CBCT Projections Research Articles

TU‐C‐M100J‐02: Direct Detection of the Tumor Trajectory Using Raw Cone Beam CT Projections

Purpose: To develop and evaluate a template matching algorithm for direct detection of a lung tumor trajectory in the raw projections acquired from a cone beam CT. Method and Materials: A planning respiration‐correlated CT (pRCCT) was used to construct digitally‐reconstructed radiographic (DRR) templates for registration with raw projection data of a free‐breathing cone beam CT (FB‐CBCT). The pRCCT and FB‐CBCT were first registered to obtain the approximate mean 3D position of the tumor. This position was applied as a shift to the planning isocenter in the pRCCT. Next, 72 DRRs were generated at 5 degree angular increments around this shifted position, for each phase in the pRCCT. The appropriate phase DRR template for registration was selected using the measured phase of the FB‐CBCT projection. This phase was measured by tracking the diaphragm apex position in the projections. Two template matching algorithms were compared: normalized cross correlation and a block normalized cross correlation. A dynamic phantom was induced to move reproducibly 2cm during planning and 3cm during online CBCT imaging. The phantom was imaged at isocenter and with a 1cm translational shift from isocenter. Results: The tumor excursion measured with the normalized cross correlation method was 28.7 mm, and 28.5 mm with the block normalized cross correlation. The error in detecting the daily standard deviation of tumor position was 0.1mm for both methods. The mean absolute difference in detected tumor position was 1.07 mm, with a 95% confidence value of 2.43 mm. Block normalized cross correlation reduced the maximum error in tumor position detection slightly, in relation to normalized cross correlation. Conclusion: Direct detection of the tumor position from cone beam CT projections is feasible with template matching, provided that appropriate and accurate templates are used. Conflict of Interest: Supported in part by NIH R01 CA116249 and Elekta, Inc.

WE-E-M100F-01: 3D Interfraction Position Verification for Patients Undergoing Partial Breast Irradiation: Comparing Digital Tomosynthesis to Cone-Beam CT

Purpose: This clinical study evaluates digital tomosynthesis (DTS) technology for daily imaging guidance for partial breast irradiation (PBI) and compares its positioning accuracy to the 3D CBCT technique. Compared to CBCT, DTS offers lower imaging dose and the geometrical flexibility that particularly suits PBI. Methods and Material: Ten patients undergoing PBI were imaged using an on‐board‐imager mounted on a Varian 21EX linear accelerator. Following the initials setup using skin markers and 2D KV/MV radiographs, a CBCT scan was acquired to provide 3D positioning guidance. A subset of the CBCT projections were used to reconstruct a stack of DTS image slices using the Feldkamp filtered back‐projection algorithm. To optimize soft tissuecontrast, the DTS images were reconstructed for the 45‐degree oblique view along which the tumor bed, breast tissues, bones, and lung were well separated. Coronal and sagittal DTS views were also reconstructed for comparison. The inter‐fraction position deviations between the 1st fraction and each of the subsequent fractions were measured by coronal‐DTS, sagittal‐DTS, oblique‐DTS and CBCT as four independent technologies. Differences between these technologies and their clinical impact were evaluated. The evaluator was well trained for DTS technology. Results: Eighty‐five imaging datasets were obtained from 10 patients. Surgical clips (when present) were visible in all three DTS views. The tumor bed had the best contrast in the oblique‐DTS. One‐dimensional positioning differences between DTS (averaged over 3 DTS views) and CBCT were ∼0.1 cm when surgical clips were used in the registration and ∼0.2 cm when the tumor bed were used in the registration. Conclusion: DTS is equivalent to CBDT as a 3D imaging technique for daily patient positioning of PBI but with less mechanical constraints and imaging dose. Partially supported by a research grant from Varian Medical Systems.

