Kilovoltage Imaging Research Articles

SU‐FF‐J‐29: Assessment of Patient Setup Variations with a Commercial On‐Board Imaging System: Is There a Benefit to the Patient?

Purpose: Kilovoltage imaging with a commercial on‐board imager (OBI) has been implemented in our treatment room for image‐guided radiation therapy (IGRT). It holds great potential to improve treatment accuracy by reducing the setup errors. However, performing patient setup with OBI costs extra time and effort. Thus, it is important to demonstrate which patient will benefit from this new technique. We have assessed patient setup variations and its dosimetric effect. Method and Materials: We perform daily OBI setup on the prostate, head‐and‐neck (H&N), and central‐nerve‐system (CNS) IMRT patients. Our setup procedure: (1) Patients were aligned to their skin marks. (2) Orthogonal kV images were acquired with OBI. (3) Setup corrections were made by registering the kV images with DRR's based on bony landmark (H&N and CNS patients) or fiducial markers (prostate patients). Shifts larger than PTV margins were verified with MV portal imaging. Setup data from 2 CNS, 9 H&N, and 10 prostate patients were analyzed. For H&N and CNS patients, dose redistributions that incorporate each day's actual shifts were calculated by rigid‐body translation. Results: (1) H&N and CNS patients: the random setup variations (standard deviation of the daily data) are 0.15cm, 0.32cm and 0.42cm in LR, AP, and SI direction. The systematic setup variations (the average) are 0.5cm. The largest shift is 1.2cm. (2) Prostate patients: average interfractional variations are 0.26 cm (LR), 0.24 (SI) and 0.39 cm (AP). (3) Dose redistribution in the postplans demonstrated that OBI setup corrections typically don't modify the CTV coverage but frequently affect sparing of the critical organs. Conclusion: Our random setup variations were found to be smaller than the PTV margin (5mm). With OBI setup corrections, we can further reduce the PTV margin safely. Our postplans have shown that OBI setup corrections can dramatically improve sparing of the normal structures.

MO‐A‐224C‐01: Daily Localization I: KV/CBCT

Daily target localization is a critical step to secure accurate delivery for 3‐D conformal radiation therapy and intensity‐modulated radiation therapy. One emerging technology using in‐room kilovoltage (kV) imaging has shown to be very promising for targeting treatment volumes. The major advantages of using kV imaging are 1) its comparable contrast between soft tissue and bone structure to that in simulation images, 2) real‐time high resolution fluoroscopic imaging, 3) lower radiation dose to patient compared to conventional MV imaging, and 4) availability of reconstructing tomographic images (for example, cone‐beam CT) using 2‐ D kV projection images. At present, different types of in‐room kV imaging systems are being developed for different applications such as in‐room CT on rail, ceiling‐mounted dual‐source/detector configuration, and gantrymounted single source/detector system. In terms of clinical applications, some generate 2‐D radiographic images for target verification based on bony marks or implanted surrogator while others generate 3‐D tomographic images for target verification based on both soft tissues and bony structures. Some could be used for viewing organ motion to verify applied margin while others could be used for monitoring target motion to verify dynamic delivering or gated treatment. Issues related to the risk and benefit between imaging information and radiation dose using 2‐D or 3‐D imaging, the geometric accuracy of imaging systems, trade‐off between treatment accuracy vs. time required to carry on imaging and manipulation process, early image‐guided protocols for basic clinical applications, etc. will be discussed. Some new developments related to effective and efficient use of in‐room kV and MV imaging, tomographic image reconstruction using limited number and angle projections have shown very promising for future clinical applications.This lecture will describe the commercially avaialbe kV imaging systems, as we as their applicable clinical protocols, acceptance testing and commissioning processes, basic QA requirements, system limitations, and potential future developments.Educational Objectives:1. Understand the latest commercial available technologies for in‐room kV and CBCT systems.2. Understand the basic functionalities of in‐room kV imaging system.3. Understand the basic imaging applications.4. Understand the basic system limitations and QA.

