CBCT Acquisition Research Articles

SU-GG-I-151: Background Subtraction From X-Ray Projection Images Using Prior Information: A Phantom Study

Purpose: Kilovoltage image guidance is increasingly available for daily imaging in radiotherapy departments. Improving image quality on the projection of a section of the anatomy can be of interest for some imaging techniques. Our purpose is to investigate a proof-of-principle background subtraction technique that improves contrast resolution in x-ray projection images, by isolating the structures belonging to a selected slice of the anatomy, using prior CT information. Method and Materials: we acquired X-ray projection images of a phantom with a CBCT Varian On-Board Imager over a full gantry turn, using a standard CBCT acquisition technique. The CBCT scanner was equipped with antiscatter grid, without a bowtie filter. The dataset was comprised of a reconstructed CBCT image and the set of 2D projections used to reconstruct the CBCT image using the standard FDK algorithm. In the reconstructed CT image we selected sets of slices parallel to the axis of gantry rotation on the CBCT reconstructed images, and obtained a background image by forward projecting the full CBCT image minus the voxels belonging to the selected slice. No denoising was applied to the background image. The background was subtracted from the full projection to give an attenuation map of the selected slices. Results: the background-subtracted X-ray projections of selected slices of the Catphan phantom show a higher contrast than the projections of the full phantom. The CNR of the imaged slices improved by a factor of two with respect to the full projection. Conclusion: Background-subtracted X-ray projection images could show enhanced low contrast resolution when imaging pre-selected slices of a previously imaged anatomy. Investigation supported by NCI grant R01CA117997.

SU-GG-J-20: A Trajectory Estimation Method of Moving Target From Cone Beam CT Projection Images

Purpose: The aim of this work was to develop and investigate a method for trajectory estimation of a moving target from cone beam CT projection images. It can be useful for assessment of respiratory tumor motion just before or during treatment, and for developing further respiratory compensation strategies. Method and Materials: Six lungtumor trajectories with range of motion larger than 10 mm, acquired using a CyberKnife system, were used to simulate CBCT acquisition. To reconstruct 3D positions from projected positions, the two nearest angular projection images with the same SI‐projection positions were used for triangulation. With the triangulated positions, a linear correlation was assumed for tumor motion. LR and AP positions were then derived from SI positions using the linear model. The estimated 3D tumor positions were compared with the known positions. To demonstrate the real simulation, CBCT projection images for a seed‐embedded moving phantom were acquired using a Varian OBI system. A 3D motion platform was used to drive the phantom with one of six trajectories. The marker position from each projection image was extracted using Varian RPM‐Fluoro application. Results: The mean estimation errors were within 0.3±1.8 mm while errors of up to 4 mm were observed. In general, higher correlation coefficients with SI‐motion provide better estimation of AP and LR positions. Applying the center offset of OBI imager (xOffset=−2 pixel, yOffset=0 pixel), the remaining uncertainty associated with gantry sag and marker extraction (∼1 pixel) was within ∼0.5 mm. Conclusion: The proposed method provides useful information for tumor motion characteristics such as the mean position, range, and principal direction of motion which can be useful for respiratory compensation treatments. Conflict of Interest: Research supported by Varian Medical Systems.

SU‐GG‐J‐72: Development of Megavoltage Cone Beam Image Guided Radiation Therapy (IGRT) for Mini‐Multileaf Collimator (mMLC) Based IMRT and Radiosurgery Applications

