Feasibility and limitations of quantitative SPECT for ²²³Ra

Abstract

The aim of this paper is to investigate the feasibility and limitations of activity-concentration estimation for 223Ra using SPECT. Phantom measurements are performed using spheres (volumes 5.5 mL to 26.4 mL, concentrations 1.6 kBq mL−1 to 4.5 kBq mL−1). Furthermore, SPECT projections are simulated using the SIMIND Monte Carlo program for two geometries, one similar to the physical phantom and the other being an anthropomorphic phantom with added lesions (volumes 34 mL to 100 mL, concentrations 0.5 kBq mL−1 to 4 kBq mL−1). Medium-energy and high-energy collimators, 60 projections with 55 s per projection and a 20% energy window at 82 keV are employed. For the Monte Carlo simulated images, Poisson-distributed noise is added in ten noise realizations. Reconstruction is performed (OS-EM, 40 iterations, 6 subsets) employing compensation for attenuation, scatter, and collimator-detector response. The estimated concentrations in the anthropomorphic phantom are also corrected using recovery coefficients. Errors for the largest sphere in the physical phantom range from −25% to −34% for the medium-energy collimator and larger deviations for smaller spheres. Corresponding results for the high-energy collimator are −15% to −31%. The corresponding Monte Carlo simulations show standard deviations of a few percentage points. For the anthropomorphic phantom, before application of recovery coefficients the bias ranges from −16% to −46% (medium-energy collimator) and −10% to −28% (high-energy collimator), with standard deviations of 2% to 14% and 1% to 16%. After the application of recovery coefficients, the biases range from −3% to −35% (medium energy collimator) and from 0% to −18%. The errors decrease with increasing concentrations. Activity-concentration estimation of 223Ra with SPECT is feasible, but problems with repeatability need to be further studied.

Practical considerations for quantitative clinical SPECT/CT imaging of alpha particle emitting radioisotopes

“…Recently, several studies have shown the feasibility of quantitative single-photon emission computed tomography (SPECT) 23 - 25 using physical-phantom measurements. For example, Gustafson et al 24 showed that quantification of a 26.4 mL sphere in a NEMA IEC Body phantom was achievable with errors ranging from -25% to -34% with a GE Infinia Hawkeye SPECT/CT gamma-camera (GE, Fairfield, CT, USA) equipped with a medium-energy collimator.…”

Dosimetry in Clinical Radiopharmaceutical Therapy of Cancer: Practicality Versus Perfection in Current Practice

“…Such data are often inaccurate when translated into human beings, probably because of different kinetics and affinity profiles, greater in vivo heterogeneity, and nonuniformity of RPT within lesions based on size, location, and tumor microenvironment. For instance, quantitation of 223 Ra-chloride using phantoms has been shown to be feasible, but significant challenges remain, including validation and reproducibility (129).…”

A projection-domain low-count quantitative SPECT method for alpha-particle emitting radiopharmaceutical therapy

“…For example, when imaging with 223 Ra, the number of detected counts can be as low as 5,000 counts per projection slice. This is multiple folds lower than conventional SPECT studies and results in high bias in estimated activity (19%-35%) even with highly fine-tuned protocols [14]- [16]. Another major concern is the low precision in the estimated activity with these methods [16].…”