Use of beam shapers for cone-beam CT with off-centered flat detector

While cone-beam CT using flat x-ray detectors has gained increased popularity in the past years, the 3D imaging quality is still limited by a large amount of scatter, low dynamic range, and small field of view of the detector. Especially for large objects, the high dynamic range of the projections is a common source for detector specific artifacts. In conventional CT, the application of beam shapers (or bowtie filters) to decrease the signal dynamic in the projections is quite common. In this paper we investigate the use of a beam shaper for cone-beam CT with an off-centered flat detector by means of Monte-Carlo (MC) simulations and test-bench experiments. The shift of the detector out of the central axis increases the field of view and allows the imaging of larger patients, but in turn leads to a very high dynamic signal range and poor scatter-to-primary ratios (SPR). The impact of a half bowtie filter on key performance parameters of the imaging chain is investigated with MC simulations. It is demonstrated that a beam shaper significantly improves the peak SPR especially for large patients and that the reshaping of the SPR has a dominant impact on the homogeneity of the reconstructed image. The use of beam shapers for CBCT requires a modified pre-processing chain that also accounts for secondary effects introduced by the beam modulation filter. Beside patient scatter correction, the inhomogeneous spectral hardening of the x-ray beam and scattered radiation from the beam shaper itself have to be corrected. A comparison of phantom scans with and without beam shaper after pre-processing demonstrates the potential of beam shapers for dose reduction and SNR improvement in flat detector cone-beam CT.