Effect of differential tracer washout during SPECT acquisition

A central assumption in SPECT is that the projection data are "consistent," that is, the camera views an unchanging distribution during acquisition. Several new radiotracers of interest, including 99mTc-teboroxime (Cardiotec), have rapid clearance from the myocardium. Furthermore, the washout is different in normal and ischemic tissues. We used computer simulations to estimate the effect of this differential washout on quantification of the severity of ischemia. We simulated defect-to-normal myocardial activity ratios of 1 (no defect), 0.8, 0.6, 0.4, 0.2, and 0 (complete defect), with single defects placed either in the lateral wall or apex, and SPECT acquisitions of 1, 3, 6, 12, and 24 total minutes. We modeled washout with a monoexponential curve whose clearance half-time was 5.9 min for "normal myocardium" and 9.3 min for "ischemic myocardium." We found that differential washout from normal and ischemic zones produced image artifacts and errors in defect quantification for acquisitions longer than 3 min. With longer acquisitions, the degree of ischemia was significantly underestimated, with increasing error at longer acquisition times. In addition, in the "no defect" situation an apparent small lateral wall defect (relative to the apex) was present. Finally, lateral wall defects produced artifacts (streaks and reduced apparent activity) in the opposite (medial) wall. Differential normal/ischemic zone washout during SPECT acquisition produces artifacts and errors in quantification, whose severity is dependent on acquisition length, actual defect severity, and defect location.