Experimental evaluation of polychromatic reconstruction for quantitative CBCT

Abstract

Polychromatic reconstruction is a promising technique for quantitative cone-beam computed tomography in radiation therapy. In this study, we have implemented polychromatic forward projection into our reconstruction framework to directly reconstruct relative electron density volumes without the need for additional HU calibration. The underlying spectral model takes beam hardening into account by design. Thereby this extended reconstruction framework is a natural step in the direction of spectral imaging, albeit without any hardware modifications. Reconstructed relative electron density volumes from phantom scans show sufficiently good agreement with ground truth for photon dose calculation; relative errors for most inserts are below 3%. We also demonstrate beam hardening artifact reduction in virtual monoenergetic images obtained from polychromatic reconstruction as compared to an established iterative reconstruction using water-based correction. Similarly, polychromatic reconstruction shows potential for mitigating metal artifacts in a clinical scan acquired for a patient with bilateral hip implants.