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Comparison of Americium-241 and Technetium-99m as Transmission Sources for Attenuation Correction of Thallium-201 SPECT Imaging of the Heart

Department of Internal Medicine, Division of Nuclear Medicine, The University of Michigan Medical Center, Ann Arbor, Michigan

Correspondence: For correspondence or reprints contact: Edward Ficaro, PhD, The University of Michigan Hospitals, 1500 E. Medical Center Dr., Ann Arbor, MI 48109.

ABSTRACT

This study compares the ability of ²⁴¹Am and ^99mTc to estimate ²⁰¹Tl attenuation maps while minimizing the loss in the precision of the emission data. Methods: A triple-head SPECT system with either an ²⁴¹Am or ^99mTc line source opposite a fan-beam collimator was used to estimate attenuation maps of the thorax of an anthropomorphic phantom. Linear attenuation values at 75 keV for ²⁰¹Tl were obtained by linear extrapolation of the measured values from ²⁴¹Am and ^99mTc. Results: Lung and soft-tissue estimates from both isotopes showed excellent agreement to within 3% of the measured values for ²⁰¹Tl. Linear extrapolation did not yield satisfactory estimates for bone from either ²⁴¹Am (+11.7%) or ^99mTc (-15.3%). Patient data were used to estimate the dependence of crosstalk on patient size. Contamination from ²⁰¹Tl in the transmission window was 5–6 times greater for ²⁴¹Am compared to ^99mTc, while the contamination in the ²⁰¹Tl data in the transmission-emission detector head (head 1) was 4–5 times greater for ^99mTc compared to ²⁴¹Am. No contamination was detected in the ²⁰¹Tl emission data of heads 2 and 3 from ²⁴¹Am, whereas the ^99mTc produced a small crosstalk component giving a signal-to-crosstalk ratio near 20:1. Measurements with a fillable chest phantom estimated the mean error introduced into the data from the removal of the crosstalk. Conclusion: Based on the measured data, ²⁴¹AM is a suitable transmission source for simultaneous transmission-emission tomography for ²⁰¹Tl cardiac studies.

Key Words: transmission tomography • SPECT • attenuation correction • cardiac imaging

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