Establishing measurement traceability for quantitative SPECT imaging.

Background: Single Photon Emission Computed Tomography (SPECT) is increasingly used as a quantitative modality, especially in the context of Molecular Radiotherapy, where the measurements are used as input to absorbed dose calculations for patient-specific dosimetry. Establishing measurement traceability is an essential step in providing confidence in quantitative measurements. This requires an unbroken chain of calibrations where uncertainties must be reported in all stages of calibration and for the final measurement result. Traceability ensures that a measurement result can be related to an underlying standard, allowing harmonisation of data, and facilitating comparison of results between sites.

Methods: The process of establishing measurement traceability for quantitative SPECT is demonstrated for the therapeutic radionuclide ¹⁷⁷Lu using a common, phantom based, calibration method. Phantoms with activities of ¹⁷⁷Lu, measured using a traceably calibrated radionuclide calibrator, were used to perform the calibration. The calibration was validated using 3D-printed anthropomorphic organ phantom inserts mimicking clinically relevant geometries. For all measurements, traceability to primary standards for radioactivity is demonstrated along with an accompanying calibration chain and statement of uncertainty.

Results: For all activity measurements the dominant component in the activity uncertainty budget was the uncertainty on the radionuclide calibrator calibration factor, resulting in an average combined standard uncertainty of 1.57%. The resulting uncertainty on the SPECT Image Calibration Factor was 1.6%. An optional additional correction was included in the calibration to provide volume-based partial volume correction (PVC). Measurement traceability was extended for measurands using this additional correction. The activity recovery in the organ phantoms with PVC applied was 96(7)% for both the kidney and spleen.

Conclusions: A manufacturer independent methodology for establishing measurement traceability for quantitative SPECT is demonstrated for ¹⁷⁷Lu, using a radionuclide calibrator previously calibrated against national standards. The ability to establish measurement traceability for quantitative SPECT using standard clinical equipment, and the limitations of traceability are presented.