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

Abstract

Objective: Cadmium-zinc-telluride (CZT) SPECT cameras offer high energy resolution. This allows simultaneous dual-isotope (SDI) acquisition even when ¹²³I and ^99mTc photon peaks are close. This study aimed to reveal the impact of image quality for ^99mTc/¹²³I ratios on SDI images.

Methods: We created normal and inferior wall defect myocardial models using an anthropomorphic myocardial phantom. The doses of ^99mTc/¹²³I 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 × 10⁶ left ventricular ¹²³I 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 ^99mTc/¹²³I 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 ^99mTc and ¹²³I. In SC images, ^99mTc counts in the inferoseptal myocardial wall slightly decreased along with the ^99mTc/¹²³I ratio, whereas ¹²³I counts in the inferolateral walls declined as the ^99mTc/¹²³I ratio increased to 6.5. Higher kernel and Gaussian standard settings resulted in lower %CV values in both NC and SC ^99mTc and ¹²³I 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 ^99mTc/¹²³I 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 ^99mTc/¹²³I ratios are very high or low.