Cone-beam CT Guidance Research Articles

Prone accelerated partial breast irradiation using cone-beam CT (CBCT) guidance.

Abstract Abstract #5136 Introduction: Prone external beam partial breast irradiation when used for a carefully selected subset of patients has been shown to have excellent long term control and cosmesis (Formenti et al. IJROBP 60(2): 493-504 2004). We used Cone-beam CT (CBCT) to visualize the tumor bed target prior to each daily fraction of partial breast irradiation. The purpose of this study is to measure the residual error in soft tissue positioning after alignment based on skin marks and MV portal imaging for patients treated on a prospective prone APBI study (3000cGy in 5 fractions over 5 days).
 Methods: Twenty-five post-menopausal women with pT1 breast cancer have consented on our NYU partial breast protocol using CBCT. Twenty-three patients have been simulated and treated in the prone position on a specially designed mattress. After initial skin mark setup and alignment, MV portal imaging with shift using the tangent beams eye view was used to optimize setup for the first 6 patients (Group I). For all daily fractions, CBCT was performed with the Varian On-Board Imager kV imaging system with a 35cm field of view and a 2.5mm slice thickness. CBCT was compared with the planning CT to shift the patient and evaluate the residual error in setup. The residual error from CBCT setup after optimal portal imaging was recorded for each patient. For the subsequent 17 patients (Group II), portal imaging was performed without shift to calculate the residual error from CBCT setup representing skin mark setup.
 Results: At the time of this report, 23 patients have been treated with all 5 fractions. The values for the residual error detected after cone-beam CT for Group I (after skin mark set up and portal imaging) and for Group II (after skin mark setup alone) are compared and shown in Table 1.
 Conclusions: In this preliminary study with a small number of patients, the residual error was minimal, around 2 mm. Therefore, for patients immobilized on our customized mattress, a reproducible and accurate prone APBI setup can be achieved with use of CBCT. However, given the very small residual error detected clinicians should be weary of indiscriminant use of CBCT for PBI. Concerns over contralateral breast dose and carcinogenic risk should limit the routine use of CBCT imaging for breast radiotherapy unless larger studies demonstrate improved accuracy of treatment delivery with CBCT.
 
 Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 5136.

Image-guided small animal radiation research platform: calibration of treatment beam alignment

Small animal research allows detailed study of biological processes, disease progression and response to therapy with the potential to provide a natural bridge to the clinical environment. The small animal radiation research platform (SARRP) is a portable system for precision irradiation with beam sizes down to approximately 0.5 mm and optimally planned radiation with on-board cone-beam CT (CBCT) guidance. This paper focuses on the geometric calibration of the system for high-precision irradiation. A novel technique for the calibration of the treatment beam is presented, which employs an x-ray camera whose precise positioning need not be known. Using the camera system we acquired a digitally reconstructed 3D ‘star shot’ for gantry calibration and then developed a technique to align each beam to a common isocenter with the robotic animal positioning stages. The calibration incorporates localization by cone-beam CT guidance. Uncorrected offsets of the beams with respect to the calibration origin ranged from 0.4 mm to 5.2 mm. With corrections, these alignment errors can be reduced to the sub-millimeter range. The calibration technique was used to deliver a stereotactic-like arc treatment to a phantom constructed with EBT Gafchromic films. All beams were shown to intersect at a common isocenter with a measured beam (FWHM) of approximately 1.07 mm using the 0.5 mm collimated beam. The desired positioning accuracy of the SARRP is 0.25 mm and the results indicate an accuracy of 0.2 mm. To fully realize the radiation localization capabilities of the SARRP, precise geometric calibration is required, as with any such system. The x-ray camera-based technique presented here provides a straightforward and semi-automatic method for system calibration.