How image quality affects determination of target displacement when using kilovoltage cone‐beam computed tomography

The advent of kilovoltage cone‐beam computed tomography (CBCT) integrated with a linear accelerator allows for more accurate image‐guided radiotherapy (IGRT). The IGRT technique corrects target displacement based on internal body information obtained by acquiring the CBCT image set and registering it with the simulation CT image set just before the beam is delivered. In the present paper, we compare registration results by CBCT reconstruction quality (either high or medium). We analyzed data from a total of 56 CBCT projections from 6 patients. The translational vector differences were within 1 mm in all but 3 cases. The rotational displacement differences considered components of all three axes, and in 3 of 168 cases (56 projections by 3 axes), showed more than 1 degree of difference.PACS number: 87.57.Nk

Four‐dimensional cone‐beam computed tomography using an on‐board imager

On-board cone-beam computed tomography (CBCT) has recently become available to provide volumetric information of a patient in the treatment position, and holds promises for improved target localization and irradiation dose verification. The design of currently available on-board CBCT, however, is far from optimal. Its quality is adversely influenced by many factors, such as scatter, beam hardening, and intra-scanning organ motion. In this work we quantitatively study the influence of organ motion on CBCT imaging and investigate a strategy to acquire high quality phase-resolved [four-dimensional (4D)] CBCT images based on phase binning of the CBCT projection data. An efficient and robust method for binning CBCT data according to the patient's respiratory phase derived in the projection space was developed. The phase-binned projections were reconstructed using the conventional Feldkamp algorithm to yield 4D CBCT images. Both phantom and patient studies were carried out to validate the technique and to optimize the 4D CBCT data acquisition protocol. Several factors that are important to the clinical implementation of the technique, such as the image quality, scanning time, number of projections, and radiation dose, were analyzed for various scanning schemes. The general references drawn from this study are: (i) reliable phase binning of CBCT projections is accomplishable with the aid of external or internal marker and simple analysis of its trace in the projection space, and (ii) artifact-free 4D CBCT images can be obtained without increasing the patient radiation dose as compared to the current 3D CBCT scan.

SU-CC-ValA-01: Automatic Comparison Between Reference and On Board Digital Tomosynthesis for Target Localization

Purpose: Digital tomosynthesis (DTS) is a method for reconstructing 3D images from cone-beam projection data acquired with limited angulation (e.g., 40°) of an x-ray source, and is much faster and lower dose than full cone-beam CT (CBCT). We previously developed a method for generating reference DTS images from a planning CT for registration with actual on-board DTS images. This study examines the accuracy of 3D-3D registration of reference and on-board DTS images to assess the potential of DTS for image-guided radiation therapy (IGRT). Method and Materials: We simulated the online positioning of an anthropomorphic chest phantom with 6 noncoplanar reference BBs attached. Planning CT data of the phantom were acquired with a G.E. Lightspeed RT scanner. On-board CBCT projection data were acquired with a Varian 21EX Clinac, equipped with a kV on-board imager. On-board DTS images were reconstructed from a subset of the CBCT projection data (81 projections, 44°). True alignment of planning and on-board image data was achieved according to a 3D point-based registration of the 6 reference BBs in the CT and CBCT images. Single-axis rotations up to +/− 10° and 3-axis translations up to +/− 10 mm were simulated in the planning CT, prior to the generation of reference DTS images. A 67.5mm × 162.5mm × 20.8mm region of interest surrounding the spinal cord was extracted for registration. Mutual information-based 3D-3D registration of reference and on-board DTS images was performed, and residual registration error was recorded. Results: Registration errors are within 0.7mm and 0.1 degree in all cases. The average registration error was 30% less for translations along the dimension of tomographic motion than for the other two dimensions. Conclusions: 3D-3D rigid-body registration of reference and on-board DTS images is highly accurate, suggesting that DTS may be an effective IGRT technique. Partially supported by a Varian research grant.

SU‐EE‐A1‐04: Verifying Internal Target Volume Using Cone‐Beam CT for Stereotactic Body Radiotherapy Treatment

