Room: Room 202
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 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, 2-10mCi I-131 is administered. To avoid stunning effects, 2mCi is suggested. However, with 2mCi, images are noisy. Compared with standard single pinhole SPECT systems, 7-pinhole systems may provide greater geometric efficiency 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 and attenuation distributions were computer simulated using modified XCAT phantoms. 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 resolution, noise, and RMS error. Gamma camera sensitivity and spatial resolution were also assessed.
Results: Compared with single pinhole, the 7-pinhole system had better spatial resolution at a given sensitivity and provided better conspicuity, uniformity, contrast, and RMS error.
Conclusion: Compared with single pinhole, seven pinholes may offer higher sensitivity and better resolution. Higher sensitivity may allow lower diagnostic I-131 dose and potentially cause less stunning effect. Better resolution could better resolve the remnant/lymph node metastasis and thereby improve dosimetry.