Principal component analysis (PCA)-based radionuclide separation in simultaneous dual-isotope ²⁰¹Tl/¹²³I cardiac imaging

Abstract

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Objectives: Dual-isotope ²⁰¹Tl chloride/¹²³I MIBG scanning allows simultaneous assessment of myocardial perfusion and innervation. However, accuracy is limited by cross-talk between the two radionuclides. This study investigated a PCA-based method of creating ²⁰¹Tl and ¹²³I photopeak images with suppression of cross-talk contamination. Methods: A six-energy-window (SEW) acquisition was defined, to capture photopeak (¹²³I 159 keV γ, ²⁰¹Hg X-ray) events, and contamination events (Compton scatter, downscatter, septal penetration, Pb X-rays) present in the photopeak images (in keV: 50-60, 60-80, 80-95, 96-142, 143-175 and 200-300). Scans of a torso phantom containing myocardial inserts filled with either ²⁰¹Tl, ¹²³I or both, were acquired for 10 minutes into 256×256 matrices, through low-energy (LEHR) and medium-energy (MELP) collimation. For each collimator, PCA of the SEW images from a scan with two spatially separated inserts, each filled with one of the radionuclides, was used to determine the linear combinations of PCs that produced images of either the ²⁰¹Tl insert or ¹²³I insert alone. PCA was then applied to SEW scans of the same insert filled with either ²⁰¹Tl alone, ¹²³I alone, or both in roughly equivalent amounts, to assess whether those linear combinations produced separated ²⁰¹Tl and ¹²³I images. Results: PCA yielded four significant components for both collimators. For LEHR, ²⁰¹Tl was generated from the mean image + 1.0×PC1 + 1.0×PC2 - 0.3×PC3 - 1.2×PC4, and ¹²³I from the mean + 0.9×PC1 - 2.0×PC2 - 3.0×PC3 + 2.0×PC4. For MELP, the images were mean + 1.0×PC1 + 0.32×PC2 + 1.0×PC4 and mean + 0.13×PC1 - 1.0×PC2, respectively. As expected, for the ²⁰¹Tl-only scans, reconstructions of ¹²³I contained comparatively few counts, and vice versa. The ²⁰¹Tl and ¹²³I reconstructions were of comparable intensity for the scans containing both. Compared to the original photopeak images, contamination events appeared substantially reduced. Conclusions: PCA of multiple-energy-window, dual-isotope ²⁰¹Tl/¹²³I scans appears capable of producing highly-separated and contamination-suppressed ²⁰¹Tl and ¹²³I images, potentially improving accuracy of the technique.