Quantitative SPECT (QSPECT) at high count rates with contemporary SPECT/CT systems.

BACKGROUND: Accurate QSPECT is crucial in dosimetry-based, personalized radiopharmaceutical therapy with 177Lu and other radionuclides. We compared the quantitative performance of three NaI(Tl)-crystal SPECT/CT systems equipped with low-energy high-resolution collimators from two vendors (Siemens Symbia T6; GE Discovery 670 and NM/CT 870 DR). METHODS: Using up to 14 GBq of 99mTc in planar mode, we determined the calibration factor and dead-time constant under the assumption that these systems have a paralyzable behaviour. We monitored their response when one or both detectors were activated. QSPECT capability was validated by SPECT/CT imaging of a customized NEMA phantom containing up to 17 GBq of 99mTc. Acquisitions were reconstructed with a third-party ordered subset expectation maximization algorithm. RESULTS: The Siemens system had a higher calibration factor (100.0 cps/MBq) and a lower dead-time constant (0.49 µs) than those from GE (75.4-87.5 cps/MBq; 1.74 µs). Activities of up to 3.3 vs. 2.3-2.7 GBq, respectively, were quantifiable by QSPECT before the observed count rate plateaued or decreased. When used in single-detector mode, the QSPECT capability of the former system increased to 5.1 GBq, whereas that of the latter two systems remained independent of the detectors activation mode. CONCLUSION: Despite similar hardware, SPECT/CT systems' response can significantly differ at high count rate, which impacts their QSPECT capability in a post-therapeutic setting.