An empirical method for lag correction in cone‐beam CT

Image lag degrades image quality in cone-beam CT (CBCT), resulting in contrast reduction, lack of CT number accuracy and uniformity, and skin-line artifacts. The magnitude of such degradation and the extent to which imaging performance can be improved by means of a lag correction method were investigated. Measurements were performed using a radiotherapy CBCT guidance system (Elekta Synergy XVI, Elekta Oncology Systems, Atlanta, GA), for which the imaging system is based upon a RID1640-AL1 flat-panel imager (Perkin Elmer, Wiesbaden, Germany). Image lag and its relationship to various parameters including signal magnitude, photon energy, and frame number were investigated, and an empirical lag correction method was developed to manage lag artifacts. The correction method was simply the subtraction from the current frame by previous frames weighted by the temporal response function. The CatPhan 500 phantom (The Phantom Laboratory, Salem, NY) within an irregularly shaped body annulus was used to demonstrate the magnitude of artifacts with and without lag correction. CBCT images after correction demonstrated improvement in skin-line reconstruction, CT number accuracy, image uniformity, and contrast-to-noise ratio. Lag artifacts can be reduced by means of algorithmic correction of the projection images. Lag correction is most important for all shapes of objects having contrast inserts. For circular/cylindrical objects, lag correction does not improve the skin-line artifact but can improve low contrast visibility adjacent to high contrast objects.

SU‐GG‐T‐468: Image‐Guided Stereotactic Spinal Radiosurgery with a Conventional Linear‐Accelerator

Purpose: As large experiences of spinal radiosurgery were reported by using CyberKnife, this study evaluates our experiences with spinal radiosurgery given by a conventional linear‐accelerator, utilizing the features of KV‐OBI and CBCT. Method and Materials: Image‐guided stereotactic radiosurgery is used in our clinic to treat spinal metastases with 16–18 Gy in a single‐fraction, or 20–30 Gy in 5‐fractions. Treatment is delivered using nine co‐planar IMRT‐beams in 20–25 minutes. The success of spinal radiosurgery requires sparing of the proximal spinal cord and high precision of dose delivery. With proper beam arrangement and dose optimization, the maximal cord dose can be minimized to 60–65% of the prescription dose. Stereotactic localization of the metastatic vertebrae‐body is achieved by KV‐OBI and CBCT guidance before and during the treatment. The shifts based on OBI♯1 are made from the skin marks after initial setup. Next, CBCT further verifies the patient alignment. Two more OBIs before every three fields ensure patient immobility and allow iso shifts if necessary. Results: Couch shifts were recorded for a sample of 12 patients, a total of 27 fractions in this evaluation. It is found that the ranges of the shifts guided by OBI♯1 from skin marks are: 1.4 cm longitudinal, 0.9 cm vertical, 0.7 cm lateral, and 2° in‐plane couch rotation, while CBCT and consecutive OBI♯2 and ♯3 before every three fields found little patient movements. Conclusion: The conventional linear‐accelerator is able to deliver stereotactic radiosurgery treatment of spinal metastases with a high degree of accuracy, as guided by OBI and CBCT image techniques, in combination with proper patient setup, immobilization, and skin marks. No body‐wrap or fiducials are necessary in our approach. The study also finds that OBI and CBCT are equivalent due to the proper patient setup procedures in our approach. CBCT is beneficial for localizing C7/T1 for treatment.

SU-GG-T-223: Multiple Imaging Modality Isocentricity (MIMI) Test

Purpose: Develop a simple and fast means for cross‐testing the isocentricity of four systems, cone‐beam‐CT (CBCT), on‐board imaging (OBI), optical guidance, and in‐room lasers. We have employed the Standard Imaging MIMI phantom for this test along with a Varian iX Linac with CBCT, OBI, and Varian's optical guidance system.Method and Materials: We placed a passive fiducial localizer with 4 infrared light reflecting markers on top of the MIMI phantom, which has 5 rods that run through it in various directions. A CT scan of the phantom was acquired and a simple treatment plan was created which was then sent to the optical guidance system. To enable localization of the phantom using optical guidance the phantom was registered in that system. The calibration of the optical guidance system with respect to in‐room lasers was then verified and the phantom was aligned to isocenter adjusting 3 translational and 3 rotational degrees of freedom to within 0.3 mm root mean square error and less than 0.3 degrees rotational offset in each rotational degree of freedom. After aligning the phantom to the isocenter localization predicted by the optical guidance system orthogonal MV OBI images were acquired and a 2D/2D match was performed. Lastly, a CBCT of the phantom was acquired and a manual match was performed. Results: The 4 methods agreed with each other to within 1 mm or less in any one direction. Conclusion: This is a simple and effective means for QA of optical guidance, CBCT, and OBI which can be performed either weekly or monthly. This is not a complete test of the localization accuracy of any of these systems but could be part of the arsenal of quick and routine QA checks to ensure coincidence between the different isocenter locations predicted by each of these localization systems.

