Cone-beam Computed Tomography Acquisition Research Articles

Poster - Wed Eve-55: Automated Image Guidance for Prostate IMRT Using Low Dose Cone Beam CT

Currently, we are using Varian's On Board Imager to acquire cone beam computed tomography(CBCT)images prior to prostate intensity modulated radiotherapy(IMRT). The images are used to determine the post‐setup prostate position, and thus, to shift the couch towards the situation assumed during treatment planning. This “manual forward” image guided radiotherapy(IGRT) technique is time consuming and requires significant imagingdose over the course of treatment. Therefore, we present two low‐dose IGRT methods that are entirely automatic. The dose is reduced by lowering the mAs during CBCT acquisition at the cost of increased noise in the final images. In the “auto forward” IGRT technique, the CBCTimage is registered to the planning CT and the result is used to shift the patient on the couch. The “auto reverse” technique involves non‐rigid registration of the planning CT to the CBCT. Both techniques were evaluated using a retrospective analysis of data from ten patients, including one planning CT and five CBCTimages acquired at evenly spaced fractions per patient. At the standard imagingdose,IGRT error was 4.3 ± 1.6, 3.9 ± 1.2, 3.8 ± 1.8, and 3.1 ± 1.1 mm for no correction, manual forward, auto forward, and auto reverse techniques, respectively. Errors exceeded seven mm in 4% of the fractions when employing the auto reverse technique, highlighting a possible need for manual surveillance. The imagingdose could be reduced to 20% without an appreciable loss in accuracy of the automatic methods.

Three-dimensional C-arm Cone-beam CT: Applications in the Interventional Suite

C-arm cone-beam computed tomography (CT) with a flat-panel detector represents the next generation of imaging technology available in the interventional radiology suite and is predicted to be the platform for many of the three-dimensional (3D) roadmapping and navigational tools that will emerge in parallel with its integration. The combination of current and unappreciated capabilities may be the foundation on which improvements in both safety and effectiveness of complex vascular and nonvascular interventional procedures become possible. These improvements include multiplanar soft tissue imaging, enhanced pretreatment target lesion roadmapping and guidance, and the ability for immediate multiplanar posttreatment assessment. These key features alone may translate to a reduction in the use of iodinated contrast media, a decrease in the radiation dose to the patient and operator, and an increase in the therapeutic index (increase in safety-vs-benefit ratio). In routine practice, imaging information obtained with C-arm cone-beam CT provides a subjective level of confidence factor to the operator that has not yet been thoroughly quantified.

SU‐FF‐J‐16: Reference Dosimetry During CBCT Acquisition Using Radiochromic Film

Purpose: We describe a 2D reference dosimetry technique for measuring dose from cone‐beam computed tomography (CBCT) scans using on‐board imager (OBI) on Varian Clinac‐iX linear accelerator that employs XR‐QA radiochromic film model, specifically designed for dose measurements at low energy photons. We report on surface dose and percent depth dose (PDD) measurements during clinical CBCT procedures carried out on a humanoid Rando phantom. Method and Materials: Response of XR‐QA model GAFCHROMIC™ film reference dosimetry system was calibrated in terms of air kerma in air. To measure surface dose during CBCT acquisitions, pieces of XR‐QA films were taped on surface of the Rando phantom. Also, film strips were placed between Rando slices in vertical and lateral direction in order to measure PDDs during CBCT scans for different sites and different CBCT protocols available on our imaging system. Spatial dosimetry was performed using Epson Expression 10000XL document scanner. Change in optical reflectance of the unexposed film piece was subtracted from the exposed one to obtain final netR, which was converted to dose using previously determined calibration curve. Results: Our measurements show that skin dose can range from 0.07 cGy in Low Dose Head, to 4.64 cGy in Pelvis Spot Light CBCT protocol with uncertainty of 2% at higher doses, rising to 4% at 0.5 cGy and 12% at the lowest measured dose. Profiles obtained from film strip measurements show different dose distributions depending on the CBCT technique used and the anatomical site imaged. Conclusions: In this work, we describe an air kerma in air based technique of measuring dose during CBCT scans using XR‐QA model GAFCHROMIC™ film in combination with flat‐bed document scanner. Main advantage of this system is that it does not require a priori knowledge of the backscatter factors and it provides 2D reference dosimetry at low kVp photon beams.

