Artifacts In CBCT Research Articles

SU‐E‐T‐340: Estimation of the Delivered Fractional and Accumulative Patient Dose in IMRT and VMAT

Purpose: To estimate the delivered dose after each fraction of treatment as well as the delivered accumulative dose distribution. The goal of this project is to develop an automatic verification system using daily images and machine log files. Methods: First, we perform deformable registration from CT to CBCT and correct CBCT artifacts and CT numbers. Due to the limitation of the current scanning protocol, CBCT may not cover the whole treatment region needed for dose reconstruction. In that case, we use deformed CT for dose calculation which has correct CT number as well as daily patient geometry within whole treatment region. With machine log files, we generate actually delivered fluence maps at all delivery angles for that fraction. Then a GPU‐based Monte Carlo dose calculation package, gDPM, is employed to get delivered fractional dose on deformed CT or corrected CBCT. To compare the planned dose with the delivered dose, the delivered dose on new daily image is transferred back to the original CT geometry using the deformation vector fields obtained in the first step. This procedure is employed for each fraction to get the accumulative dose on original CT. Results: We demonstrate this treatment verification system on VMAT patients. We plot the DVHs for delivered and planned dose distributions and summarize the absolute errors in dose matrix report. Conclusions: We have proposed a procedure to estimate the delivered fractional and accumulative patient doses in IMRT and VMAT. If big errors are observed in the dose matrix report, then a warning should be sent out to responsible personnel, so the error can be indentified and corrected promptly.

WE-G-BRCD-01: A Procedure for Efficient Large-Scale Retrospective Clinical Studies for Online Adaptive Radiotherapy.

Online adaptive radiotherapy (ART) is promising for handling inter-fraction variations of patient's geometry. Before a clinical implementation of this advanced technology, it is necessary to study its potential clinical gains and optimal frequencies to be used for various tumor sites. The goal of this work is to establish and examine a procedure for efficient large-scale retrospective clinical studies for online ART using a GPU-based re-planning platform. The proposed procedure utilizes an in-house developed GPU-based replanning software called SCORE. SCORE starts by applying deformable registration from CT to CBCT and correcting CBCT artifacts and intensities. However, the CBCT image may not cover the whole treatment region due to the limited field of view. In that case, we use deformed CT for replanning and dose calculation. The final optimized fractional dose is calculated using the optimized fluence maps and a finite size pencil beam model. We also use the deformed CT to calculate the delivered fractional dose using the fluence maps from the original plan. The delivered fractional dose is compared to the optimized fractional dose to estimate the daily gain of replanning. To compare accumulated optimized dose and delivered dose, the delivered and optimized doses are mapped back to the original CT geometry using the deformation vector fields. We tested this procedure using prostate cancer IMRT cases and found that the re-optimized and delivered DVHs and dose distributions can be generated in a couple of minutes. We have developed a procedure using a GPU-based replanning software to retrospectively study the clinical gains of online ART in an efficient and large scale manner.

Removal of Motion Artifacts in CBCT of Lung Tumors Based on Tracked Tumor Motion during CBCT Acquisition

Removal of Motion Artifacts in CBCT of Lung Tumors Based on Tracked Tumor Motion during CBCT Acquisition

TH-D-351-01: High Temporal Resolution and Streak-Free Four-Dimensional Cone-Beam Computed Tomography

Purpose: Respiratory gated CBCT (e.g. 4D CBCT) with an on-board imager has been introduced to track tumor motion for lung cancer patients. However, the image quality is comprised by aliasing artifacts and lower CNR, due to the limited number of projections. The temporal resolution of 4D CBCT is often chosen to be around 500∼1000ms to increase the sampling at each phase. In this paper, we present a method to simultaneously achieve high temporal resolution and streaking artifacts-free images in 4D CBCT. Method and Materials: The enabling technique is an image reconstruction method called Prior Image Constrained Compressed Sensing (PICCS). By incorporating a prior image into the reconstruction, PICCS enables accurate image reconstruction from few projections. Home made physical motion phantoms were scanned on a Varian Trilogy system. Projection data were retrospectively gated based on the phase information obtained by synchronizing the ‘x-ray on’ signal and the motion signal. Results: The gating window was chosen so that only one projection was selected in each respiratory cycle. Streak artifacts free 4D CBCT images were achieved using PICCS from these 11–14 selected projections, while other reconstruction algorithms like FDK and conventional Compressed Sensing failed to generate meaningful images. Temporal information was well preserved in these images and the temporal resolution in these images was primarily limited by the detector read out speed which was about 100ms. Moving objects were segmented out from images at each phase and the motion trajectory was extracted. Agreement was observed between the extracted motion profile and the programmed motion profile. Conclusion: A new algorithm, PICCS, was proposed to mitigate undersampling streaking artifacts in 4D CBCT. High temporal resolution was achievable by using fewer projections. The trajectory of moving objects could be extracted from the high quality 4D CBCT images.

