The impact of time-of-flight acquisition on PET/CT standardized uptake values

591 Objectives The improvement of PET/CT image quality provided by time-of-flight (TOF) acquisition also expands its usefulness in initial staging of disease and in response assessment. However the direct impact of the addition of TOF on quantification may affect the accuracy of such tumor response evaluations. We investigated the variation in quantification among TOF and non-TOF acquisition and reconstruction in clinical data sets. Methods PET/CT scans were collected for 100 consecutive patients on a Siemens Biograph 64 mCT PET/CT and reconstructed according to the standard local protocol, a 3D OSEM reconstruction with 3 iterations and 24 subsets with the exclusion of TOF information. The scan data were then reconstructed a second time with the inclusion of TOF information. Identical and matched regions of interest (ROIs) were placed on target lesions, as well as background tissues, of both reconstructed PET data sets. SUVmax was recorded for each ROI. Results For target lesions the average change in SUVmax with the addition of TOF was a 10.3 ± 16.2% increase from standard reconstructed scan values. However there was markedly wide variation among individual lesion measurements, with the differences ranging from a 103.9% decrease from the non-TOF value to an 8.7% increase. The discrepancy decreased with increasing lesion uptake levels as well as tumor volume. The average percent differences in each of the background tissues, heart (5.5%), bladder (3.0%), and liver (4.7%) were considerably lower than in target lesions, and also appeared less dependent on volume and activity concentration for these tissues. Conclusions PET/CT images acquired with TOF information compared to without demonstrated wide variation when comparing individual target lesion measurements. These variations were dependent upon both lesion volume and uptake. While TOF acquisition appears to be the standard going forward, careful consideration must then be given when comparing differently acquired and reconstructed data sets in order to ensure accurate quantitative and therapy response assessment. Research Support Wright Project TECH 10-012 and OIRAIN TECH 09-028