SU‐FF‐T‐276: Utilization of Conebeam CT to Accurately Assess TLD Placement for In‐Vivo Dosimetry

Purpose: Assessing the predicted dose at the thermoluminescent dosimeter (TLD) location is one of difficulties when utilizing in‐vivo TLD dosimetry. Accurate dose prediction at the exact TLD location increases confidence in the measured TLD results. Method and Materials: Prior to localization and treatment, TLDs are placed according to physician's direction. A conebeam CT (CBCT) image set, in treatment position and encompassing all TLD locations, is obtained. The patient's treatment is delivered and the TLDs are sent for analysis (University of Wisconsin, Radiation Calibration Laboratory). The acquired CBCT image set is imported into the Eclipse treatment planning computer and the data set is rigidly fused with the initial treatment planning simulation CT data set. The TLDs are visible on the CBCT data set, allowing for visualization of the TLD location on initial planning CT which contains the calculated dose distribution. The calculated dose at the exact location of the TLD is obtained for comparison to the measured TLDs. Thus, the calculated dose at the exact TLD position is obtained for comparison to the measured TLD results. From 2008 – 2009, three patients underwent treatment for scalp lesions, resulting in twelve TLD measurements from the above described technique. Results: All TLD results were corrected for contribution from the CBCT acquisition. The percentage agreement of measured vs. predicted TLD doses was 0.2 ± 7.5 %. Conclusion: These results indicate that CBCT can be used to predict the dose at the exact TLD location during treatment within 10%. Additionally, CBCT removes uncertainty in TLD location on the patient at the time of exposure.

SU‐FF‐J‐10: Converting Cone‐Beam CT to In‐Room Conventional CT by a Deformable Image Registration Method

Purpose: Poor image quality in Cone‐beam CT (CBCT) usually poses a challenge for soft tissue based target localization or accurate dose calculation for adaptive radiotherapy. The purpose of this study is to develop an effective post‐processing method to drastically improve CBCT image quality without losing anatomical and geometric information it provides. Method and Materials: We used an image‐based deformable registration algorithm to establish dense correspondence among voxels of the daily CBCT and the planning CT images. Once the voxel‐by‐voxel correspondence is obtained, conventional CT numbers from the planning CT can be mapped to the geometrically aligned voxel locations of the daily CBCT. The converted CBCT (Mapped‐CBCT) maintains the geometrical information of the daily CBCT while retaining the more accurate electron density information from the original planning CT. A previously validated, in‐house developed “demons” algorithm was used in combination with an intensity pre‐processing method. Results: Soft tissue contrast is greatly enhanced in each of the prostate, lung, and head & neck cases we tested. In addition, various CBCT imaging artifacts, caused by beam hardening, detector response lag, scatter, and organ motion during CBCT acquisition etc., were all disappeared. Histogram distribution of voxel intensities in the soft tissue region showed a better tissue/fat separation in the mapped CBCT than the original CBCT. For a prostate case, we observed a 41% reduction in entropy, which further demonstrates improved soft‐tissue discrimination. Correspondence ambiguity in some anatomical objects is one limitation for this method. For example, the daily variation of rectal gas will not be presented if the rectum is empty in the planning CT. Conclusion: We demonstrated an effective voxel mapping procedure to match the CBCT image to the quality of a conventional CT, which should improve online image evaluation or provide a more accurate CT data set for adaptive replanning or dose accumulation.

