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Nonuniform Transmission in Brain SPECT Using ²⁰¹Tl, ¹⁵³Gd, and ^99mTc Static Line Sources: Anthropomorphic Dosimetry Studies and Influence on Brain Quantification

Division of Nuclear Medicine, Ghent University Hospital, Ghent; Medical Imaging and Signal Processing Department and Laboratory for Standard Dosimetry, Ghent University, Ghent, Belgium; and Department of Clinical Physiology and Nuclear Medicine, Kuopio University Hospital, Kuopio, Finland

Nonuniform attenuation correction in brain SPECT can be done routinely by means of additional transmission CT (TCT) measurements, using different commercially available line-source isotopes. ²⁰¹Tl, ¹⁵³Gd, and ^99mTc are among the most commonly used isotopes, depending on practical and cost-effectiveness issues. We have measured additional radiation burden from static uncollimated brain SPECT transmission sources for these isotopes. The influence of the transmission isotope on brain quantification was also measured and compared with uniform attenuation correction for phantom and human data. Full iterative transmission and emission reconstruction were compared with filtered backprojection techniques. Methods: Rod sources with ²⁰¹Tl, ¹⁵³Gd, and ^99mTc were used on a triple-head gamma camera. Dosimetry was performed using LiF TLD-100 pellets and an anthropomorphic RANDO phantom. Effective dose equivalents were calculated on the basis of measured and extrapolated 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 maximization [OSEM]). To study the effect of inhomogeneous bone attenuation, realistic measurements were performed on 10 young, healthy volunteers with ¹⁵³Gd TCT. After stereotactic image realignment, a volume-of-interest analysis normalized to total counts was performed. Results: Brain SPECT-TCT using ²⁰¹Tl, ¹⁵³Gd, and ^99mTc produced total effective dose-rate equivalents of 50.3 ± 11.2, 32.0 ± 2.7, and 71.1 ± 7.1 µSv/GBq x h, respectively, representing dose equivalents of 18.6, 11.9, and 26.3 µSv for a typical 20-min brain SPECT scan at maximal used source strength. Standardized quantification resulted in insignificant differences between the isotopes and methods (Chang versus OSEM) used for nonuniform correction. Iterative reconstruction enhanced image contrast and provided more accurate gray-to-white matter ratios. Between nonuniform and uniform attenuation with an optimized attenuation coefficient, slight central discrepancies were found for volunteer studies. Significantly lower intersubject variation was found for nonuniform corrected values in infratentorial and posterior brain regions. Conclusion: Brain transmission scanning using ²⁰¹Tl, ¹⁵³Gd, or ^99mTc results in limited effective radiation dose equivalents compared with the typical radiation burden. Relative brain perfusion quantification is not significantly different for the various nonuniform TCT isotopes. Iterative reconstruction improves gray-to-white contrasts but has no significant influence on brain perfusion semiquantification. Nonuniform attenuation correction decreases intersubject variability in the posterior brain regions that were compared, which may lead to improved sensitivity toward clinical applications.

Key Words: transmission CT • dosimetry • brain SPECT • quantification

Received Dec. 31, 1999; revision accepted May 16, 2000.

For correspondence or reprints contact: Koen Van Laere, MD, DSc, Nuclear Medicine Division, P7, Ghent University Hospital, De Pintelaan 185, 9000 Ghent, Belgium.

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