Purpose: The internal target volume (ITV) could be determined using 4D CT simulation images and be verified in the treatment room using on‐board cone‐beam CT (CBCT) since the CBCT projection images are acquired over approximately 10 breathing cycles. This study used a 4D dynamic phantom to verify the accuracy of this technique and also to develop a procedure for using CBCT to clinically verify ITV in stereotactic body radiotherapy (SBRT) treatment. Method and Materials: A CIRS 4D dynamic phantom, with a target ball and precisely controlled motion, was imaged using a 4D CT scanner. A Varian RPM system was used for respiratory gating. Ten 3‐D image sets were generated corresponding to 10 breathing phases. The ITV was determined based on the phase images. To assess concordance, on‐board CBCT images of the target ball were compared with the 4D‐CT defined ITV. SBRT patient with tumor targets located in the thorax and upper abdomen were similarly scanned using phase gating 4D CT. The ITVs were compared between simulation CT and CBCT scans to identify localization error. Results: The dynamic phantom motion was 20mm along the inferior‐superior direction, 5mm along the anterior‐posterior direction, and 2mm along the left‐right direction with a cycle time of 4 seconds. The concordance of the CBCT and ITV matching was within 1 mm. For the lung SBRT patient, the target volume based on the CT images without respiratory gating was 0.7 cc. The ITV was 2 cc. The ITV matches well with the CBCT images. The localization errors between free‐breathing CT and CBCT were 2 mm to the right, 1 mm to the anterior, and 2 mm to the superior. Conclusion: CBCT provides an accurate assessment of the ITV for targets affected by respiratory motion.

TH‐C‐ValB‐07: Enhanced 4D CBCT Imaging for Slow‐Rotating On‐Board Imager

Purpose: Four‐dimensional (4D) cone‐beam CT (CBCT) is currently obtained by respiratory phase binning of the acquired projections followed by independent CBCT reconstruction of the rebinned projections for each phase. Due to the significantly reduced number of projections in a single phase bin, the quality of 4D‐CBCT images is severely degraded. In this work, we develop a technique to combine the data from all phases for greatly enhanced 4D CBCT. Method and Materials: 4D‐CBCT data were acquired with “extremely‐slow‐gantry‐rotation” protocol (1 deg/sec) on a Varian Acuity system. The respiratory phase binning was achieved by automatic texture matching of the CBCT projections, and 4D‐CBCT images were subsequently obtained by Feldkamp reconstruction of the phase‐binned data. Finally, to enhance the quality, the 4D‐CBCT images were superimposed via deformable registration based on maximum mutual information and BSpline model. The method was quantitatively evaluated with numerical and physical phantom experiments. Three clinical cases are also being investigated. Results: The numerical and physical phantom studies showed that 4D CBCT resulted in less motion artifacts than 3D CBCT, however, the view‐aliasing artifacts are visible in 4D CBCT images due to limited number of projections being used for reconstructing each phase, this led to large fluctuation of the CT numbers in any uniform region and generally reduced contrast‐to‐noise ratio (CNR). In comparison, the proposed 4D CBCT technique scientifically improved the CT number accuracy, the image uniformity, and the CNRs. Conclusion: A novel technique for 4D CBCT imaging with a slow‐rotating onboard imager has been developed. It integrates information of different respiratory phases to achieve an enhanced image quality. The improvement on the CT number accuracy is extremely important to image‐guide radiation therapy for online treatment planning and off‐line dose verification.

WE‐C‐ValA‐10: 4D Cone‐Beam CT (CBCT) Using An On‐Board Imager

Purpose: To develop an artifact‐free four‐dimensional (4D) cone‐beam CT (CBCT) imaging technique for image‐guided radiotherapy, and to optimize the image quality, scanning time, and patient radiation dose with respect to acquisition parameters such as the number of gantry rotations, gantry‐rotation speed, and x‐ray tube current. Method and Materials: A Varian Trilogy™ system was employed for this study. To sort CBCT projections in terms of patient breathing phase, a CT‐opaque fiducial was adhered to the patient skin and tracked automatically in the projection space, and the phases of the fiducial trajectory were used to tag the projections. Projections after phase‐binning were subsequently reconstructed to yield 4D‐CBCT images. To reduce/eliminate view‐aliasing artifacts due to limited number of projections in each phase, both “multiple‐rotation” and “slow‐gantry‐rotation” strategies were investigated. Quantitative evaluation and comparison were performed with a motion phantom for the following acquisition settings: (i)varying number of gantry rotations (1∼8) with all other scanning parameters kept the same; (ii)varying x‐ray tube current together with the number of rotations while keeping the radiation dose constant, namely, 1‐rotation‐80mA, 2‐rotation‐40mA, 4‐rotation‐20mA, and 8‐rotation‐10mA; (iii)varying gantry‐rotation speed (1deg/sec∼8deg/sec). Three patient cases were used in the study. Results: The image quality, represented by relative error (RE), varied nonlinearly with the number of rotations: RE reduction became less pronounced as the number of rotations increased. After 3∼5 rotations, the benefit resulting from more rotations started saturating. When the dose level was kept the same, the images of 1‐rotation‐80mA and 8‐rotation‐10mA acquisitions had the largest and smallest REs, respectively. Similar RE reduction behaviors were found when slowing down the gantry‐rotation speed. For the patient cases, 4D images were obtained with negligible motion or view‐aliasing artifacts. Conclusion: We have successfully demonstrated that the commercially available OBI system can be utilized to acquire artifacts‐free 4D‐CBCT images without increasing the patient radiation dose.

