Comparison of MTFs in x-ray CT images between measured by current method and considered linearity in low contrast

Abstract

Generally, the modulation transfer function (MTF) of a computed tomography (CT) scanner is calculated based on the CT value. However, it is impossible to measure the MTF directly because the CT value is defined as a nonlinear function of the X-ray intensity. Due to this characteristic, the MTF varies with the subject's contrast. Therefore, we measured the MTFs of a CT scanner using high- and low-contrast wire phantoms. We selected thick copper wire in water as the high-contrast subject and thin copper wire in water as the low-contrast subject. The MTF measured with the low-contrast subject was decreased relative to that measured with the high-contrast subject because the CT value was nonlinear. Thus, to evaluate the spatial resolution in a low-contrast subject such as the human body, we should measure the MTF with a low-contrast wire phantom. In addition, by using low-contrast subjects, we can approximately determine the CT value using a linear function.