Estimation of tumor activities from simulated I-123 SPECT studies

A method for estimating activities within small tumors from quantitative filtered backprojection reconstructions has been developed and evaluated for simulated I-123 SPECT studies. Tumor activity in a reconstructed SPECT image is modeled as blurred version of uniform activity in concentric spherical shells about the tumor center. The shell activities are estimated by a least squares procedure. This method was evaluated for 1.25 and 2.0 cm radius tumors in a water-filled cylinder using Monte Carlo simulations of SPECT acquisitions with a medium energy collimator. Shell:background and shell:core activity ratios were 4:1. Images were reconstructed by filtered backprojection with compensation for attenuation, scatter and detector response. Spatial resolution in the reconstructed images is about 1.5 cm. For noise-free projection data, shell activities are better recovered with blurring compensation. For projection data at a clinical count level, shell activity estimation with blurring compensation is beneficial for the 2.0 cm radius tumor. Blurring compensation is of little benefit for the 1.25 cm radius tumors because the noise causes the shell and core activity estimates to vary greatly