Left anterior oblique projection and peak-to-scatter ratio for attenuation compensation of gastric emptying studies

Compensation for soft-tissue attenuation is necessary for accurate quantitation of nuclear gastric emptying studies. We sought an attenuation compensation method that would require the acquisition of images from a single projection, thus allowing for continuous dynamic acquisition. We investigated the use of the left anterior oblique (LAO) projection and the peak-to-scatter ratio (P/S) method. Phantom studies indicated that the P/S was not only a function of the amount of overlying attenuating material but also a function of the activity in the small intestine. Two models for the P/S method were developed, one that considered the activity in the small intestine and one that did not. A series of 27 patients (21 for solid gastric emptying and six for liquid gastric emptying) were studied comparing the results using the geometric mean (GM) method with the two P/S methods, the LAO and the uncorrected anterior (ANT) projection. The uncorrected ANT view for solid gastric emptying underestimated the percent emptying at 60 min by approximately 7%. The P/S method did not adequately compensate for attenuation. The use of the LAO projection yielded results that were highly correlated and unbiased when compared to the GM method. Accurate estimates of liquid gastric emptying can be obtained without attenuation compensation.