C Pelizzari

WE-G-BRF-07: Non-Circular Scanning Trajectories with Varian Developer Mode

PURPOSE Cone-beam CT (CBCT) in image-guide radiation therapy (IGRT) typicallyacquires scan data via the circular trajectory of the linearaccelerators (linac) gantry rotation. Though this lends itself toanalytic reconstruction algorithms like FDK, iterative reconstructionalgorithms allow for a broader range of scanning trajectories. Weimplemented a non-circular scanning trajectory with Varians TrueBeamDeveloper Mode and performed some preliminary reconstructions toverify the geometry. METHODS We used TrueBeam Developer Mode to program a new scanning trajectorythat increases the field of view (FOV) along the gantry rotation axiswithout moving the patient. This trajectory consisted of moving thegantry in a circle, then translating the source and detector along theaxial direction before acquiring another circular scan 19 cm away fromthe first. The linear portion of the trajectory includes an additional4.5 cm above and below the axial planes of the sources circularrotation. We scanned a calibration phantom consisting of a lucite tubewith a spiral pattern of CT spots and used the maximum-likelihoodalgorithm to iteratively reconstruct the CBCT volume. RESULTS With the TrueBeam trajectory definition, we acquired projection dataof the calibration phantom using the previously described trajectory.We obtained a scan of the treatment couch for log normalization byscanning with the same trajectory but without the phantom present.Using the nominal geometric parameters reported in the projectionheaders with our iterative reconstruction algorithm, we obtained acorrect reconstruction of the calibration phantom. CONCLUSION The ability to implement new scanning trajectories with the TrueBeamDeveloper Mode enables us access to a new parameter space for imagingwith CBCT for IGRT. Previous simulations and simple dual circle scanshave shown iterative reconstruction with non-circular trajectories canincrease the axial FOV with CBCT. Use of Developer Mode allowsexperimentally testing these and other new scanning trajectories. Support was provided in part by the University of Chicago Research Computing Center, Varian Medical Systems, and NIH Grants 1RO1CA120540, T32EB002103, S10 RR021039 and P30 CA14599. The contents of this work are solely the responsibility of the authors and do not necessarily represent the official views of the supporting organizations.

TH-A-18C-10: Dynamic Intensity Weighted Region of Interest Imaging

PURPOSE For image guidance tasks full image quality is not required throughout the entire image. With dynamic filtration of the kV imaging beam the noise properties of the CT image can be locally controlled, providing a high quality image around the target volume with a lower quality surrounding region while providing substantial dose sparing to the patient as well as reduced scatter fluence on the detector. METHODS A dynamic collimation device with 3mm copper blades has been designed to mount in place of the bowtie filter on the On-Board Imager (Varian Medical Systems). The beam intensity is reduced by 95% behind the copper filters and the aperture is controlled dynamically to conformally illuminate a given ROI during a standard cone-beam CT scan. A data correction framework to account for the physical effects of the collimator prior to reconstruction was developed. Furthermore, to determine the dose savings and scatter reduction a monte carlo model was built in BEAMnrc with specifics from the Varian Monte Carlo Data Package. The MC model was validated with Gafchromic film. RESULTS The reconstructed image shows image quality comparable to a standard scan in the specified ROI, with higher noise and streaks in the outer region but still sufficient information for alignment to high contrast structures. The monte carlo modeling showed that the scatter-to-primary ratio was reduced from 1.26 for an unfiltered scan to 0.45 for an intensity weighted scan, suggesting that image quality may be improved in the inner ROI. Dose in the inner region was reduced 10-15% due to reduced scatter and by as much as 75% in the outer region. CONCLUSION Dynamic intensity-weighted ROI imaging allows reduction of imaging dose to sensitive organs away from the target region while providing images that retain their utility for patient setup and procedure guidance. Funding was provided in part by Varian Medical Systems and NIH Grants 1RO1CA120540, T32EB002103, S10 RR021039 and P30 CA14599. The contents of this work are solely the responsibility of the authors and do not necessarily represent the official views of any of the supporting organizations.