SU‐FF‐J‐115: Registering the Planning CT to the Treatment Geometry in IGRT, Using a Limited Reconstructed Volume Derived From Planar Images

A set of megavoltage (MV) and kilovoltage (kV) images are obtained during a patient's image‐guided radiotherapy treatment (IGRT). The kV images are acquired prior to treating the fields using an on‐board x‐ray imager. However, The MV images can be acquired both prior and during the actual treatments. Fan‐beam reconstruction of back‐projected kV images obtained at four or more gantry angles can be used to orient the planning CT to match the treatment geometry. Registering the planning CT to the treatment geometry is necessary if one is to perform dose reconstruction using the multi‐planar images and the planning CT, in the absence of a cone beam CT. The MV and kV images provide the exit fluence information for 3D dose reconstruction. Using these 2D treatment images along with the 3D planning CT volume provides adequate information about the treatment anatomy that is needed for the dose reconstruction process. This system might alleviate the need for cone beam CT for patient positioning and/or dose reconstruction. The planar treatment images provide adequately sampled sinogram planes in the spatial dimension, but not in the radial direction. Nonetheless, there is enough data in the sinogram space for registering the treatment geometry to the planning CT. Canny edge finding method is used to decipher anatomical structure in the limited reconstructed volume as well as in the CT volume. This technique can detect both strong and weak edges; and it is less likely than the other edge finding techniques to be fooled by noise. The resulting edge maps are registered to each other by affine transformations. The transformation matrices are then applied to the CT volume in order to register it to the treatment geometry.

SU‐EE‐A4‐03: Spatial Resolution‐Matched Comparison Between Fan‐Beam and Cone‐Beam X‐Ray CT Images

Purpose: Cone Beam CT (CBCT) kilovoltage imaging devices are increasingly available for daily imaging in radiotherapy departments. Flat‐panel based CBCT scanners present a distinctive set of artifacts due mostly to increased scatter, longer data acquisition time and reduced detector quantum efficiency as compared to helical Fan Beam CT (FBCT) systems. Our purpose is to characterize image quality from FBCT and CBCT scanners based on noise, contrast and dose, using FBCT as a benchmark. Method and Materials: we acquired phantom and clinical patient images with a CBCT Varian On‐Board Imager as well as with a FBCT Picker PQ5000 single‐row helical scanner. The CBCT scanner was equipped with antiscatter grid and bowtie filter. By comparing CBCT and FBCT images of a high contrast resolution insert, the CBCT reconstruction voxel size and filter were adjusted until the spatial resolution of the FBCT and CBCT images was approximately matched. Dose was measured with standard CTDI and Farmer chambers. Noise, contrast and SNR were evaluated and compared. Results: CBCT images of both phantom and patient were relatively free of streaking and cupping artifacts, indicating that the grid had successfully attenuated most of the scatter. Low contrast detectability threshold is similar for the two modalities, when CBCT dose is about twice as large as FBCT. Noise and non‐uniformities are more prevalent in patient CBCT images, but pelvic soft tissue structures are well discernible. For patient and phantom images Dose×SNR2 is about 4 times lower for FBCT than in CBCT, which is about 1.5–2 times larger than expected, given the measured grid transmission and detector quantum efficiency. Conclusion: In this study, resolution‐matched CBCT and FBCT images could exhibit similar SNRs and contrast‐to‐noise ratios through a combination of increased imaging dose and reduced spatial resolution.

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.

Image-guided radiotherapy using a mobile kilovoltage x-ray device

A mobile isocentric C-arm kilovoltage imager has been evaluated as a potential tool for image-guided radiotherapy. The C-arm is equipped with an amorphous silicon flat panel for high-quality image acquisition. Additionally, the device is capable of cone beam computed tomography (CT) and volumetric reconstruction. This is achieved through the application of a modified Feldkamp algorithm with acquisition over a 180 degrees scan arc. The number of projections can be varied from 100 to 1000, resulting in a reconstructed volume 20 cm in diameter by 15-cm long. While acquisition time depends upon number of projections, acceptable quality images can be obtained in less than 60 seconds. Image resolution and contrast of cone-beam phantom images have been compared with images from a conventional CT scanner. The system has a spatial resolution of > or = 10 lp/cm and resolution is approximately equal in all 3 dimensions. Conversely, subject contrast is poorer than conventional CT, compromised by the increased scatter and underlying noise inherent in cone beam reconstruction, as well as the absence of filtering prior to reconstruction. The mobility of the C-arm makes it necessary to determine the C-arm position relative to the linear accelerator isocenter. Two solutions have been investigated: (1) the use of fiducial markers, embedded in the linac couch, that can subsequently be registered in the image sets; and (2), a navigation approach for infrared tracking of the C-arm relative to the linac isocenter. Observed accuracy in phantom positioning ranged from 1.0 to 1.5 mm using the navigation approach and 1.5 to 2.5 mm using the fiducial-based approach. As part of this work, the impact of respiratory motion on cone-beam image quality was evaluated, and a scheme for retrospective gating was devised. Results demonstrated that kilovoltage cone beam CT provides spatial integrity and resolution comparable to conventional CT. Cone-beam CT studies of patients undergoing radiotherapy have demonstrated acceptable soft tissue contrast, allowing assessment of daily changes in target anatomy. Of the 2 approaches developed to register images to the linac isocenter, the navigation method demonstrated superior accuracy for daily patient positioning relative to the fiducial-based method. Finally, significant image degradation due to respiratory motion was observed. It was demonstrated that this could be improved by correlating the acquisition of individual 2D projections with respiration for retrospective reconstruction of phase-based volumetric datasets.

