Quantitative accuracy of ¹⁷⁷Lu SPECT/CT imaging using ring-shaped CZT versus dual-head NaI systems

Ring-shaped systems based on CZT are promising for quantitative imaging, but their accuracy may be affected by septal penetration of high-energy photons-given the fixed collimator-and by limitations in scatter correction due to low-energy tailing. This study compares the quantitative performance of a ring-shaped CZT system with that of a conventional dual-head NaI SPECT/CT system (StarGuide™ and NM/CT 870 DR, both GE HealthCare) using the 208 keV photopeak of ¹⁷⁷Lu.

A NEMA phantom with 3D-printed anthropomorphic inserts, including kidneys and lumbar vertebrae, as well as standard spherical inserts, was filled with activity concentrations mimicking the distribution observed in patients 1 day post-injection of 7.4 GBq of [¹⁷⁷Lu]Lu-PSMA-I&T. The phantom was scanned repeatedly on both systems (scan duration/bed position: 7 min for StarGuide™ and 20 min for NM/CT 870 DR). Quantitative accuracy and reproducibility were evaluated using the mean error and the coefficient of variation (CV) over repeated measurements, respectively. Additionally, a group of patients treated with [¹⁷⁷Lu]Lu-PSMA-I&T underwent sequential SPECT/CT on both systems to confirm the phantom-based results.

The total activity measured within the phantom deviated from the expected value by means of 13% for the StarGuide™ and 0% for the NM/CT 870 DR. After partial-volume-effect correction based on recovery coefficients, the activity concentration in kidney inserts was measured with ~ 5% accuracy on both systems. For the StarGuide™, mean errors in tumour inserts (volumes: 2.6-28 ml) ranged from - 8% to 3%, whereas a consistent underestimation by an average of - 11% was observed for the NM/CT 870 DR. The StarGuide™ significantly overestimated the background activity concentration (59%), while the NM/CT 870 DR showed a smaller error (- 8%). Reproducibility was lower for the StarGuide™, with maximum CV values of 7%, compared with 3% for the NM/CT 870 DR. Clinical findings were consistent with phantom results.

The StarGuide™ provided slightly more accurate activity quantification in regions of high relative uptake, such as tumours and kidneys, compared with the NM/CT 870 DR. However, quantification in regions of low relative uptake and total activity estimation were less accurate, likely due to septal penetration. The lower reproducibility suggests that longer scan durations may be considered.