Assessment of quantitative accuracy of SPECT/CT in dosimetric studies for 177Lu therapeutic radionuclide

Abstract Objective Quantitative SPECT/CT imaging for targeted radionuclide therapy with 177 Lu holds excellent potential for developing imaging-based personalised dose assessment. The study aims to evaluate the quantitative accuracy of SPECT/CT and its effect on image quality parameters for 177 Lu using image quality phantom at four hot spheres to background ratio. Material and Methods The study was performed on a dedicated SPECT/CT system using a NEMA standard image quality phantom having 6 sphere inserts (inner diameter 10, 13, 17, 22, 28, and 37 mm) and a lung insert. Except for the largest 37 mm sphere (filled with water), all other spheres were filled with an activity 177 Lu concentration of 440 kBq/mL. At a minimum of three-degree angle and for a minimum of 30 s per projection, the phantom images were collected with a four spheres-to-background activity concentration ratio (no background,1:4, 1:8, 1:16). Also, the accuracy of attenuation correction was determined using lung insert. The attenuation-corrected images reconstructed via the OSEM interactive reconstruction-based method were analysed. The region of interest (ROI) was drawn in a transverse image slice containing the sphere's center and adjacent slices, calculating average counts in the sphere. Percent contrast recovery was calculated for each sphere-to-background ratio. 12 ROI for each sphere diameter were drawn in central and adjacent slices for background variability assessment for each hot sphere to background ratio. Result The highest percent contrast recovery(in the hot sphere) was observed in the 28 mm sphere for a 16:1 hot sphere to background ratio, whereas the 10 mm sphere shows the least per cent recovery ranges from 4.14% to 5.07% in all sphere to background ratios. The background variability was maximum for the 10 mm sphere diameter, which decreases as per diameter, and the least was noted for the 28 mm sphere. The 37 mm cold spheres show the highest contrast recovery and least background variability in all sphere volumes. The highest lung error was seen in no background concentration ratio images. The maximum lung error was seen in the 16:1 sphere-to-background concentration ratio. Conclusion The study results show that percent contrast increases with an increase in the sphere size where background concentration significantly affects per cent contrast recovery and background variability in SPECT CT quantification for 177 Lu.