Absolute quantitation of a spherical I-131 activity distribution using a high-resolution rotating collimator: a phantom study

SPECT data were acquired using a high-resolution rotating parallel-hole collimator designed for imaging of medium-energy radionuclides. The authors reconstructed images of I-131 activity distributions for a 2.2 cm-diameter sphere inside a water-filled cylinder, which approximates the activity distribution and attenuation medium encountered in the I-131 monoclonal antibody treatment of brain tumors. Reconstructions were performed using FBP, OSEM and MAP/ICD methods including detector response modeling in the iterative algorithms. The MAP/ICD with an edge-preserving convex prior and OSEM reconstructions provided the more accurate absolute activity distributions with better resolution (FWHM of 0.5 cm). Once the image resolution was accounted for in the determination of volumes of interest, the sphere´s activity and activity density were reproduced to within ±5% of their known values. The activity estimation based on MAP/ICD and OSEM reconstructions showed relatively little dependence on the size of the volume of interest compared to FBP images