Megavoltage Cone-beam CT Research Articles

Po-Poster - 02: Generation of MV cone beam CT - a feasibility study

The objective of this work was to study the feasibility of acquiring mega-voltage cone-beam CT (MV CBCT) images of sufficient quality for setup verification from 2 D projection images. The generation of mega-voltage cone-beam CT with a standard radiation therapy Primus linear accelerator and an amorphous silicon electronic portal imaging device has been attempted in this work. 2 D projection images of the head section of a Rando phantom were acquired at an interval of 2° from gantry angle of 258° to 102°. Thus 103 2D projection images of the Rando phantom were acquired with 6 MV beam with 1 MU/projection exposure at 300MU/minute dose rate. The estimated dose to the center of the phantom placed at the isocentre of the linac was about 75cGy. The cone angle was only 14.2° and hence for simplicity the images were reconstructed assuming a parallel beam geometry using the IRADON function implemented in MATLAB. High contrast objects are clearly seen on the reconstructed images thus showing the potential of MV CBCT for patient alignment during external beam radiotherapy. From the available image data set, reconstructions were also performed with fewer projections i.e with 52 and 26. Image quality was acceptable with 103 and 52 projections. Reconstruction with Feldkamp cone beam algorithm or iterative algorithm such as Algebraic Reconstruction Techniques (ART) could further improve the image quality.

TU-C-I-609-10: Preliminary Study of Pixel Pitch and Effect of Divergence On Thick Cadmium Tungstate Detector

Purpose: To theoretically study the effects of pixel pitch and beam divergence on 1 cm thick crystals for use in megavoltage cone beam computed tomography (MVCBCT). Method and Material: linear array (80-elements, each 0.275 × 0.8 × 1 cm3 in fan beam) has been proven to be a good candidate for MVCT in our lab. For this study, DOSXYZnrc Monte Carlo code was used to find the point spread function (PSF) as the distribution of energy deposited in a theoretical flat detector (30 × 30 × 10 mm3, 0.01 × 0.01 × 10 mm3 voxels). A pencil-beam of 6 MV photons was incident at a range of angles (0°, 5°, 10°, 15°, 20°) with respect to the normal. In each case, the detector PSF (ignoring optical spread) was convolved with a realistic source function to give the system PSF in the plane of the object. A realistic object to detector magnification of 1.4 was used in the system PSF calculations. The PSF at 0.01 mm pitch was re-binned into several pitches (0.5, 0.7, 1.0 mm) and Fourier transformed to obtain the system modulation transfer function (MTF). Results: This analysis suggests an optimum pitch of 0.5 mm which cannot be fabricated using because of the cleavage plane. However, the pre-sampled detector PSF is further degraded due to small optical leakage through reflective coating and scintillator-photodiode interface, and the scattered radiation from the object. So, in practice a pitch of 1 mm may be sufficient. The degradation of the system MTF with the increasing divergence is significant between 0° and 5°, and becomes large at 10°. Conclusion: Practical pitch of 1 mm is not ideal yet maybe sufficient. The use of flat panel photodiode arrays for MVCBCT is precluded due to divergence; instead a focused detector should be used.

SU‐DD‐A4‐06: Registration of Volumetric KV Or MV Cone Beam CT with Fan Beam CT

Purpose: Kilovoltage (kV) cone beam CT (CBCT) provides the patient's volumetric information and is valuable for patient positioning verification. An important step in this application is the registration of CBCT acquired at simulation or before treatment with the planning CT (PCT). The purpose of this work is to investigate several specific issues involved in CBCT‐TPCT registration and develop an effective algorithm to effectively utilize the volumetric imaging system for IGRT. Method and Materials: The PCT data were acquired using Picker PQ5000 scanner, and the CBCT images were obtained using Varian Trilogy OBI system. The image quality of CBCT is generally poor as compared with conventional CT: Low soft tissue contrast, artifacts/distortions, and limited width/length present practical problems for its clinical application. A BSpline based non‐rigid image fusion software was implemented to provide voxel‐to‐voxel matching between PCT and CBCT. Results: For the phantom study, the quality of CBCT has little effect on the registration and the CBCT‐PCT mapping was relatively straightforward because of the absence of motion artifacts. Examination of “bony” landmarks indicated that an accuracy less than 2mm was achievable. The patient registrations were complicated by artifacts and, in the cervix case, the limited field size of CBCT. While the bony landmarks can be registered to within 2mm, the motion artifacts complicated registration and caused a large uncertainty in the PCT‐CBCT mapping. Conclusion: Low soft tissue contrast and artifacts of CBCT posed a challenge for CBCT‐PCT registration. While continuous effect is needed to improve the CBCT, a robust registration tool that can cope with specific issues of CBCT is highly desirable to fully exploit the volumetric CBCT technology.

