DO PATIENT DATA EVER EXCEED THE PARTIAL VOLUME LIMIT IN GATED SPECT STUDIES

Background. Some single photon emission computed tomography (SPECT) me thods to detect percent myocardial wall thickening (%WT) assume a line ar relationship to changes in maximum myocardial counts, predicated on myocardial walls never exceeding the SPECT camera's partial volume li mit. Recent studies have challenged such assumptions, reporting that s ystolic count changes underestimate wall thickening as measured by ech ocardiography and magnetic resonance imaging. Methods and Results. To test whether clinical data ever are observed to exceed the partial vol ume limit, we examined gated tomograms of 75 patients selected at rand om and of an additional 25 patients known to have hypertension with el ectrocardiographic evidence of left ventricular hypertrophy, Image tra nsformations were performed such that for every cinematic frame, radia l counts at every angle were automatically normalized to the same maxi mum count. If no patient's myocardium ever exceeded the partial volume limit, thickness quantified from transformed images would always be t he same throughout the cardiac cycle and would just correspond to the camera's line spread function. Thickness was measured by Gaussian fitt ing of transformed myocardial counts in the epicardial direction only to exclude cavitary count contamination, %WT was computed from thickne ss differences from diastole to systole, %WT values were assessed from clinical data at lateral? inferior, septal, anterior, and apical terr itories. Resulting %WT distributions were tested against the null hypo thesis of %WT = 0 by the Z-test, Although some distributions were not actually Gaussian, the maximum mean %WT was only +3% +/- 5% for the se ptal wall, in agreement with an observer's impressions of no detectabl e wall thickening. Thus mean %WT values were trivial compared with exp ected physiologic normal values of 30% to 50%. Conclusion. No convinci ng evidence was found of thickness above the partial volume limit in t his large sample of 75 normotensive and 25 hypertensive patients. Ther efore it is likely that relations between myocardial count increases a nd wall thickening are similar throughout the cardiac cycle, even in p atients with left ventricular hypertrophy.