Quantitative SPECT
Modern gamma-camera scans can now be turned into maps of exactly how much radioactivity (in becquerels) sits in each part of the body, not just a picture, which is what lets doctors calculate a real radiation dose to a tumour or organ.
Overview
Conventional SPECT reconstructs relative counts per pixel; quantitative SPECT (qSPECT) converts those counts into absolute activity concentration, expressed in becquerels per millilitre (Bq/mL), by combining CT-based attenuation correction, scatter correction and collimator/detector resolution-recovery within an iterative reconstruction (typically ordered-subset expectation maximisation) and then applying a system-specific calibration factor. Attenuation correction requires a co-registered CT-derived attenuation map covering the full photon path length; scatter correction commonly uses dual- or triple-energy-window methods, with the choice of scatter-correction weighting shown to materially affect quantitative accuracy (activity underestimation of roughly 8-22% has been reported when using default rather than object-optimised scatter weighting). Cross-calibration between the gamma camera and an independent, traceable radionuclide (dose) calibrator, verified with a uniform cylindrical phantom, is required before patient images can be trusted quantitatively.
The 2022 EANM practice guideline for quantitative SPECT/CT recommends accuracy targets of 5-10% for diagnostic and about 5% for therapeutic radionuclides, and specifies phantom-based validation of recovery coefficients for small-volume accuracy (partial-volume effects remain the dominant source of negative bias for small lesions even with full correction). With these corrections in place, reconstructed qSPECT images at 24-iteration-equivalent settings can achieve activity-concentration accuracy within about 10% of true values for spheres down to roughly 11-12 mL. Quantitative SPECT is the imaging foundation for post-therapy dosimetry of 177Lu-, 90Y- and other radionuclide therapies and for deriving SUV-like metrics from SPECT rather than PET.
How it works
Raw SPECT counts are corrected, within the image reconstruction, for photon attenuation using a co-registered CT map, for scattered photons using multiple energy windows, and for the camera's own blurring; the corrected counts are then converted to absolute activity concentration (becquerels per millilitre) using a calibration factor measured against a traceable radionuclide calibrator and a phantom of known activity.
Dependencies
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Latest papers
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