The effect of source size on the buildup factor calculation of absolute volume

A general scheme for generating attenuation-corrected images for use in absolute volume measurements has been developed. The technique is based on a buildup factor approach for Compton scatter compensation and requires anterior and posterior count-rate measurements. The scatter correction requires evaluation of the attenuation factor (1-(1-e-mu d)B(infinity), where mu is the linear attenuation coefficient in cm-1 and B(infinity) is the buildup factor at infinite depth. The attenuation factors for four different source sizes using 99mTcO4- and a 20% scintillation camera energy window are reported. The results indicated that B(infinity) was constant while mu varied as a function of size (S) in pixels according to mu = 0.151 cm-1 X exp(-1.18 X 10(-4) pixels-1 X S). Once the appropriate value of mu was determined, a pair of anterior-posterior count rate equations was used to generate attenuation-corrected count rate data for use in absolute volume measurements. The method was validated by calculating three separate phantom volumes. The results showed that the method provides less than +/- 6.0% error for volume determinations at all investigated depths.