Simplified Quantification and Acquisition Protocol for ¹²³I-MIBG Dynamic SPECT

Previous studies have demonstrated the feasibility of absolute quantification of dynamic ¹²³I-metaiodobenzylguanidine (¹²³I-MIBG) SPECT imaging in humans. This work reports a simplified quantification method for dynamic ¹²³I-MIBG SPECT using practical protocols with shortened acquisition time and voxel-by-voxel parametric imaging. Methods: Twelve healthy human volunteers underwent five 15-min dynamic SPECT scans at 0, 15, 90, 120, and 180 min after ¹²³I-MIBG injection. List-mode SPECT data were binned into 29 frames and reconstructed with corrections for attenuation, scatter, and decay. Population-based blood-to-plasma correction and metabolite correction were applied to the image-derived input function. Likelihood estimation in graphical analysis (LEGA) was used as a simplified model to obtain volume of distribution (V_T) values, which were compared with those obtained with the reversible 2-tissue (2T) compartment model. Three simplified protocols were evaluated with 2T and LEGA using a 30-min scan started simultaneously with tracer injection plus a 15-min scan at 90, 120, or 180 min after injection. Voxel-by-voxel LEGA fitting was applied to the aligned dynamic images using both the full protocol (five 15-min scans) and the simplified protocols. Results: Correlation analysis (y = 0.955x + 0.547, R² = 0.997) and Bland-Altman plot (mean difference, -0.8 mL/cm³; 95% limits of agreement, [-2.5, 1.0] mL/cm³; normal V_T range, 29.0 ± 12.4 mL/cm³) showed that LEGA can be used as a simplified model of 2T for ¹²³I-MIBG. High-quality V_T parametric images could be obtained with LEGA. Region-of-interest (ROI) modeling and parametric imaging results were in excellent agreement as determined by correlation analysis (y = 0.999x - 1.026, R² = 0.982) and Bland-Altman plot (mean difference, -1.0 mL/cm³; 95% limits of agreement, [-4.2, 2.1] mL/cm³). V_T correlated reasonably well between all simplified protocols and the full protocol with LEGA but not with 2T. The V_T results were more reliable when there was a longer interval between the 2 acquisitions in the simplified protocols. Conclusion: For ROI-based kinetic modeling and parametric imaging, reliable quantification of dynamic ¹²³I-MIBG SPECT can be achieved with LEGA using a simplified protocol of a 30-min scan starting with tracer injection plus a 15-min scan no earlier than 180 min after injection.