Dual‐scan conformal cone‐beam CT for targeted image‐quality improvement using dynamic collimation
Yuxiang Liu, Siqi Yuan, Xin Feng, Jianrong DaiAbstract
Background
Cone‐beam computed tomography (CBCT) is routinely used for image guidance in radiation therapy, but conventional full‐field CBCT acquisition may expose anatomically irrelevant regions and generate substantial scatter, which degrades soft‐tissue contrast and limits the accuracy of target localization and adaptive radiotherapy workflows. A clinically practical imaging strategy should improve image quality in the region of clinical interest while preserving sufficient full‐field anatomical information for patient setup and dose‐related assessment.
Purpose
To improve image quality within clinically relevant regions without increasing the total imaging dose, this study developed a dual‐scan conformal cone‐beam CT method using dynamic collimation.
Methods
A dual‐scan conformal acquisition strategy using dynamic collimation was designed. Dynamic collimation was defined as angle‐dependent shaping of the X‐ray field around a preplanned target region; it was implemented with a multi‐leaf collimator model in the digital phantom and patient‐data simulations and with the movable kV collimator in the physical phantom measurement. Collimator positions were calculated from forward projections of the target region, and a target‐region‐based intensity‐optimization model was used to allocate more photons to projection angles and detector regions contributing to the target region. The dual‐scan protocol, consisting of a low‐dose full‐field‐of‐view (FOV) scan and a higher‐dose conformal scan, was reconstructed using a regularized weighted least‐squares algorithm with anisotropic total‐variation regularization. The method was evaluated using a FORBILD head phantom, patient planning CT datasets, and an anthropomorphic head phantom measured on a Varian Edge on‐board imaging system.
Results
The proposed method achieved the best overall image quality among the compared methods. In the FORBILD phantom, for target region 1 (VOI1), SSIM increased from 0.702 to 0.855, CNR increased from 0.514 to 1.463, and SNR1/SNR2 increased from 33.8/32.8 to 113.1/114.6 compared with conventional FDK reconstruction. In the prostate cancer case, CNR increased from 0.44 to 1.94, and SNR1/SNR2 increased from 25.6/23.3 to 74.6/82.4. In the measured anthropomorphic head phantom, CNR improved from 1.036 to 2.379, and SNR1/SNR2 improved from 42.8/50.1 to 89.8/98.0.
Conclusion
The proposed dual‐scan conformal CBCT strategy using dynamic collimation improves local image quality in the clinically relevant target region while maintaining full‐FOV information through the two‐scan protocol. This method may provide a practical route toward patient‐specific CBCT guidance for adaptive radiotherapy.