I-131 SPECT activity recovery coefficients with implicit or triple-energy-window scatter correction

We define a total-activity recovery coefficient, TARC, as the activity within a volume of interest applied to a set of images from single photon emission computed tomography (SPECT) divided by the true activity in that volume. Because of finite camera resolution, recovery-coefficient values are expected to be less than 1 for small objects. In this report, the TARC is measured as a function of volume, background activity and image-acquisition radius of rotation. A set of hollow spheres which vary in volume are filled with a solution of I-131 in water, and placed within a water-filled cylinder having an elliptical cross section. Tomographic-imaging measurements are carried out for two systems. The first utilizes an implicit scatter correction and the second triple-energy-window correction. Both systems employ a reconstructed-counts-to-activity conversion factor which varies with a measured background parameter and with the radius of rotation. Sphere and cylinder outlines are drawn on a CT scan and transferred to the SPECT scan via a 3-D image fusion. As expected, the inverse of the TARC increases as the sphere volume decreases. With implicit scatter correction, the TARC has no systematic dependence on either radius of rotation or background. With triple-energy-window scatter correction, there is an increase in 1/TARC when the radius of rotation is increased by 5 cm. This increase ranges from 9% to 25% depending on volume. For all conditions, 1/TARC is fit to a power-law equation as a function of sphere volume. The use of the resultant equations to correct a measured activity for a small volume is outlined. Accuracy of the present relationships for target shapes which are not spherical is not addressed.