Optimization of Bremsstrahlung imaging in Yttrium internal radiation therapy

Introduction: Treatment efficacy of radiation therapy using Yttrium-90 radionuclide is evaluated by the bremsstrahlung SPECT imaging following radiation therapy. The radioisotopic images have the ability to provide reliable activity map of 90Y distribution. But these images have a low quantitative accuracy in the 90Y bremsstrahlung SPECT imaging, therefore the optimization of   some   imaging   important indicators   i.e.   the energy window   width and collimator geometry, which heavily affect acquisition images quality, is necessary.  The aim of this paper is to estimate the effects of the energy window width and the hole diameter of a ME collimator on the image contrast of the 90Y bremsstrahlung SPECT images obtained using SIMIND Monte Carlo simulation program and improve image quality which could be used to estimate activity distribution after radiation therapy. Materials and Methods: In this paper we simulated the SPECT system and a Jaszczak phantom consist of six hot spheres in different diameters by use of the SIMIND Monte Carlo simulation program to study 90Y SPECT images by assessment of the holes diameter of a ME collimator including 2.35, 2.59, 2.82, 2.94, 3.06 and 3.3 mm with fixed values of the collimator and septa thicknesses (3.2 cm and 1.14mm, respectively) in the two energy windows including 60- 160 keV and 60-400 keV. SPECT imaging protocol is selected according to the clinical study. An activity of 1.5GBq 90Y was considered for the hot spheres without background activity. We used OS-EM algorithm to reconstruct projections and evaluated the contrast of the reconstructed images of six hot spheres of Jaszczak phantom with selection of proper ROIs. Results: The comparison of the images contrast of Jaszczak phantom spheres indicated that generally with increasing the hole diameter of collimator, the images contrast of the spheres were decreased. In order to, in comparison to the 1st energy window (60–160 keV), the 2nd energy window (60-400 keV) was increased the image contrast. The results showed that for a fixed collimator and septa thickness values (3.2 cm and 1.14mm, respectively), the optimal value of the hole diameter was found 2.35 mm. Also from the stand point of the image contrast, the optimal energy window was found ranging from 60 to 400 keV. Conclusion: By use of the evaluation of the images contrast of Jaszczak phantom by SIMIND Monte Carlo simulation study, the best contrast is obtained when by use of the wide energy window ranging from 60 to 400 keV, which increased the imaging system sensitivity and decreased variance of the activity estimation. Moreover, a ME collimator with fixed values of 3.2 cm and 1.14 mm for the collimator and septa thicknesses, respectively and a hole diameter of 2.35 mm can improve the 90Y activity estimation and provide a suitable image quality for the 90Y bremsstrahlung SPECT imaging.