DEAD-TIME OF AN ANGER CAMERA IN DUAL-ENERGY-WINDOW-ACQUISITION MODE

Two side-by-side energy windows are sometimes employed in quantitative single photon emission computed tomography (SPECT) studies. The count -rate losses at high activities for a GE400AT camera were measured in such a dual-window-acquisition mode by imaging a decaying source compo sed of a hot sphere within a warm cylinder. The data in each window wa s either kept separate or combined for purposes of dead-time correctio n and the paralyzable model was assumed. In addition, correction facto rs were derived from a ''monitor'' source at the edge of the camera. F inally, energy spectra for only the monitor-source region of interest and for the entire camera were obtained under both low- and high-count conditions and compared. With Tc-99m (1) the spectral measurements sh ow no peak shift but reveal pulse-pileup spectral degradation. (2) The monitor-source corrections do not agree well with those from the mode l, presumably because of the differential effects of such degradation. For this camera the preferred correction method for patients is one u sing the model and two, effective, phantom-derived dead times for the separate data from the two windows. The two effective dead times are n eeded to compensate for pulse pileup adding more counts to the lower-e nergy window than to the higher-energy one at high rates. For Tc-99m, the effective dead time to correct data in the 93-123 keV window is 7. 0 +/- 0.14 mus and for that in the 124-154 keV window 5.95 +/- 0.10 mu s, and for I-131, for data in the 260-332 keV window 18.8 mus and for that in the 333-405 keV window 14.3 mus.