Temporal and Axial Quantitative Uniformity Measurements of Total-body PET Systems (2021)

3041 Objectives: The ability of positron emission tomography (PET) scanners to accurately reproduce standardized uptake values (SUV) serves as an important imaging biomarker in oncologic studies. We aimed to develop a practical quality control test to verify that an uEXPLORER total-body PET system produces accurate SUVs between and after calibrations and across the 194.0 cm axial field-of-view (AFOV). A 30 cm axial length germanium-68 (Ge-68) cylindrical phantom cross-calibrated with a fluorine-18 (F-18) cylindrical uniformity phantom is used to periodically measure and flag changes in SUVs across the entire AFOV.
 Methods: Following a semi-annual calibration of the total-body PET/CT system, an F-18/Ge-68 cross calibration was performed. A 9,175 mL cylindrical uniformity phantom with a 30 cm axial length and 20 cm diameter, was filled with 48.3 MBq of F-18. The F-18 phantom was positioned horizontally on the PET/CT scanner bed, iso-centered in the X and Y plane. Five-minute images were acquired at three positions within the 194.0 cm AFOV of the uEXPLORER PET/CT scanner at ¼ FOV, ½ FOV and ¾ FOV. Images were reconstructed using a conventional clinical protocol and SUVs were measured from spherical regions of interests (ROI) for the three acquisitions. Applying the same acquisition protocol, a Ge-68 cylinder containing 52.8 MBq with the same dimensions (30 cm long x 20 cm diameter) was acquired. Cross-calibration was achieved by determining the Ge-68 volume/mass by using the equation: Ge-68 scaled SUV mass = (SUV of F18 x F-18 mass)/SUV of Ge-68. The determined Ge-68 mass and the decay corrected activity of the Ge-68 phantom can now be applied for future acquisitions. Verification of the cross-calibration between the F-18 phantom and the Ge-68 phantom is achieved by re-acquiring the Ge-68 phantom with the new Ge-68 mass. Measurements of SUVs from the Ge-68 phantom should now be identical to the F-18 phantom, and SUVs can be plotted and periodically tracked to show axial and temporal uniformity. We measured SUV at the three axial positions weekly for 16 weeks, using the Ge-68 phantom. Applying the mean SUV results from the three axial positions, an axial profile demonstrated the range of values along the AFOV in-between calibrations. The axial profile was applied to track the change and improvement due to a new normalization method implemented by the vendor. The new method uses a short cylinder extended away from the patient bed with uniform activity. This is then continuously moved through the full AFOV and results in a more uniform normalization.
 Results: In figure 1(A), temporal quantification change was plotted for all three axial positions during the 16 weeks of testing. Between weeks 7 and 8, semi-annual calibration was completed, and the new normalization method was implemented, which caused a 1.5% increase in SUV at the center position and a 3% increase at the axial ends. The temporal uniformity before calibration (7 weeks) and after calibration (9 weeks) was within 1.5% for all axial positions. In figure 1(B), the mean SUVs, before and after calibration, is plotted for 3 positions at ¼, ½ and ¾ FOV. The improved normalization method after calibration of the scanner illustrated a uniform axial profile with less than 1% deviation across the AFOV.
 Conclusions: A cross calibrated phantom protocol can provide an accurate and efficient way to periodically verify that the quantification of a total-body PET/CT scanner are within quality control standards. The observed temporal uniformity between calibrations during the first 16 weeks of measurements showed variation of ~1%. Additionally, the procedure revealed a quantification change from the calibration between the routine semi-annual F-18 calibrations when applying a new improved normalization method. The axial uniformity analysis illustrated the effect of the improved normalization method implemented during calibration. This work demonstrates the need to test calibration in more than one axial location for long axial FOV scanners.