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

⁶⁸ Ga-DOTATOC and ⁶⁸ Ga-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 ⁶⁸ Ga-DOTATOC and ⁶⁸ Ga-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 ( R ² ) and agreement between VOI-based and parametric K _i values were assessed using regression and Bland-Altman analysis. Results: The R ² 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 ⁶⁸ Ga-DOTATOC and ⁶⁸ Ga-DOTATATE, respectively. For Patlak analysis, the R ² between NLR-based and parametric-based (Patlak) tumor K _i was 0.95 (slope, 0.71) and 0.92 (slope, 0.74) for ⁶⁸ Ga-DOTATOC and ⁶⁸ Ga-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 ⁶⁸ Ga-DOTATOC and ⁶⁸ Ga-DOTATATE, respectively. Conclusion: A high R ² 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 ⁶⁸ Ga-DOTATOC and ⁶⁸ Ga DOTATATE.
(© 2017 by the Society of Nuclear Medicine and Molecular Imaging.)