Monte Carlo evaluation of quantitation in simultaneous Tc-99m/I-123 brain SPECT using constrained spectral factor analysis

Simultaneous imaging of Tc-99m and I-123 has important applications for brain imaging but is hindered by crosstalk between the two radionuclides. Monte Carlo simulations of 15 different dual-isotope studies were performed using a digital brain phantom. Several physiologic Tc-99m and I-123 uptake patterns were modeled in the brain structures. Spectral factor analysis (SFA) using constraints on the primary spectra was performed on projections to correct for crosstalk from both scattered and unscattered photons. The accuracy of reconstructed pixel values, using filtered-backprojection, within several brain structures was compared to that obtained with an energy windowing method (WSA). In I-123 images, mean bias was close to 10% in all structures for SFA and between 14% (in the caudate nucleus) and 25% (in the cerebellum) for WSA. Tc-99m activity was overestimated by 35% in the cortex and 53% in the caudate nucleus with WSA, but by less than 9% in all structures with SFA. SFA performed well even in the presence of high-energy I-123 photons; good accuracy was attained by incorporating the contamination into the model as a priori information. SFA is a promising approach to correct for crosstalk in simultaneous Tc-99m/I-123 SPECT.