SU‐FF‐J‐81: Clinical Integration of a MV Conebeam CT System for Image‐Guided Treatment

Purpose: To perform the integration of a newly developed image‐guidance system and to describe the main advantages and performance of the first Megavoltage Conebeam CT (MV CBCT) system. Method and Materials: The MV CBCT system, consisting of a new a‐Si flat panel adapted for MV imaging and an integrated workflow application allowing the automatic acquisition of projection images, conebeam CT image reconstruction, CT to CBCT image registration and couch position adjustment was recently introduced in clinic. Template protocols can be used for the acquisition of CBCT images at different dose ranging from 1 to 60 M.U. Geometrical calibration, gain image adjustment and defect pixels correction procedures are performed off‐line. Results: For a typical case, 200 projection portal images and a total exposure of 5 to 8 M.U. are acquired with the 6 MV beam in 45 seconds and the 256×256×256 MV CBCT image is reconstructed less than two minutes after the start of the acquisition. Examples of the image‐guided treatment process including the acquisition of projections images, the reconstruction of the MV CBCT image and its registration with the planning CT, followed by the couch position correction and dose delivery will be presented. Conclusion: MV CBCT provides a 3D patient anatomy volume in the actual treatment position, relative to the treatment isocenter, moments before the dose delivery, that can be tightly aligned to the planning CT, allowing verification and correction of the patient position. Research supported by Siemens Oncology Care Systems.

WE‐D‐I‐6B‐01: Soft Tissue Visualization Using a Highly Efficient Megavoltage Cone Beam CT Imaging System

Purpose: Recent developments in two‐dimensional x‐ray detector technology have made volumetric Cone Beam CT (CBCT) a feasible approach for integration with conventional medical linear accelerators. The requirements of a robust image guidance system for radiation therapy include the challenging combination of soft tissue sensitivity with clinically reasonable doses. Previously, low contrast objects have generally not been perceptible with MV energies due to relatively poor signal to noise ratio (SNR) performance. We have developed an improved imaging system that is optimized for MV CBCT and acquire CBCT images containing soft tissue contrast using a 6MV x‐ray beam. Method and Materials: Many factors, such as image noise, x‐ray scatter, improper calibration and acquisitions have a profound effect on the imaging performance of CBCT. In this study attempts were made to optimize these factors in order to maximize the SNR. A QC‐3V and contrast/resolution phantoms were used to determine the contrast to noise ratio (CNR) and f50 of a single 2‐D projection and the contrast and spatial resolution of the reconstructed images. Results: The computed f50 was 0.43 lp/mm and the CNR for a radiation dose of 0.02cGy was 43. Relative electron density as low as 1% can be resolved with clinically reasonable radiation doses. Clinical Megavoltage CBCT images acquired with this system demonstrate that anatomical structures such as the prostate and optic nerves are visible using radiation doses in range of 4 to 8cGy. Conclusion: We have developed an imaging system that is optimized for MV and acquire Megavoltage CBCT images containing soft tissue contrast using a 6MV x‐ray beam and irradiation doses in range of 4 to 8cGy. This system can also be used for routine portal imaging applications without risk of saturation for high dose/high energy treatment/verification imaging, or dosimetry applications. Conflict of Interest : Sponsored by Siemens.

