Avaliação da função de transferência de modulação para receptores de CsI e a-Se em mamografia digital utilizando simulações Monte Carlo

Abstract

The Modulation Transfer Function (MTF) is an important parameter to evaluate image quality, in mammograph since it allows evaluation of the performance of digital image receptors taking into account their spatial resolution. In this work, MTFs were obtained in digital mammography, for Cesium Iodide (CsI) and amorphous Selenium (a-Se) receptors, which perform, respectively, the indirect and direct conversion of x- rays into an electrical signal. The MTF was obtained using Monte Carlo simulations, using the PENELOPE code to simulate the transport of radiation in image receptors. The MTF was evaluated under the influence of several factors present in daily radiodiagnosis, such as: anode/filter combinations of Mo/Mo, W/Ag and W/Rh; tube potential between 24 to 36kv; receptor thickness of the receiving material, these being 200 and 300μm for a-Se and 150 and 250μm for CsI; and beam attenuation by a breast composed of 20% glandular tissue with compression thicknesses of 3cm, 5cm and 7cm. The results show that the anode/filter combination and receptor thickness are influencing factors on the MTF of both receptors. The reabsorption of characteristic radiation in the receptors is the main degrading factor of the spatial resolution and this deleterious effect is linked to the energy range used in digital mammography. The increase in the average beam energy caused by breast attenuation proved to be a factor of little influence on the MTF and the spatial resolution of the images.