WE‐G‐141‐04: Use of a Dynamic KV X‐Ray Collimator for Reduced‐Dose Fluoroscopic Fiducial Tracking

PURPOSE Intra-fraction organ motion can be problematic for highly conformal radiation therapy techniques. Fluoroscopy with the on-board imager available on many linear accelerators can provide real time tumor position information however potentially high radiation dose can be a concern. The target may only account for a few percent of the image area, thus using a dynamic collimation device to restrict the field of view to an appropriate region-of-interest and follow its motions the imaging dose can be substantially reduced. METHODS A prototype collimator has been developed which can be mounted to the kV x-ray source of a Varian Trilogy linear accelerator in place of the standard bowtie filter. The collimator tracking was tested using realistic motion profiles derived from patient data and a 3-axis motion stage with a CT spot as the target fiducial. The clinical system does not currently provide real time access to the image data so the collimator was fed the fiducial position from the motion control system with controllable lag to simulate image-processing time. RESULTS In the acquired fluoroscopic data the collimator blade edges were identified using a hough line transform on the thresholded gradient magnitude image and the fiducial position was identified with a template matching technique. Over four minutes of continuous tracking with intermittent burst of fluoroscopic imaging the fiducial was never more than 1.7mm from the aperture center and the root mean square distance was 0.8mm. CONCLUSION The dynamic collimator exhibits accuracy that could enable highly conformal fluoroscopic imaging to provide real-time target position information with greatly reduced imaging dose. This work was funded, in part, by Varian Medical Systems, Palo Alto CA. The contents of this work are solely the responsibility of the authors and do not necessarily represent the official views of any supporting organizations.

SU‐E‐J‐07: A Preliminary Study on Optimal Dose‐Allocation Parameters for Low‐Dose Cone‐Beam CT

Purpose: Cone‐beam CT(CBCT) is widely used for providing image guidance in radiotherapy and interventional procedures. Due to the patient safety concern involved in repetitive CBCT scans, significant effort has been devoted to possibly lowering imagingdose in CBCT. For a given total dose, it is also important to investigate how image quality can be affected by dose allocations over projection views and by image‐reconstruction algorithms. In this work, we investigate quantitatively how image quality changes resulted from different combinations of dose‐allocation parameters and some existing reconstruction algorithms. Methods: We performed simulation studies by using a numerical phantom containing structures ofdifferent contrast levels. We also acquired real data of a Catphan phantom (The Phantom Laboratory, Salem, NY) by using the on‐board imaging system (Varian Medical Systems, Palo Alto, CA). At three different total dose levels, we allocated each of them to different numbers of views ranging from 120 to 650. For each allocation scheme we applied the FDK and ASD‐POCS algorithms to obtain reconstructed images. A set of quantitative metrics were used to evaluate image quality based on relevant tasks. Results: Preliminary results showed that for total dose levels under study, both FDK and ASD‐POCS algorithms yield images with comparable quality when a large number of views are considered and that imagesreconstructed by the ASD‐POCS from smaller number of views generally exhibit higher quality. Overall, for each given total dose level the ASD‐POCS algorithm yields images of comparable or higher quality than does the FDK algorithm. Conclusions: We demonstrated that CBCTimage quality can be optimized for a fixed total dose by choosing an appropriate combination of dose‐ allocation parameters and reconstruction algorithm. This finding may potentially be used for improving current CBCTimage quality and for designing innovative, low‐dose CBCTimaging protocols.

SU‐GG‐I‐32: Preliminary Performance Evaluation of CBCT Image Reconstruction from Reduced Projection Data by TV‐Minimization

Purpose: Kilovoltage CBCT has become a useful clinical tool which provides in‐room image guidance for radiation therapy. Frequent use of CBCT also raises patient safety concern, because tomographic images are obtained by reconstruction from projection data acquired at hundreds of angular views, which may accumulate to an un‐negligible amount of radiationdose. It is therefore desired to develop a low‐dose CBCTimaging technique that requires smaller amount of projection data while can still yield images sufficing clinical needs. In this work, we have applied a new TV‐minimization algorithm to reconstructCBCTimages from a fraction of the amount of projection data currently used and have evaluated the image quality according to established quality‐assurance procedures.Materials and methods: We have acquired real projection data of a CATPHAN phantom using a clinical on‐board imaging system (Varian Medical Systems, Palo Alto, CA). We then applied the TV‐minimization algorithm to reconstructimages from subsets of the full, 628‐projection data set. The subsets contain from 360 down to 60 projections. We then examined the reconstructed images by calculating a number of metrics, including spatial resolution, contrast linearity, and low‐constrast resolution, and compared the results to the established quality‐assurance (QA) standards. Results: The imagesreconstructed by use of the TV‐minimization algorithm from the reduced data sets appeared to be comparable to the imagesreconstructed from the full data set by the FDK algorithm. In addition, the tested image‐quality metrics can satisfy the QA requirement for imagesreconstructed from as few as 100 projections. Conclusion: The TV‐minimization algorithm can yield CBCTimages from reduced projection data without significant sacrifice of image quality. The imagesreconstructed from about 100 projections may be potentially useful for clinical applications according to QA results.

