CBCT Image Quality Research Articles

Megavoltage planar and cone‐beam imaging with low‐ targets: Dependence of image quality improvement on beam energy and patient separation

The purpose of this study is to investigate the improvement of megavoltage planar and cone-beam CT (CBCT) image quality with the use of low atomic number (Z) external targets in the linear accelerator. In this investigation, two experimental megavoltage imaging beams were generated by using either 3.5 or 7.0 MeV electrons incident on aluminum targets installed above the level of the carousel in a linear accelerator (2100EX, Varian Medical, Inc., Palo Alto, CA). Images were acquired using an amorphous silicon detector panel. Contrast-to-noise ratio (CNR) in planar and CBCT images was measured as a function of dose and a comparison was made between the imaging beams and the standard 6 MV therapy beam. Phantoms of variable diameter were used to examine the loss of contrast due to beam hardening. Porcine imaging was conducted to examine qualitatively the advantages of the low-Z target approach in CBCT. In CBCT imaging CNR increases by factors as high as 2.4 and 4.3 for the 7.0 and 3.5 MeV/Al beams, respectively, compared to images acquired with 6 MV. Similar factors of improvement are observed in planar imaging. For the imaging beams, beam hardening causes a significant loss of the contrast advantage with increasing phantom diameter; however, for the 3.5 MeV/Al beam and a phantom diameter of 25 cm, a contrast advantage remains, with increases of contrast by factors of 1.5 and 3.4 over 6 MV for bone and lung inhale regions, respectively. The spatial resolution is improved slightly in CBCT images for the imaging beams. CBCT images of a porcine cranium demonstrate qualitatively the advantages of the low-Z target approach, showing greater contrast between tissues and improved visibility of fine detail. The use of low-Z external targets in the linear accelerator improves megavoltage planar and CBCT image quality significantly. CNR may be increased by a factor of 4 or greater. Improvement of the spatial resolution is also apparent.

SU‐FF‐J‐01: Feasibility of a Pre‐Object‐Grid to Reduce Scatter and Improve Image Quality in Cone‐Beam Computed Tomography (CBCT)

Introduction: Scattered radiation is a major contributor to the degradation of image quality in Cone‐beam Computed Tomography (CBCT). The purpose of this study is to demonstrate a novel technique that samples scattered radiation during imaging acquisition, and subsequently account for it during image processing, thereby significantly improving CBCT image quality. Methods and material: A grid composed of multiple, interspaced, lead strips is placed between the x‐ray source and the imaging object. The grid alternately blocks half of the entrance radiation and divides the imaging field into multiple strips of fan‐beam fields. In each projection, scatter is sampled in the shadowed strips, then interpolated and subtracted from the original imaging data in the fan‐beam fields. Half of the 3D‐CBCT images in an alternate pattern can be obtained from these projection data with one rotation. A complete set of CBCT images can be obtained by two rotations in axial mode, or one rotation in helical mode. We have tested the feasibility of the method by comparing an incomplete set of CBCT images of an enlarged Catphan phantom using the Varian Trilogy CBCT system using 3 settings: (A) without the grid; (B) with the grid, but without scatter correction; (C) with the grid including scatter correction. Results: CBCT image quality improved dramatically using setting C (grid with scatter correction). Streaking artifacts around the air and high‐density inserts, apparent in A and B, were almost completely removed in C. CT number linearity (R2) was improved from 0.880 (A) and 0.967 (B) to 0.998 (C). Contrast scales were 0.42, 0.71 and 1.47 for A, B and C respectively. The corresponding contrast‐to‐noise ratios were 4.29, 6.60 and 6.42, respectively. Conclusion: Preliminary results suggest that the proposed pre‐object‐grid method can be used to substantially reduce scatter in projection data and therefore improve image quality in CBCT.

