Nuclear Medicine

There is no standard protocol for conducting gastric emptying studies, as methods vary depending on the laboratory performing the test. However, some core principles remain the same. A radiolabeled meal is ingested and the gastric emptying study is acquired on computer for 90 – 120 minutes. The study may be performed with liquid or solid meals. Solid meals are more sensitive than liquid meals for detection of abnormalities. Liquid studies are always normal when solid studies are normal, however when solid studies are normal, liquid studies may appear as either normal or abnormal. Solid emptying studies demonstrate a lag phase, after which emptying occurs at a constant linear rate. Liquid studies do not have a lag phase, are mono exponential and have a normal half-time of 10-20 minutes (see Figure 1 for comparison). The gastric emptying study may be conducted with either single or dual isotope. Dual isotope studies allow the simultaneous acquisition of liquid and solid meal studies by labelling both meals with different isotopes. This method is not preferred due to the higher coast and ionizing radiation exposure and has no added benefit than a single-isotope solid gastric emptying study. 

As the ingested meal moves posteriorly to anteriorly in the stomach (from the fundus to the antrum, which are anatomically positioned as such), there needs to be attenuation correction. As the antrum is positioned more anteriorly with respect to the fundus, it is closer to the camera and, as food travels down, it becomes detected in larger amounts even though the quantity may not have changed. The gold standard for correcting the attenuation artifact is the geometric mean method, which involves taking opposite images (anterior and posterior projections) and calculating the geometric mean of the radioactive counts at each data point (see Figure 2). A study demonstrating delayed gastric emptying, as per Georgetown University Hospital protocol, is quantified as less than 30% emptying at 90 minutes (solid study).