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

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).

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.

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.

Despite similar hardware, SPECT/CT systems' response can significantly differ at high count rate, which impacts their QSPECT capability in a post-therapeutic setting.