Multi‐energy blended CBCT spectral imaging and scatter‐decoupled material decomposition using a spectral modulator with flying focal spot (SMFFS)
Authors: Deng Yifan , Zhou Hao , Wang Zhilei , Wang Adam S. , Gao Hewei 
Published: March 2024

Background

Cone‐beam CT (CBCT) has been extensively employed in industrial and medical applications, such as image‐guided radiotherapy and diagnostic imaging, with a growing demand for quantitative imaging using CBCT. However, conventional CBCT can be easily compromised by scatter and beam hardening artifacts, and the entanglement of scatter and spectral effects introduces additional complexity.

Purpose

The intertwined scatter and spectral effects within CBCT pose significant challenges to the quantitative performance of spectral imaging. In this work, we present the first attempt to develop a stationary spectral modulator with flying focal spot (SMFFS) technology as a promising, low‐cost approach to accurately solving the x‐ray scattering problem and physically enabling spectral imaging in a unified framework, and with no significant misalignment in data sampling of spectral projections.

Methods

To deal with the intertwined scatter‐spectral challenge, we propose a novel scatter‐decoupled material decomposition (SDMD) method for SMFFS, which consists of four steps in total, including (1) spatial resolution‐preserved and noise‐suppressed multi‐energy “residual” projection generation free from scatter, based on a hypothesis of scatter similarity; (2) first‐pass material decomposition from the generated multi‐energy residual projections in non‐penumbra regions, with a structure similarity constraint to overcome the increased noise and penumbra effect; (3) scatter estimation for complete data; and (4) second‐pass material decomposition for complete data by using a multi‐material spectral correction method. Monte Carlo simulations of a pure‐water cylinder phantom with different focal spot deflections are conducted to validate the scatter similarity hypothesis. Both numerical simulations using a clinical abdominal CT dataset, and physics experiments on a tabletop CBCT system using a Gammex multi‐energy CT phantom and an anthropomorphic chest phantom, are carried out to demonstrate the feasibility of CBCT spectral imaging with SMFFS and our proposed SDMD method.

Results

Monte Carlo simulations show that focal spot deflections within a range of 2 mm share quite similar scatter distributions overall. Numerical simulations demonstrate that SMFFS with SDMD method can achieve better material decomposition and CT number accuracy with fewer artifacts. In physics experiments, for the Gammex phantom, the average error of the mean values () in selected regions of interest (ROIs) of virtual monochromatic image (VMI) at 70 keV is 8 HU in SMFFS cone‐beam (CB) scan, and 19 and 210 HU in sequential 80/120 kVp (dual kVp, DKV) CB scan with and without scatter correction, respectively. For the chest phantom, the in selected ROIs of VMIs is 12 HU for SMFFS CB scan, and 15 and 438 HU for sequential 80/140 kVp CB scan with and without scatter correction, respectively. Also, the non‐uniformity among selected regions of the chest phantom is 14 HU for SMFFS CB scan, and 59 and 184 HU for the DKV CB scan with and without a traditional scatter correction method, respectively.

Conclusions

We propose a SDMD method for CBCT with SMFFS. Our preliminary results show that SMFFS can enable spectral imaging with simultaneous scatter correction for CBCT and effectively improve its quantitative imaging performance.