Bias and precision of SPECT-based ¹⁷⁷Lu activity-concentration estimation using a ring-configured solid-state versus...

The aim was to compare bias and precision for ¹⁷⁷Lu-SPECT activity-concentration estimation using a dual-headed Anger SPECT system and a ring-configured CZT SPECT system. This was investigated for imaging at 208 keV and 113 keV, respectively.

Phantom experiments were performed on a GE Discovery 670 system with 5/8'' NaI(Tl) crystal (dual-headed Anger system) and a GE StarGuide (ring-configured CZT system). Six spheres (1.2 mL to 113 mL) in a NEMA PET body phantom were filled with ^99mTc and ¹⁷⁷Lu, separately. Mean relative errors and coefficients of variation (CV) in estimated sphere activity concentration were studied over six timeframes of 10 min each for the two systems. For ¹⁷⁷Lu, similar acquisitions were also performed for an anthropomorphic phantom with two spheres (10 mL and 25 mL) in a liver with non-radioactive background and a sphere-to-background ratio of 15:1. Tomographic reconstruction was performed using OS-EM with 10 subsets with compensation for attenuation, scatter, and distance-dependent spatial resolution. For the Anger system, up to 40 iterations were used and for the ring-configured CZT system up to 30 iterations were used.

The two systems showed similar mean relative errors and CVs for ¹⁷⁷Lu when using an energy window around 208 keV, while the ring-configured system demonstrated a lower bias for a similar CV compared to the Anger system for ^99mTc and for ¹⁷⁷Lu when using an energy window around 113 keV. However, total activity in the phantom tended to be overestimated in both systems for these cases.

The ring-configured CZT system is a viable alternative to the dual-headed Anger system equipped with medium-energy collimators for ¹⁷⁷Lu-SPECT and shows a potential advantage for activity-concentration estimation when operated at 113 keV. However, further consideration of the preservation of total activity is warranted.