TH‐E‐330A‐07: Evaluation of Scatter Mitigation Strategies for X‐Ray Cone‐Beam CT

Purpose: Linac‐mounted cone‐beam CT (CBCT) imaging is a promising approach for improving the soft‐tissue targeting accuracy of external‐beam radiotherapy. However, the large proportion of signal due to scattered radiation results in large cupping artifacts and reduced contrast‐to‐noise ratio (CNR), hampering delineation of soft‐tissue structures. The goal of this investigation is evaluate the impact of three scatter mitigation strategies: scatter removal (SR), subtractive scatter corrections (SSC), and anti‐scatter grids (ASG) on CBCT image CNR.Method and Materials: A one‐dimensional model was developed that to predict image noise, intensity, and contrast from the photon flux incident upon the flat‐panel detector. Using previously published scatter‐to‐primary ratio (SPR) data, the impact of Poisson signal statistics, Gaussian readout noise, reconstruction filter, and total air‐kerma on image uniformity, contrast, and CNR was evaluated. Using previously measured CBCT scatter and primary grid transmission; the impact of ASGs, SSR, and SR (e.g., via bowtie filters) was evaluated. In addition, synthetic polyenergetic CBCT projections of cylindrical low‐contrast threshold phantoms, including compound Poisson process noise, were used to evaluate the impact of scatter mitigation on filtered backprojection (FBP) images. Results: Relative to complete SR, typical CBCT scatter reduces CNR by 30%–70% depending on patient diameter. Both 1D analysis and simulated images demonstrate that SSC effectively reduces cupping artifacts but does not improve CNR. ASGs with high scatter selectivity (>5) or large primary transmission (>0.7) modestly increase (20–30%) CNR for thick subjects. However, ASGs diminish CNR for smaller body sections or for exposure levels where additive noise dominates sinogram signal statistics. Conclusion: Correcting CBCT sinograms for scatter radiation is effective in reducing structural image artifacts but does not improve CNR. Antiscatter grids effectively suppress scatter artifacts but improve CNR only in selected clinical settings. This project was supported in part by a grant from Varian Medical Systems.

TH‐C‐ValB‐05: Rapid Low‐Dose 3D Image‐Guided Treatment Verification of Sites Prone to Respiratory Motion Using Breath‐Hold On‐Board Digital Tomosynthesis (DTS)

Purpose: This study examines the potential use of on‐board digital tomosynthesis (DTS) for image‐guided treatment verification of sites prone to respiratory motion. DTS is a fast, low‐dose method for reconstructing 3‐ D images from 2‐D projection data, acquired with a limited scan angle. In the treatment room, a 45° DTS scan can be acquired in less than 10 seconds, making rapid breath‐hold DTS a simple method for acquiring 3‐D treatment verification images, devoid of respiratory motion. Method and Materials: On‐board DTS images of ten human liver, pancreas, and lung subjects were reconstructed from kV CBCT projection data, acquired on a Varian 21EX Clinac equipped with an on‐board imager (OBI), either during a breath‐hold or while subjects were freely breathing. Corresponding reference DTS (RDTS) images were reconstructed from breath‐hold planning CT data. Soft‐tissue visibility was compared between breath‐hold DTS and free‐breathing DTS and CBCT, to assess the potential efficacy of the breath‐hold DTS strategy for 4‐D image‐guided treatment verification. Results: Breath‐hold DTS markedly improved the rendering of soft‐tissue abdominal and thoracic anatomy, compared with free‐breathing DTS or CBCT. Organ structures were clearly defined, and even low‐contrast target malignancies were often visible in breath‐hold DTS reconstructions. Free‐breathing DTS and CBCT reconstructions, on the other hand, often exhibited artificially enlarged target volumes and poor visibility of soft‐tissue anatomy, due to motion averaging effects. Conclusion: Rapid breath‐hold DTS enhances the visibility of bony and soft‐tissue anatomy in sites prone to respiratory motion, facilitating daily localization of soft‐tissue targets. Breath‐hold DTS localization is superior to free‐breathing on‐board CBCT for thoracic and abdominal image‐guidance. Conflict of Interest: This research was supported in part by a grant from Varian Medical Systems.

