The CT motion quantitation of lung lesions and its impact on PET-measured SUVs.

UNLABELLED: We previously reported that respiratory motion is a major source of error in quantitation of lesion activity using combined PET/CT units. CT acquisition of the lesion occurs in seconds, rather than the 4-6 min required for PET emission scans. Therefore, an incongruent lesion position during CT acquisition will bias activity estimates using PET. In this study, we systematically analyzed the range of activity concentration changes, hence SUV, for lung lesions.

METHODS: Five lung cancer patients were scanned with PET/CT. In CT, data were acquired in correlation with the real-time positioning. CT images were acquired, in cine mode, at 0.45-s intervals for slightly longer (1 s) than a full respiratory cycle at each couch position. Other scanning parameters were a 0.5-s gantry rotation, 140 kVp, 175 mA, 10-mm couch increments, and a 2.5-mm slice thickness. PET data were acquired after intravenous injection of about 444-555 MBq of (18)F-FDG with a 1-h uptake period. The scanning time was 3 min per bed position for PET. Regularity in breathing was assisted by audio coaching. A commercial software program was then used to sort the acquired CT images into 10 phases, with 0% corresponding to end of inspiration (EI) and 50% corresponding to end of expiration (EE). Using the respiration-correlated CT data, images were rebinned to match the PET slice locations and thickness.

RESULTS: We analyzed 8 lesions from 5 patients. Reconstructed PET emission data showed up to a 24% variation in the lesion maximum standardized uptake values (SUVs) between EI and EE phases. Examination of all the phases showed an SUV variation of up to 30%. Also, in some cases the lesion showed up to a 9-mm shift in location and up to a 21% reduction in size when measured from PET during the EI phase, compared with during the EE phase.

CONCLUSION: Using respiration-correlated CT for attenuation correction, we were able to quantitate the fluctuations in PET SUVs. Because those changes may lead to estimates of lower SUVs, the respiratory phase during CT transmission scanning needs to be measured or lung motion has to be regulated for imaging lung cancer in routine clinical practice.