SU-FF-I-53: The Impact of Portal Imager Shifts and the Assumption of Rigid Isocentricity On Megavoltage Cone Beam CT Images

Purpose: Linear accelerators and the current flat panel positioners do not represent a rigid isocentric imaging system. To correct for this effect in the reconstruction process of Megavoltage Cone Beam CT, the group uses projection matrices that are obtained from geometric calibration. This project studies the effect of deviations in detector position from the calibrated geometry which may occur over time. Method and Materials: To simulate the effect of portal imager shifts, we translated projection images before performing the reconstruction. Synthetic projection matrices that assume perfect isocentricity were also produced to study the utility of our geometric calibration. To accentuate the observed effects, we first used noise-free simulated projections of a CT phantom as well as reconstructions of small, high-contrast ball bearings. We also acquired anatomical images of a Rando head with an acceptable clinical dose (8 MU) to verify our capacity to identify the previously observed artifacts. Results: A pure flat panel shift of 2 mm along the longitudinal direction causes the same shift in the reconstruction volume. A 2 mm shift in the lateral direction however, greatly degrades the image quality with streaking and half-moon shadow artifacts. The orientation of those artifacts depends on the start and end angle of the acquisition. Since most of the flat panel flex was previously measured to be in the longitudinal direction, the use of synthetic projection matrices caused a blurring of the image in the longitudinal direction. Conclusion: The calibrated projection matrices play an important role at conserving the image quality around high contrast objects. A 2 mm lateral shift away from the calibration will likely be detectable in high contrast regions of anatomical images. Longitudinal shifts will not degrade the image but will cause a positioning error. Conflict of Interest: This research is supported by Siemens OCS.

SU‐FF‐J‐60: Effectiveness of MVCBCT for Patients with Implanted High‐Z Material

Purpose: To exploit the penetrability of high‐energy photons of Megavoltage ConeBeam CT system (MVCBCT) to obtain 3D images of the anatomy in presence of “Non‐compatible CT” objects made of high‐Z material. Methods and Material: A new MVCBCT system integrated onto a clinical Linac was used to acquire 3D images of different phantoms and patients. The grey levels of different electron density inserts (lung to dense bone) in a CT phantom were measured with and without the presence of a small Cerrobend rod (10×15mm) on a regular CT and with the MVCBCT. MVCBCT of a Rando phantom with implanted gold markers and tooth fillings as well as patients with dental implants or with gold markers implanted in prostate were also obtained. Results: The presence of the Cerrobend object in the CT phantom scanned with a regular CT creates strong artifacts around the object and disturbs the quality of the entire image, modifying the Hounsfield numbers by an average of 10%, even 15 cm away from the rod. The grey levels of the density inserts in the CT phantom remain unchanged within 3% in presence of the Cerrobend rod for MVCBCT. Similarly, gold markers appear with the typical star pattern artifact on CT images where a well‐defined dot is seen on MVCBCT. The tooth fillings in the MVCBCT Rando phantom do not disturb the soft tissue information around the teeth. Conclusion: Compared to the kV energy range, the presence of high‐Z material has relatively little impact on image quality of MVCBCT. Therefore, MVCBCT can complement missing information for planning or patient position verification purposes when high‐Z materials such as gold markers, tooth fillings, dental implants or hip prostheses are present. Clinical examples of each of these items will be presented. Conflict of Interest: Siemens supports this Research.

SU-FF-J-72: The Effect of MV Cone-Beam CT Cupping Artifacts On Dose Calculation Accuracy