SU‐FF‐I‐15: An Algorithm for Metal Streaking Artifact Reduction in Cone Beam CT

Purpose: To develop a metal streaking artifact correction for cone beam CT (CBCT) and to evaluate its effect on image quality. Method and Materials: A small water phantom with steel cylinders, a large solid water phantom with contrast inserts and steel cylinders and a GYN patient with a metallic vaginal cylinder were scanned on a CBCT scanner with an amorphous silicon flat panel detector. CBCT images were reconstructed in three different ways: using the CBCT scanner reconstruction algorithm, using an in‐house reconstruction algorithm, both on the basis of the original projections and using our reconstruction algorithm with based on modification of projection data. First, metal traces were identified in either the projection space or the image space. The projection areas corresponding to the metals were deleted and replaced by interpolated data from the neighboring projections. Results: Identification of metal traces worked very well for the phantoms and the patient. In the case of the small phantom, the scanner reconstructed images suffered from severe artifacts that were reduced when the CBCT images were reconstructed by the in‐house reconstruction algorithm. CBCT images using the interpolated projections were nearly artifact free in both phantoms. The corrected image of the large phantom closely resembled the CBCT image without metals. The original patient CBCT images showed streaking artifacts in the proximity of the vaginal cylinder that were significantly reduced after the correction algorithm was applied. A coronal slice at the position of the cylinder clearly demonstrated the improvement in image quality. Conclusions: The quality of CBCT images was greatly improved using the projection interpolation algorithm in both the phantoms and the patient. The successful test of the correction algorithm applied on CBCT images of the large phantom and in the complex patient anatomy offers the possibility to use the correction for patients with hip prostheses.

TH-D-BRC-02: Dosimetric Characterization of An Imaging Beam Line with a Carbon Electron Target for Megavoltage Cone Beam Computed Tomography

Purpose: Megavoltage cone beam CT image quality can be significantly improved with an imaging beam line (IBL) with no flattener and a carbon electron target. The IBL imaging dose is non-negligible, however, and the high keV-range x-ray fluence makes beam modeling nontrivial. An IBL was modeled with the Philips Pinnacle3 treatment planning system, verified experimentally, and applied to clinical cases. Method and Materials: The IBL was modeled and the image acquisition dose was verified in a customized acrylic cylindrical phantom with 196 ion chamber measurements. Agreement between the measured and modeled IBL dose was quantified with the 3D γ-index. Representative IBL and TBL imaging dose distributions were calculated for head and neck and prostate patients and included in treatment plans using the imaging dose incorporation (IDI) method. Surface dose was measured for the TBL and IBL for four head and neck cancer patients with metal oxide field effect transistors. Results: The depth dose and profile curves had 97% passing γ-indices for dosimetric and distance acceptance criteria of 3%, 3 mm, and 100% passed for 5.2%, 5.2 mm. For the ion chamber measurements of phantom image acquisition dose, the IBL model had 93% passing γ- indices for acceptance criteria of 3%, 3 mm, and 100% passed for 4%, 4 mm. Differences between the IBL- and TBL-based IMRT treatment plans created with the IDI method were dosimetrically insignificant. IBL surface dose was greater than TBL by 18% (p = 2×10minus;6). Conclusion: The IBL can be modeled with acceptable accuracy using a standard TPS, and accounting for IBL dose in treatment plans with the IDI-method is straightforward. The resulting composite dose distributions, assuming similar imaging doses, are negligibly different from those of the TBL. The increased IBL surface dose relative to the TBL is likely clinically insignificant. Conflict of Interest: Partially funded by Siemens.

SU‐FF‐I‐25: Quantitative Analysis of the Influence of KV Beam Characteristics and Image Acquisition Parameters On Image Quality in Cone‐Beam CT

Purpose: Image quality in cone‐beam CT (CBCT) imaging is influenced by a variety of factors associated with the selection of image acquisition parameters like kVp, tube current, field size, beam filter, number of projections, and reconstruction filter. An improper selection of scan protocols may lead to unoptimized image quality and unnecessary dose to the patient. Therefore, a rigorous study of CBCT image quality and its dependence on beam characteristics is vital. In this work, the relative influences of pertinent image acquisition parameters are evaluated for a clinical Elekta XVI CBCT system. Method and Materials: Image quality measurements were conducted using a conventional CatPhan phantom, wherein the line‐pair bar inserts and uniform slices were used to characterize spatial resolution and noise via modulation transfer function (MTF) and noise power spectrum (NPS) respectively for several combinations of kVp/current, field size, beam filters, and gantry rotation arcs. CTDI dose estimates were also acquired for each scan protocol. Overall image quality was evaluated in terms of the detective quantum efficiency (DQE), obtained from MTF and NPS spectra and normalized using the CTDI doses and a fluence — CTDI dose conversion factor obtained from Monte Carlo simulations. Results: The MTF, NPS and DQE measurements indicated that overall image quality was severely affected by the increased scatter from larger field sizes, and from the use of incomplete projection arcs. The use of beam filters led to some reduction in scatter and improved image quality, while also reducing patient dose. Conclusions: The image quality measurements indicated that the appropriate choice of scan parameters can lead to improved image quality and sparing of patient dose. However, more detailed investigations may be necessary to optimize CBCT image quality, including a better choice of beam compensators and possible use of anti‐scatter grids.

