SEGMENTATION OF LUNG LESION VOLUME BY ADAPTIVE POSITRON-EMISSION-TOMOGRAPHY IMAGE THRESHOLDING

BACKGROUND. It is common protocol in radionuclide therapies to adminis ter a tracer dose of a radiopharmaceutical, determine its lesion uptak e and biodistribution by gamma imaging, and then use this information to determine the most effective therapeutic dose. This treatment plann ing approach can be used to quantitate accurately the activity and vol ume of lesions and organs with positron emission tomography (PET). In this article, the authors focus on the specification of appropriate vo lumes of interest (Vol) using PET in association with computed tomogra phy (CT). METHODS. The authors have developed an automatic image segme ntation schema to determine the Vol of metastases to the lung from PET images, under conditions of variable background activity. An elliptic al Jaszczak phantom containing a set of spheres with volumes ranging f rom 0.4 to 5.5 mL was filled with F-18 activity (2-3 mu Ci/mL) corresp onding to activities clinically observed in lung lesions. Images were acquired with a cold background and then with variable source-to-backg round (S/B) ratios of: 7.4, 5.5, 3.1, and 2.8. Lesion Vol analysis was performed on 10 patients with 17 primary or metastatic lung lesions, applying the optimum threshold values derived from the phantom experim ents. Initial volume estimates for lung lesions were determined from C T images. Approximate S/B ratios were obtained for the corresponding l esions on F-18-fluoro-2-deoxy-D-glucose ((18)FDG)-PET images. From the CT estimate of the lesion size and the PET estimate of the S/B ratio, the appropriate optimum threshold could be chosen. The threshold was applied to the PET images to obtain lesion activity and a final estima te of the lesion volume. RESULTS. Phantom data analysis showed that im age segmentation converged to a fixed threshold value (from 36% to 44% ) for sphere volumes larger than 4 mL, with the exact value depending on the S/B ratios. For patients, the use of optimum threshold schema d emonstrated a good correlation (r = 0.999) between the initial volume from CT and the final volume derived from the (18)FDG-PET scan (P < 0. 02). The mean difference for those volumes was 8.4%. CONCLUSIONS. The adaptive thresholding method applied to PET scans enables the definiti on of tumor Vol, which hopefully leads to accurate tumor dosimetry. Th is method can also be applied to small lesions (<4 mL). It should enab le physicians to track objectively changes in disease status that coul d otherwise be obscured by the uncertainties in the region-of-interest drawing, even when the scans are delineated by the same physician. (C ) 1997 American Cancer Society.