WE-D-351-09: Target Delineation, Reposition, and Dose Delivering Accuracies in CBCT-Guided Stereotactic Radiotherapy of Small Lung Tumors

Purpose: To use daily cone‐beam computed tomography(CBCT) for assessment of image‐guidedstereotactic radiotherapy (IGSR) of early‐stage non‐small cell lungcancer(NSCLC).Method and Materials: Patients were CT simulated in a body‐frame with or without an abdominal compression device. PET‐CT scans, if available, were fused with the simulation CT for target delineation. Daily pre‐treatment CBCTs were acquired with the Elekta XVI system, and the lungtumors were manually aligned with the simulation CTimages at the treatment machine. The shifts required to align lungtumors were recorded. Later, the CBCTs were imported into our treatment planning system and the treatment isocenter was registered with the planning CT isocenter, accounting for the daily pretreatment table shifts. GTVs and PTVs for every treatment session were redefined on the daily CBCTimage sets. Individual and cumulative DVHs for the daily treatments were calculated and compared to the original planning DVHs. Results: Using CBCT‐guidance, daily setup errors ranging from 1 to 3 cm had been corrected, that would have altered the mean dose to GTV by >30%. We found that even with daily CBCT‐guided table shifts to correct setup error, the dose to GTV could vary by 20% due to daily changes in shape of the lungtumor, when no margin was added from GTV to PTV. This daily dose variability was reduced to < 3% if PTV included a 5 mm margin around GTV. Conclusion: The interfractional setup errors caused the greatest amount of potential geographic miss of GTV, which was almost entirely eliminated by daily CBCT guidance. The underdosing of GTV due to deformation of the lungtumor could be overcome by expanding GTV by 5mm in all directions to define the PTV.

Quality Assurance for the Geometric Accuracy of Cone-Beam CT Guidance in Radiation Therapy

The introduction of volumetric X-ray image-guided radiotherapy systems allows improved management of geometric variations in patient setup and internal organ motion. As these systems become a routine clinical modality, we propose a daily quality assurance (QA) program for cone-beam computed tomography (CBCT) integrated with a linear accelerator. The image-guided system used in this work combines a linear accelerator with conventional X-ray tube and an amorphous silicon flat-panel detector mounted orthogonally from the accelerator central beam axis. This article focuses on daily QA protocols germane to geometric accuracy of the CBCT systems and proposes tolerance levels on the basis of more than 3 years of experience with seven CBCT systems used in our clinic. Monthly geometric calibration tests demonstrate the long-term stability of the flex movements, which are reproducible within +/-0.5 mm (95% confidence interval). The daily QA procedure demonstrates that, for rigid phantoms, the accuracy of the image-guided process can be within 1 mm on average, with a 99% confidence interval of +/-2 mm.

Different Styles of Image-Guided Radiotherapy

To account for geometric uncertainties during radiotherapy, safety margins are applied. In many cases, these margins overlap organs at risk, thereby limiting dose escalation. The aim of image-guided radiotherapy is to improve the accuracy by imaging tumors and critical structures on the machine just before irradiation. The availability of high-quality imaging systems and automatic image registration on the machine leads to many new clinical applications, such as high-precision hypofractionated treatments of brain metastases and solitary long tumors with online tumor position corrections. In this review, the prerequisites for image guidance in terms of planning, image acquisition, and processing are first described. Then, the various methods of correction are discussed such as table shifts and rotation and direct adaptation of machine parameters. Then, online, offline, and intrafraction correction strategies are discussed. Finally, some imaging dose issues are discussed showing that kilovoltage cone-beam computed tomography guidance has a net positive impact on the integral dose; the gain caused by margin reduction is larger than the image dose. We can conclude that image-guided radiotherapy is very much a clinical reality and that the development of optimal clinical protocols should currently be the focus of research.

The Effect of Registration Volumes on Positional Residual Errors Assessed Using Cone-Beam Computed Tomography in Radiation Treatment of Head and Neck Cancer

The use of cone-beam computed tomography (CBCT) for the purpose of patient position assessment in head and neck cancer is complicated by patient setup errors that are often not simple translations. We have retrospectively evaluated the effect of varying the inferior extent of the registration volume in patients undergoing daily online CBCT guidance.