Changes in the respiratory pattern during radiotherapy for cancer in the lung

Background and purpose To quantify changes in patients' diaphragm motion pattern over the course of radiotherapy and to evaluate the implications of these changes for 4D radiotherapy. Patients and methods From January 2004 to October 2004, 10 patients with lung malignancies treated at our department underwent weekly respiratory motion verification during the course of external beam radiation. An onboard kilovoltage imaging system was used to acquire fluoroscopy weekly for patients with lung neoplasms. The diaphragm position as a function of time was extracted automatically from the fluoroscopy and used to calculate the daily mean and daily SD of motion. The diaphragm position was related to both a bony reference point and machine isocenter. Changes in the daily mean and daily SD in relation to the reference (first day) daily mean and reference daily SD were measured. Results The mean change in the daily mean was 0.32 mm±6.11 mm in relation to the bony reference point and 0.38 mm±6.28 mm in relation to isocenter. The mean change in the daily SD was 0.91 mm±1.81 mm. The mean systematic change in the daily mean was 4.97 mm, and the mean random change in the daily mean was 3.61 mm. Conclusions Daily verification of 4D radiotherapy techniques to assess the necessity of online set-up correction may be required due to the large change in the mean diaphragm position observed for these patients. However, the variation of the daily SD was small for most patients. Adaptive adjustment of the margin may be necessary for those patients with larger variation of the daily SD.

Performance evaluation of an automated image registration algorithm using an integrated kilovoltage imaging and guidance system

Image-guided radiation therapy delivery may be used to assess the position of the tumor and anatomical structures within the body as opposed to relying on external marks. The purpose of this manuscript is to evaluate the performance of the image registration software for automatically detecting and repositioning a 3D offset of a phantom using a kilovoltage onboard imaging system. Verification tests were performed on both a geometric rigid phantom and an anthropomorphic head phantom containing a humanoid skeleton to assess the precision and accuracy of the automated positioning system. From the translation only studies, the average deviation between the detected and known offset was less than 0.75 mm for each of the three principal directions, and the shifts did not show any directional sensitivity. The results are given as the measurement with standard deviation in parentheses. The combined translations and rotations had the greatest average deviation in the lateral, longitudinal, and vertical directions. For all dimensions, the magnitude of the deviation does not appear to be correlated with the magnitude of the actual translation introduced. The On-Board Imager™ (OBI) system has been successfully integrated into a feasible online radiotherapy treatment guidance procedure. Evaluation of each patient's resulting automatch should be performed by therapists before each treatment session for adequate clinical oversight. PACS numbers: 87.53.-j, 87.53.Xd, 87.57.Gg

Clinical Implementation and Efficiency of Kilovoltage Image-Guided Radiation Therapy

This paper describes measurements of clinical efficiency and time requirements associated with image-guided radiation therapy (IGRT). In June 2004, the authors' institution installed an integrated kilovoltage (kV) imaging system attached to a medical linear accelerator for radiographic target localization. Over the past year, 242 patients have been localized with the kV radiographic imaging system for a total of 2,700 fractions. Data were analyzed by reviewing the time required for each patient's IGRT session, broken into both image acquisition and image analysis time. Average IGRT procedure time was reviewed pertaining to months, treatment sessions, disease sites, and radiation therapists. Results showed that the average IGRT procedure time was reduced from 450 to 237 seconds from June 2004 to June 2005. Further analysis revealed that each therapist showed improvement in reducing the IGRT procedure time from the first month of use to the month of June 2005. The routine use of IGRT may ultimately be performed within 3 to 4 minutes, with minimal disruption to the clinical treatment process.

