Estimation of the lower limits for feasible Ra-223 SPECT imaging: a Monte Carlo simulation study.

Abstract

Ra-223 is a promising radionuclide for the treatment of skeletal metastases in castration-resistant prostate cancer patients. This study aims to estimate the lower limits for feasible Ra-223 single-photon emission computerized tomography (SPECT) imaging using a Monte Carlo simulation study. The SPECT images were produced on a homemade code: the Monte Carlo simulation of electrons and photons for SPECT (MCEP-SPECT). The National Electrical Manufacturers Association (NEMA) phantom with six hot spheres of diameters of 37, 28, 22, 17, 13, and 10 mm installed inside, was used. The background activity concentration was 0.6 kBq/mL, and the ratios of hot concentrations to the background (R HB) were 25, 20, 15, 10, and 5. When R HB was 15, the background concentrations of 1.5, 0.9, 0.3, and 0.15 kBq/mL were also tested. The energy window was 84 keV±10%. The number of projections was 60/360°, and the acquisition time was 60 s per projection. Two kinds of collimators: middle-energy general-purpose (MEGP) and high-energy general-purpose (HEGP), were examined. The SPECT images were evaluated based on two quantitative indexes: contrast-to-noise ratio (CNR) for detectability and contrast recovery coefficient (CRC) for quantitative accuracy. The CRC for the HEGP collimator was 35-40%, while the CRC for the MEGP collimator was 25-30%. The CNRs for the MEGP collimator were larger than those for the HEGP collimator. The CNRs of the hot spheres with diameters less than 22 mm were lower than 5.0 for both collimators, when R HB and the background concentration were 15 and 0.6 kBq/mL, respectively. Based on the obtained results, it was estimated that the lower limit of R HB for the detection of the hot sphere with a diameter of 37 mm would be approximately 20 if the background concentration is 0.05 kBq/mL. The MEGP collimator is superior in terms of detectability, while the HEGP collimator is superior in terms of quantitative accuracy. When the lesion size is small, the MEGP collimator may be favorable. Based on these results, the estimated lower limit of the activity concentration would be approximately 1 kBq/mL if the background concentration is 0.05 kBq/mL for a large lesion.