Cone Beam CT In Radiation Therapy Research Articles

Improving Image Quality of On-Board Cone-Beam CT in Radiation Therapy Using Image Information Provided by Planning Multi-Detector CT: A Phantom Study

PurposeThe aim of this study was to improve the image quality of cone-beam computed tomography (CBCT) mounted on the gantry of a linear accelerator used in radiation therapy based on the image information provided by planning multi-detector CT (MDCT).MethodsMDCT-based shading correction for CBCT and virtual monochromatic CT (VMCT) synthesized using the dual-energy method were performed. In VMCT, the high-energy data were obtained from CBCT, while the low-energy data were obtained from MDCT. An electron density phantom was used to investigate the efficacy of shading correction and VMCT on improving the target detectability, Hounsfield unit (HU) accuracy and variation, which were quantified by calculating the contrast-to-noise ratio (CNR), the percent difference (%Diff) and the standard deviation of the CT numbers for tissue equivalent background material, respectively. Treatment plan studies for a chest phantom were conducted to investigate the effects of image quality improvement on dose planning.ResultsFor the electron density phantom, the mean value of CNR was 17.84, 26.78 and 34.31 in CBCT, shading-corrected CBCT and VMCT, respectively. The mean value of %Diff was 152.67%, 11.93% and 7.66% in CBCT, shading-corrected CBCT and VMCT, respectively. The standard deviation within a uniform background of CBCT, shading-corrected CBCT and VMCT was 85, 23 and 15 HU, respectively. With regards to the chest phantom, the monitor unit (MU) difference between the treatment plan calculated using MDCT and those based on CBCT, shading corrected CBCT and VMCT was 6.32%, 1.05% and 0.94%, respectively.ConclusionsEnhancement of image quality in on-board CBCT can contribute to daily patient setup and adaptive dose delivery, thus enabling higher confidence in patient treatment accuracy in radiation therapy. Based on our results, VMCT has the highest image quality, followed by the shading corrected CBCT and the original CBCT. The research results presented in this study should be able to provide a route to reach a high level of image quality for CBCT imaging in radiation oncology.

Imaging Doses and Secondary Cancer Risk From Kilovoltage Cone-beam CT in Radiation Therapy

The authors assessed the radiation-induced cancer risk due to organ doses from kilovoltage (kV) cone beam computed tomography (CBCT), a verification technique in image-guided radiotherapy (IGRT). CBCTs were performed for three different treatment sites: the head and neck, chest, and pelvis. Using a glass dosimeter, primary doses versus depth were measured inside a homemade phantom, and organ doses were measured at various locations inside an anthropomorphic phantom. The excess relative risk (ERR), excess absolute risk (EAR), and lifetime attributable risk (LAR) for cancer induction were estimated using the BEIR VII models based on dose measurement. The average primary (i.e., in-field) doses at the center of the phantom for standard imaging options were 1.9, 5.1, and 16.7 cGy for the head and neck, chest, and pelvis, respectively. The average secondary dose per scan for the pelvis measured 20-50 cm from the isocenter and ranged from 0.67-0.02 cGy, whereas the secondary dose per scan for the head and neck ranged from 0.07-0.003 cGy, indicating that CBCT for treatment of the head and neck is associated with a smaller secondary radiation dose than CBCT for treatment of the pelvis. The estimation of LAR from CBCT in IGRT indicated that the lifetime cancer risk for major organs can reach approximately 400 per 10,000 persons if 30 CBCT scans are performed to position a patient during radiation treatment of the pelvis site.

TH-D-351-05: Scatter Correction for Cone-Beam CT in Radiation Therapy

Purpose: Cone-beam CT (CBCT) is being increasingly used in radiation therapy. However, as compared to conventional CT, the degraded image quality of CBCT hampers its applications. Due to the large volume of x-ray illumination in CBCT, scatter is considered as one of the fundamental limitations of CBCT image quality. Many scatter correction algorithms have been developed, while drawbacks still exist. Here, we propose a new scatter correction method which is particularly useful in radiation therapy. Method and Materials: Since the same patient is scanned repetitively during one radiation treatment course, we measure the scatter distributions in one scan, and use the measured scatter distributions to estimate and correct scatter in the following scans. A partially blocked CBCT is used in the scatter measurement scan. The x-ray beam blocker has a strip pattern, such that the whole-field scatter distribution can be estimated from the detected signals in the shadow region and the patient rigid transformation can be determined from the reconstructed image using the illuminated detector projection data. From the derived patient transformation, the measured scatter is then modified and used for scatter correction in the following regular CBCT scans. Results: The proposed method has been evaluated using Monte Carlo simulations and physical experiments on an anthropomorphic chest phantom. The results show a significant suppression of scatter artifacts using the proposed method. Using the reconstruction in a narrow collimator geometry as a reference, the comparison also shows that the proposed method reduces reconstruction error from 13.2% to 0.8%. Conclusion: This work indicates that much improved CBCT image quality is achievable using the proposed scatter correction method in radiation therapy. Our method is very attractive in applications where high CBCT reconstruction accuracy is critical, for example, dose calculation in adaptive radiation therapy.

Low-dose megavoltage cone-beam CT for radiation therapy

The objective of this work was to demonstrate the feasibility of acquiring low-exposure megavoltage cone-beam CT (MV CBCT) three-dimensional (3D) image data of sufficient quality to register the CBCT images to kilovoltage planning CT images for patient alignment and dose verification purposes. A standard clinical 6-MV Primus linear accelerator, operating in arc therapy mode, and an amorphous-silicon (a-Si) flat-panel electronic portal-imaging device (EPID) were employed. The dose-pulse rate of 6-MV Primus accelerator beam was windowed to expose an a-Si flat panel by using only 0.02 to 0.08 monitor unit (MUs) per image. A triggered image-acquisition mode was designed to produce a high signal-to-noise ratio without pulsing artifacts. Several data sets were acquired for an anthropomorphic head phantom and frozen sheep and pig cadaver head, as well as for a head-and-neck cancer patient on intensity-modulated radiotherapy (IMRT). For each CBCT image, a set of 90 to 180 projection images incremented by 1 degree to 2 degrees was acquired. The two-dimensional (2D) projection images were then synthesized into a 3D image by use of cone-beam CT reconstruction. The resulting MV CBCT image set was used to visualize the 3D bony anatomy and some soft-tissue details. The 3D image registration with the kV planning CT was performed either automatically by application of a maximization of mutual information (MMI) algorithm or manually by aligning multiple 1D slices. Low-noise 3D MV CBCT images without pulsing artifacts were acquired with a total delivered dose that ranged from 5 to 15 cGy. Acquisition times, including image readout, were on the order of 90 seconds for 180 projection images taken through a continuous gantry rotation of 180 degrees. The processing time of the data required an additional 90 seconds for the reconstruction of a 256(3) cube with 1.0-mm voxel size. Implanted gold markers (1 mm x 3 mm) were easily visible or all exposure levels without artifacts. In general, the presence of high Z materials such as tooth fillings or implanted markers did not result in visible streak artifacts. The registration of structures such as the spinal canal and the nasopharynx in the MV CBCT and kV CT data sets was possible with millimeter and degree accuracy as assessed by displacement simulations and subsequent visual evaluation. We believe that the quality of these images, along with the rapid acquisition and reconstruction times, demonstrates that MV CBCT performed by use of a standard linear accelerator equipped with a flat-panel imager can be applied clinically for patient alignment.