SU‐FF‐J‐56: Integrating Real‐Time Tracking Into Image Guided Radiation Therapy for Prostate Cancer Treatment

Purpose: To investigate how a real‐time tracking technique, based on implanted transponders, could be effectively integrated into the current process for image‐guided radiation treatment of prostate cancer. Method and Materials: Current prostate IGRT process includes MR‐imaging for prostate delineation, CT simulation for treatment planning, and daily on‐board kV‐ and CBCT‐imaging for target alignment. The real‐time tracking technique uses implanted radiofrequency transponders and an external detector array (Calypso4D) to monitor prostate motion. This study assess (1) magnetic field induced displacement of transponders during MR imaging, (2) image artifacts introduced by transponders in CT and MR images, (3) image artifacts caused by the transponder detector antenna array in in‐room kV and CBCT images. A tissue‐equivalent phantom mimicking prostate tissue stiffness was constructed and implanted with 3 operational transponders prior to phantom solidification. CT scans were performed before and immediately after 1.5T and 3.0T MR imaging of the phantom, which used prostate‐typical pulse sequences. Transponder positions were independently localized by the Calypso system and by CT/CT image registration and evaluated for MR induced displacement. Image artifacts by modality were assessed in phantom and patient cases. Results: MR‐induced position displacements are minimal (<1.0mm) for both 1.5T and 3T scanners. The temperature variation due to MRI RF heating is <0.1°C. MR image artifacts are most significant with the gradient echo sequence, and have radii ∼1.5cm and length∼4.0cm. The detector array caused significant deterioration in the CBCT image quality. The visibility of transponders and bony anatomy was not affected on the kV images. Major changes of the IGRT protocol: (a) MR and CT imaging back‐to‐back for treatment planning is changed to MR prior‐ and CT post‐implant; (b) Daily CBCT prior‐ is changed to post‐treatment and with detector array removed. Conclusion: There are tradeoffs in the clinical IGRT flow to accommodate and minimize the substantial imaging artifacts.

SU‐FF‐J‐13: Decreasing CBCT Dose Without Compromising Its Clinical Fidelity

Purpose: The objective of this study is to evaluate the effect of KV cone‐beam CT registration accuracy and image quality with reduced number of planar projections used in 3D reconstruction. The ultimate goal is to reduce patient dose while maintaining registration accuracy for different clinical disease sites. Methods and Materials: An Elekta Synergy Linear accelerator with onboard CBCT system (XVI) is used in this study. The image quality of XVI CBCT images reconstructed with decreasing number of projections was evaluated using a CAT phantom that provides quantitative data on spatial resolution, low contrast visibility, uniformity, and geometrical accuracy. Subsequently, we acquired volumetric imaging data on an anthropomorphic phantom with different rectilinear shifts and rotations. We used standard scanning protocol for CBCT imaging. However, we reconstructed images with reduced number of projections (as low as 1/6th). We evaluated image registration accuracy between planning CT and CBCT data generated with decreased number of projections used in reconstruction. We also reconstructed imaging data for 3 real patients with limited number of projections. Results: The CBCT images of CAT phantom reconstructed with half as many projections satisfied all image quality criteria defined by Elekta in its acceptance test document. The images reconstructed with 1/6th of the original number of projections failed only in low contrast resolution while satisfying all other quantitative image quality specifications. The CBCT images get noisier with reduced number of projections but their spatial resolution is hardly affected. The maximum registration error for both the phantom and real patients was found to be within 1mm and 1° Conclusions: This study demonstrates that the image registration accuracy is not compromised even when the numbers of planar projections are reduced significantly. This means that the patient dose from CBCT study can potentially be reduced by a factor of approximately six.

