Reference Digital Tomosynthesis Research Articles

A software tool of digital tomosynthesis application for patient positioning in radiotherapy.

Digital Tomosynthesis (DTS) is an image modality in reconstructing tomographic images from two‐dimensional kV projections covering a narrow scan angles. Comparing with conventional cone‐beam CT (CBCT), it requires less time and radiation dose in data acquisition. It is feasible to apply this technique in patient positioning in radiotherapy. To facilitate its clinical application, a software tool was developed and the reconstruction processes were accelerated by graphic processing unit (GPU). Two reconstruction and two registration processes are required for DTS application which is different from conventional CBCT application which requires one image reconstruction process and one image registration process. The reconstruction stage consists of productions of two types of DTS. One type of DTS is reconstructed from cone‐beam (CB) projections covering a narrow scan angle and is named onboard DTS (ODTS), which represents the real patient position in treatment room. Another type of DTS is reconstructed from digitally reconstructed radiography (DRR) and is named reference DTS (RDTS), which represents the ideal patient position in treatment room. Prior to the reconstruction of RDTS, The DRRs are reconstructed from planning CT using the same acquisition setting of CB projections. The registration stage consists of two matching processes between ODTS and RDTS. The target shift in lateral and longitudinal axes are obtained from the matching between ODTS and RDTS in coronal view, while the target shift in longitudinal and vertical axes are obtained from the matching between ODTS and RDTS in sagittal view. In this software, both DRR and DTS reconstruction algorithms were implemented on GPU environments for acceleration purpose. The comprehensive evaluation of this software tool was performed including geometric accuracy, image quality, registration accuracy, and reconstruction efficiency. The average correlation coefficient between DRR/DTS generated by GPU‐based algorithm and CPU‐based algorithm is 0.99. Based on the measurements of cube phantom on DTS, the geometric errors are within 0.5 mm in three axes. For both cube phantom and pelvic phantom, the registration errors are within 0.5 mm in three axes. Compared with reconstruction performance of CPU‐based algorithms, the performances of DRR and DTS reconstructions are improved by a factor of 15 to 20. A GPU‐based software tool was developed for DTS application for patient positioning of radiotherapy. The geometric and registration accuracy met the clinical requirement in patient setup of radiotherapy. The high performance of DRR and DTS reconstruction algorithms was achieved by the GPU‐based computation environments. It is a useful software tool for researcher and clinician in evaluating DTS application in patient positioning of radiotherapy.PACS number(s): 87.57.nf

WE-AB-303-01: FEATURED PRESENTATION: A Dual-Detector Phase-Matched Digital Tomosynthesis (DTS) Imaging Scheme Using Aggregated KV and MV Projections for Intra-Treatment Lung Tumor Tracking

Purpose: To develop a dual-detector phase-matched DTS technique for continuous and fast intra-treatment lung tumor localization. Methods: Tumor localization accuracy of limited-angle DTS imaging is affected by low inter-slice resolution. The dual-detector DTS technique aims to overcome this limitation through combining orthogonally acquired beam's eye view MV projections and kV projections for intra-treatment DTS reconstruction and localization. To aggregate the kV and MV projections for reconstruction, the MV projections were linearly converted to synthesize corresponding kV projections. To further address the lung motion induced localization errors, this technique uses respiratory phase-matching to match the motion information between on-board DTS and reference DTS to offset the adverse effects of motion blurriness in tumor localization.A study was performed using the CIRS008A lung phantom to simulate different on-board target variation scenarios for localization. The intra-treatment kV and MV acquisition was achieved through the Varian TrueBeam Developer Mode. Four methods were compared for their localization accuracy: 1. the proposed dual-detector phase-matched DTS technique; 2. the single-detector phase-matched DTS technique; 3. the dual-detector 3D-DTS technique without phase-matching; and 4. the single-detector 3D-DTS technique without phase-matching. Results: For scan angles of 2.5°, 5°, 10°, 20° and 30°, the dual-detector phase-matched DTS technique localized the tumor with average(±standard deviations) errors of 0.4±0.3 mm, 0.5±0.3 mm, 0.6±0.2 mm, 0.9±0.4 mm and 1.0±0.3 mm, respectively. The corresponding values of single-detector phase-matched DTS technique were 4.0±2.5 mm, 2.7±1.1 mm, 1.7±1.2 mm, 2.2±0.9 mm and 1.5±0.8 mm, respectively. The values of dual-detector 3D-DTS technique were 6.2±1.7 mm, 6.3±1.2 mm, 5.3±1.3 mm, 2.0±2.2 mm and 1.5±0.5 mm, respectively. And the values of single-detector 3D-DTS technique were 9.7±8.9 mm, 9.8±8.8 mm, 10.0±9.7 mm, 3.9±2.7 mm and 2.2±1.3 mm, respectively. Conclusion: The dual-detector phase-matched DTS technique substantially improves the tumor localization accuracy, which can be applied to real-time intra-treatment lung tumor localization. The research was funded by the National Institutes of Health Grant No. R01-CA184173 and a grant from Varian Medical Systems.

