Simultaneous dual-isotope SPECT imaging using a cardiac cadmium-zinc-telluride camera with various Technetium-99 m to Iodine-123 ratios

Objective Cadmium-zinc-telluride (CZT) SPECT cameras offer high energy resolution. This allows simultaneous dual-isotope (SDI) acquisition even when 123 I and 99m Tc photon peaks are close. This study aimed to reveal the impact of image quality for 99m Tc/ 123 I ratios on SDI images. Methods We created normal and inferior wall defect myocardial models using an anthropomorphic myocardial phantom. The doses of 99m Tc/ 123 I ratios were set to 1.0, 3.5, 5.0 and 6.5 (referred to as Tc/I_1.0, Tc/I_3.5, Tc/I_5.0 and Tc/I_6.5 conditions). We acquired SPECT images using D-SPECT Cardio. The acquisition time was adjusted to achieve 1.5 × 10 6 left ventricular 123 I counts as a clinical reference. Short-axis images were reconstructed with (SC) and without (NC) scatter correction. Kernel and Gaussian-standard values were set to 1 and 1.0 as default conditions. The filter parameters were changed to 1, 3 and 5 for the kernel settings, and 0.25–1.0 for Gaussian standards. The image quality of normal and defective myocardia at different 99m Tc/ 123 I ratios was evaluated as cavity contrast on short-axis images, the percent coefficient of variance (%CV) and %uptake on polar maps. In addition, these reconstruction conditions were applied to a patient with acute coronary syndrome, supporting the phantom-based findings. Results In the normal myocardial phantom, %uptake in the inferior and inferolateral myocardial walls was significantly lower in SC images than in NC images for both 99m Tc and 123 I. In SC images, 99m Tc counts in the inferoseptal myocardial wall slightly decreased along with the 99m Tc/ 123 I ratio, whereas 123 I counts in the inferolateral walls declined as the 99m Tc/ 123 I ratio increased to 6.5. Higher kernel and Gaussian standard settings resulted in lower %CV values in both NC and SC 99m Tc and 123 I images, improving image uniformity. Overall, NC images showed more homogeneous myocardial distribution in the inferior wall compared with SC images. Conclusion Image quality and correction effects are influenced by 99m Tc/ 123 I ratios, resulting in heterogeneity in the inferior wall of the normal myocardium due to SC. Adjusting kernel parameters of the Gaussian filter or using NC images may help improve image quality when 99m Tc/ 123 I ratios are very high or low.