Abstract
68Ga-DOTATOC and 68Ga-DOTATATE are radiolabelled somatostatin analogs used for diagnosis of somatostatin receptor expressing neuroendocrine tumors (NETs) and SUV –measurements are suggested for treatment monitoring. However, changes in net-influx rate (Ki) may better reflect treatment effects than those of the SUV, and accordingly there is a need to compute parametric images showing Ki at the voxel level. The aim of this study was to evaluate parametric methods for computation of parametric Ki images by comparison to volume of interest 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 post injection of 68Ga-DOTATOC and 68Ga-DOTATATE on consecutive days. Parametric Ki images were computed using a basis function method (BFM) implementation of the two tissue irreversible compartment model and the Patlak method using a descending aorta image-derived input function, and mean tumor Ki values were determined for 50% isocontour VOIs and compared to Ki values based on non-linear regression (NLR) of the whole-VOI time-activity curve. A subsample of healthy liver was delineated in the whole-body and Ki images and tumor-to-liver ratios were calculated in order to evaluate image contrast. Correlation and agreement between VOI-based and parametric Ki values were assessed using regression and Bland-Altman analysis. Results: Correlation (R2) between NLR-based and parametric image-based (BFM) tumor Ki values was 0.98 (slope 0.81) and 0.97 (slope 0.88) for 68Ga-DOTATOC and 68Ga DOTATATE, respectively. For Patlak analysis, correlation between NLR-based and parametric based (Patlak) tumor Ki were 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 Ki-values. Tumor-to-liver contrast was 1.6 and 2.0 times higher in the parametric BFM-Ki 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 correlation and agreement between NLR- and parametric based Ki values was found, showing that parametric net influx rate images are quantitatively accurate. In addition, tumor-to-liver contrast was superior in the parametric Ki images compared to whole-body images both for 68Ga-DOTATOC and 68Ga DOTATATE.
- Image Processing
- Neuroendocrine
- PET/CT
- 68Ga-DOTATATE
- 68Ga-DOTATOC
- Net influx rate
- Neuroendocrine tumors
- Parametric images
- Copyright © 2016 by the Society of Nuclear Medicine and Molecular Imaging, Inc.