Sub-millimeter spine position monitoring for stereotactic body radiotherapy using offline digital tomosynthesis

PurposeSpine stereotactic radiotherapy (SBRT) requires intrafraction motion <1–2mm. We evaluated the accuracy and precision of digital tomosynthesis (DTS) in combination with triangulation for spine position tracking. Materials/methodsSingle-slice DTS images were generated from kV cone beam CT (CBCT) projection images. They were registered to reference DTS images reconstructed from the planning CT-scan to determine 2D shifts between actual patient position and treatment plan position. 3D spine position was obtained by triangulation of each registration with a previous registration, for every 1° of data. For 7 patients who underwent spine SBRT, the standard deviation (SD) of DTS+triangulation over one entire dataset was evaluated for different DTS angles (2–16°) and triangulation angles (1–46°). For 32 CBCT datasets, acquired before or after treatment of the 7 patients, using 4° DTS and 18° triangulation angle, SDs were determined and average positions were compared to clinically performed CBCT registrations. ResultsMean SDs were 0.29±0.10mm for lateral (range 0.1–0.55mm), 0.14±0.08 for longitudinal (0.05–0.39) and 0.24±0.10 for the vertical direction (0.10–0.57). Lateral and vertical SDs for thoracic spine were higher than for lumbar spine. Differences between clinical CBCT registration and DTS+triangulation were 0.1±0.26, 0.02±0.33 and −0.07±0.21mm. ConclusionThe combination of DTS and triangulation allows for monitoring spine position with sub-mm accuracy and precision.

SU-D-BRF-04: Digital Tomosynthesis for Improved Daily Setup in Treatment of Liver Lesions

Purpose: Daily localization of liver lesions with cone-beam CT (CBCT) is difficult due to poor image quality caused by scatter, respiratory motion, and the lack of radiographic contrast between the liver parenchyma and the lesion(s). Digital tomosynthesis (DTS) is investigated as a modality to improve liver visualization and lesion/parenchyma contrast for daily setup. Methods: An in-house tool was developed to generate DTS images using a point-by-point filtered back-projection method from on-board CBCT projection data. DTS image planes are generated in a user defined orientation to visualize the anatomy at various depths. Reference DTS images are obtained from forward projection of the planning CT dataset at each projection angle. The CBCT DTS image set can then be registered to the reference DTS image set as a means for localization. Contour data from the planning CT's associate RT Structure file and forward projected similarly to the planning CT data. DTS images are created for each contoured structure, which can then be overlaid onto the DTS images for organ volume visualization. Results: High resolution DTS images generated from CBCT projections show fine anatomical detail, including small blood vessels, within the patient. However, the reference DTS images generated from forward projection of the planningmore » CT lacks this level of detail due to the low resolution of the CT voxels as compared to the pixel size in the projection images; typically 1mm-by-1mm-by-3mm (lat, vrt, lng) for the planning CT vs. 0.4mm-by-0.4mm for CBCT projections. Overlaying of the contours onto the DTS image allows for visualization of structures of interest. Conclusion: The ability to generate DTS images over a limited range of projection angles allows for reduction in the amount of respiratory motion within each acquisition. DTS may provide improved visualization of structures and lesions as compared to CBCT for highly mobile tumors.« less

Respiration‐phase‐matched digital tomosynthesis imaging for moving target verification: A feasibility study

