An Investigation in Quantitative Accuracy in Preablation I-131 Scans: 7-pinhole system compared with single-pinhole system.
Purpose: Early detection and prevention of differentiated thyroid cancer using thyroidectomy and ablation therapy can reduce disease persistence and recurrence. A preablation I-131 scan performed between the thyroidectomy and ablation therapy may improve patient management. Although I-123 would involve less radiation dose, I-131 is widely available, and its long half-life enables imaging 24-72 hours after injection, which is crucial for dosimetry and which enhances visibility of distant metastasis. Typically, 2-10 mCi I-131 is administered. To avoid stunning effects, 2 mCi is suggested. However, with 2 mCi, images are noisy. Compared with standard single pinhole SPECT systems, 7-pinhole systems may provide greater geometric efficiency when using smaller pinhole diameter and thereby reduce noise. Due to the size of 7-pinhole systems, collision constraints may, however, increase the pinhole radius of rotation (ROR), thereby reducing efficiency. Herein we assess the competing effects of more pinholes and larger ROR to determine whether 7 pinholes could meaningfully improve efficiency, at a comparable or better spatial resolution.
Methods: Radiotracer distributions and attenuation were computer simulated using modified XCAT phantom. Single-pinhole and 7-pinhole trajectories were developed to provide minimal RORs while avoiding collision with the patient. Reconstructed images were computer simulated, modeling attenuation, spatial resolution, and Poisson noise. Single-pinhole and 7-pinhole were compared for a range of lesion sizes and activity concentrations. Comparison metrics included lesion conspicuity, uniformity, and contrast; and image quality in terms of noise, contrast recovery and RMSE. Gamma camera sensitivity and spatial resolution were also assessed.
Results: In this study, seven-pinhole configurations were compared to a clinically typical single-pinhole system. In the low-count study, it was found that the seven-pinhole system with 4-5 mm pinhole diameter could outperform the benchmark single-pinhole system. In the high-count study, it was found that seven-pinhole system with 3 mm pinhole diameter could outperform the benchmark single-pinhole system. However, ROR increases are great enough to substantially decrease the benefit of seven pinholes, for the pinhole configuration considered here.
Conclusion: Seven pinhole maybe suitable for preablation scan because high sensitivity allows better detect the lesion with low activity concentration and smaller pinhole diameter allows better resolve the metastasis.
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