SU-FF-J-170: Impact of 4D Cone Beam CT View-Aliasing Artifact On Nonrigid Registration Accuracy

Purpose: To evaluate the effect of 4D cone beam CT view‐aliasing artifact on nonrigid registration accuracy. Method and Materials: End exhalation (EE) and end inhalation (EI) volumes from a 4D multi‐slice CTimage (4DCT) of a research subject were registered using a small deformation inverse consistent linear elastic (SICLE) registration algorithm to produce a reference displacement vector field (DVF) between the two images. Artificial cone beam CT projections were generated for the EE and EI volumes at several different angular sampling patterns, mimicking 4D cone beam CT (4DCBCT) acquisition in a free‐breathing patient. The projections were reconstructed into EE and EI 4DCBCT phase images containing view‐aliasing artifact due to angular undersampling, but without CBCT scatter and beam hardening artifacts. The EE and EI 4DCBCT images were registered using the SICLE algorithm to produce a test DVF. Test DVFs were generated at a range of angular frequencies from 0.17 projections per degree to 1.83 projections per degree (full angular sampling on a commercial cone beam CT system). The norm of the voxel by voxel vector difference between the test DVF and reference DVF within the patient's delineated lung volume was calculated. Results: The mean absolute error was significantly associated with the projection angular frequency (p < 0.05, exponential regression, R2=0.996). The median error was below 0.2 cm and 90% of the error was below 0.3 cm for the lowest frequency evaluated here of 60 projections per phase. However, above a sampling frequency of 0.33 projections per degree (120 projections per phase), 90% of the error was less than 0.1 cm in relation to the 4DCT registration. Conclusion: Using at least 120 projections per phase, view‐aliasing artifact had a minimal effect on the registration accuracy for the SICLE algorithm due to inherent elastic and smoothing constraints in the algorithm.

SU‐FF‐J‐12: HU and Dosimetric Comparison of Six Cone‐Beam CT Acquisition Techniques for Radiation Therapy Planning

Purpose: It has been shown that cone‐beam computed tomography (CBCT) can be used in dose calculation for treatment planning. This study investigates the HU and dosimetric properties of six CBCT techniques in comparison with conventional CT to determine which techniques are proper for planning. Method and Materials: HU of material disks in Catphan and HU profiles for homogeneous and inhomogeneous phantoms for six different CBCT techniques (Low‐Dose Head, Standard‐Dose Head, High Quality Head, Pelvis Spotlight, Pelvis, and Low‐Dose Thorax) were compared to CT images. Plans with a single photon beam based on CBCT techniques and conventional CT of phantoms were compared using dose values at the isocenter, isodose distributions, and dose volume histograms. Results: HU values of all the CBCT techniques are very close to those of CT in Catphan and in homogeneous phantoms except for Low‐Dose Thorax (LDTH) technique, which overestimates HU by 85. However, the LDTH CBCT showed the HU profile in the lung area matched well with the CT compared to the other five techniques. The dose distribution based on all CBCT techniques in the homogeneous phantoms matched well with the CT except for LDTH, which underestimated the dose. For the lung area, the LDTH dose distribution matched well with CT and the other CBCT techniques did not. For the spine area, the dose distribution of the Pelvis technique matched best with CT, but the other techniques matched relatively well with CT too except for LDTH. Conclusion: This study illustrates HU values and dose calculation based on six different CBCT techniques compared to conventional CT for the purpose of radiation therapy treatment planning. The dose calculation accuracy depends on combination of CBCT acquisition technique and treatment site. A CBCT technique should be properly selected for a treatment site if CBCT‐based dose calculation is to be performed.

Cone Beam Computed Tomography, A Low-Dose Imaging Technique in the Postoperative Assessment of Cochlear Implantation

Cone beam computed tomography (CBCT) has become an extremely useful technique for dentomaxillofacial imaging because it provides clear images of highly contrasted structures. Previous studies evaluating the applicability of this technique in otologic imaging were very encouraging but were only performed in vitro on temporal bone specimens. The intracochlear positioning of the individual electrodes after cochlear implantation by means of CBCT has not yet been shown in vivo. We describe a protocol for in vivo postoperative imaging of cochlear implants by CBCT. Moreover, the effective dose was measured and compared with the effective dose used on 4- and 16-slice multislice computed tomography (MSCT) by using a RANDO-phantom. Developing a protocol for in vivo postoperative imaging of cochlear implants by CBCT. CBCT provides high-resolution and almost artifact-free multiplanar reconstruction images allowing assessment of the precise intracochlear position of the electrode and visualization of each of the individual contacts. The calculated effective dose of the used CBCT and MSCT acquisitions is 80 musv for the CBCT, 3,600 musv for the 16-slice computed tomography, and 4,800 musv for the 4-slice computed tomography. These preliminary results suggests that, for in vivo postoperative evaluation of cochlear implants, CBCT can provide at least the same information as conventional radiography, digital radiograph, and MSCT but in a more comfortable and a much more safer way.