To develop a respiration-phase-matched digital tomosynthesis (DTS) technique to monitor moving targets, and to evaluate its accuracy for various imaging parameters and anatomical characteristics. Previously developed 3D-DTS techniques, registering onboard DTS (OB-DTS, reconstructed from onboard projections) to reference DTS (R-DTS, reconstructed from DRRs of 3D reference CT), are inadequate to monitor moving targets. The authors' proposed respiration-phase-matched DTS technique registers OB-DTS to R-DTS reconstructed from DRRs generated by the same phase images of 4D reference CT as the corresponding onboard projections. To evaluate the improved accuracy of the author's technique, the authors performed thoracic phantom studies using (1) simulation with the 4D digital extended-cardiac-torso (XCAT) phantom, and (2) experiments with an anthropomorphic motion phantom. The studies were performed for various: respiratory cycle (RC), scan angle, and fraction of RC contained therein. Also, the authors assessed the accuracy of their technique relative to target size/location, and respiration inconsistencies from the R-DTS to OB-DTS. In both simulation and experimental studies, the respiration-phase-matched DTS technique is significantly more accurate in determining moving target positions. For 324 different scenarios simulated by XCAT, the respiration-phase-matched DTS technique localizes the 3D target position to errors of 1.07 ± 0.57 mm (mean ± S.D.), as compared to (a) 2.58 ± 1.37 and (b) 7.37 ± 4.18 mm, for 3D-DTS using 3D reference CT of (a) average intensity projection and (b) free-breathing CT. For 60 scenarios evaluated through experimental study, the uncertainties corresponding to those above are 1.24 ± 0.87, 2.42 ± 1.80, and 5.77 ± 6.45 mm, respectively. For a given scan angle, the accuracy of respiration-phase-matched DTS technique is less dependent on RC and the fraction of RC included in the scan. Increasing scan angle improves its accuracy. For different target locations, the targets near the chest wall or in the middle of lung provide higher registration accuracy compared to those near the mediastinum and diaphragm. Larger targets provide higher registration accuracy than small targets. Different respiratory cycle inconsistencies from R-DTS to OB-DTS minimally affect the registration accuracy. Increasing the respiratory amplitude inconsistencies will decrease the accuracy. The respiration-phase-matched DTS is more accurate and robust in determining moving target positions than 3D-DTS. It has potential application in pretreatment setup, post-treatment analysis, and intrafractional target verification.

WE‐A‐134‐04: Comprehensive Evaluation of a Respiration‐Phase‐Matched Digital Tomosynthesis (DTS) Imaging Technique for Monitoring Moving Targets

Purpose: To develop a respiration‐phase‐matched DTS technique to monitor moving targets, and to evaluate its accuracy for various imaging parameters and anatomical characteristics Methods: Conventional methods, registering on‐board DTS(OB‐DTS, reconstructed from on‐board projections) to reference DTS(R‐DTS, reconstructed from DRRs of 3D‐planning‐CT), are inadequate to monitor moving targets. Our proposed technique registers OB‐DTS to R‐DTS reconstructed from DRRs generated by the same phase images of 4D‐planning‐CT as the corresponding on‐board projections. To evaluate the improved accuracy of our technique, we performed thoracic phantom studies using (1)simulation with the 4D Digital Extended‐cardiac‐torso(XCAT) phantom, and (2)experiments with an anthropomorphic motion phantom. The studies were performed for various: respiratory‐cycle(RC), scan angle and fraction of RC contained therein. Also, we assessed the accuracy of our technique relative to target size/location, and respiration changes from the planning‐CT scan to on‐board volume. Results: In both simulation and experimental studies the respiration‐phase‐matched DTS technique is significantly more accurate in determining moving target positions. For 324 different scenarios simulated by XCAT, the respiration‐phase‐matched DTS technique localizes the 3D target position to within 1.07±0.57mm(mean±S.D.), as compared to (a)2.58±1.37mm and (b)7.37±4.18mm, for traditional DTS using 3D‐planning‐CT of (a)average‐intensity‐projection(AIP) and (b)free‐breathing‐CT(FB‐CT). For the 60 scenarios evaluated through experimental study, the uncertainties corresponding to those above are 1.24±0.87mm, 2.42±1.80mm, and 5.77±6.45mm, respectively. For a given scan angle, the accuracy of respiration‐phase‐matched DTS technique is less dependent on RC and the fraction of RC included in the scan. Increasing scan angle improves its accuracy. Its accuracy is also minimally dependent on different tumor size/location combinations, or different respiratory cycle changes from planning‐CT to on‐board volume. Increasing the respiratory amplitude change will decrease its accuracy. Conclusion: The respiration‐phase‐matched DTS is more accurate and robust in determining moving target positions than traditional DTS. It has potential application in pre‐treatment setup, post‐treatment analysis and intra‐fractional target verification. Research partially supported by grant from Varian Medical Systems