MO‐D‐M100F‐05: Latest Clinical Development for H&amp;N and CNS

Daily target localization is a critical step to ensure accurate delivery of 3‐D conformal radiation therapy and intensity‐modulated radiation therapy. Current in‐room localization technologies for HN and CNS treatment include 2D kilovoltage (kV) or megavoltage (MV) radiography, CT on‐the‐rail, cone beam CT (CBCT), megavoltage CT (MVCT), and optical guidance systems.This lecture will provide an overview of the current in‐room target localization technologies for HN/CNS treatment, including the system design and characteristics, patient alignment process, and their integrations in clinical workflow. The accuracies of these technique and consequently the margin considerations will be discussed.Educational Objectives:1. Understand the latest commercial available in‐room localization techniques for HN treatment.2. Understand the accuracies of these localization systems.3. Understand the margin considerations associated with using these systems.

SU‐FF‐I‐21: An Empirical Method for Lag Correction in Cone‐Beam CT

Purpose: Image lag degrades image quality in cone‐beam CT (CBCT), resulting in contrast reduction, lack of CT number accuracy and uniformity, and skin‐line artifacts. This work investigates the magnitude of such degradation and demonstrates a lag correction method based on measured data from a system for CBCT guidance of radiotherapy (Elekta Synergy XVI). Method and Materials: Image lag and its relationship with various parameters including signal strength, photon energy and frame number was investigated for a PerkinElmer (RID1640) flat‐panel imager. An empirical lag correction based on measurements of image lag was developed to manage lag artifacts. The CatPhan phantom within an irregular body annulus was used to demonstrate the magnitude of artifacts before and after correction. Each projection was corrected for lag effects by subtracting previous projections weighted by the magnitude of image lag. The subtraction of previous projections was limited up to 51 frames due to uncorrelated noise. The reconstructed images before and after correction were analyzed for skin‐line and radar type artifacts, CT♯ accuracy, uniformity and contrast of several inserts. Reconstructed images were transformed into polar coordinates for analysis. Results: Experimental results illustrate that the first frame lag of the imager is less than 2.3%, and is independent of the kVp value (70–120 kV) and pixel saturation (20–80%). Lag correction improves the skin‐line artifact and reduces radar artifact by 40%. Contrast enhances by 20% and improves the CT♯ accuracy for water by 2.5%. It improves water uniformity by 5% compared to the nominal images. Conclusion: Lag artifacts can be reduced by correction of the projection images. Lag correction is most important for high contrast and irregularly shaped objects. For a circularly shaped object, lag correction does not improve the skin‐line artifact but can be useful for soft‐tissue visibility adjacent to bony anatomy.This research is sponsored by Elekta.

SU‐FF‐T‐252: Improvement of Localization Accuracy by Using 3D Cone Beam CT for Stereotactic Body Radiation Therapy of Liver, Lung and Spine Lesions

Purpose: To compare treatment isocenter placement based on OBI 2D kV imaging and OBI 3D‐CBCT imaging for patients undergoing SBRT, and compare the CBCT based isocenter shifts among liver, lung and spine lesions. Material and Methods: 119 SBRT fractions were delivered to 40 lesions of 32 patients. The patients were initially localized with 2D orthogonal on‐board kV images. 3D on‐board CBCT images were then acquired to localize the isocenter for treatment. The shifts from 2D to 3D localization were recorded. Histograms, mean values and standard deviation of the isocenter shifts were calculated. Results: The isocenter shifts based on the 3D CBCT for all 119 SBRT fractions were 0.27 ± 0.25 cm, 0.19 ± 0.23 cm, and 0.23 ± 0.27 cm in the A‐P, C‐C, and M‐L direction, respectively. The mean values and standard deviations of the magnitudes of the shifts along AP, CC and ML directions were 0.30 ± 0.26 cm, 0.21 ± 0.20 cm, and 0.31 ± 0.35 cm for liver patients, 0.31 ± 0.29 cm, 0.30 ± 0.31 cm, and 0.28 ± 0.33 cm for lung patients, and 0.22 ± 0.21 cm, 0.13 ± 0.16 cm, and 0.18 ± 0.18 cm for spine patients. Conclusion: CBCT guidance enhances the setup accuracy of SBRT treatment by using 3D anatomical information.Partly supported by Varian Research Grant.

