Evaluation of the sensitivity of SPECT/CT systems for 225Ac

Radiopharmaceuticals labelled with alpha-emitting radionuclides, particularly 225Ac, are the most promising and interesting radiopharmaceuticals for the cancer treatment. Evaluation of the radiopharmaceutical distribution in the patient body using single photon emission computed tomography (SPECT) is one of the necessary steps during radiopharmaceutical therapy. The quantitative results of the accumulated activity of a radionuclide in an organ or lesion on SPECT images must be accurate and reproducible. To obtain accurate and reproducible results, SPECT quality control and calibration procedures should be performed including evaluation of the sensitivity and accuracy of the system reproducibility. The aim of the study is to evaluate the sensitivity of SPECT for 225Ac with different detection systems, data acquisition and image reconstruction parameters using three SPECT systems and the anthropomorphic phantom. The sensitivity of SPECT for 225Ac was evaluate for three SPECT systems by different manufactures using Kyoto Kagaku PET/SPECT Thorax Phantom PH-63 which simulated patient body (torso with liver, kidneys, heart, lungs) with lesions in the region of the sternum and spine. The organs and lesions were filled with 225Ac solution. Phantom images were obtained with different collimators and different image reconstruction parameters with variable number of subsets and width of the Gaussian post-reconstruction filter. The sensitivity of SPECT was defined as ratio of measured counts (average and maximum values) on SPECT image in the region of interest (organ or lesion) and administered activity concentration in the region of interest multiplied by acquisition time. The system with detectors based on cadmium-zinc-tellurium crystals and the system with scintillation detectors and a high-energy collimator had the best sensitivity. The high-energy collimator is the most appropriate for 225Ac imaging. The increase of the number of subsets leads to the increase of the sensitivity of SPECT for 225Ac; the increase of the width of the Gaussian filter leads to the decrease of the sensitivity of SPECT for 225Ac. Hence, the optimal reconstruction parameters for 225Ac SPECT imaging were determined as three-dimensional OSEM (ordered subset expectation maximization) reconstruction algorithm with 8 iterations and 32 subsets using a 4 mm Gaussian filter. The sensitivity coefficients evaluated in the current work using anthropomorphic phantom can be applied as calibration coefficients to clinical images of patients with 225Ac obtained by the same SPECT systems.