Improvement of cardiac CT reconstruction using local motion vector fields

The motion of the heart is a major challenge for cardiac imaging using CT. A novel approach to decrease motion blur and to improve the signal to noise ratio is motion compensated reconstruction which takes motion vector fields into account in order to correct motion. The presented work deals with the determination of local motion vector fields from high contrast objects and their utilization within motion compensated filtered back projection reconstruction. Image registration is applied during the quiescent cardiac phases. Temporal interpolation in parameter space is used in order to estimate motion during strong motion phases. The resulting motion vector fields are during image reconstruction. The method is assessed using a software phantom and several clinical cases for calcium scoring. As a criterion for reconstruction quality, calcium volume scores were derived from both, gated cardiac reconstruction and motion compensated reconstruction throughout the cardiac phases using low pitch helical cone beam CT acquisitions. The presented technique is a robust method to determine and utilize local motion vector fields. Motion compensated reconstruction using the derived motion vector fields leads to superior image quality compared to gated reconstruction. As a result, the gating window can be enlarged significantly, resulting in increased SNR, while reliable Hounsfield units are achieved due to the reduced level of motion artefacts. The enlargement of the gating window can be translated into reduced dose requirements.

A Method to Estimate Mean Position, Motion Magnitude, Motion Correlation, and Trajectory of a Tumor From Cone-Beam CT Projections for Image-Guided Radiotherapy

To develop a probability-based method for estimating the mean position, motion magnitude, and trajectory of a tumor using cone-beam CT (CBCT) projections. CBCT acquisition was simulated for more than 80 hours of patient-measured trajectories for thoracic/abdominal tumors and prostate. The trajectories were divided into 60-second segments for which CBCT was simulated by projecting the tumor position onto a rotating imager. Tumor (surrogate) visibility on all projections was assumed. The mean and standard deviation of the tumor position and motion correlation along the three axes were determined with maximum likelihood estimation based on the projection data, assuming a Gaussian spatial distribution. The unknown position component along the imager axis was approximated by its expectation value, determined by the Gaussian distribution. Transformation of the resulting three-dimensional position to patient coordinates provided the estimated trajectory. Two trajectories were experimentally investigated by CBCT acquisition of a phantom. The root-mean-square error of the estimated mean position was 0.05 mm. The root-mean-square error of the trajectories was <1 mm in 99.1% of the thorax/abdomen cases and in 99.7% of the prostate cases. The experimental trajectory estimation agreed with the actual phantom trajectory within 0.44 mm in any direction. Clinical applicability was demonstrated by estimating the tumor trajectory for a pancreas cancer case. A method for estimation of mean position, motion magnitude, and trajectory of a tumor from CBCT projections has been developed. The accuracy was typically much better than 1 mm. The method is applicable to motion-inclusive, respiratory-gated, and tumor-tracking radiotherapy.

