Underestimation of ⁶⁸Ga PET/CT SUV caused by activity overestimation using default calibrator settings

A PET/CT scan of a uniformly filled ⁶⁸Ga phantom resulted in an unexpectedly low SUV_mean of 0.88. A potential contributing cause of underestimation of ⁶⁸Ga SUV is overestimation of ⁶⁸Ga activity in the Radionuclide Calibrator associated with the PET/CT scanner. To investigate this, a Radionuclide Calibrator cross-calibration exercise was performed.

A source of ⁶⁸Ga was measured in 5 Capintec CRC-55tR calibrators using the pre-set ⁶⁸Ga calibrator factor of 416, and a Veenstra VDC-505 calibrator using a factor recommended by the manufacturer. The source was then measured in an externally located Fidelis Secondary Standard Radionuclide Calibrator. Manual adjustments were made to the Capintec calibrator factors to match the decay corrected Fidelis measurement, followed by a repeat PET/CT scan of a uniform ⁶⁸Ga phantom.

The cross-calibration results showed that the 5 Capintec calibrators systematically overestimated ⁶⁸Ga activity by 7.8-9.4% (mean 8.5%) compared to the Fidelis. The calibrator factors were adjusted to 456-464 to match the Fidelis measurement, and the repeat phantom scan resulted in a SUV_mean of 0.97, within the local tolerance of 1.00 ± 5%. The result for the Veenstra calibrator was within the tolerance of ±5%.

Underestimation of ⁶⁸Ga SUV was primarily caused by overestimation of ⁶⁸Ga activity using the pre-set calibrator factor setting on a Capintec CRC-55tR. Improvement in quantification accuracy was achieved by adjusting the ⁶⁸Ga calibrator factor based upon a cross-calibration exercise. We recommend SUV checks using a uniform phantom and regular calibrator cross-calibration exercises for all isotopes used for quantitative PET/CT imaging.