350 POSTER Visualization of fiducial markers for setup verification of partial breast irradiation with kilovoltage imaging

350 POSTER Visualization of fiducial markers for setup verification of partial breast irradiation with kilovoltage imaging

Measurement of IMRT Head and Neck Setup Error Using an On-board Kilovoltage Imager

Purpose/Objective: Kilovoltage imaging of patients on the treatment couch is a promising new tool for image guided radiotherapy. One use of kV imaging is to passively monitor day to day setup, facilitating evaluations of various aspects of treatment such as setup reproducibility and intra-fractional motion. Setup data collected for individual patients can also be analyzed offline to yield setup corrections. The purpose of this study was to evaluate the performance of a commercial kV imaging system in the context of head and neck radiotherapy.

286 Registration of Megavoltage and Kilovoltage images for automated setup verification on Electronic Portal Imaging Device (EPID) Images

286 Registration of Megavoltage and Kilovoltage images for automated setup verification on Electronic Portal Imaging Device (EPID) Images

SU‐FF‐J‐80: Clinical Procedures for Daily Patient Setup Verification Using On‐Board Imager (OBI)

Purpose: The newly installed OBI system consists of a kilo‐voltage (KV) X‐ray source and a flat‐ panel detector mounted orthogonally to the megavoltage portal imager (MV) in Varian 21EX machine. These devices are linked to the OBI workstation which provides a platform for acquisition/analysis of images and auto couch motion for repositioning the patient. This paper presents our clinical procedures for patient setup verification using OBI. Method and Materials: In phase I, two sets of orthogonal MV/MV and KV/KV images were acquired to confirm the accuracy of the KV system by comparing daily KV images to traditional MV images and corresponding Digital Reconstructed Radiographs (DRRs). The phase II study acquired only one set of orthogonal MV/KV portal images for daily patient setup verification. An anterior MV and lateral KV images were obtained without gantry rotation. The OBI workstation provided the tools to match the images with DRRs and to analyze the shifts to be made for patient setup correction based on either implanted and/or anatomical markers. The treatment couch was moved automatically based on this analysis. Post‐shift MV/KV images were acquired to reconfirm patient positioning. All images and positioning data were electronically saved and reviewed. Results: Four prostate IMRT patients were included in phase I study. The isocenter alignment of the two sets of images (MV/MV and KV/KV) was found within 2mm relative to DRRs during the entire treatment courses. The phase II study included 30 prostate IMRT patients. The review of post‐shift MV/KV images showed an average of 2 mm deviation relative to DRRs. All procedures could be completed within 3–5 minutes. Conclusion: Phase I study proves that KV images are as reliable as MV portal images with better image quality. Phase II study verifies that matching/analysis tools and auto‐couch motion provide fast and accurate patient positioning.

SU‐DD‐A4‐02: Fast Multimodal Image Registration Using Mutual Information and Variable Step‐Size Grid Search Algorithm

Purpose: A robust optimization algorithm, referred to as variable step‐size grid search (VSGS), is presented for 2D/2D multimodal image registration, and evaluated using phantom and clinical imaging. Method and Materials: The VSGS optimization algorithm is based on recursive iterations of a geometrically scaled step‐size, and is more likely to converge to the global optimum and be less insensitive to the local optima compared to traditional gradient techniques (i.e. Powell, quasi‐Newton). As an optimization technique, it can be used with any objective function. Here mutual information was used as the similarity measure, and a multiresolution strategy was adopted to accelerate computation without adversely affecting robustness. VSGS was applied to the problem of automatic patient positioning in radiotherapy. The multimodal imaging consists of digitally reconstructed radiographs (DRRs), portal images acquired with electronic portal imaging devices (EPID), and the Varian linac‐based on‐board kilovoltage imager. EPID images were collected using the Siemens BEAMVIEW video EPID and the Varian AS500 flat‐panel imager. Results: Validation was carried out using phantom and clinical imaging data. Measurements indicated that VSGS has an accuracy of 0.5 mm (or 1 pixel) and <0.5°, and required about 8 seconds per registration on a 2 GHz PC. VSGS has an accuracy equivalent to or better than the traditional optimization techniques depending on histogram or Parzen‐windowing based mutual information calculations, but is more robust and faster computationally. This allows VSGS to be used for online real‐time image guided patient positioning without requiring frequent user intervention to recognize shifts associated with registrations trapped in local extrema. Conclusion: VSGS can be used for online real‐time image guided patient positioning to achieve more accurate patient positioning than using visual verification, and is sufficiently robust compared to traditional optimization techniques to make algorithm‐based registrations sufficiently robust for routine clinical use.