Purpose: MV cone‐beam (CB) CTimaging can be used to position the patient prior to external‐beam radiotherapy. If calibrated, these images may also be used in dose calculations, opening the possibility to reconstruct the delivered dose or re‐optimize the treatment plan before delivery. This study investigates the effect of imaging artifacts on dosimetric accuracy. Method and Materials: Two water cylinders of 13 cm and 22.5 cm diameters were imaged using A) a MV CBCT system integrated with a Siemens accelerator and B) a conventional kV CTimager. Cupping trends observed in the MV CBCT were imposed on the kV CTimages to create artificial data with simulated artifacts. Using the Phillips Pinnacle planning system, a 6 MV 10 × 16 cm2 field was applied to 1) the original kV CT of the smaller cylinder and 2) the kV CT with simulated artifacts. Similarly, a 18 × 18 cm2 field was applied to the original and artificial kV CT of the larger cylinder. Results: For the smaller cylinder, the MV CBCTwater signal relative to air at the cylinder center differs from the radial and axial extremities by approximately 14% and 17% respectively. For the larger cylinder, the differences are 38% and 20% in radial and axial directions. The dose distributions for the artificial images show a systematic deviation which increases with depth. In the high‐dose, low‐gradient regions, the largest deviation for the smaller cylinder is 1% of the maximum delivered dose. For the larger cylinder, the largest deviation is 4%. Conclusion: Cupping artifacts in water phantoms produce dosimetric errors much smaller in magnitude than the cupping trend itself but as large as 4% for large phantoms. These results suggest that rough correction of MV CBCT artifacts may be sufficient for dosimetric accuracy. Conflict of Interest: Research supported by Siemens OCS.

Developments in megavoltage cone beam CT with an amorphous silicon EPID: Reduction of exposure and synchronization with respiratory gating

We have studied the feasibility of a low-dose megavoltage cone beam computed tomography (MV CBCT) system for visualizing the gross tumor volume in respiratory gated radiation treatments of nonsmall-cell lung cancer. The system consists of a commercially available linear accelerator (LINAC), an amorphous silicon electronic portal imaging device, and a respiratory gating system. The gantry movement and beam delivery are controlled using dynamic beam delivery toolbox, a commercial software package for executing scripts to control the LINAC. A specially designed interface box synchronizes the LINAC, image acquisition electronics, and the respiratory gating system. Images are preprocessed to remove artifacts due to detector sag and LINAC output fluctuations. We report on the output, flatness, and symmetry of the images acquired using different imaging parameters. We also examine the quality of three-dimensional (3D) tomographic reconstruction with projection images of anthropomorphic thorax, contrast detail, and motion phantoms. The results show that, with the proper choice of imaging parameters, the flatness and symmetry are reasonably good with as low as three beam pulses per projection image. Resolution of 5% electron density differences is possible in a contrast detail phantom using 100 projections and 30 MU. Synchronization of image acquisition with simulated respiration also eliminated motion artifacts in a moving phantom, demonstrating the system's capability for imaging patients undergoing gated radiation therapy. The acquisition time is limited by the patient's respiration (only one image per breathing cycle) and is under 10 min for a scan of 100 projections. In conclusion, we have developed a MV CBCT system using commercially available components to produce 3D reconstructions, with sufficient contrast resolution for localizing a simulated lung tumor, using a dose comparable to portal imaging.

Low-dose megavoltage cone-beam CT for radiation therapy

The objective of this work was to demonstrate the feasibility of acquiring low-exposure megavoltage cone-beam CT (MV CBCT) three-dimensional (3D) image data of sufficient quality to register the CBCT images to kilovoltage planning CT images for patient alignment and dose verification purposes. A standard clinical 6-MV Primus linear accelerator, operating in arc therapy mode, and an amorphous-silicon (a-Si) flat-panel electronic portal-imaging device (EPID) were employed. The dose-pulse rate of 6-MV Primus accelerator beam was windowed to expose an a-Si flat panel by using only 0.02 to 0.08 monitor unit (MUs) per image. A triggered image-acquisition mode was designed to produce a high signal-to-noise ratio without pulsing artifacts. Several data sets were acquired for an anthropomorphic head phantom and frozen sheep and pig cadaver head, as well as for a head-and-neck cancer patient on intensity-modulated radiotherapy (IMRT). For each CBCT image, a set of 90 to 180 projection images incremented by 1 degree to 2 degrees was acquired. The two-dimensional (2D) projection images were then synthesized into a 3D image by use of cone-beam CT reconstruction. The resulting MV CBCT image set was used to visualize the 3D bony anatomy and some soft-tissue details. The 3D image registration with the kV planning CT was performed either automatically by application of a maximization of mutual information (MMI) algorithm or manually by aligning multiple 1D slices. Low-noise 3D MV CBCT images without pulsing artifacts were acquired with a total delivered dose that ranged from 5 to 15 cGy. Acquisition times, including image readout, were on the order of 90 seconds for 180 projection images taken through a continuous gantry rotation of 180 degrees. The processing time of the data required an additional 90 seconds for the reconstruction of a 256(3) cube with 1.0-mm voxel size. Implanted gold markers (1 mm x 3 mm) were easily visible or all exposure levels without artifacts. In general, the presence of high Z materials such as tooth fillings or implanted markers did not result in visible streak artifacts. The registration of structures such as the spinal canal and the nasopharynx in the MV CBCT and kV CT data sets was possible with millimeter and degree accuracy as assessed by displacement simulations and subsequent visual evaluation. We believe that the quality of these images, along with the rapid acquisition and reconstruction times, demonstrates that MV CBCT performed by use of a standard linear accelerator equipped with a flat-panel imager can be applied clinically for patient alignment.

