Parametric Net Influx Rate Images of ⁶⁸Ga-DOTATOC and ⁶⁸Ga-DOTATATE: Quantitative Accuracy and Improved Image...

⁶⁸Ga-DOTATOC and ⁶⁸Ga-DOTATATE are radiolabeled somatostatin analogs used for the diagnosis of somatostatin receptor-expressing neuroendocrine tumors (NETs), and SUV measurements are suggested for treatment monitoring. However, changes in net influx rate (K_i) may better reflect treatment effects than those of the SUV, and accordingly there is a need to compute parametric images showing K_i at the voxel level. The aim of this study was to evaluate parametric methods for computation of parametric K_i images by comparison to volume of interest (VOI)-based methods and to assess image contrast in terms of tumor-to-liver ratio. Methods: Ten patients with metastatic NETs underwent a 45-min dynamic PET examination followed by whole-body PET/CT at 1 h after injection of ⁶⁸Ga-DOTATOC and ⁶⁸Ga-DOTATATE on consecutive days. Parametric K_i images were computed using a basis function method (BFM) implementation of the 2-tissue-irreversible-compartment model and the Patlak method using a descending aorta image-derived input function, and mean tumor K_i values were determined for 50% isocontour VOIs and compared with K_i values based on nonlinear regression (NLR) of the whole-VOI time-activity curve. A subsample of healthy liver was delineated in the whole-body and K_i images, and tumor-to-liver ratios were calculated to evaluate image contrast. Correlation (R²) and agreement between VOI-based and parametric K_i values were assessed using regression and Bland-Altman analysis. Results: The R² between NLR-based and parametric image-based (BFM) tumor K_i values was 0.98 (slope, 0.81) and 0.97 (slope, 0.88) for ⁶⁸Ga-DOTATOC and ⁶⁸Ga-DOTATATE, respectively. For Patlak analysis, the R² between NLR-based and parametric-based (Patlak) tumor K_i was 0.95 (slope, 0.71) and 0.92 (slope, 0.74) for ⁶⁸Ga-DOTATOC and ⁶⁸Ga-DOTATATE, respectively. There was no bias between NLR and parametric-based K_i values. Tumor-to-liver contrast was 1.6 and 2.0 times higher in the parametric BFM K_i images and 2.3 and 3.0 times in the Patlak images than in the whole-body images for ⁶⁸Ga-DOTATOC and ⁶⁸Ga-DOTATATE, respectively. Conclusion: A high R² and agreement between NLR- and parametric-based K_i values was found, showing that K_i images are quantitatively accurate. In addition, tumor-to-liver contrast was superior in the parametric K_i images compared with whole-body images for both ⁶⁸Ga-DOTATOC and ⁶⁸Ga DOTATATE.