MO‐F‐WAB‐03: Trajectory Imaging: Exploring Options of Kilovoltage Digital Tomosynthesis During Intensity Modulated Radiation Therapy

Purpose: To integrate novel kilovoltage (kV) digital tomosynthesis (DTS) sequences into intensity modulated radiation therapy (IMRT) to determine 3D fiducial positions for intrafractional motion monitoring. Method: In one method, we converted a static gantry IMRT beam into a series of arcs in which dose index and multileaf collimator positions for all control points were kept intact, but gantry angles were modified to oscillate +/−3° around the original angle. kV projections were acquired continuously throughout delivery and reconstructed to provide a series of 6° arc DTS images which were used to evaluate in‐plane positions of gold fiducials embedded in an anthropomorphic phantom. To obtain out‐of‐plane positions via triangulation, a 20° gantry rotation with beam held‐off was inserted during delivery to produce a pair of 6° DTS images separated by 14°. In a second method, the gantry remained stationary, but both kV source and detector moved over a 15° longitudinal arc via pitch and translational adjustment of the robotic arms. Evaluation of localization accuracy in phantom during simulated intrafractional motion used programmed couch translations from customized scripts. In‐house software was used to reconstruct DTS images, register them to reference DTS images and calculate 3D fiducial positions. Result No significant dose difference (&lt;0.5%) was found between the original and converted IMRT beams. For a typical hypofractionated spine treatment, 200 single DTS (6° arc) and 10 paired DTS (20° arc) were acquired for each IMRT beam, providing in‐plane (IP) and out‐of‐plane (OP) monitoring every 1.5 and 30 seconds, respectively. Mean (standard deviation) error in predicted position was −0.3(0.2)mm, −0.1(0.1)mm IP, and 0.2(0.4)mm OP with rotational gantry, −0.1(0.1)mm, 0.7(0.3)mm IP and 1.9(1.1)mm OP with translational source/detector. Conclusion: Acquiring 3D fiducial positions from kV‐DTS during IMRT delivery is technically feasible. Enabling movement of kV‐source and detector during delivery could further reduce imaging time overhead. Part of the research is sponsored by Varian Medical System.

TU‐A‐BRA‐09: Phase‐Matched Digital Tomosynthesis (DTS) Imaging for Simultaneous Target Verification during Volumetric Modulated Arc Therapy (VMAT) Treatment

Purpose: To develop a phase‐matched DTS technique to allow tomographic verification of moving targets simultaneous to VMAT delivery. Methods: This study proposes to verify moving targets by comparing phase ‐matched reference DTS (Ref ‐DTS) generated from simulation 4DCT to corresponding on ‐board DTS (OBI ‐DTS) acquired during VMAT delivery. The 4D Digital Extended Cardiac Torso (XCAT) Phantom was used for demonstration. A normal patient breathing pattern was input into the XCAT phantom to generate on ‐board CBCT projections, simulation 4DCT, average intensity projection (AIP) of the 4DCT and free ‐breathing CT (FBCT).These simulation CT sets (FBCT, AIP and 4DCT) were used to project digitally reconstructed radiographs (DRRs): DRR ‐FBCT and DRR ‐AIP (both with no respiratory phase information), and DRR ‐4DCT matched to phases of onboard projections. These DRR sets were further used to reconstruct three Ref ‐DTS sets: Ref ‐DTS ‐FBCT, Ref ‐DTS ‐AIP, and phase ‐matched Ref ‐DTS ‐4DCT. Each Ref ‐DTS set was compared with OBI ‐DTS (generated from on ‐board projections) to identify relative differences between both. To quantitatively assess the feasibility of comparing each Ref ‐DTS set with OBI ‐DTS for target verification, the simulation CT sets were shifted by a given amount to simulate target motion/setup variation, and the resulting Ref ‐DTS sets were registered to OBI ‐DTS automatically. The accuracy of target verification was evaluated by the discrepancies between simulated and registered shifts. Results: For a 1mm discrepancy criterion between simulated and registered shifts, the success rates between OBI‐DTS /Ref‐DTS‐FBCT, OBI‐DTS /Ref‐DTS‐AIP, and OBI‐DTS /Ref‐DTS‐4DCT registrations are 14.3%, 28.6%, and 100% for 2mm shifts, 0%, 14.3%, 100% for 5mm shifts, and 42.9%, 28.6%, 100% for 10mm shifts, respectively. Conclusions: The simulation study indicated that the phase‐matched Ref‐DTS‐4DCT technique considerably improved the target verification accuracy than that provided by non‐phase‐information Ref‐DTS‐FBCT or Ref‐DTS‐AIP. Therefore, it has the potential to validate VMAT treatment of moving targets. (Research partially supported by grant from Varian Medical Systems) Research partially supported by grant from Varian Medical Systems.

