, to the antrum, an anterior structure. The attenuation effect is therefore one of the technical factors that can cause underestimation of gastric emptying and, therefore, a false-positive result. This is most commonly corrected with the accepted gold standard for correcting attenuation, the geometric mean measure. Frequent image acquisition increases accuracy in determining gastric emptying. An alternative method to decrease false-positive results is to acquire images in the left anterior oblique position.
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