A New Scatter Correction Method for KV and MV Cone Beam CT

Kilo-voltage(KV) and mega-voltage(MV) cone beam computed tomography (CBCT) play an important role in image-guided adaptive radiotherapy. However, the cupping effect, introduced by Compton scatter contribution due to a large cone angle of CBCT acquisition, significantly degrades the CBCT image quality and hampers its clinical application. The objective of this study is to develop a new CBCT scatter correction method in the image domain. The hypotheses of the proposed method are that the image intensity of different kind of materials in CBCT image should be globally uniform and the cupping effect can be described as a bias field, a low frequency signal across the whole 3-D images. Based on above assumptions, we proposed a maximum a posteriori probability (MAP) framework to estimate the bias field contribution. The well-known fuzzy C-mean model is extended to define the likelihood function of the CBCT image. A markov random field model is used to describe the prior probability of the mixture defined in FCM model. The bias field is characterized by a Gaussian prior probability with a banded covariance matrix representing a low-pass filter. An iterated conditional mode (ICM)-like approach is utilized to minimize the objective function. The corrected CBCT image can be obtained by subtracting the bias field from original CBCT image. The performance of correcting the cupping effect of CBCT image with the proposed MAP framework was tested using both contrast phantoms and clinical CBCT images. In the cylindrical shape-contrast phantom the higher density material is surrounded by the lower density material. In the contrast phantom study, the intensities of higher density material in both KV and MV CBCT images are even lower than that of lower density material due to the cupping effect. The corrected CBCT images showed almost same intensity distribution as the original contrast phantom images. In a real patient study, it is clearly shown that there is a shadow inside the brain soft tissue in the original brain KV and MV CBCT images. The corrected CBCT images significantly improved the uniformity of intensity distribution of brain soft tissue. The proposed image-based scatter correction method showed a promising result to reduce the cupping effect commonly encountered in KV and MV CBCT image.

Analysis of Online Cone-beam CT Images for Non-small Cell Lung Cancer to Define an Appropriate CTV-to-PTV Margins

Kilovoltage cone-beam computed tomography (CBCT) has been developed to provide accurate soft tissue and bony setup information. We evaluated the magnitude of interfraction and intrafraction tumor displacement using online CBCT for non-small-cell lung cancer (NSCLC) patients and estimated the appropriate planning target volume (PTV) margins. Ten NSCLC patients treated with three-dimensional conformal radiotherapy (3D-CRT) were investigated. The study procedure involved (1) initial patient setup using laser alignment to skin marks, (2) initial CBCT acquisition and analysis of these images with planning CT images using automatic and manual registration to assess the interfraction tumor displacement (setup error), (3) couch translation based on the registration results accordingly and treatment delivery, (4) a second CBCT were acquired on couch immediately after treatment to assess intrafraction tumor displacement. Online-guided correction data were recorded in the left-right (LR), cranio-caudal (CC), and anterior-posterior (AP) directions respectively. Each patient underwent this procedure 2∼3 times per week over the course of their treatment. The precorrection and postcorrection PTV margins were calculated using the 2.0 Σ+0.7σ recipe. The criterion to derive the recipe was that on average more than 99% of the CTV should at least get 95% of the dose. Interfraction (Σ inter, σ inter)and intrafraction errors (Σ intra, σ intra) were all involved in the calculation of the precorrection PTV margins, while postcorrection PTV margins accounts for intrafraction errors only. The complete procedure took approximately 6∼8 min, including deployment of CBCT components, scanning, reconstruction, images registration, and couch correction. Total of 159 sets of CBCT images were analyzed. The precorrection PTV margins required to account for total errors are 7.2 mm, 8.8 mm and 10.1 mm in the LR, CC and AP directions, respectively. When the tumor was corrected every fraction, the postcorrection PTV margins required to account for intrafraction errors are 2 mm,1.6 mm,1.6 mm in the LR, CC and AP directions respectively. There exists some extent of errors from setup and organ motion in 3D CRT for NSCLC. The approach based on the CBCT measurements can be used to reduce the impact of setup errors obviously and to estimate the PTV margins. When no CBCT correction was applied, the PTV margins should be 7 mm, 9 mm and 10 mm in the LR, CC and AP directions respectively, and if there is correction before treatment, the PTV margins were 2 mm in every direction.

TU‐C‐351‐04: Cone Beam CT Acquisition During Volumetric Arc Radiotherapy Delivery: Correction for Induced Artifacts

