Collimator and energy window optimization for yttrium-90 imaging

Purpose: Yttrium-90 is used in the treatment of targeted radionuclide, primary hepatic and metastatic tumors (radioembolization with microspheres). Yttrium-90; is a pure beta-emitter isotope with a physical half-life of 2.67 days. The emitted particles have a maximum energy of 2.27 MeV and their average energy is 0.93 MeV. In conventional SPECT imaging, gamma-transmitting radioisotopes that have obvious photopeak such as Tc-99m (140 keV energy photopeak) are used. In contrast, Yttrium-90 bremsstrahlung photons are produced from the interaction of beta particles with the patient's body and have prominent non-photopeak continuous and broad energy spectrum. Accordingly, the image quality is highly dependent on the selected energy window and also collimator. In this study, the effects of the collimators, source-detector distance and energy window widths on image quality were investigated in order to determine the optimum bremsstrahlung imaging conditions with Yttrium-90 source. Methods: In the study, Philips Forte JET stream branded gamma camera system, 1.0 mL glass vials enclosed in an acrylic shield and the TheraSphere Yttrium-90 glass spheres with an average size of 20-30 μm were used. The planar images were acquired consecutively with the LEHR, MEGP and HEGP collimators for the two different source-detectors distances (25 and 40 cm) in 20 different energy windows ranging from 55 to 285 keV. SPECT data were obtained for 28 s/view for 64 projections and size of the 128x128 matrix with two rotations. The reconstruction of the images was performed by filtered back-projection and iterative methods. Image quality parameters (sensitivity, %FOV, contrast, and S/B) were investigated for different energy windows and collimator combinations. Results: Compared to basic image quality parameters, the highest S/B, %FOV and contrast values were found 460, %71, 3578 in the SPECT images whereas the HEGP collimator together with a source to detector distance of 25 cm and the planar images were 34, %50 and 436, respectively. Conclusions: It was concluded that the best quality image of Yttrium-90 vial was taken with HEGP collimator and choice of energy window with a width of 90-110 keV at the sourcedetector distance of 25 cm, Butterworth filter, 0.5 cutoff frequency, and 8 order parameters using the Filtered Back Projection method. Key Words: Yttrium-90, SPECT, Imaging, Signal/Background Ratio, Contrast, %FOV.