Motion correction for improved target localization with on-board cone-beam computed tomography

On-board imager (OBI) based cone-beam computed tomography (CBCT) has become available in radiotherapy clinics to accurately identify the target in the treatment position. However, due to the relatively slow gantry rotation (typically about 60 s for a full 360° scan) in acquiring the CBCT projection data, the patient's respiratory motion causes serious problems such as blurring, doubling, streaking and distortion in the reconstructed images, which heavily degrade the image quality and the target localization. In this work, we present a motion compensation method for slow-rotating CBCT scans by incorporating into image reconstruction a patient-specific motion model, which is derived from previously obtained four-dimensional (4D) treatment planning CT images of the same patient via deformable registration. The registration of the 4D CT phases results in transformations representing a temporal sequence of three-dimensional (3D) deformation fields, or in other words, a 4D model of organ motion. The algorithm was developed heuristically in two-dimensional (2D) parallel-beam geometry and extended to 3D cone-beam geometry. By simulations with digital phantoms capable of translational motion and other complex motion, we demonstrated that the algorithm can reduce the motion artefacts locally, and restore the tumour size and shape, which may thereby improve the accuracy of target localization and patient positioning when CBCT is used as the treatment guidance.

Respiratory correlated cone-beam computed tomography on an isocentric C-arm

A methodology for 3D image reconstruction from retrospectively gated cone-beam CT projection data has been developed. A mobile x-ray cone-beam device consisting of an isocentric C-arm equipped with a flat panel detector was used to image a moving phantom. Frames for reconstruction were retrospectively selected from complete datasets based on the known rotation of the C-arm and a signal from a respiratory monitor. Different sizes of gating windows were tested. A numerical criterion for blur on the reconstructed image was suggested. The criterion is based on minimization of an Ising energy function, similar to approaches used in image segmentation or restoration. It is shown that this criterion can be used for the determination of the optimal gating window size. Images reconstructed from the retrospectively gated projection sequences using the optimal gating window data showed a significant improvement compared to images reconstructed from the complete projection datasets.

Respiratory Signal Extraction for 4D CT Imaging of the Thorax from Cone-Beam CT Projections

Current methods of four-dimensional (4D) CT imaging of the thorax synchronise the acquisition with a respiratory signal to restrospectively sort acquired data. The quality of the 4D images relies on an accurate description of the position of the thorax in the respiratory cycle by the respiratory signal. Most of the methods used an external device for acquiring the respiratory signal. We propose to extract it directly from thorax cone-beam (CB) CT projections. This study implied two main steps: the simulation of a set of CBCT projections, and the extraction, selection and integration of motion information from the simulation output to obtain the respiratory signal. A real respiratory signal was used for simulating the CB acquisition of a breathing patient. We extracted from CB images a respiratory signal with 96.4% linear correlation with the reference signal, but we showed that other measures of the quality of the extracted respiratory signal were required.

SU-FF-J-84: Accuracy and Feasibility of Cone Beam Computed Tomography (CBCT) for Stereotactic Radiosurgery (SRS) Setup

