A Novel Method for Measuring the MTF of CT Scanners: A Phantom Study

The modulation transfer function (MTF) is well known as a crucial parameter in quality assurance of computed tomography (CT) scanners, which provides detailed information of both contrast and resolution of CT images. Different methods have been introduced and developed to calculate the MTF of CT scanners. However, a robust methodology which accurately estimates the MTF of CT scanners under the use of every range of object electron density and tube current-time product (mAs) has not been reported so far. To this aim, a new wavelet-based circular edge method for MTF measurement has been presented in this work. Owning to the edge spread function (ESF) susceptibility to noise, the approach was based on the assumption that the ESF can be decomposed into approximate and detailed information containing different noise levels. To evaluate the performance of our method, an in-house fabricated phantom containing various disk objects covering a range of electron densities from low to high values was scanned by a volumetric 64-slice clinical CT scanner under a range of low tube current from 50 to 100 mAs where image noise levels are higher than those of normal-dose scan protocols. Measurements have shown that our proposed method yielded an accurate estimation of MTF for high-density as well as low-density disk objects and it is valid and stable over the variations of noise levels.