Evaluation the effect of different collimators and energy window on Y-90 bremsstrahlung SPECT imaging by SIMIND Monte Carlo program

BACKGROUND: Recently, the treatment efficiency of Yttrium 90 (Y-90) and providing reliable estimates of activity by single photon emission computed tomography (SPECT) imaging of bremsstrahlung radiation released during beta therapy have been evaluated. In the Y-90 bremsstrahlung SPECT imaging, the resulting energy spectrum is very complex and continuous, which creates many difficulties in the imaging protocol and image reconstruction. Furthermore, image quality and quantitative accuracy in the bremsstrahlung SPECT imaging are affected by collimator penetration and scatter. So, the collimator type and its geometry have impressive effects on the spatial resolution, system sensitivity and image contrast. MATERIAL AND METHODS: Hereby, in this paper, we evaluated the effect of the energy window (three energy windows: 60 to 160 keV, 160 to 400 keV, and 60 to 400 keV) and the commercial parallel-hole collimators with different geometric parameters on the Y-90 bremsstrahlung spectrum and the image quality of the liver tumors based on criteria such as system sensitivity and image contrast. SIMIND Monte Carlo simulation code was used to generate the Y-90 bremsstrahlung SPECT images of the liver tumor with different diameters: 1.36, 2.04, 2.72, 3.4, 4.08, and 4.76 cm by use of the digital Zubal phantom. Furthermore, the tumor size was estimated by evaluating pixel intensity profile on the line drawn through the activity distribution image. RESULTS: Our results showed that the collimator choice and energy window setting in the bremsstrahlung SPECT imaging have significant effects on the image quality and tumor size estimation. Optimal image quality could be acquired by the energy window of 60 to 400 keV and the SPECT system equipped with a Medium-Energy General-Purpose (MEGP) collimator of Millennium VG Kameran (GV) Company. Moreover, the estimation of distribution size was close to the actual value for tumor sizes larger than 2.04 cm, especially by using the SPECT system equipped with the GV-MEGP collimator in the wide energy window. CONCLUSIONS: We found an optimal collimator to be more appropriate for improving the imaging quality of Y-90 bremsstrahlung photons, which can be used for reliable activity distribution estimates after radiation therapy