  Background: Dual-energy imaging on cone-beam CT (CBCT) scanners has great
potential in different clinical applications, including image-guided surgery
and adaptive proton therapy. However, the clinical practice of dual-energy CBCT
(DE-CBCT) has been hindered by the requirement of sophisticated hardware
components. Purpose: In this work, we aim to propose a practical solution for
single-scan dual-energy imaging on current CBCT scanners without hardware
modifications, using two complementary limited-angle scans with a
projection-consistent diffusion model. Methods: Our approach has two major
components: data acquisition using two complementary limited-angle scans, and
dual-energy projections restoration with subsequent FDK reconstruction. Two
complementary scans at different kVps are performed in a single rotation by
switching the tube voltage at the middle of the source trajectory, acquiring
the mixed-spectra projection in a single CBCT scan. Full-sampled dual-energy
projections are then restored by a projection-consistent diffusion model in a
slice-by-slice manner, followed by the DE-CBCT reconstruction using the FDK
algorithm. Results: The proposed method was evaluated in a simulation study of
digital abdomen phantoms and a study of real rat data. In the simulation study,
the proposed method produced DE-CBCT images at a mean absolute error (MAE) of
20 HU. In the small-animal study, reconstructed DE-CBCT images using the
proposed method gave an MAE of 25 HU. Conclusion: This study demonstrates the
feasibility of DE-CBCT imaging using two complementary limited-angle scans with
a projection-consistent diffusion model in both half-fan and short scans. The
proposed method may allow quantitative applications of DE-CBCT and enable
DE-CBCT-based adaptive proton therapy.
