A demonstration of the effect of time of flight in clinical PET imaging.

3058 Objectives: Primarily limited to the research environment until approximately the last decade, the development of faster scintillators, improved light collection, ever faster electronics and computing power have brought commercially available Time of Flight (TOF) Positron Emission Tomography (PET) systems into routine clinical practice.TOF technique utilizes differences in gamma photon detection timing to calculate a more accurate localization of the point of annihilation along a line-of-response. The length of the resulting segment-of-response decreases in proportion to the improvement in timing resolution. The net effect is a reduction in image noise and increased image contrast due to increased effective sensitivity - TOF PET requires fewer ‘counts’ to provide the same image contrast as non-TOF imaging. The benefits of this method are especially relevant in larger patients, where the segment-of-response is shorter than the diameter of the patient. The objective of this study is to demonstrate the benefits of the TOF technique by directly comparing images reconstructed with and without TOF. Methods: Three patients’ exams, having been clinically imaged on a Siemens Biograph Vision PET-CT system, were identified as having metastatic, F-18 FDG avid lesions. The patients’ BMIs ranged from 25-36. Per clinical protocol, each was injected with approximately 8mCi of FDG and imaged one hour post injection. BMI based total imaging time ranged from 20-25 minutes and all data sets were reconstructed with a 3D-OSEM algorithm, 4 iterations, 5 subsets and TOF (4i5s-TOF). To demonstrate the benefits of TOF, these data sets were reconstructed without TOF (4i5s-noTOF). Considering that in the absence of TOF, image quality may be improved through modification of reconstruction parameters, a third reconstruction was performed - 8i5s-noTOF. As well as a visual assessment of image quality and lesion contrast, a metastatic lesion for each patient data set was chosen for quantitative analysis (Standardized Uptake Value - SUVmax) across each of the reconstructions. To further demonstrate the enhanced image contrast provided by TOF, a background measurement (SUVavg) of adjacent normal soft tissue was included for the patient with the largest BMI. Results: For each patient, differences in reconstruction methods are immediately apparent. TOF images demonstrate increased lesion contrast (detectability) and decreased background and noise (i.e. reduced lung counts). Lesion intensity (SUVmax) with TOF is increased for each patient as counts are more accurately localized. For the largest patient, a lesion clearly visible on the TOF reconstruction becomes nearly indistinguishable from background on noTOF constructions (lesion to background ratio 89% greater with TOF versus 8i5s-noTOF). The lesion contrast with TOF was less significant for the smallest BMI patient, but still visually apparent. Results from the SUV measurements can be found in Table 1. Conclusions: In each patient, lesion contrast was considerably enhanced with TOF. This is most significantly demonstrated with the largest patient. Without TOF, the lesion nearly disappears into the background.