Development of high quantum efficiency, flat panel, thick detectors for megavoltage x-ray imaging: A novel direct-conversion design and its feasibility

Most electronic portal imaging devices (EPIDs) developed to date, including recently developed flat panel systems, have low x-ray absorption, i.e., low quantum efficiency (QE) of 2%-4% as compared to the theoretical limit of 100%. A significant increase of QE is desirable for applications such as a megavoltage cone-beam computed tomography (MVCT) and megavoltage fluoroscopy. However, the spatial resolution of an imaging system usually decreases significantly with an increase of QE. The key to the success in the design of a high QE detector is therefore to maintain the spatial resolution. Recently, we demonstrated theoretically that it is possible to design a portal imaging detector with both high QE and high resolution [see Pang and Rowlands, Med. Phys. 29, 2274 (2002)]. In this paper, we introduce such a novel design consisting of a large number of microstructured plates (made by, e.g., photolithographic patterning of evaporated or electroplated layers) packed together and aligned with the incident x rays. On each plate, microstrip charge collectors are focused toward the x-ray source to collect charges generated in the ionization medium (e.g., air or gas) surrounded by high-density materials that act as x-ray converters. The collected charges represent the x-ray image and can be read out by various means, including a two-dimensional (2-D) active readout matrix. The QE, spatial resolution, and sensitivity of the detector have been calculated. It has been shown that the new design will have a QE of more than an order of magnitude higher and a spatial resolution equivalent to that of flat panel systems currently used for portal imaging. The new design is also quantum noise limited down to very low doses (approximately 1-2 radiation pulses of the linear accelerator).

132 The role of MV Cone Beam CT and Dose-Guided Radiotherapy

132 The role of MV Cone Beam CT and Dose-Guided Radiotherapy

Towards optimization of megavoltage cone-beam CT for image-guided radiotherapy

Towards optimization of megavoltage cone-beam CT for image-guided radiotherapy

Towards optimization of megavoltage cone-beam CT for image-guided radiotherapy

Soft tissue imaging in the treatment room is one of the main challenges faced today in high-precision radiation therapy. Megavoltage cone beam CT (MV-CBCT) is a promising imaging technique for image-guided radiotherapy due to its simplicity and its potentially higher accuracy than kV-CBCT (i.e., cone beam CT with a kilovoltage x-ray source mounted on a Linac machine). MV-CBCT uses an electronic portal imaging device (EPID) attached to a Linac to acquire CT images by rotating the MV x-ray source (emitting a cone beam) and the EPID around the patient.

Integrating respiratory gating into a megavoltage cone-beam CT (CBCT) system

Integrating respiratory gating into a megavoltage cone-beam CT (CBCT) system

Integrating respiratory gating into a megavoltage cone-beam CT (CBCT) system

We have carried out preclinical testing of a new low-dose MV CBCT system. The purpose of this work is to integrate the imaging system with respiratory gating, to enable us to use it in verifying the positioning of lung cancer patients undergoing gated radiotherapy.