Purpose: To evaluate the feasibility of kV cone beam CT (CBCT) acquisition simultaneously with volumetric modulated arc therapy (VMAT) delivery, and to test a method to correct for degradation of image quality due to VMAT delivery. Method and Materials: A commercial CBCT system was modified to enable simultaneous CBCT acquisition with VMAT delivery. CBCT scans of an image quality phantom were acquired during VMAT delivery while varying the VMAT parameters. Dose rate, energy, and field size of the VMAT delivery, and phantom size, were varied to evaluate the effect on image quality. The mean and standard deviation of the signal in a known location was quantified both in the raw 2D projection images and also in the reconstructed 3D CBCT images. A nonlinear filter was tested to remove structural artifact and noise. An analytical scatter correction model was developed and used to remove scatter generated by the VMAT beam. Results: Structural artifact was reduced in the CBCT projections with a nonlinear filter. Scatter generated from the VMAT delivery varied with field size and dose rate, and minimally with phantom size. An analytical scatter model was constructed based on the VMAT fluence (field area times dose rate) for each CBCT projection, and applied to reduce scatter per projection. Applying the model improved uniformity from 7.9% to 3.0% and improved the contrast to noise ratio from 0.97 to 1.84. Conclusion: Megavoltage scatter, and its per projection variation, is the largest component contributing to degradation of CBCT image quality during VMAT delivery. The degradation was reduced with a scatter model based on the VMAT delivery. A secondary component was structural artifact related to the repetition rate of the megavoltage beam and the readout mechanism of the kV detector. Conflict of Interest: Research sponsored by Elekta, Inc.

SU-GG-T-217: Test of Laser Position, Isocenter Congruence Between KV Imager, Cone Beam CT and MV Beam with a Phantom

Purpose: On board imager (OBI) in the form of kV imaging and cone beam CT(CBCT) is gaining wide clinical application. A fast QA method that can be used to evaluate geometric accuracy and congruence with the therapy beams is lacking. We have developed a QA phantom for weekly and monthly QA of the OBI. The purpose of this study is to evaluate the accuracy of the method and its time requirement. Method and Materials: A phantom containing 10 fiducial markers were designed to check laser position, isocenter congruence between the kV imager, the CBCT and the MV beam in one time setup. Four markers are attached to each side of the 15‐cm plastic cube, one marker is located in the center of the 8 markers and the last one is located to next to the center marker. The phantom is aligned with laser and 2 sets of MV‐kV, 2D‐2D matching images are acquired, followed by the CBCT acquisition with same setup. Then the centers of the laser, the kV imager, the CBCT can be calculated from a formula from simple geometric relation. Results: Total QA time including setup, image acquisition, and evaluation was not more than 20 min. The phantom can detect less than 0.5 mm variations and misalignments. The most time consuming step is image enhancement of the MV images, a process necessary to find the fiducial markers. Further improvements in the material and design of the fiducial markers to eliminate this step and significantly reduce the QA time are possible. Discussion and Conclusion: The phantom is appropriate for both acceptance tests the routine QA of the OBI system. Comprehensive QA for OBI systems can be performed if inserts for assessing image quality, such as different density materials and spatial resolution inserts are incorporated into the phantom.

WE-D-351-02: Feasibility Study of Frameless Angiographic Localization of Arteriovenous Malformations (AVMs) for Image-Guided Radiosurgical Interventions

Purpose: We investigated the feasibility of novel approach for frameless 3D AVM localization within the coordinate space of an angiographic cone‐beam CT(CBCT) dataset. The localization is based on characterization and calibration of 2D digital subtraction angiography(DSA) and CBCT acquisition modes available on a commercial flat panel detector C‐arm neuroangiography system. This localization method in combination with image‐guided Cyberknife delivery could provide consistent approach to frameless AVM radiosurgery.Method and Materials: We introduced preset AVM localization protocols comprising anterior‐posterior (AP) and lateral (LAT) DSA series combined with CBCT without couch displacement between acquisitions. The AP and LAT C‐arm positions were selected by matching AP/LAT phantom images to corresponding projections from the CBCT series. We subsequently evaluated the accuracy and the reproducibility of the 2D–3D correspondence for the various protocols by imaging a CBCTcalibration phantom with embedded markers. Paired marker centers were automatically extracted in the AP/LAT images and the corresponding CBCT acquisition frames. The maximum and the mean distance between the marker centers were calculated as a metrics for 2D‐to‐3D correspondence accuracy. The reproducibility was investigated by repeating the imaging protocols in different sessions and displacing the C‐arm between the sessions. Results: The accuracy of the DSA‐CBCT correspondence depended on the protocol and the C‐arm position for the DSA acquisitions within the protocol. For further clinical investigations we retained a protocol with an AP and left LAT DSA modes that reproducibly resulted in maximum/mean paired marker distance of 0.68 mm and 0.4 mm (AP) and 0.29 mm and 0.15 mm (left LAT) in the C‐arm isocenter plane. Conclusion: With proper patient immobilization, frameless AVM localization within the angiographic CBCT dataset based on calibrated AP and left LAT views is feasible with an uncertainty of 0.7–0.8 mm. Conflict of Interest: This work is supported by Siemens Medical Solutions.

