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

Abstract

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 (<i>K</i>_i) may better reflect treatment effects than those of the SUV, and accordingly there is a need to compute parametric images showing <i>K</i>_i at the voxel level. The aim of this study was to evaluate parametric methods for computation of parametric <i>K</i>_i images by comparison to volume of interest (VOI)-based methods and to assess image contrast in terms of tumor-to-liver ratio. <b>Methods:</b> 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 <i>K</i>_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 <i>K</i>_i values were determined for 50% isocontour VOIs and compared with <i>K</i>_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 <i>K</i>_i images, and tumor-to-liver ratios were calculated to evaluate image contrast. Correlation (<i>R</i>²) and agreement between VOI-based and parametric <i>K</i>_i values were assessed using regression and Bland-Altman analysis. <b>Results:</b> The <i>R</i>² between NLR-based and parametric image-based (BFM) tumor <i>K</i>_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 <i>R</i>² between NLR-based and parametric-based (Patlak) tumor <i>K</i>_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 <i>K</i>_i values. Tumor-to-liver contrast was 1.6 and 2.0 times higher in the parametric BFM <i>K</i>_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. <b>Conclusion:</b> A high <i>R</i>² and agreement between NLR- and parametric-based <i>K</i>_i values was found, showing that <i>K</i>_i images are quantitatively accurate. In addition, tumor-to-liver contrast was superior in the parametric <i>K</i>_i images compared with whole-body images for both ⁶⁸Ga-DOTATOC and ⁶⁸Ga DOTATATE.