Purpose: The purpose of this work was to investigate the technical feasibility of using the widely deployed Megavoltage Cone Beam CT (MVCBCT) for 3D IGRT applications with mMLC based treatment delivery. Method and Materials: In most clinical applications of MVCBCT IGRT, the largest possible field of view (FOV) is sought for evaluating adjacent organs‐at‐risk. However, an add‐on mMLC with maximum field size of 10cm×12cm is preferred for precise small‐field hypofractionated treatments. mMLC based treatments utilizing 2.5mm leaf widths are usually performed for SRS, SRT and SBRT applications, where the higher accuracy of 3D IGRT localization is also warranted. Current clinical systems do not allow restricting the field size of CBCT acquisition in the X direction. Performing imaging with the mMLC mounted and the field size opened to ∼27cm results in major truncation artifacts rendering the images unusable, as well as the irradiation of the mMLC electronics during image acquisition, which is undesirable. The current study was conducted with MVision™ and Moduleaf™ mMLC (Siemens Medical Systems), using anthropomorphic head and neck, and chest phantoms. A custom cerrobend aperture block was fabricated to shield the mMLC from radiation damage during imaging with the X Jaw open. The clinically used reconstruction algorithm was set to only reconstruct a smaller field of view corresponding to the clinically useful restricted treatment field size, although the data acquisition field size was set to the required 27.4 cm in the X direction. Results: With the changes described above, we were able to accomplish high quality images with the mMLC. The image quality was better than images obtained for the full field acquisition in the clinical mode for the same phantoms. Conclusion: We believe this to be the first study demonstrating high quality 3D IGRT through a small mMLC collimator designed for SRT.

TU‐A‐BRA‐01: EPID, MVCT

The Mega‐Voltage Cone‐Beam CT (MVCBCT) system provides a 3D image of the patient anatomy in the actual treatment position that can be tightly aligned to the planning CT, allowing daily verification and correction of the patient position moments before the dose delivery. Integrated onto a linear accelerator, the system consists of a new a‐Si flat panel adapted for MV imaging and a workflow application allowing the automatic acquisition of projection images at low dose rate, CBCT image reconstruction, CT to CBCT image registration and couch position adjustment. MVCBCT provides accurate electron density and allows studies of dosimetrical impact of setup error, anatomical changes or in presence of implanted metallic objects.This lecture will provide an overview of the physics characteristics of MV and MV CBCT imaging, acquisition and reconstruction, image registration, alignment precision and quality assurance procedures. A description of the workflow and a survey of the clinical applications, indications and protocols will be presented. Finally, current challenges and future developments will be addressed.Educational objectives1. Understand the basics of MV Cone‐Beam CT imaging.2. Understand the possibilities of daily 3D Imaging for patient alignment and other applications.3. Understand the clinical roles of Image‐Guided (IGRT) and Dose‐ Guided (DGRT) Radiation Therapy.This work was supported by Siemens Oncology Care Systems.

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 & 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 (< 10 sec.) BH‐DTS is more efficient than BH‐CBCT (> 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.

Patient dose from kilovoltage cone beam computed tomography imaging in radiation therapy

Kilovoltage cone-beam computerized tomography (kV-CBCT) systems integrated into the gantry of linear accelerators can be used to acquire high-resolution volumetric images of the patient in the treatment position. Using on-line software and hardware, patient position can be determined accurately with a high degree of precision and, subsequently, set-up parameters can be adjusted to deliver the intended treatment. While the patient dose due to a single volumetric imaging acquisition is small compared to the therapy dose, repeated and daily image guidance procedures can lead to substantial dose to normal tissue. The dosimetric properties of a clinical CBCT system have been studied on an Elekta linear accelerator (Synergy RP, XVI system) and additional measurements performed on a laboratory system with identical geometry. Dose measurements were performed with an ion chamber and MOSFET detectors at the center, periphery, and surface of 30 and 16-cm-diam cylindrical shaped water phantoms, as a function of x-ray energy and longitudinal field-of-view (FOV) settings of 5,10,15, and 26 cm. The measurements were performed for full 360 degrees CBCT acquisition as well as for half-rotation scans for 120 kVp beams using the 30-cm-diam phantom. The dose at the center and surface of the body phantom were determined to be 1.6 and 2.3 cGy for a typical imaging protocol, using full rotation scan, with a technique setting of 120 kVp and 660 mAs. The results of our measurements have been presented in terms of a dose conversion factor fCBCT, expressed in cGy/R. These factors depend on beam quality and phantom size as well as on scan geometry and can be utilized to estimate dose for any arbitrary mAs setting and reference exposure rate of the x-ray tube at standard distance. The results demonstrate the opportunity to manipulate the scanning parameters to reduce the dose to the patient by employing lower energy (kVp) beams, smaller FOV, or by using half-rotation scan.