PET Based Volume Segmentation with Emphasis on the Iterative TrueX Algorithm

Purpose: To assess the influence of reconstruction algorithms for positron emission tomography (PET) based volume quantification. The specifically detected activity in the threshold defined volume was investigated for different reconstruction algorithms as a function of volume size and signal to background ratio (SBR), especially for volumes smaller than 1ml. Special attention was given to the Siemens specific iterative reconstruction algorithm TrueX.

Methods: Measurements were performed with a modified in-house produced IEC body phantom on a Siemens Biograph 64 True Point PET/CT scanner (Siemens, Medical Systems) for six different SBRs (2.1, 3.8, 4.9, 6.7, 8.9, 9.4 and without active background (BG)). The phantom consisted of a water-filled cavity with built-in plastic spheres (0.27, 0.52, 1.15, 2.57, 5.58 and 11.49ml). The following reconstruction algorithms available on the Siemens Syngo workstation were evaluated: Iterative OSEM (OSEM) (4 iterations, 21 subsets), iterative TrueX (TrueX) (4 iterations, 21 subsets) and filtered backprojection (FBP). For the threshold based volume segmentation the software Rover (ABX, Dresden) was used.

Results: For spheres larger than 2.5ml a constant threshold (standard deviation (SD) 10%) level was found for a given SBR and reconstruction algorithm and therefore a mean threshold for the largest three spheres was calculated. This threshold could be approximated by a function inversely proportional to the SBR. The threshold decreased with increasing SBR for all sphere sizes. For the OSEM algorithm the threshold for small spheres with 0.27, 0.52 and 1.15ml varied between 17% and 44% (depending on sphere size). The threshold for the TrueX algorithm was substantially lower (up to 17%) than for the OSEM algorithm for all sphere sizes. The maximum activity in a specific volume yielded the true activity for the OSEM algorithm when using a SBR independent correction factor C, which depended on sphere size. For the largest three volumes a constant factor C=1.10±0.03 was found. For smaller volumes, C increased exponentially due to the partial volume effect. For the TrueX algorithm the maximum activity overestimated the true activity.

Conclusion: The threshold values for PET based target volume segmentation increased with increasing sphere size for all tested algorithms. True activity values of spheres in the phantom could be extracted using experimentally determined correction factors C. The TrueX algorithm has to be used carefully for quantitative comparison (e.g. follow-up) and multicenter studies.