Automatic Registration Between Reference and On-Board Digital Tomosynthesis for Target Localization of Head and Neck Treatment

Digital tomosynthesis (DTS) is being investigated as a new on-board target localization technique. We previously developed an automatic method for registering reference DTS with on-board DTS images. This study is aimed to assess the accuracy and robustness of using this automatic technique for head and neck patient treatment.

Accelerating reconstruction of reference digital tomosynthesis using graphics hardware

The successful implementation of digital tomosynthesis (DTS) for on-board image guided radiation therapy (IGRT) requires fast DTS image reconstruction. Both target and reference DTS image sets are required to support an image registration application for IGRT. Target images are usually DTS image sets reconstructed from on-board projections, which can be accomplished quickly using the conventional filtered backprojection algorithm. Reference images are DTS image sets reconstructed from digitally reconstructed radiographs (DRRs) previously generated from conventional planning CT data. Generating a set of DRRs from planning CT is relatively slow using the conventional ray-casting algorithm. In order to facilitate DTS reconstruction within a clinically acceptable period of time, we implemented a high performance DRR reconstruction algorithm on a graphics processing unit of commercial PC graphics hardware. The performance of this new algorithm was evaluated and compared with that which is achieved using the conventional software-based ray-casting algorithm. DTS images were reconstructed from DRRs previously generated by both hardware and software algorithms. On average, the DRR reconstruction efficiency using the hardware method is improved by a factor of 67 over the software method. The image quality of the DRRs was comparable to those generated using the software-based ray-casting algorithm. Accelerated DRR reconstruction significantly reduces the overall time required to produce a set of reference DTS images from planning CT and makes this technique clinically practical for target localization for radiation therapy.

Evaluation of three types of reference image data for external beam radiotherapy target localization using digital tomosynthesis (DTS)

Digital tomosynthesis (DTS) is a fast, low-dose three-dimensional (3D) imaging approach which yields slice images with excellent in-plane resolution, though low plane-to-plane resolution. A stack of DTS slices can be reconstructed from a single limited-angle scan, with typical scan angles ranging from 10 degrees to 40 degrees and acquisition times of less than 10 s. The resulting DTS slices show soft tissue contrast approaching that of full cone-beam CT. External beam radiotherapy target localization using DTS requires the registration of on-board DTS images with corresponding reference image data. This study evaluates three types of reference volume: original reference CT, exact reference DTS (RDTS), and a more computationally efficient approximate reference DTS (RDTSapprox), as well as three different DTS scan angles (22 degrees, 44 degrees, and 65 degrees) for the DTS target localization task. Three-dimensional mutual information (MI) shared between reference and onboard DTS volumes was computed in a region surrounding the spine of a chest phantom, as translations spanning +/-5 mm and rotations spanning +/-5 degrees were simulated along each dimension in the reference volumes. The locations of the MI maxima were used as surrogates for registration accuracy, and the width of the MI peaks were used to characterize the registration robustness. The results show that conventional treatment planning CT volumes are inadequate reference volumes for direct registration with on-board DTS data. The efficient RDTSapprox method also appears insufficient for MI-based registration without further refinement of the technique, though it may be suitable for manual registration performed by a human observer. The exact RDTS volumes, on the other hand, delivered a 3D DTS localization accuracy of 0.5 mm and 0.50 along each axis, using only a single 44 degrees coronal on-board DTS scan of the chest phantom.

