Nonuniform transmission in brain SPECT using Tl-201, Gd-153, and Tc-99m static line sources: Anthropomorphic dosimetry studies and influence on brainquantification

Nonuniform attenuation correction in brain SPECT can be done routinely by m eans of additional gamma transmission CT (TCT) measurements, using differen t commercially available line-source isotopes. Tl-201, Gd-153, and Tc-99m a re among the most commonly used isotopes, depending on practical and cost-e ffectiveness issues. We have measured additional radiation burden from stat ic uncollimated brain SPECT transmission sources for these isotopes. The in fluence of the transmission isotope on brain quantification was also measur ed and compared with uniform attenuation correction for phantom and human d ata. Full iterative transmission and emission reconstruction were compared with filtered backprojection techniques. Methods: Rod sources with Tl-201, Gd-153, and Tc-99m were used on a triple-head gamma camera. Dosimetry was p erformed using LIF TLD-100 pellets and an anthropomorphic RANDO phantom. Ef fective dose equivalents were calculated on the basis of measured and extra polated absorbed doses. For brain activity measurements, a Hoffman phantom was used. Images were corrected for scatter (triple-energy window) and were reconstructed by Chang attenuation correction and filtered backprojection as well as full iterative reconstruction (ordered-subsets expectation maxim ization [OSEM]). To study the effect of inhomogeneous bone attenuation, rea listic measurements were performed on 10 young, healthy volunteers with Gd- 153 TCT. After stereotactic image realignment, a volume-of-interest analysi s normalized to total counts was performed. Results: Brain SPECT-TCT using Tl-201, Gd-153, and Tc-99m produced total effective dose-rate equivalents o f 50.3 +/- 11.2, 32.0 +/- 2.7, and 71.1 +/- 7.1 mu Sv/GBq x h, respectively , representing dose equivalents of 18.6, 11.9, and 26.3 mu Sv for a typical 20-min brain SPECT scan at maximal used source strength. Standardized quan tification resulted in insignificant differences between the isotopes and m ethods (Chang versus OSEM) used for nonuniform correction. Iterative recons truction enhanced image contrast and provided more accurate gray-to-white m atter ratios. Between nonuniform and uniform attenuation with an optimized attenuation coefficient, slight central discrepancies were found for volunt eer studies. Significantly lower intersubject variation was found for nonun iform corrected values in infratentorial and posterior brain regions. Concl usion: Brain transmission scanning using Tl-201 Gd-153, Or Tc-99m results i n limited effective radiation dose equivalents compared with the typical ra diation burden. Relative brain perfusion quantification is not significantl y different for the various nonuniform TCT isotopes. Iterative reconstructi on improves gray-to-white contrasts but has no significant influence on bra in perfusion semiquantification. Nonuniform attenuation correction decrease s intersubject variability in the posterior brain regions that were compare d, which may lead to improved sensitivity toward clinical applications.