Segmentation of PET Volumes by Iterative Image

The segmentation of metastatic volumes in PET is usually per-formed by thresholding methods. In a clinical application, the optimum threshold obtained from the adaptive thresholding method requires a priori estimation of the lesion volume from anatomic images such asCT.Wedescribe an iterative threshold-ing method (ITM) used to estimate the PET volumes without an-atomic a priori knowledge and its application to clinical images. Methods: The ITM is based on threshold-volume curves at varying source-to-background (S/B) ratio acquired from a body phantom. The spheres and background were filled either with 18F-FDG or Na124I (124I). These calibrated S/B-threshold-volume curves were used in estimating the volume by applying an itera-tive procedure. The ITM was validated with a PET phantom con-taining spheres andwith 39 PET tumors that were discernable on CT by using whole-body 18F-FDG (15 patients) and 124I PET/CT (9 patients): The measured S/B ratios of the lesions were esti-mated from PET images, and their volumes were iteratively cal-culated using the calibrated S/B-threshold-volume curves. The resulting PET volumes were then compared with the known sphere inner volume and CT volumes of tumors that served as gold standards. Results: Phantom data analysis showed that the S/B-threshold-volume curves of 18F-FDG and 124I were sim-ilar. The average absolute deviation (expressed as a percentage of the expected volume) obtained in the PET validation phantom was 10 % for volumes larger than 1.0 mL; sphere volumes of 0.5 mL showed a significantly larger deviation. For patients, the average absolute deviation for volumes between 0.8 and 7.5 mL was about 9 % (31 lesions), whereas volumes larger than 7.5 mL showed an average volume mismatch of 15 % (8 le-sions). Conclusion: The ITM sufficiently estimated the clinical volumes in the range of 0.8–7.5 mL; volumes larger than 7.5 mL showed greater deviations that were still acceptable. These findings are associated with the limitation of the ITM. The ITM is especially useful for lesions that are only visible on PET. As a consequence, the lesion dosimetry is feasible with suf-ficient accuracy using PET images only