SU‐FF‐I‐46: Accurate Image Reconstruction From Incomplete Kilovoltage Cone‐Beam CT Data in Radiation Therapy

Purpose: Kilovoltage cone‐beam CT(CBCT) is becoming an increasingly important clinical tool for image‐guidedradiation therapy. Current CBCTimaging procedures acquire data from hundreds of angular views, during which considerable amount of radiationdose is deposited in the patient body. This raises patient safety concerns as CBCT is often used for frequent image acquisitions.Dose reduction can be achieved in part by acquiring data collected from a considerably reduced number of views. In this work, we develop a total‐variation‐minimization‐based image reconstruction algorithm (TV algorithm) to reconstructimages for situations when only a fraction of full‐scan CBCT data is acquired in radiation therapy.Materials and methods: The TV algorithm seeks among all the image candidates satisfying a given data‐fidelity tolerance the one with minimum total variation. Projection onto convex sets (POCS) and gradient‐descent techniques were used in the algorithm to achieve this goal. To test the algorithm, we acquired 856‐view CBCT data of a home‐built phantom with the on‐board imaging (OBI) system mounted on a Trilogy linear accelerator (Varian Medical Systems, Palo Alto, CA). We then extracted 32‐, 62‐, and 96‐view data for reconstruction using TV and other analytical and iterative algorithms. Results: Among all the tested reconstruction algorithms, the reconstructed images using TV algorithm is least contaminated with artifacts caused by insufficient angular sampling, and the low‐contrast object in the phantom is most distinguishable from background in TV reconstructed images.Conclusion: The proposed TV algorithm produces more accurate reconstruction images than other algorithms in the tested cases where only a fraction of the full dataset is used. The TV algorithm may be potentially useful in reducing radiationdose in CBCTimage‐guidedradiation therapy.

TH‐C‐303A‐05: Development of a Dynamic KV Collimator for Low Diagnostic Dose Real‐Time 3D Motion Tracking During Radiation Therapy by Combined MV‐KV Imaging

Purpose: Currently, real‐time 3D MV‐kV monitoring requires the use of continuous kV imaging throughout the treatment process leading to high diagnosticdose costs. For MV‐kV tracking purposes the only needed kV imageinformation are the projected images of the metallic fiducial markers. Generally these markers are small (3mm in length and 0.8mm in diameter), and for a standard 40cm × 30cm kV image comprise less than 1% of the total area. The proposed technique here uses a dynamic kV aperture to confine the kV exposure to a small region of interest (ROI) encompassing only the markers. As the internal markers move, the aperture is dynamically updated using feedback information provided by the last known marker positions. Method: A Varian Trilogy equipped with both an EPID and a kV imaging system was used. The kV collimator was mounted over the kV source and consists of four lead blades placed orthogonally on low friction linear guide rails. The position of each blade was controlled independently using a servomotor. MV‐kV imaging was performed and software was used to calculate a suitable ROI that will be used as an input for the kV collimator. As the internal markers move, the aperture will be dynamically updated using the last known marker positions. Results: The combination of controller circuitry and chosen servomotors allows for blade travel speed of up to 11mm/s, depending on orientation, with an accuracy of 0.2mm in the collimator plane. This should be sufficient to keep the ROI properly centered on nearly any fiducial cluster. Conclusion: The technique proposed here would potentially lower the kV exposure by a factor of 50–500 depending on the speed, number, and spatial separation of the fiducials. The technology is directly applicable to any kV imaging system where only selective ROI information is required.

