Optimized Energy Window Configuration for ²⁰¹Tl Imaging

¹ Department of Medical Physics, School of Medical Sciences, Tarbiat Modares University, Tehran, Iran
² Department of Nuclear Medicine, Rajaei Heart Center, Iran University of Medical Sciences, Tehran, Iran

^* To whom correspondence should be addressed. E-mail: hrajabi{at}modares.ac.ir.

	Abstract

A poor signal-to-noise ratio attributable to a low injected dose of thallium and the presence of scattered photons are the major impediments in the use of thallium as an imaging agent. Thallium decays in a complicated way and emits photons in a wide range of energies (68–82 keV). To increase the ratios of primary photons to scatter photons (primary-to-scatter ratios) and possibly increase system sensitivity, a new energy window for thallium was investigated. Methods: The NCAT phantom was used to simulate the distribution of activity and the attenuation coefficient in a typical patient torso. The phantom was imaged with a SPECT simulator in different energy window configurations. The energy spectra for primary photons and scatter photons were generated, and the most suitable energy windows were investigated. To evaluate the results of the simulation study, a physical phantom was imaged in different energy windows with a SPECT system. The images of the physical phantom were analyzed for the best-quality image and the corresponding window setting. To evaluate the windows determined in the simulation and phantom studies, SPECT images of 7 patients who had angiographically confirmed myocardial defects were acquired in different energy windows. The images were quantitatively compared on the basis of the calculated contrast, scatter-to-noise ratio, and sensitivity. The images were also qualitatively evaluated independently by 4 nuclear medicine specialists. Results: The simulation study showed that the conventional window setting (68 ± 10% keV) is not the most suitable window configuration for ²⁰¹Tl imaging and that the optimum energy window is 77 ± 15% keV. The images acquired in the latter window configuration yielded higher primary-to-scatter ratios, higher sensitivity (total counts), and better contrast than the images acquired in the conventional window configuration. The phantom study confirmed the results of the simulation study. In the clinical study, the images acquired in the suggested window showed a considerable increase in myocardium-to-defect contrast (1.541 ± 0.368) and myocardium-to-cavity contrast (1.171 ± 0.099) than those acquired in the conventional window configuration. Conclusion: The energy window configuration of 77 ± 15% keV yields higher-quality images than the conventional window configuration.