Complete MTF evaluation of two cone beam CT systems

Abstract

The Modulation Transfer Function (MTF) of any system is the frequency response to a delta signal. This response is degraded by several factors such as the inherent veiling glare of the detector and the focal spot size among others. Consequently, the MTF has been one of the physical characteristics that are commonly used to quantitatively measure the physical performance of a system. In this article, the MTF of two flat panel detectors (FPD) and of two Cone Beam CT systems is evaluated. First, the MTF of PaxScan 2520 and of PaxScan 4030CB is evaluated. One of the standard techniques to evaluate the MTF of a FPD is by using an edge of a metal with high atomic number. For instance, it has been suggested by IEC 62220-1 to use an opaque edge to evaluate the MTF of a FPD. Yet, it was found that different metals yield slightly different MTF. In this study, the effects on the MTF evaluation of different metals and exposure parameters was studied and analyzed. First, the MTF was evaluated using different kVps and exposures levels. Second, the MTF was evaluated using aluminum edges of different thickness. Third, the MTF was evaluated using the following four different metals: Aluminum, Copper, Steel and Lead. Finally, the MTF obtained previously were compared to the MTF obtained by using a pinhole. In the second part of this study, the MTF of two systems using these two FPDs were also evaluated using different wires, filters and acquisition modes. The preliminary results demonstrate that the MTF is independent of kVp and exposure level. Yet, it is dependent on the material used to evaluate it.