Simulation and analysis of image quality impacts from single source, ultra-wide coverage CT scanner.

Future generations of CT systems would need a mean to cover an entire organ in a single rotation. A way to accomplish this is to physically increase detector size to provide, e.g., 120∼160 mm z (head-foot) coverage at iso-center. The x-ray cone angle of such a system is usually 3∼4 times of that of a 64-slice (40 mm) system, which leads to more severe cone beam artifacts in cardiac scans. In addition, the extreme x-ray take-off angles for such a system cause severe heel effect, which would require an increase in anode target angle to compensate for it. One shortcoming of larger target angle is that tube output likely decreases because of shorter thermal length. This would result in an increase of image noise. Our goal is to understand from a physics and math point of view, what is the theoretical entitlement of artifacts, resolution, and noise impact of such a system. The image artifacts are assessed through computer simulation of a helical body phantom and visual comparison of reconstructed images between a 140 mm system and a 64-slice system. The IQ impact from target angle increase is studied analytically and experimentally by first finding the proper range of target angles that give the acceptable heel effect, then estimating the impact on peak power (flux) and z resolution using an empirical model of heel effect for given target angle and analytical models of z resolution and tube current loading factor for given target thermal length. The results show that, for a 140 mm system, 24.5% of imaging volume exhibits more severe cone beam artifacts than a 64-slice system, which also poses a patient dose concern. In addition, this system may suffer from a 36% peak power (flux) loss, which is equivalent to about 20% image noise increase. Therefore, a wide coverage CT system using a single x-ray source is likely to face some severe challenges in IQ and clinical accuracy.