Multienergy element‐resolved cone beam CT ( MEER ‐ CBCT ) realized on a conventional CBCT platform

Purpose Cone beam CT ( CBCT ) has been widely used in radiation therapy. However, its main application is still to acquire anatomical information for patient positioning. This study proposes a multienergy element‐resolved ( MEER ) CBCT framework that employs energy‐resolved data acquisition on a conventional CBCT platform and then simultaneously reconstructs images of x‐ray attenuation coefficients, electron density relative to water ( rED ), and elemental composition ( EC ) to support advanced applications. Methods The MEER ‐ CBCT framework is realized on a Varian TrueBeam CBCT platform using a kV p‐switching scanning scheme. A simultaneous image reconstruction and elemental decomposition model is formulated as an optimization problem. The objective function uses a least square term to enforce fidelity between x‐ray attenuation coefficients and projection measurements. Spatial regularization is introduced via sparsity under a tight wavelet‐frame transform. Consistency is imposed among rED , EC , and attenuation coefficients and inherently serves as a regularization term along the energy direction. The EC is further constrained by a sparse combination of EC s in a dictionary containing tissues commonly existing in humans. The optimization problem is solved by a novel alternating‐direction minimization scheme. The MEER ‐ CBCT framework was tested in a simulation study using an NCAT phantom and an experimental study using a Gammex phantom. Results MEER ‐ CBCT framework was successfully realized on a clinical Varian TrueBeam onboard CBCT platform with three energy channels of 80, 100, and 120  kV p. In the simulation study, the attenuation coefficient image achieved a structural similarity index of 0.98, compared to 0.61 for the image reconstructed by the conventional conjugate gradient least square ( CGLS ) algorithm, primarily because of reduction in artifacts. In the experimental study, the attenuation image obtained a contrast‐to‐noise ratio ≥60, much higher than that of CGLS results (~16) because of noise reduction. The median errors in rED and EC were 0.5% and 1.4% in the simulation study and 1.4% and 2.3% in the experimental study. Conclusion We proposed a novel MEER ‐ CBCT framework realized on a clinical CBCT platform. Simulation and experimental studies demonstrated its capability to simultaneously reconstruct x‐ray attenuation coefficient, rED , and EC images accurately.