SU-FF-J-54: A Hybrid Multiresolution Method for Automatic Registration Between Reference and On Board Digital Tomosynthesis

Purpose: Automatic registration between reference and on‐board digital tomosynthesis (DTS) is a critical step for target localization using DTS. We previously developed an approximate method for automatic registration between reference and on‐board DTS, in which reference DTS was not generated iteratively to match with the anisotropic resolution of on‐board DTS. This study aims at developing an accurate method for automatic DTS registration with improved efficiency, robustness and accuracy. Method and Materials: The accurate DTS registration method was realized mainly by hardware acceleration of DRR generation, which was achieved by implementing a 3D texturemap based volume rendering process in graphic card using OpenGL language. The alpha blending mode was chosen to simulate ray casting algorithm, and three color channels of graphic card were used for data storage. With hardware acceleration, the accurate method changes the CT pose and regenerates DRR and reference DTS iteratively to match reference DTS with on‐board DTS based on mutual information. A hybrid multiresolution method was developed to combine approximate and accurate methods in a multiresolution scheme. It was tested on both anthropomorphic chest phantom and H&N patient data with simulated translations and rotations. The registration efficiency, attraction range and accuracy were recorded and compared with previous results. Results: The hybrid multiresolution method is 4 times faster than approximate method. Its registration attraction range is 2–5 times larger than that of approximate method for single‐axis translations and rotations, and 1.5–2 times larger for combined 6‐D rotations and translations. The registration accuracy is within 1.2 deg/mm for phantom data and 1.5deg/mm for patient data through the whole attraction range. Conclusions: The hybrid multiresolution method is much more efficient, robust, and accurate than approximate method. It is potentially a practical and reliable method to clinically implement DTS for target localization. Partially supported by a Varian research grant.

SU‐FF‐J‐58: Clinical Evaluation Using Digital Tomosynthesis for Positioning Verification of Breath‐Hold Liver Treatment

Purpose: To demonstrate clinical feasibility of generating digital tomosynthesis (DTS) in a single breath‐hold (BH) period compared to multiple BH periods for CBCT acquisition and to examine on‐board BH‐DTS as an efficient alternative to BH‐CBCT for target localization of breath‐hold liver treatment. Methods &amp; Materials: 3 patients receiving 3D conformal or IMRT treatments to the liver were retrospectively studied. Reference DTS images (RDTS) were generated using the BH planning CT and BH‐DTS images were reconstructed using the projection images acquired with BH‐CBCT. Matching between on‐board image set and reference image set was performed by (1) aligning bony anatomy near isocenter and (2) aligning target soft tissue using all image sets. Localization variations were calculated as the relative couch shifts in the vertical, longitudinal and lateral directions, as well as the relative couch rotations in pitch, yaw, and roll directions. Results: 10 – 40 degrees DTS images can be acquired within 1 single BH. Localization accuracy is in the range of 0.2 cm and 1.0 degree using the coronal‐view DTSs and is in the range of 0.35 cm and 1.6 degree using the for sagittal view DTS. The localization accuracy is comparable either using bony anatomy or using soft tissue. Conclusion: Rapid (&lt; 10 sec.) BH‐DTS is more efficient than BH‐CBCT (&gt; 5 min.) for daily 3D on‐board image guidance and localization of liver under breath‐hold radiation therapy.Partially supported by research grant from Varian Medical Systems.

WE‐E‐M100F‐05: Evaluation of Respiration‐Correlated Digital Tomosynthesis in the Thorax and Abdomen for Soft Tissue Visualization and Patient Positioning

