title: An improved iterative thresholding method to delineate PET volumes using the delineation-averaged signal instead of the enclosed maximum signal.
abstract: An improved thresholding method for segmentation of PET volumes is proposed that uses the delineation-averaged activity concentration (AC) within an iterative procedure instead of the enclosed maximum AC.
  
  
  METHODS
  The average-based iterative thresholding method (avg-ITM) needs the background-corrected relative boundary-reproducing threshold (BRT) curve applied to segment the tumor. On the basis of a previous study, which developed an iterative thresholding method using the maximum AC (max-ITM), the average-based BRT curve was derived from the AC profiles of a sphere model. Numerous phantom scans (using glass spheres, wax spheres, and bihemispheres) under different conditions were obtained to verify and assess the avg-ITM. Clinically, the avg-ITM was tested in 2 patients bearing tumors, and the interobserver and intraobserver variability in the volume segmentation was assessed using 5 tumors analyzed by 5 observers. In the entire study, the max-ITM was also applied to compare the respective results.
  
  
  RESULTS
  The phantom measurements verified the average-based BRT curve and demonstrated that spheric tumors down to a diameter equaling the spatial resolution could be delineated. In contrast to the max-ITM, the avg-ITM yielded reliable volumes (within the 95% confidence intervals) for standardized uptake values and signal-to-background ratios larger than 3. The volumes derived using the max-ITM were significantly underestimated for object sizes with diameters considerably larger than the spatial resolution. Phantom measurements using bihemispheres with decreasing AC ratios demonstrated that the avg-ITM was robust down to an AC ratio of 0.5 and, therefore, is less prone to nonuniformity than the max-ITM, which was corroborated by real tumor imaging. The observer-related variability was small (mean absolute relative SD < 4%).
  
  
  CONCLUSIONS
  Compared with the max-ITM, the avg-ITM improves the segmentation results for large objects and is less sensitive against image noise and nonuniformity. A clinical comparison study is warranted to assess the benefits of the avg-ITM method compared with other segmentation methods.
authors: Klinik für Nuklearmedizin, Universität Duisburg-Essen, Essen, Germany walter.jentzen@uni-duisburg-essen.de.
authors: Walter
authors: W
authors: Jentzen
mesh_terms: D000008:Abdominal Neoplasms / Q000000981:diagnostic imaging; D005240:Feasibility Studies; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D015588:Observer Variation; D019047:Phantoms, Imaging; D049268:Positron-Emission Tomography / Q000379:methods*; D015203:Reproducibility of Results; D013964:Thyroid Neoplasms / Q000000981:diagnostic imaging; D014057:Tomography, X-Ray Computed; D035501:Uncertainty
keywords: PET; delineation; segmentation; thresholding