The Influence of X-ray Spectra Filtration on Image Quality and Patient Dose in the GE VCT 64-Slice Cardiac CT Scanner

The polyenergetic x-ray spectra used during CT imaging makes it subject to beam hardening artefacts caused by the absorption of low energy x-rays as they pass through the patient. The direct consequence is that the linear attenuation coefficient calculated for thick body regions is lower than in thin regions. This effect generates cupping and streak artefacts in the reconstructed CT images thus compromising their use in clinical diagnosis. A possible solution for reduction of beam hardening artefact and furthermore minimizing patient dose is, to use suitable x-ray spectra filtration of aluminium (Al) or cupper (Cu) before penetration of the x-ray photons into the patient's body. In this study, we quantified the effect of using different x-ray spectra filtration on image quality, patient dose and modulation transfer function (MTF) in the GE Light Speed VCT 64-slice cardiac CT scanner using experimental measurements. Different thicknesses of Cu filters ranging from 0.13 to 1.6 mm were installed on the collimator aperture. A cylindrical phantom comprising 16 different materials for modelling of different tissues and the GE performance phantom were scanned at different tube voltages and thicknesses of Cu filters. The absorbed dose in the centre of the phantom was measured using a pencil dosimeter placed in the central part of the phantom. Then the images acquired using different filters were analyzed through comparison of the CT numbers (contrast) and standard deviation (noise) of different ROIs in the phantom with the values obtained using the default filtration of the scanner fixed in the factory. To assess the influence of x-ray spectra filtration on MTF, the GE performance phantom was used. The additional filtration of 0.6 mm Cu was considered as the optimal filter resulting in acceptable image quality with minimum dose (50% reduction). This study is being continued by employing different filter thicknesses to accurately find the optimum filtration required for the GE VCT scanner to achieve the best image quality with the least possible patient absorbed dose.