The Importance of Region-of-Interest Placement in Gastric Emptying Studies with Small Bowel Activity at Time 0.

Gastric emptying is assessed in nuclear medicine by having patients consume a standardized radioactive meal, followed by imaging after meal ingestion (time 0) and at each hour up to 4 h. Usually, all of the meal is in the stomach at time 0. However, in some patients, a portion is in the small bowel at time 0. This study evaluates the effects of region-of-interest (ROI) placement on gastric retention percentages when meal is present in small bowel at time 0.

Methods

In 624 patients who underwent gastric emptying scintigraphy, images were examined retrospectively for small bowel activity at time 0 (small bowel sign). We analyzed gastric retention by comparing time 0 ROIs drawn around the stomach and those drawn around the stomach plus small bowel. ROIs were drawn around the stomach at 1-4 h after meal ingestion. An equal number of age- and sex-matched patients without the small bowel sign were evaluated. Qualitative assessment was used to determine whether most counts were in the gastric fundus or antrum at time 0.

Results

The small bowel sign was present in 8.3% of patients (52/624). Over 1-4 h, the mean initial ROIs without bowel analysis were significantly higher than those with bowel analysis (38% ± 36% vs. 30% ± 30%, P < 0.0001). A normal gastric retention value of no more than 10% at 4 h was found in 94.2% of patients (49/52) using the initial ROI without bowel analysis and in 100% of patients (52/52) using the initial ROI with bowel analysis. Rapid gastric transit (retention of ≤30% at 1 h) was seen in 3.8% of cases (2/52) using the initial ROI without bowel analysis and in 9.6% of cases (5/52) using the initial ROI with bowel analysis (P not significant). At time 0, most counts were in the antrum in 33% of patients with the small bowel sign versus 8% in patients without the small bowel sign (P = 0.001).

Conclusion

In patients with a portion of the meal in the small bowel on time 0 images, the exclusion of small bowel counts on ROI analysis can lead to spuriously high 1- and 4-h gastric retention values. This can lead to underdiagnosis of rapid gastric transit at 1 h and overdiagnosis of delayed gastric emptying at 4 h.