Monte Carlo simulations and analysis of transmitted gamma ray spectra through various tissue phantoms.

Studies of radiation interactions with tissue equivalent material find importance in efforts that seek to avoid unjustifiable dose to patients, also in ensuring quality control of for instance nuclear medicine imaging equipment. Use of the Monte Carlo (MC) simulation tool in such characterization processes allows for the avoidance of costly experiments involving transmitted X- and γ-ray spectrometry. Present work investigates MC simulations of γ-ray transmission through tissue equivalent solid phantoms. Use has been made of a range of radionuclide gamma ray sources, 99mTc, 131I, 137Cs, 60Co (offering photons in the energy range from a few keV up to low MeV), popularly applied in medicine and in some cases for gauging in industry, obtaining the transmission spectra following their interaction with various phantom materials and thicknesses. In validation of the model, the simulated values of mass attenuation coefficients (µ/ρ) for different phantom materials and thicknesses were found to be in good agreement with reference values (NIST, 2004) to within 1.1% for all material compositions. For all of the primary photon energies and medium thicknesses of interest herein, results show that multiple scattering peaks are generally located at energies lower than 100 keV, although for the larger phantom thicknesses it is more difficult to distinguish single, double and multiple scattering in the gamma spectra. Transmitted photon spectra investigated for water, soft tissue, breast, brain and lung tissue slab phantoms are demonstrated to be practically independent of the phantom material, while a significant difference is observed for the spectra transmitted through bone that was proved to be due to the density effect and not material composition.

Gamma spectrometry efficiency calibration using Monte Carlo methods to measure radioactivity of 137Cs in food samples.

A simple method of efficiency calibration for gamma spectrometry was performed. This method, which focused on measuring the radioactivity of (137)Cs in food samples, was based on Monte Carlo simulations available in the free-of-charge toolkit GEANT4. Experimentally, the efficiency values of a high-purity germanium detector were calculated for three reference materials representing three different food items. These efficiency values were compared with their counterparts produced by a computer code that simulated experimental conditions. Interestingly, the output of the simulation code was in acceptable agreement with the experimental findings, thus validating the proposed method.

High-resolution magnetic resonance imaging and diffusion tensor imaging of the porcine temporomandibular joint disc.

OBJECTIVES: Diffusion tensor imaging (DTI) is an MRI modality for characterizing the property, microstructural organization and function in tissues such as the brain and spinal cord. Prior to this investigation, DTI had not been adapted for studies of the temporomandibular joint (TMJ) disc. Objectives were to test the feasibility of DTI to evaluate the porcine TMJ disc and to use DTI to observe differences in magnitude of anisotropy of water diffusion between TMJ disc regions. METHODS: Five adult pig TMJs were scanned on a 9.4 Tesla horizontal bore MRI scanner using an inductively coupled surface coil. High-resolution gradient-echo and diffusion-weighted spin-echo based images were obtained. The mean diffusivity and fractional anisotropy (FA) were computed in different regions of the disc. Two observers were calibrated to review the two-dimensional and three-dimensional images. Polarized light microscopy was used as the gold standard for collagen fibre orientation. RESULTS: In the sagittal plane, the mean diffusivity was higher in the posterior (1.28+/-0.10 x 10(-3) mm(-2) s(-1)) and anterior (1.27+/-0.08 x 10(-3) mm(-2) s(-1)) bands compared with the intermediate zone (0.96+/-0.01 x 10(-3) mm(-2) s(-1)), and the FA index was also lowest in the intermediate zone. In the coronal plane, the mean diffusivity was higher in the medial (1.42+/-0.01 x 10(-3) mm(-2) s(-1)) and lateral (1.21+/-0.12 x 10(-3) mm(-2) s(-1)) aspects than in the centre (1.09+/-0.08 x 10(-3) mm(-2) s(-1)), and the FA index was also lowest in the centre. CONCLUSIONS: DTI is a useful method for non-invasively characterizing the structure/property relationships of the porcine TMJ disc.