Low-dose megavoltage cone-beam CT for dose-guided radiation therapy

Low-dose megavoltage cone-beam CT for dose-guided radiation therapy

3D tomographic reconstruction from portal imaging for patient positioning

External beam radiation therapy is an effective method of treating cancer in which the lesion is irradiated with high-energy X-ray beams. The first step in accurate delivery of radiation is accurate positioning of patient on the treatment table. Typically, reference images from the planning systems are matched with portal images acquired during patient setup prior to treatment. Matching of anatomical landmarks between the reference images and the portal images is attempted for patient positioning. This method has a number of shortcomings that influence the accuracy of patient positioning. We are proposing the use of Mega-Voltage Cone Beam CT (MVCT) acquired at the treatment time for patient positioning. The MVCT may be compared and matched with the planning CT for accurate determination of patient offset. We have demonstrated that acquisition of MVCT may be done with delivering a few Monitor Units (MU).

Optimization of conformal thoracic radiotherapy using cone-beam CT imaging for treatment verification

Purpose: Megavoltage cone-beam computed tomography (MVCBCT) has been proposed for treatment verification in conformal radiotherapy. However, the doses required for such imaging may compromise the quality of the delivered dose distribution. The present paper explores the effect of cone-beam imaging on dose homogeneity and critical organ dose and the use of our new tool, adapted intensity-modulated radiation therapy (AIMRT). Methods and Materials: Three types of treatment plans were devised (3D-CRT [three-dimensional conformal radiotherapy], IMRT [intensity-modulated radiotherapy], and AIMRT) based on 4 patients with thoracic malignancies. MVCBCT fields were then integrated into the plans. The MVCBCT technique used 21 imaging portals at 10° intervals. The MVCBCT apertures were shaped to conform to the planning target volume with a 6-mm margin. In a second set of plans, the field size was expanded by a further 2 cm. The unoptimized MVCBCT dose distribution was incorporated into the IMRT plan using AIMRT. Results: Normal-tissue complication probability with MVCBCT is acceptable for all plans at the 66.6 Gy level, but exceeds tolerance for both 3D-CRT alone and 3D-CRT with MVCBCT at higher doses. In contrast, the use of AIMRT planning with MVCBCT allowed safe dose escalation to 85 Gy. Expanding the MVCBCT aperture provided better anatomic visibility with an acceptable lung dose. The results using IMRT with MVCBCT fell between the values measured for 3D-CRT and AIMRT with MVCBCT. Conclusion: The present study is the first to demonstrate that MVCBCT can be incorporated into 3D-CRT and IMRT planning with minimal effect on planning target volume homogeneity and dose to critical structures. This paves the way for highly conformal radiotherapy at greater doses delivered with increased confidence and safety.

Cone-beam CT with megavoltage beams and an amorphous silicon electronic portal imaging device: potential for verification of radiotherapy of lung cancer.

We investigate the potential of megavoltage (MV) cone-beam CT with an amorphous silicon electronic portal imaging device (EPID) as a tool for patient position verification and tumor/organ motion studies in radiation treatment of lung tumors. We acquire 25 to 200 projection images using a 22 x 29 cm EPID. The acquisition is automatic and requires 7 minutes for 100 projections; it can be synchronized with respiratory gating. From these images, volumetric reconstruction is accomplished with a filtered backprojection in the cone-beam geometry. Several important prereconstruction image corrections, such as detector sag, must be applied. Tests with a contrast phantom indicate that differences in electron density of 2% can be detected with 100 projections, 200 cGy total dose. The contrast-to-noise ratio improves as the number of projections is increased. With 50 projections (100 cGy), high contrast objects are visible, and as few as 25 projections yield images with discernible features. We identify a technique of acquiring projection images with conformal beam apertures, shaped by a multileaf collimator, to reduce the dose to surrounding normal tissue. Tests of this technique on an anthropomorphic phantom demonstrate that a gross tumor volume in the lung can be accurately localized in three dimensions with scans using 88 monitor units. As such, conformal megavoltage cone-beam CT can provide three-dimensional imaging of lung tumors and may be used, for example, in verifying respiratory gated treatments.