Collimator and energy window optimization for 90Y bremsstrahlung SPECT imaging: A SIMIND Monte Carlo study

Abstract

Treatment efficacy of radioembolization using Yttrium-90 (90Y) microspheres is assessed by the 90Y bremsstrahlung single photon emission computed tomography (SPECT) imaging following radioembolization. The radioisotopic image has the potential of providing reliable activity map of 90Y microspheres distribution. One of the main reasons of the poor image quality in 90Y bremsstrahlung SPECT imaging is the continuous and broad energy spectrum of the related bremsstrahlung photons. Furthermore, collimator geometry plays an impressive role in the spatial resolution, sensitivity and image contrast. Due to the relatively poor quality of the 90Y bremsstrahlung SPECT images, we intend to optimize the medium-energy (ME) parallel-hole collimator and energy window. The Siemens e.cam gamma camera equipped with a ME collimator and a voxelized phantom was simulated by the SImulating Medical Imaging Nuclear Detectors (SIMIND) program. We used the SIMIND Monte Carlo program to generate the 90Y bremsstrahlung SPECT projection of the digital Jaszczak phantom. The phantom consist of the six hot spheres ranging from 9.5 to 31.8mm in diameter, which are used to evaluate the image contrast. In order to assess the effect of the energy window on the image contrast, three energy windows ranging from 60 to 160KeV, 160 to 400KeV, and 60 to 400KeV were set on a 90Y bremsstrahlung spectrum. As well, the effect of the hole diameter of a ME collimator on the image contrast and bremsstrahlung spectrum were investigated. For the fixed collimator and septa thickness values (3.28cm and 1.14mm, respectively), a hole diameter range (2.35–3.3mm) was chosen based on the appropriate balance between the spatial resolution and sensitivity. The optimal energy window for 90Y bremsstrahlung SPECT imaging was extended energy window from 60 to 400KeV. Besides, The optimal value of the hole diameter of ME collimator was obtained 3.3mm. Geometry of the ME parallel-hole collimator and energy window are indeed important indicators of the image quality in 90Y bremsstrahlung imaging. The obtained optimal ME collimator and optimal energy window have the potential to improve the image contrast of 90Y bremsstrahlung images. Subsequently, high quality 90Y bremsstrahlung images can provide reliable estimate of the 90Y microsphere activity distribution after radioembolization.