Cone Beam Computed Tomography Guidance for Setup of Patients Receiving Accelerated Partial Breast Irradiation

To evaluate the role of cone-beam CT (CBCT) guidance for setup error reduction and soft tissue visualization in accelerated partial breast irradiation (APBI). Twenty patients were recruited for the delivery of radiotherapy to the postoperative cavity (3850 cGy in 10 fractions over 5 days) using an APBI technique. Cone-beam CT data sets were acquired after an initial skin-mark setup and before treatment delivery. These were registered online using the ipsilateral lung and external contours. Corrections were executed for translations exceeding 3 mm. The random and systematic errors associated with setup using skin-marks and setup using CBCT guidance were calculated and compared. A total of 315 CBCT data sets were analyzed. The systematic errors for the skin-mark setup were 2.7, 1.7, and 2.4 mm in the right-left, anterior-posterior, and superior-inferior directions, respectively. These were reduced to 0.8, 0.7, and 0.8 mm when CBCT guidance was used. The random errors were reduced from 2.4, 2.2, and 2.9 mm for skin-marks to 1.5, 1.5, and 1.6 mm for CBCT guidance in the right-left, anterior-posterior, and superior-inferior directions, respectively. A skin-mark setup for APBI patients is sufficient for current planning target volume margins for the population of patients studied here. Online CBCT guidance minimizes the occurrence of large random deviations, which may have a greater impact for the accelerated fractionation schedule used in APBI. It is also likely to permit a reduction in planning target volume margins and provide skin-line visualization and dosimetric evaluation of cardiac and lung volumes.

187: Cone-Beam CT Guidance for Daily Setup of Patients Receiving Accelerated Partial Breast Irradiation

To evaluate the role of cone-beam CT (CBCT) guidance for set-up error reduction and normal tissue visualization in accelerated partial breast irradiation (APBI). Ethics approval for the accrual of 20 patients for the delivery of radiation therapy using an APBI technique was obtained. Ten patients have completed treatment to date. The clinical target volume (CTV) is defined as the post-surgical seroma plus 1-2cm, with a further 1cm expansion for the planning target volume (PTV). A dose of 3850cGy was delivered in 10 fractions over 5 days (2 fractions per day). Initial set-up was performed using skin marks for alignment. CBCT datasets were acquired prior to delivery of each fraction and were assessed online. Manual registrations using the ipsilateral lung and external contours were performed. Table corrections were executed for translations exceeding 3mm in any cardinal direction. If a shift from the initial CBCT was implemented a second CBCT dataset was acquired and analyzed. The random and systematic errors associated with setup using skin marks and setup using CBCT guidance were calculated and compared. A margin formula was used to estimate the required PTV margins for each setup method. CBCT datasets were successfully acquired for all fractions for the ten patients treated to date. Retrospective analysis showed that heart and lung could be visualized on all datasets. The CTV was visible on the CBCT images in 9 of the 10 patients. An adjustment to the isocentre was made for 52% of the fractions across all 10 patients. The largest shift applied was 15mm in the AP direction. The random errors for set-up using skin marks were 2.4, 2.3, and 3.3mm in the right-left (RL), anterior-posterior (AP), and superior-inferior (SI) directions respectively. These were reduced to 1.8, 1.8, and 1.7mm in the RL, AP, and SI directions when guidance using CBCT was performed. The systematic errors for set-up using skin marks were 2.4, 1.3, and 1.9mm in the RL, AP, and SI directions respectively. These were subsequently reduced to 0.7, 1.3, and 1.0mm when CBCT guidance was employed. Isotropic PTV margins were calculated to be 8.3mm for skin marks alone and 4.5mm for CBCT guidance. The use of skin marks for the set-up of APBI patients is sufficient for current PTV margins. Online CBCT guidance minimizes the occurrence of large random deviations, which may have a greater impact for accelerated fractionation schedules. CBCT guidance may also permit a reduction in PTV margins and provides additional benefits that include skin-line visualization and dosimetric evaluation of cardiac and lung volumes.

Investigation of C‐Arm Cone‐Beam CT‐Guided Surgery of the Frontal Recess

A cone-beam CT (CBCT) imaging system based on a mobile C-arm (Siemens PowerMobil) incorporating a high-performance flat-panel detector (Varian PaxScan) has been developed in our laboratory. We hypothesize that intraoperative C-arm CBCT provides image quality and guidance performance sufficient to assist surgical approach to the frontal recess. A preclinical prospective study was conducted using six cadaver heads to assess the performance characteristics and the potential clinical utility of this imaging system. The mobile C-arm was employed for intraoperative CBCT guidance of the endoscopic approach to twelve frontal recesses. The imaging system is capable of sub-mm 3D spatial resolution with bone and soft-tissue visibility and a field of view sufficient for guidance of head and neck surgery. The system can generate intraoperative, volumetric CT images rapidly with an acceptably low radiation exposure to the patient and with image quality sufficient for most surgical tasks. Moreover, the system is portable and compatible with the surgical setup, providing excellent access to the patient. Finally, the accuracy of the system is not bound to a registration process. The ability to create updated images as surgery progresses introduces the concept of 'near-real-time' CT guidance for head and neck surgery. We found that the use of CBCT increased surgical confidence in accessing the frontal recess, resolved ambiguities with anatomical variations, and provided valuable teaching information to surgeons in training in both preoperative planning and correlation between tri-planar CT scans and intraoperative endoscopic findings.

