Monte Carlo study of the effect of collimator thickness on 99mTc sources responses in SPECT

Introduction: In SPECT the collimator is a crucial element of the imaging chain and controls the noise resolution tradeoff of the collected data. The detection of photons in SPECT is seriously affected by collisions of the photons with atoms inside the patient (photon attenuation and scatter), and the inevitable inaccuracy of the collimator used (collimator blurring). The images are also severely degraded because of noise, partly due to the Poisson nature of the photon emission. Accordingly, it is very difficult to obtain high quality and quantitative accurate SPECT images. The lead x-rays, collimator scatter and partial energy deposition in the detector crystal are the effects that contribute to the 99mTc camera down-scatter in the energy range of 100±10%keV or 72±10%.The current study is an evaluation of the effects of low energy high resolution collimator thickness, commonly used in SPECT, on tomographic spatial resolution. Methods: SIMIND Monte Carlo program was used for simulation of a Siemen's dual-head variable angle scintillation gamma camera and also a related Low Energy High Resolution (LEHR) collimator. For this study a point source of 99mTc and also an acrylic cylindric Jaszczack phantom, with cold spheres and rods, and a NCAT phantom were used. Quantitative and qualitative studies were performed for obtained acquired projections and also reconstructed images. Results: Results for calculated detector parameters, contribution of Compton scattering, photoelectric reactions, and also peak to Compton (P/C) area in the obtained energy spectrums from scanning of the sources with 11 collimator thickness, ranged from 0.400 to 0.410 cm, were tabulated. The Image quality analyses by SSIM algorithm and also by eye interpretation were provided. Conclusion: There was a suitable quality and also performance parameters analysis results for the projections and reconstructed images prepared with a 0.405 mm LEHR collimator thickness compared to other thicknesses.