SU‐GG‐T‐524: Initialization Strategy and Concurrent Imaging for Single 360 Degree Arc Intensity Modulated Treatment Delivery

Purpose: Single 360 degree arc treatment with variable MU per degree and changing MLC apertures provides an efficient means for delivering highly conformal radiation therapy. Additionally, the treatment geometry is amenable to CT image acquisition during the delivery enabling verification of patient position. The challenge with the technique is finding the optimal set of apertures over the arc. Aperture based optimization (MLC leaf positions) is the natural approach, but suffers for complex targets if the initial starting condition is simply the fields exposing the target around the arc. The problem occurs when the optimal solution would have the MLC closing off a larger portion of the target requiring large positional changes from the initial condition to the optimal position. In this case gradient based optimizations fall short in that the derivative of the dose distribution with respect to the leaf position is flat for locations further from the leaf tip. In order to remedy this problem, improved initialization of the initial MLC positions around the arc based on the target geometry are proposed. Method and Materials: A research version of the Pinnacle3 RTPS was used to plan the single arc delivery. The approach uses 36 equispaced beams over a 360 degree arc, and optimizes the plan using a single MLC aperture for each beam and the direct machine parameter optimization (DMPO) technique in the system. The Elekta Synergy linear accelerator was used to deliver the treatment and for cone beam CT acquisition. Results: Initial results show dose conformality similar to that of traditional intensity modulated radiotherapy with the 360 degree delivery using geometry based pre‐initialization of the apertures. Additionally, kilovolt image acquisition during delivery was possible. Scattered radiation and the interplay between image acquisition frequency and the pulse rate of the treatment beam influence the image quality but not significantly.

SU‐GG‐J‐106: Improved Planar and Megavoltage Cone‐Beam Imaging Using Low‐Z Linear Accelerator Targets

Purpose: to examine the effect of using low atomic number (Z) target materials on megavoltage portal and cone‐beam CT (CBCT) image quality. Method and materials: for experimental measurements, four low‐Z targets were installed in a linear accelerator carousel (Varian 2100EX) for the generation of experimental spectra for imaging. The targets were composed of beryllium or aluminum with thicknesses set to approximately 60% of the CSDA range of either 4 MeV or 6 MeV electrons. For CBCT acquisition, the beam and detector were fixed, and a rotation stage was controlled by software to acquire multiple angular projections of phantoms. To examine photon energy spectra and to provide a means for optimizing the imaging system, the beam generation and detector panel were modeled using BEAMnrc/DOSXYZ Monte Carlo package. For 6MV, the beam/detector model was validated by comparing experimental and MC‐generated images of open fields. Results: The modeled, experimental spectra demonstrate the recovery of photons below 150 keV. For the 4 MeV/Be beam, for example, approximately 1/3rd of photons have energies below 60 keV. MC models of planar imaging show significant improvement of image contrast compared to the standard 6MV beam, and suggest that i) of the four energy/target combinations studied, the 4 MeV/Be combination provides the greatest contrast improvement and ii) a modest additional increase in contrast is achieved by removing the copper buildup layer from the detector. Initial low‐Z target CBCT images show improved image contrast; full quantitative results will be presented. Conclusion: The use of megavoltage electron beams combined with low‐Z targets offers the potential for improved image quality, particularly in terms of image contrast. This approach may be promising for improved and highly‐integrated on‐board megavoltage imaging.Conflict of interest: This research has been sponsored by Varian Medical Systems, Incorporated.