MO-D-I-6B-03: Cone-Beam CT with Flat-Panel Detectors: Applications in Diagnostic and Image-Guided Procedures

Flat-panel detectors featuring real-time readout, distortionless image capture, and high imaging performance at low dose offer a breakthrough technology for fully 3D x-ray imaging. Used in conjunction with techniques for 3D reconstruction, flat-panel cone-beam computed tomography provides soft-tissue imaging capability in combination with nearly isotropic, sub-millimeter 3D spatial resolution. Depending on the application and implementation, the volumetric field of view achieved in a single rotation about the patient is large (e.g., 40 × 40 × 25 cm3), and the imaging dose is low (e.g., lower than that of a diagnostic CT scan). Applications of flat-panel cone-beam CT represent a broad spectrum of medical imaging research. In breast imaging, dedicated cone-beam CT scanners could dramatically increase the sensitivity and specificity of subtle lesion detection. In small animal imaging, systems offering high-resolution, high-speed 3D imaging will likely form an important technology for imaging therapeutic response. In image-guided radiation therapy, flat-panel cone-beam CT is being used to guide precise radiation delivery based on the position of soft-tissue targets at the time of therapy. And in image-guided surgery, systems are under development for multi-mode fluoroscopic / cone-beam CT guidance in a broad range of procedures, including head-and-neck surgery, orthopedic and spinal surgery, and interventional radiology. The basic process of 3D image formation, the physical factors that challenge imaging performance, the scope of applications in diagnostic and image-guided procedures, and important areas for future research are discussed. Educational Objectives: 1. Understand the means by which volumetric cone-beam CT images are reconstructed from multiple projections acquired using a flat-panel detector. 2. Understand the physical factors (e.g., x-ray scatter) that limit image quality in cone-beam CT. 3. Discuss the latest applications of flat-panel cone-beam CT in medical imaging, including: a. Diagnostic imaging b. Small animal imaging c. Image-guided radiation therapy d. Image-guided surgery 4. Discuss the near and long-term areas of research in fully 3D digital x-ray imaging.

The influence of antiscatter grids on soft‐tissue detectability in cone‐beam computed tomography with flat‐panel detectors

The influence of antiscatter x-ray grids on image quality in cone-beam computed tomography (CT) is evaluated through broad experimental investigation for various anatomical sites (head and body), scatter conditions (scatter-to-primary ratio (SPR) ranging from approximately 10% to 150%), patient dose, and spatial resolution in three-dimensional reconstructions. Studies involved linear grids in combination with a flat-panel imager on a system for kilovoltage cone-beam CT imaging and guidance of radiation therapy. Grids were found to be effective in reducing x-ray scatter "cupping" artifacts, with heavier grids providing increased image uniformity. The system was highly robust against ring artifacts that might arise in CT reconstructions as a result of gridline shadows in the projection data. The influence of grids on soft-tissue detectability was evaluated quantitatively in terms of absolute contrast, voxel noise, and contrast-to-noise ratio (CNR) in cone-beam CT reconstructions of 16 cm "head" and 32 cm "body" cylindrical phantoms. Imaging performance was investigated qualitatively in observer preference tests based on patient images (pelvis, abdomen, and head-and-neck sites) acquired with and without antiscatter grids. The results suggest that although grids reduce scatter artifacts and improve subject contrast, there is little strong motivation for the use of grids in cone-beam CT in terms of CNR and overall image quality under most circumstances. The results highlight the tradeoffs in contrast and noise imparted by grids, showing improved image quality with grids only under specific conditions of high x-ray scatter (SPR> 100%), high imaging dose (Dcenter> 2 cGy), and low spatial resolution (voxel size > or = 1 mm).

Calibration and targeting performance of a cone-beam computed tomography guidance system for radiation therapy

Calibration and targeting performance of a cone-beam computed tomography guidance system for radiation therapy