Estimating 4D‐CBCT from prior information and extremely limited angle projections using structural PCA and weighted free‐form deformation for lung radiotherapy

Abstract

Purpose To investigate the feasibility of using structural-based principal component analysis (PCA) motion-modeling and weighted free-form deformation to estimate on-board 4D-CBCT using prior information and extremely limited angle projections for potential 4D target verification of lung radiotherapy. Methods A technique for lung 4D-CBCT reconstruction has been previously developed using a deformation field map (DFM)-based strategy. In the previous method, each phase of the 4D-CBCT was generated by deforming a prior CT volume. The DFM was solved by a motion-model extracted by global PCA and free-form deformation (GMM-FD) technique, using a data fidelity constraint and deformation energy minimization. In this study, a new structural-PCA method was developed to build a structural motion-model (SMM) by accounting for potential relative motion pattern changes between different anatomical structures from simulation to treatment. The motion model extracted from planning 4DCT was divided into two structures: tumor and body excluding tumor, and the parameters of both structures were optimized together. Weighted free-form deformation (WFD) was employed afterwards to introduce flexibility in adjusting the weightings of different structures in the data fidelity constraint based on clinical interests. XCAT (computerized patient model) simulation with a 30 mm diameter lesion was simulated with various anatomical and respirational changes from planning 4D-CT to onboard volume to evaluate the method. The estimation accuracy was evaluated by the Volume-Percent-Difference (VPD)/Center-of-Mass-Shift (COMS) between lesions in the estimated and “ground-truth” on board 4D-CBCT. Different onboard projection acquisition scenarios and projection noise levels were simulated to investigate their effects on the estimation accuracy. The method was also evaluated against 3 lung patients. Results The SMM-WFD method achieved substantially better accuracy than the GMM-FD method for CBCT estimation using extremely small scan angles or projections. Using orthogonal 15° scanning angles, the VPD/COMS were 3.47±2.94% and 0.23±0.22mm for SMM-WFD and 25.23±19.01% and 2.58±2.54mm for GMM-FD among all 8 XCAT scenarios. Compared to GMM-FD, SMM-WFD was more robust against reduction of the scanning angles down to orthogonal 10° with VPD/COMS of 6.21±5.61% and 0.39±0.49mm, and more robust against reduction of projection numbers down to only 8 projections in total for both orthogonal-view 30° and orthogonal-view 15° scan angles. SMM-WFD method was also more robust than the GMM-FD method against increasing levels of noise in the projection images. Additionally, the SMM-WFD technique provided better tumor estimation for all three lung patients compared to the GMM-FD technique. Conclusion Compared to the GMM-FD technique, the SMM-WFD technique can substantially improve the 4D-CBCT estimation accuracy using extremely small scan angles and low number of projections to provide fast low dose 4D target verification.

A Novel method to generate on‐board 4D MRI using prior 4D MRI and on‐board kV projections from a conventional LINAC for target localization in liver SBRT

“…A previous technique was developed using prior images, PCA‐based motion modeling, and on‐board fiducial tracking based on x‐ray imaging to reconstruct respiratory motion for on‐board dose reconstruction or tracking for proton therapy . However, the method is potentially limited by the accuracy of the PCA modeling since patient breathing patterns can change from simulation to treatment, as shown in our previous studies . Furthermore, it required the invasive procedure of fiducial marker implantation, which may not always be applicable for patients.…”

“…[22][23][24] However, the method is potentially limited by the accuracy of the PCA modeling since patient breathing patterns can change from simulation to treatment, as shown in our previous studies. 4,6,20 Furthermore, it required the invasive procedure of fiducial marker implantation, which may not always be applicable for patients. The marker migration would also affect the accuracy of the algorithm.…”

“…This is especially important for liver stereotactic body radiation therapy (SBRT) treatments since (a) respiratory motion can cause treatment errors and (b) SBRT is more prone to significant dosimetric errors due to high dose per fraction delivery over a longer temporal duration. Therefore, on‐board 4D verification of the liver tumor before and during the SBRT treatment is critical …”

“…Therefore, on-board 4D verification of the liver tumor before and during the SBRT treatment is critical. [4][5][6][7] Conventionally, cone-beam CT (CBCT) has been used for on-board target localization in liver SBRT. However, its localization accuracy is limited due to the poor soft tissue contrast and limited image quality when motion is involved.…”

Image acquisition optimization of a limited‐angle intrafraction verification (LIVE) system for lung radiotherapy

“…Through PCA motion modeling, the problem of solving the deformation field map D is converted into solving coefficients of the deformation modes, which substantially reduces the degree of freedom to find D . A gradient descent method is used as the optimizer to solve the coefficients, as was used in our previous studies …”