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

PURPOSE

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

METHODS

A source of Ga was measured in 5 Capintec CRC-55tR calibrators using the pre-set Ga 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 Ga phantom.

RESULTS

The cross-calibration results showed that the 5 Capintec calibrators systematically overestimated Ga 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 of 0.97, within the local tolerance of 1.00 ± 5%. The result for the Veenstra calibrator was within the tolerance of ±5%.

CONCLUSIONS

Underestimation of Ga SUV was primarily caused by overestimation of Ga activity using the pre-set calibrator factor setting on a Capintec CRC-55tR. Improvement in quantification accuracy was achieved by adjusting the Ga 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.