SU‐FF‐J‐62: Image‐Guided Radiotherapy Feasibility Study Using Kilo‐Voltage Cone‐Beam CT Images for Patient Alignment Verification

Purpose: The objective of this work was to evaluate the feasibility to acquire the kilo‐voltage (KV) cone‐beam computer tomography (CBCT) images and to register the CBCT images with the conventional planning CT images for patient alignment verification. Method and Materials: A Varian high precision accelerator with on‐board KV imaging (OBI) system and CBCT was used in conjunction with a QA head phantom for patient setup position verification. In addition, the in‐house image registration and verification software was used for the analyses of patient alignment accuracy. First, we acquired the same reference image data sets of the QA head phantom from the CBCT and the LINAC/CT‐on‐rails unit. The conventional CT image set was used as the planning CT images. Applied the translation and/or the rotation to the QA head phantom to imitate the daily patient setup variation and then, acquired the daily CBCT images. Results: No differences were observed on the registration between the reference image sets obtained from the conventional planning CT and CBCT. Similar findings were also observed on the registration of the daily CBCT with the planning CT images when the translations were applied to the setup of the daily QA head phantom deviated from the planning QA head phantom setup. When the rotations were applied to the daily setup of the QA head phantom, the treatment target was registered well between the daily CBCT images and the planning CT images, but the difference of 1.3 mm was observed on the registration in the structure relatively far away from the target. Conclusion: Phantom studies indicated that the daily pretreatment CBCT images should be suitable to use for patient alignment verification with the planning CT images. Conflict of Interest: supported by Varian Medical Systems.

SU‐FF‐T‐200: Image‐Guided IMRT Dose Verification with a Plastic Photosensitive Dosimeter

Purpose: To verify an IMRT treatment, multiple point dose measurements are time‐consuming to acquire. This investigation used a kilovoltage imaging system to localize an optical dosimeter to acquire rapidly three‐dimensional IMRT quality control measurements. Method and Materials: A plastic photosensitive dosimeter was used as a dosimetric verification tool for IMRT. The plastic was formed into a right cylinder 8cm in height and 9cm in diameter that fit precisely into a water equivalent phantom. The system was used to verify an IMRT dose distribution computed by inverse treatment planning and delivered using a medical linear accelerator equipped with a multi‐leaf collimator and a kilovoltage electronic portal imaging device (KV‐EPID). The KV‐EPID was used to determine the location of the dosimeter cylinder within the accelerator coordinate system. Small wires (1mm diameter) inserted into the cylinder at three locations were used as fiducial markers. An auto‐matching algorithm integral to the accelerator control system was used to assure that the position of the dosimeter matched the position in a phantom plan. The Root‐Mean‐Square localization error was measured to be 1.1mm. The dose distribution within the dosimeter was measured by means of an optical cone‐beam imaging system. The measured dose distribution was compared with the calculated dose distribution based on the known locations of the fiducial markers. Results: The KV‐EPID was easily able to visualize the fiducials in the dosimeter and to localize the dosimeter with a RMS error of 1.1mm. Conclusion: The dose distribution delivered by IMRT to a surrogate phantom can be aligned with a mean error of 1mm using the KV‐EPID. This allows optical reconstruction of a photosensitive plastic dosimeter to be aligned with the computed three‐dimensional dose distribution for dosimetric verification of an IMRT treatment plan.

Dose and image quality. How does the choice of kilovoltage affect patient dose, scattered radiation, and image quality in CT examinations? Submitted by Glen Thomson, Middlemore Hospital, New Zealand.

Published online: 5 December 2003 Springer-Verlag 2003 How does the choice of KV affect patient dose, scattered radiation, and image quality in CT examinations? When the x-ray tube voltage (kV) in CT is reduced while keeping the mAs constant, the patient dose is decreased. Dropping the x-ray tube voltage from 120 to 80 kV at a constant current (mAs) typically reduces the patient dose by about 60% because of the large reduction in x-ray tube output. The choice of kV dose not affect the amount of ‘‘scatter’’—CT has negligible image scatter since the thin detector (up to 2–3 cm) and the data acquisition geometry ensure that most scattered photons miss the detector. The contrast of a lesion relative to the surrounding background generally increases as kV is reduced, but the noise (mottle) level also increases since there are fewer photons at the lower kV (hence the lower patient dose). The lesion contrast-tonoise ratio (CNR) generally gets worse as the kV is reduced at a constant mAs in CT—the increase in contrast is less than the corresponding increase in noise [1]. If one reduces the kV and increases the mAs, however, it may be possible to maintain image quality (CNR) while reducing patient doses. CT optimization with respect to x-ray tube voltage is being investigated by several researchers, with initial findings suggesting that the use of lower kV values is promising for CT performed using iodinated contrast material [2, 3]. To summarize, reducing the CT kV at a constant mAs will always reduce the patient dose, and would probably reduce the lesion CNR. When iodine is administered to a patient, reducing the kV is likely to reduce patient doses with no adverse effect on CNR.

