Respiratory-gated PET/CT versus delayed images for the quantitative evaluation of lower pulmonary and hepatic lesions

Respiratory motion degrades fluorodeoxyglucose positron emission tomography (FDG PET) images of the lower chest and upper abdomen, as the blur introduced by breathing motion increases the apparent size of the moving tumour lesions and decreases their apparent uptake, reducing the sensitivity of PET in detection of small lesions. We assessed the role of delayed and respiratory-gated PET acquisition in the quantitative evaluation of lung and liver lesions. A retrospective analysis of 64 lesions was performed. After initial non-gated whole-body PET/CT, respiratory gating was performed with 15 min in list mode. Non-gated delayed images were obtained by summing all list mode data. SUVmax adjusted for lean body mass (SULmax) was measured in the initial whole-body scan, the delayed non-gated scans and the individual gated bins for each lesion. The axial z-position of SULmax for each lesion in five respiratory-gated bins was determined. The mean SUL of the non-pathological liver parenchyma was also recorded for each patient. Tumour lesion SULmax increased by an average of 34% in the delayed non-gated scan as compared with the whole-body initial scan and further by an additional 17.2% in respiratory-gated images. The maximum lesion displacement was 6.2 ± 5.0mm. Delayed imaging alone substantially increases the magnitude of the SUL of liver and lung lesions as compared with standard whole-body images and may allow for a more accurate definition of the lesion's volume and localisation and improve tracer quantitation in malignant lesions in the lungs or upper abdomen. While respiratory gating provides more optimal imaging with greatest increase in SULmax, the benefit is small, and delayed imaging appears sufficient in most cases.