TH-D-351-03: Streaking Artifacts Reduction in Four-Dimensional Cone-Beam Computed Tomography

Purpose: In order to track tumor motion for patients with lungcancer four‐dimensional CBCT (4D CBCT) techniques have been introduced to deal with respiratory motion by gating projections into several phases. However, due to the limited gantry rotation speed, fewer than 100 projections are available for the image reconstruction at each phase. Thus, severe undersampling streaking artifacts plague 4D CBCTimages. In this presentation, we propose a simple scheme to significantly reduce the streaking artifacts. Method and Materials: A prior image is first reconstructed using all of the cone‐beam projection data without gating. This image volume is then reprojected to generate a synthesized projection data set. The difference projections generated from these two data sets are then gated and reconstructed to generate a difference image for each respiratory phase. The difference image is added back to the prior image to generate the final 4D CBCTimage volume for each phase. A home‐made motion phantom was built and scanned on a Varian Trilogy system. Projection data were retrospectively gated based on the phase information. Results: For a given phase, only ∼12 projections were selected (i.e. one projection for each respiratory cycle) to reconstruct the 4D CBCTimages. The fidelity of stationary objects was preserved and descent reconstructions of moving objects were obtained. Streak artifacts were significantly reduced in the reconstructed images. A figure of merit to characterize the streak artifacts strength was introduced and about 70% streak artifacts reduction was achievable compared with traditional FDK reconstruction.Conclusion: An algorithm has been proposed to reduce undersampling streaking artifacts in 4D CBCT. In physical phantom studies, we demonstrated that the streaking artifacts were effectively mitigated (70% reduction compared with FDK reconstruction). This correction scheme enables gating of the 4D CBCT data in a very narrow window (12∼95ms) which significantly improves the temporal resolution.

SU-FF-J-72: The Effect of MV Cone-Beam CT Cupping Artifacts On Dose Calculation Accuracy

Purpose: MV cone‐beam (CB) CTimaging can be used to position the patient prior to external‐beam radiotherapy. If calibrated, these images may also be used in dose calculations, opening the possibility to reconstruct the delivered dose or re‐optimize the treatment plan before delivery. This study investigates the effect of imaging artifacts on dosimetric accuracy. Method and Materials: Two water cylinders of 13 cm and 22.5 cm diameters were imaged using A) a MV CBCT system integrated with a Siemens accelerator and B) a conventional kV CTimager. Cupping trends observed in the MV CBCT were imposed on the kV CTimages to create artificial data with simulated artifacts. Using the Phillips Pinnacle planning system, a 6 MV 10 × 16 cm2 field was applied to 1) the original kV CT of the smaller cylinder and 2) the kV CT with simulated artifacts. Similarly, a 18 × 18 cm2 field was applied to the original and artificial kV CT of the larger cylinder. Results: For the smaller cylinder, the MV CBCTwater signal relative to air at the cylinder center differs from the radial and axial extremities by approximately 14% and 17% respectively. For the larger cylinder, the differences are 38% and 20% in radial and axial directions. The dose distributions for the artificial images show a systematic deviation which increases with depth. In the high‐dose, low‐gradient regions, the largest deviation for the smaller cylinder is 1% of the maximum delivered dose. For the larger cylinder, the largest deviation is 4%. Conclusion: Cupping artifacts in water phantoms produce dosimetric errors much smaller in magnitude than the cupping trend itself but as large as 4% for large phantoms. These results suggest that rough correction of MV CBCT artifacts may be sufficient for dosimetric accuracy. Conflict of Interest: Research supported by Siemens OCS.