SU‐FF‐I‐22: Enhancement of Image Quality From Cone Beam Computed Tomography Images Using Entropy Criteria

Purpose: To improve the image quality of small low contrast objects and overall image quality from Cone Beam Computed Tomography using a new entropy technique. Materials and Methods: Cone Beam CT (CBCT) images were acquired using the Catphan phantom in an Acuity Simulator. The images were acquired at 125 kVp, 80mA with 2.5 mm slice thickness. All the images were acquired in a full fan configuration. A new criterion for image enhancement was developed based on entropy behavior. For some of the images we have applied a noise reduction technique before applying the criteria. Results and discussion: Visualization of small low contrast objects is of significant importance for clinical treatment planning and verification. In this paper, we propose a new technique to enhance the quality of images obtained from CBCT. The technique involves measuring the entropy of the image using its histogram at different contrast levels using the definition , where Pn represents the relative frequencies of occurrence of gray levels in the image with . The behavior of the entropy has been analyzed and studied on the phantom CATPHAN 500/600 and various types of CT images for the quality of image. Significant improvement in the quality of image was observed at the point of maximum entropy. We were able to observe low contrast objects which were not observed in the original image. Quantitative analysis of detectability, signal to noise ratio, contrast to noise ratio shows significant changes for the processed image. The method operates directly on the raw data with significantly lower computational time of 5–10 seconds per slice. We are hoping to improve this computational time. Conclusion: We have demonstrated that using entropy criteria it is possible to improve the CBCT image quality.

SU‐FF‐I‐17: Methodology for Image Quality Characterization of Cone‐Beam CT Systems Using a Novel Adaptation of Fundamental Imaging Metrics

Purpose: A methodology to characterize reconstructed image quality in cone‐beam computed tomography (CBCT) is presented in which, imaging performance is characterized using fundamental imaging metrics: modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE). Traditionally, imaging MTF, NPS and DQE have only been used to describe spatial resolution, noise and contrast in 2D imaging. In a novel adaptation to 3D imaging, the MTF and NPS are measured using a conventional CatPhan phantom. An effective DQE parameter is then evaluated by normalization to incident beam flunece characteristics using a dose index. Method and Materials: The modulations from CatPhan line‐pairs are used to calculate the MTF using a technique that we recently validated for megavoltage imaging. Blank/uniform slices are used to calculate the NPS, while the DQE is obtained from MTF and NPS measurements normalized by a quantum fluence term. While the fluence normalization is relatively straightforward with unscattered beams in 2D imaging, its evaluation in CBCT imaging is complicated by the effects of collimators, compensators, and scatter. In our calculations, we obtain an effective incident fluence term for each beam combination based on its weighted CT dose index (CTDIw) and by using Monte Carlo simulations to generate a transmitted fluence to CTDI dose ratio. The methodology was implemented for clinical CBCT systems to observe the influence of image acquisition parameters. Results: CatPhan based MTF, NPS and DQE measurements of clinical CBCT systems were found to be useful in evaluating the effects of various image acquisition and reconstruction parameters. The technique was extended to near automated metric analysis with minimal hindrances from scatter and associated CBCT artifacts. Conclusions: The use of fundamental imaging metrics to characterize CBCT image quality can be effective in optimizing image acquisition parameters and ensure optimal imaging performance at the lowest possible dose.

TU‐E‐BRC‐07: A New Reconstruction Algorithm for Improved Cone‐Beam CT Image Quality

Purposes: Scatter and noise are well‐known factors that degrade the cone‐beam CT (CBCT) image quality. In addition to these factors, the fundamental reconstruction process adopted by most commercial systems is also imperfect. The reconstructions are mostly based on the Feldkamp formula. Such algorithm gives the accurate solution for the central slice but approximations for the remaining slices. The error increases when the slices are further away from the center due to “incompleteness” of the projection data. To combat this problem, we developed in this work a new reconstruction algorithm, which improved the CBCT image quality significantly. Methods and Materials: The reconstruction algorithm integrated the Feldkamp formula into an iterative optimization process. For validation, a three‐dimensional (3D) digital thoracic phantom was constructed based on a patient CT image and used to simulate CBCT projections. Noise and scatter were excluded from the simulation in order to isolate the basic theoretical issues in reconstruction. The projections were reconstructed with the Feldkamp algorithm, Simultaneous Algebraic Reconstruction Technique (SART), and our proposed “FDK‐ART” algorithm, respectively. The images were then systematically evaluated. Results: Artifacts became apparent for outer slices when Feldkamp algorithm was used. These artifacts were significantly reduced with the proposed algorithm. Although Feldkamp gives an accurate solution for the central slice, the root mean square error was shown to be reduced greatly by the proposed algorithm. By visual and profile comparison, it was found that the image resolution using our new method was clearly superior to that obtained with traditional Feldkamp algorithm or SART algorithm. The algorithm converged within 5 iterations for all sample cases tested. Conclusions: A new reconstruction algorithm has been developed for CBCT imaging. The new algorithm not only reduced the errors for outer slices, but also significantly improved the image resolution and the overall quantitative accuracy for every slice.