SU‐GG‐I‐27: Image Artifacts Caused by the Extra‐Focal Spot of An X‐Ray Tube in Cone‐Beam Computed Tomography

Purpose: We investigate the reconstruction artifacts induced by extra‐focal spot of x‐ray tube in cone‐beam computed tomography(CBCT).Method and Materials: The extra‐focal radiation problem of an x‐ray tube has been investigated for more than thirty years. However, these investigations have been mostly limited to fan‐beam computed tomography (FBCT) where only the quasi‐one‐dimensional source distribution matters. In contrast, CBCT is more vulnerable to the problem of extra‐focal field than FBCT because the quasi‐two‐dimensional distribution of source is significant. Extra‐focal radiation has been attributed to two physical processes: secondary electrons and field emission. Specifically, field emission of electrons from a cold cathode may create a well‐defined extra‐focal spot that can emit a substantial amount of x‐rays. We have observed a non‐negligible amount of x‐ray flux from an off‐focal spot in the x‐ray tube of an on‐board imager used for image‐guidedradiation therapy, and have performed a numerical study to simulate the image artifacts induced by this strong extra‐focal spot. The 3‐D Shepp‐Logan phantom was used as an imaging object and circular cone‐beam projections were made with two focal source points separated by 8 mm along the rotation axis direction. The off center focal spot had an intensity of 10% that of the central spot. Results: We observed image artifacts such as overlaid structure of the object components in the coronal slice images and ghost objects in some transverse slice images.Conclusion: In addition to the well‐known image artifacts, due to spread extra‐focal field of an x‐ray tube, which include blurring of the images, well‐defined extra‐focal spots can introduce additional image artifacts such as overlaying structures and ghost objects.

SU‐GG‐J‐189: Use of a Clinically Relevant Digital PET Chest Phantom to Investigate the Threshold Required to Define a PET‐Based Target Volume

Purpose: To examine the relationship between the threshold required to segment PET‐based target volumes placed in inhomogeneous background provided by a digital PET chest phantom. Method and Materials: We placed four spheres of uniform activity and sizes ranging from 2–13ml in the lung, mediastinum and lung/mediastinum boundary in the chest of a digital PET phantom. The intensity of the spheres was adjusted to obtain different target to background (T/B) ratios and the threshold as a percent of the maximum image intensity required to get the true tumor volume was estimated. We also constructed a spherical target of non‐uniform activity by placing a spatial Gaussian activity distribution in the larger (13ml) sphere and adjusting its intensity to obtain various T/B ratios. Results: The results suggest that a threshold varying from 37–55% may be required to segment the true target volume depending on the location and size of the target. Smaller targets and ones placed in higher background activities (such as the mediastinum) require higher thresholds than larger targets or the ones placed in lower intensity background (such as the lung). Adding inhomogeneity in the intensity of the target results in lower thresholds required and smaller difference between the threshold required to segment targets in higher and lower image intensity backgrounds. Conclusion: For the first time to our knowledge we present a study investigating PET‐based threshold segmentation in a realistic situation, where the targets background activity is similar to what is seen in clinical cases. The most important finding of this study is the indication that lesions in the mediastinum require higher thresholds than the ones in normal lung or at the boundary between lung and mediastinum.

SU‐GG‐T‐400: Dosimetric Characterization and Biological Validation of a Phantom for Three Dimensional IMRT‐Based In Vitro Experiments

Purpose: To characterize the dosimetry of a previously described cylindrical phantom for use in 3‐dimensional intensity modulated radiation therapy (IMRT)‐based in vitro cell experiments and validate the phantom by comparing its performance in vitro to a standard experimental setup. Method and Materials: The phantom was loaded with a stack of three 6‐well tissue‐culture plates. An IMRT plan with a single PTV encompassing all plates was created and delivered in the phantom. Calculated doses were compared to those measured using both an array of thermoluminescencedosimeters(TLDs) and film placed in the phantom. In vitro validation was performed by delivering an array of doses from 0–10 Gy to two human cancer cell lines (A549 and SCC116) using both the phantom and a standard experimental setup employing a single open field. Percentage of viable cells post‐irradiation (%Via) was compared for both setups using the diphenylamine (DPA) assay. Results: The percent differences between TLD measurements and corresponding points in the treatment plan ranged from −1.3%–2.9% (p>0.05 for all cases). Average point‐by‐point percent dose difference (%Ddiff) between each film and the corresponding calculated dose plane ranged from 1.6%–3.1%, while the %Ddiff at which 95% of the film points agreed to ⩽3.0% ranged from 2.8%–4.1%. These results show good general agreement between measured and predicted dose. Comparison of the two experimental setups revealed average differences in %Via of 1.28% and 3.26% for SCC116 and A549, respectively (p>0.05 for all cases). Conclusion: Good general agreement between calculated and TLD and film measured dose within the phantom under experimental conditions, along with strong agreement in cell response when using the phantom versus a standard experimental setup show that the phantom is a useful, efficient, and dynamic tool for 3‐dimensional in vitro cell experiments. Conflict of Interest: Supported by a grant from MedImmune.