A MONTE-CARLO INVESTIGATION OF DUAL-ENERGY WINDOW SCATTER CORRECTION FOR VOLUME-OF-INTEREST QUANTIFICATION IN TC-99(M) SPECT

Using Monte Carlo simulation of Tc-99(m) single-photon-emission comput ed tomography (SPECT), we investigate the effects of tissue-background activity, tumour location, patient size, uncertainty of energy window s, and definition of tumour region on the accuracy of quantification. The dual-energy-window method of correction for Compton scattering is employed and the multiplier which yields correct activity for the VOI as a whole calculated. The model is usually a sphere containing radioa ctive water located within a cylinder filled with a more dilute soluti on of radioactivity. Two simulation codes are employed. Reconstruction is by ML-EM algorithm with attenuation compensation. The scatter mult iplier depends only slightly on the sphere location or the cylinder di ameter. It also depends little on whether correction is before or afte r reconstruction. At low background level, it changes with vor size, b ut not at higher background. For a geometrical VOI, it is 1.25 at zero background, decreases sharply to 0.56 for equal concentrations, and i s 0.44 when the background concentration is very large. Quantification is accurate (less than 9% error) if the test background is reasonably close to that used in setting the universal scatter-multiplier value, or if the test backgrounds are always large and so is the universal-v alue background, but not if the test backgrounds cover a large range o f values including zero. Results largely agree with those from experim ent after the experimental data with background is re-evaluated with p rejudice.