Ring artifact reduction in photon counting CT using redundant sampling and autocalibration

Ring artifacts in CT are caused by uncalibrated variations in detector pixels and are especially prevalent with emerging photon counting detectors (PCDs). Control of ring artifacts is conventionally accomplished by improving either hardware manufacturing or software correction algorithms. An alternative solution is detector autocalibration, in which two redundant samples of each line integral are acquired and used to dynamically calibrate the PCD. Autocalibration was first proposed by Hounsfield in 1977 and was demonstrated on the EMI Topaz prototype scanner in 1980, but details surrounding this implementation are sparse. We investigate a form of autocalibration that requires just two redundant acquisitions, which could be acquired using flying focal spot on a clinical scanner but is demonstrated here with a detector shift. We formulated autocalibration as an optimization problem to determine the relative gain factor of each pixel and tested it on scans of a chicken thigh specimen, resolution phantom, and a cylindrical phantom. Ring artifacts were significantly reduced. Some residual artifacts remained but could not be discriminated from the intrinsic temporal instability of our PCD modules. Autocalibration could facilitate the adoption of widespread photon counting CT by reducing ring artifacts, thermal management requirements, or stability requirements that are present today. Demonstration of autocalibration on a rotating gantry with flying focal spot remains future work.