Multi-Isotope Capabilities of a Small-Animal Multi-Pinhole SPECT System

The quantitative accuracy and image quality of multi-isotope SPECT is affected by various hardware-related perturbations. The present study evaluates the simultaneous acquisition of multiple isotopes using a multiplexed multi-pinhole SPECT system, assesses the extent of different error sources, and proposes experimental procedures for its objective characterization. Methods: Phantom measurements with single-, dual- and triple-isotope combinations of ^99mTc, ¹¹¹In, ¹²³I, ¹⁷⁷Lu, and ²⁰¹Tl were performed with the NanoSPECT/CT^PLUS to evaluate system energy resolution, count rate performance, sensitivity, collimator penetration, hardware versus object scatter, spectral crosstalk, spatial resolution, spatial registration accuracy, image uniformity, image noise, and image quality. Results: The intrinsic detector properties were suitable for the simultaneous acquisition of up to 3 isotopes with limitations for count rates exceeding 104 kcps and γ-energies lower than 75 keV. Spectral crosstalk between isotopes was more likely mediated by hardware than by source scatter and was strongly dependent on the isotope combination. Simultaneous multi-isotope acquisitions slightly degraded spatial resolution and image uniformity for spatially superimposed but not for spatially separated activity distributions while the background noise level was increased for all multi-isotope studies. For particular isotopes, collimator penetration and x-ray fluorescence contributed a significant portion of error. Conclusion: The NanoSPECT/CT^PLUS enables the simultaneous acquisition of 3 radioisotopes with high quantitative accuracy and only little loss of image quality when the activity ratio is adapted to isotope-specific count rate sensitivities and when the system calibration is performed with phantoms of appropriate size.