Purpose: To investigate the accuracy and feasibility of cone beam computed tomography (CBCT) for stereotactic radiosurgery (SRS) setup. Method and Materials: A stereotactic BRW frame was attached to a Rando head phantom with imbedded radio-opaque markers (1mm in diameter) simulating the target locations. The head phantom with a CT localizer was first simulated for radiosurgery planning on a conventional CT scanner and then scanned on a treatment machine using CBCT. The differences between the conventional and CBCT localizations of the target positions were computed using a radiosurgery planning software. The translational corrections for target positions were calculated as the differences between the CBCT coordinates of the machine isocenter and the planned isocenters, which were the planned BRW coordinates from the conventional CT scan multiplied by the transformation matrix between the BRW and CBCT coordinate systems on the CBCT scan. The setup accuracy of CBCT was assessed from the analysis of orthogonal projection images for each radio-opaque target at the machine isocenter. Results: All nine fiducial markers of BRW localizer were successfully identified on all but one slice of the CBCT scan. The average localization difference between the conventional CT and CBCT BRW target coordinates was 0.28mm (SD 0.10mm). The mean distance error for all the radio-opaque targets localized using the CBCT and orthogonal projection images was 1.28mm (SD 0.61 mm). The major contributing factor to this cumulative setup error was the uncertainty in the superior-inferior direction due to the 2mm slice thickness in conventional CT. Conclusion: The CBCT image guidance can be used to setup SRS patients within accuracy comparable to the current SRS standard using an external fiducial system. The technique described here can serve as a gold standard for evaluating the accuracy of alternative immobilization and setup devices for SRS.

TU‐D‐I‐611‐05: Monte Carlo Investigation of Scatter Contribution to Kilovoltage Cone‐Beam Computed Tomography Images

Purpose: In this work, we evaluate the scatter radiation present in projection images produced by kilovoltage (kV) cone‐beam computed tomography (CBCT) using Monte Carlo (MC) calculations. We also demonstrate that MC simulations can be used to reduce scatter in kV CBCT reconstructed images. Method and Materials: All images were acquired using a kV CBCT bench‐top system composed of an x‐ray tube, a rotation stage and a CsI flat‐panel imager. The EGSnrc MC code was used to fully model the flat‐panel imager and the x‐ray source for energies of 80, 100, and 120 kVp. The x‐ray source model was validated using first and second half‐value layer (HVL) and profile measurements. MC simulations and measured images were obtained for a cylindrical water phantom and for the head of the anthropomorphic Rando phantom. A modified version of the DOSXYZnrc program was used to score phase space files with identified scattered and non‐scattered particles after the phantoms. The cone angle and the source‐to‐detector distance (SDD) were varied during the simulations to determine their effect on scatter radiation in projection images. MC predictions of the scatter distribution were subtracted from the CBCT projection images prior to the reconstruction to improve the reconstructed image quality. Results: The simulated and measured first and second HVL agree within 2% except for the 80 kVp beam where the agreement is within 9%. The measured and simulated profiles agree within 4%. Simulated and measured projection images are in good agreement. Decreasing the cone angle and increasing the SDD decrease the amount of scatter. On the other hand, the phantom geometry has little effect on the scatter radiation distribution. Conclusion: MC simulations were used to characterize the scatter radiation from different geometries. It was shown that it is possible to use MC simulations to reduce the scatter in the reconstructed image.

WE‐C‐T‐617‐10: Geometry Calibration of An On‐Board KV Imaging System

Purpose: We have developed a method of geometrically calibrating the MV and kV radiographic isocenters of a Varian linear accelerator equipped with an on‐board kV imaging system. Method and Materials: The calibration uses a cylindrical phantom containing 16 BB's placed in a spiral pattern and ∼400–600 radiographs of the phantom acquired using both kV and MV imaging systems. Since the phantom has a known geometry, the 7 degrees of freedom (DOF) needed to fit the predicted projections of the BB's to the measured projections can be calculated. During MV imaging a partial transmission plate is placed in the accelerator wedge slot to project a reference shape onto the EPID images of the phantom. The reference shape accounts for motion of the EPID and generates the radiographic isocenter for the MV x‐ray beam alone. Geometry correction parameters can be calculated for the kV imaging system that map the center of radiographic images or CBCT projections to the radiographic isocenter of the MV beam. The method has been tested with linear accelerator and lab bench imaging systems. Results: BB identification accuracy is ∼0.3 pixels and is limited by noise in the images. While the method is insensitive to changes in SID (0.3 pixels ∼= 2.5 cm SID change), the method can measure known in‐plane imager displacements to an accuracy of better than 0.1 mm. Imager displacements versus gantry angle are small − 0.75×0.50 mm. CBCT reconstruction using parameters from this method results in spatial frequencies as high as 18 lp/cm and adjustments in parameters of ±0.1 mm reduce the reconstructed spatial resolution. Conclusion: Our method to match MV and kV radiographic isocenters for on‐board imaging systems shows that accelerator flex is quite modest but that the calibration method can improve the spatial resolution of the CBCT reconstructions.