Accurate patient dosimetry of kilovoltage cone‐beam CT in radiation therapy

The increased utilization of x-ray imaging in image-guided radiotherapy has dramatically improved the radiation treatment and the lives of cancer patients. Daily imaging procedures, such as cone-beam computed tomography (CBCT), for patient setup may significantly increase the dose to the patient's normal tissues. This study investigates the dosimetry from a kilovoltage (kV) CBCT for real patient geometries. Monte Carlo simulations were used to study the kV beams from a Varian on-board imager integrated into the Trilogy accelerator. The Monte Carlo calculated results were benchmarked against measurements and good agreement was obtained. The authors developed a novel method to calibrate Monte Carlo simulated beams with measurements using an ionization chamber in which the air-kerma calibration factors are obtained from an Accredited Dosimetry Calibration Laboratory. The authors have introduced a new Monte Carlo calibration factor, fMCcal, which is determined from the calibration procedure. The accuracy of the new method was validated by experiment. When a Monte Carlo simulated beam has been calibrated, the simulated beam can be used to accurately predict absolute dose distributions in the irradiated media. Using this method the authors calculated dose distributions to patient anatomies from a typical CBCT acquisition for different treatment sites, such as head and neck, lung, and pelvis. Their results have shown that, from a typical head and neck CBCT, doses to soft tissues, such as eye, spinal cord, and brain can be up to 8, 6, and 5 cGy, respectively. The dose to the bone, due to the photoelectric effect, can be as much as 25 cGy, about three times the dose to the soft tissue. The study provides detailed information on the additional doses to the normal tissues of a patient from a typical kV CBCT acquisition. The methodology of the Monte Carlo beam calibration developed and introduced in this study allows the user to calculate both relative and absolute absorbed doses.

Cone-Beam Micro-CT System Based on LabVIEW Software

Construction of a cone-beam computed tomography (CBCT) system for laboratory research usually requires integration of different software and hardware components. As a result, building and operating such a complex system require the expertise of researchers with significantly different backgrounds. Additionally, writing flexible code to control the hardware components of a CBCT system combined with designing a friendly graphical user interface (GUI) can be cumbersome and time consuming. An intuitive and flexible program structure, as well as the program GUI for CBCT acquisition, is presented in this note. The program was developed in National Instrument's Laboratory Virtual Instrumentation Engineering Workbench (LabVIEW) graphical language and is designed to control a custom-built CBCT system but has been also used in a standard angiographic suite. The hardware components are commercially available to researchers and are in general provided with software drivers which are LabVIEW compatible. The program structure was designed as a sequential chain. Each step in the chain takes care of one or two hardware commands at a time; the execution of the sequence can be modified according to the CBCT system design. We have scanned and reconstructed over 200 specimens using this interface and present three examples which cover different areas of interest encountered in laboratory research. The resulting 3D data are rendered using a commercial workstation. The program described in this paper is available for use or improvement by other researchers.

Optimizing 4D cone-beam CT acquisition protocol for external beam radiotherapy

Four-dimensional cone-beam computed tomography (4D-CBCT) imaging is sensitive to parameters such as gantry rotation speed, number of gantry rotations, X-ray pulse rate, and tube current, as well as a patient's breathing pattern. The aim of this study is to optimize the image acquisition on a patient-specific basis while minimizing the scan time and the radiation dose. More than 60 sets of 4D-CBCT images, each with a temporal resolution of 10 phases, were acquired using multiple-gantry rotation and slow-gantry rotation techniques. The image quality was quantified with a relative root mean-square error (RE) and correlated with various acquisition settings; specifically, varying gantry rotation speed, varying both the rotation speed and the number of rotations, and varying both the rotation speed and tube current to keep the radiation exposure constant. These experiments were repeated for three different respiratory periods. With similar radiation dose, 4D-CBCT images acquired with low current and low rotation speed have better quality over images obtained with high current and high rotation speed. In general, a one-rotation low-speed scan is superior to a two-rotation double-speed scan, even though they provide the same number of projections. Furthermore, it is found that the image quality behaves monotonically with the relative speed as defined by the gantry rotation speed and the patient respiratory period. The RE curves established in this work can be used to predict the 4D-CBCT image quality before a scan. This allows the acquisition protocol to be optimized individually to balance the desired quality with the associated scanning time and patient radiation dose.