Purpose: To find optimal parameters for digital tomosynthesis (DTS) image reconstruction, to evaluate ability of respiration correlated DTS to reduce blur caused by respiratory motion, and to assess DTS imaging for soft tissue localization and patient positioning. Methods and Materials: Image acquisition for DTS used a gantry‐mounted kV on‐board imaging system (Varian Medical Systems). We did not acquire DTS separately, but instead simulated DTS acquisition by using projection images acquired for CBCT. DTS reconstruction consisted of backprojection followed by a deblurring operation removing out‐of‐plane objects. For tumors subject to respiratory motion we selected projection images according to an external respiratory monitor signal (Real‐time Position Management System, Varian Medical Systems). Reconstruction and registration of DTS images used vender research software. Results: Optimal DTS quality is achieved with a 6–9 cm long deblurring volume in the direction perpendicular to the image and 20–30° reconstruction arc lengths. Image blur increases with longer arc lengths, while for shorter arcs out of plane objects become more pronounced. RC DTS reconstruction from disjoint arcs containing 2–3 respiratory cycles is feasible, yields images with less motion blur, and allows visualization of tumor movement. Generally, DTS was capable to visualize lung tumors, bronchi, liver, kidneys and abdominal lymph nodes. Estimated 3D registration error of DTS to reference DTS generated from the plan CT DRRs was 3 mm, relative to cone‐beam CT as a standard. Registration of RC DTS of lung tumors was possible to within 2 mm. Conclusions: DTS is capable of soft tissue visualization and patient positioning, including tumors subject to respiratory motion. With several advantages over full rotation CBCT scan, such as much shorter acquisition time, smaller dose and relaxed clearance requirement, DTS can become an efficient imaging modality for image‐guided radiotherapy. Conflict of Interest: Research sponsored by NCI Grant P01‐CA59017 and Varian Medical Systems.

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.

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.

TH‐C‐ValB‐04: Evaluation of the Quality of 3D‐3D Mutual Information (MI) Shared Between Reference and On‐Board DTS Images

Purpose: Digital tomosynthesis (DTS) is a fast, low‐dose method for reconstructing 3‐D slices from 2‐D cone‐beam x‐ray projection data acquired with limited source angulation (e.g., 40°). We previously developed a method for reconstructing reference DTS (RDTS) images from a planning CT volume, for registration with on‐board DTS image data. This study examines 3D‐3D mutual information (MI) shared between RDTS and DTS volumes of an anthropomorphic chest phantom. Method and Materials: Planning CT and on‐board CBCT volumes of an anthropomorphic chest phantom were aligned by a point‐registration of 6 metal fiducials. Misregistration (+/− 5mm and +/− 5°) was simulated along each of the six possible translational and rotational dimensions of the planning CT volume. For scan angles spanning 10°–60°, RDTS images were reconstructed from each of the misregistered CT volumes and DTS image sets were computed from actual on‐board projections. 3D‐3D MI between each RDTS and DTS volume was computed for a 4cm × 10cm × 7.5cm central region of interest containing the spine. MI was plotted as a function of each of the six possible translations and rotations, for each DTS scan angle. Results: For all scan angles greater than 10°, residual error in the location of global MI maxima was less than 1.0 mm, and 0.5°. MI resolution to shifts in the depth dimension (normal to individual DTS planes) improved noticeably with increasing scan angle. MI sensitivity to all other shifts and rotations showed only minor improvement with increasing scan angle. Conclusion: MI between planning CT‐generated RDTS and actual on‐board DTS image data is sufficient to be used for rigid‐body registration purposes. The addition of an orthogonal DTS acquisition (i.e., acquiring sagittal along with coronal DTS) is probably unnecessary for rigid‐body DTS registration. Conflict of Interest: This research was supported in part by a grant from Varian Medical Systems.

Digital tomosynthesis with an on-board kilovoltage imaging device

To generate on-board digital tomosynthesis (DTS) and reference DTS images for three-dimensional image-guided radiation therapy (IGRT) as an alternative to conventional portal imaging or on-board cone-beam computed tomography (CBCT). Three clinical cases (prostate, head-and-neck, and liver) were selected to illustrate the capabilities of on-board DTS for IGRT. Corresponding reference DTS images were reconstructed from digitally reconstructed radiographs computed from planning CT image sets. The effect of scan angle on DTS slice thickness was examined by computing the mutual information between coincident CBCT and DTS images, as the DTS scan angle was varied from 0 degrees to 165 degrees . A breath-hold DTS acquisition strategy was implemented to remove respiratory motion artifacts. Digital tomosynthesis slices appeared similar to coincident CBCT planes and yielded substantially more anatomic information than either kilovoltage or megavoltage radiographs. Breath-hold DTS acquisition improved soft-tissue visibility by suppressing respiratory motion. Improved bony and soft-tissue visibility in DTS images is likely to improve target localization compared with radiographic verification techniques and might allow for daily localization of a soft-tissue target. Breath-hold DTS is a potential alternative to on-board CBCT for sites prone to respiratory motion.