The validity of clips as a radiographic surrogate for the biopsy cavity in image-guided accelerated partial breast irradiation

The validity of clips as a radiographic surrogate for the biopsy cavity in image-guided accelerated partial breast irradiation

Flat-panel cone-beam computed tomography for image-guided radiation therapy

Purpose : Geometric uncertainties in the process of radiation planning and delivery constrain dose escalation and induce normal tissue complications. An imaging system has been developed to generate high-resolution, soft-tissue images of the patient at the time of treatment for the purpose of guiding therapy and reducing such uncertainties. The performance of the imaging system is evaluated and the application to image-guided radiation therapy is discussed. Methods and Materials : A kilovoltage imaging system capable of radiography, fluoroscopy, and cone-beam computed tomography (CT) has been integrated with a medical linear accelerator. Kilovoltage X-rays are generated by a conventional X-ray tube mounted on a retractable arm at 90° to the treatment source. A 41 × 41 cm 2 flat-panel X-ray detector is mounted opposite the kV tube. The entire imaging system operates under computer control, with a single application providing calibration, image acquisition, processing, and cone-beam CT reconstruction. Cone-beam CT imaging involves acquiring multiple kV radiographs as the gantry rotates through 360° of rotation. A filtered back-projection algorithm is employed to reconstruct the volumetric images. Geometric nonidealities in the rotation of the gantry system are measured and corrected during reconstruction. Qualitative evaluation of imaging performance is performed using an anthropomorphic head phantom and a coronal contrast phantom. The influence of geometric nonidealities is examined. Results : Images of the head phantom were acquired and illustrate the submillimeter spatial resolution that is achieved with the cone-beam approach. High-resolution sagittal and coronal views demonstrate nearly isotropic spatial resolution. Flex corrections on the order of 0.2 cm were required to compensate gravity-induced flex in the support arms of the source and detector, as well as slight axial movements of the entire gantry structure. Images reconstructed without flex correction suffered from loss of detail, misregistration, and streak artifacts. Reconstructions of the contrast phantom demonstrate the soft-tissue imaging capability of the system. A contrast of 47 Hounsfield units was easily detected in a 0.1-cm-thick reconstruction for an imaging exposure of 1.2 R (in-air, in absence of phantom). The comparison with a conventional CT scan of the phantom further demonstrates the spatial resolution advantages of the cone-beam CT approach. Conclusions : A kV cone-beam CT imaging system based on a large-area, flat-panel detector has been successfully adapted to a medical linear accelerator. The system is capable of producing images of soft tissue with excellent spatial resolution at acceptable imaging doses. Integration of this technology with the medical accelerator will result in an ideal platform for high-precision, image-guided radiation therapy.

Characterization of a fluoroscopic imaging system for kV and MV radiography.

An on-line kilovoltage (kV) imaging system has been implemented on a medical linear accelerator to verify radiotherapy field placement. A kV x-ray tube is mounted on the accelerator at 90 degrees to the megavoltage (MV) source and shares the same isocenter. Nearly identical CCD-based fluoroscopic imagers are mounted opposite the two x-ray sources. These systems are being used in a clinical study of patient setup error that examines the advantage of kV imaging for on-line localization. In the investigation reported here, the imaging performance of the kV and MV systems are characterized to provide support to the conclusions of the studies of setup error. A spatial-frequency-dependent linear systems model is used to predict the detective quantum efficiencies (DQEs) of the two systems. Each is divided into a series of gain and spreading stages. The parameters of each stage are either measured or obtained from the literature. The model predicts the system gain to within 7% of the measured gain for the MV system and to within 10% for the kV system. The systems' noise power spectra (NPSs) and modulation transfer functions (MTFs) are measured to construct the measured DQEs. X-ray fluences are calculated using modeled polyenergetic spectra. Measured DQEs agree well with those predicted by the model. The model reveals that the MV system is well optimized, and is x-ray quantum noise limited at low spatial frequencies. The kV system is suboptimal, but for purposes of patient positioning yields images superior to those produced by the MV system. This is attributed to the kV system's higher DQE and to the inherently higher contrasts present at kV energies.