SU‐FF‐J‐10: Converting Cone‐Beam CT to In‐Room Conventional CT by a Deformable Image Registration Method

Purpose: Poor image quality in Cone‐beam CT (CBCT) usually poses a challenge for soft tissue based target localization or accurate dose calculation for adaptive radiotherapy. The purpose of this study is to develop an effective post‐processing method to drastically improve CBCT image quality without losing anatomical and geometric information it provides. Method and Materials: We used an image‐based deformable registration algorithm to establish dense correspondence among voxels of the daily CBCT and the planning CT images. Once the voxel‐by‐voxel correspondence is obtained, conventional CT numbers from the planning CT can be mapped to the geometrically aligned voxel locations of the daily CBCT. The converted CBCT (Mapped‐CBCT) maintains the geometrical information of the daily CBCT while retaining the more accurate electron density information from the original planning CT. A previously validated, in‐house developed “demons” algorithm was used in combination with an intensity pre‐processing method. Results: Soft tissue contrast is greatly enhanced in each of the prostate, lung, and head & neck cases we tested. In addition, various CBCT imaging artifacts, caused by beam hardening, detector response lag, scatter, and organ motion during CBCT acquisition etc., were all disappeared. Histogram distribution of voxel intensities in the soft tissue region showed a better tissue/fat separation in the mapped CBCT than the original CBCT. For a prostate case, we observed a 41% reduction in entropy, which further demonstrates improved soft‐tissue discrimination. Correspondence ambiguity in some anatomical objects is one limitation for this method. For example, the daily variation of rectal gas will not be presented if the rectum is empty in the planning CT. Conclusion: We demonstrated an effective voxel mapping procedure to match the CBCT image to the quality of a conventional CT, which should improve online image evaluation or provide a more accurate CT data set for adaptive replanning or dose accumulation.

SU‐FF‐T‐08: Morphological Seed Identification and Removal of the Post‐Implant Prostate Brachytherapy Patients in Cone‐Beam CT Sinogram Projections

Purpose: To digitally extract and remove elongated seed features from measured 2D cone‐beam CT (CBCT) sinogram projections of the post‐implant prostate brachytherapy patients. Methods and materials: The method identifies the seeds at the projections and removes them by 2D spatial‐interpolation. Theragenics Model‐200 seed images were identified and removed from 2D patient projections by (a) normalizing the image intensity by finding its maximum and minimum values in the image, (b) top‐hat‐filtering, (c) thresholding using the 3σ‐value of the pixel intensity histogram, and (d) labeling to create a binary mask of each seed in each projection. The binary masks were subtracted from the corresponding raw projection. The subtracted images were then interpolated in the seed regions using natural‐neighbor interpolation method to recover the missing soft‐tissues information's obscured by the implanted seed images. Four post‐implant patients were scanned using an Acuity‐digital‐simulator with full‐660 projections CBCT (in half/full‐fan‐mode) for post‐implant dosimetry. In‐house filtered‐back projection algorithm was used to reconstruct CBCT images before and after seed removal applications. To quantify this method, we compared the reconstructed CBCT images before and after corrections, image‐profiles and a difference image on a single central slice to demonstrate the effects of seeds removal. Results: For the example cases, the streaking artifact is reduced by a factor of 3.5 and artifact spatial extend. The corrected images exhibit significantly improved image quality in and around the prostate. Discussion: Our preliminary results indicate that this method can be used to mitigate metal‐streaking artifacts specific to the brachytherapy seed implant geometry. By reducing streak and associated noise propagation artifacts, significant clinical value can be added to brachytherapy CBCT imaging. Improving the auto‐segmentation method and applying scatter‐subtraction corrections will be further study to improve the CBCT image quality for the intra/postoperative brachytherapy patient's images.

TH-C-BRC-08: Integration of Cone Beam CT Imaging and a Small Animal Conformal RT Device Using a 6DOF Robotic Arm

Purpose: To integrate cone beam computed tomography(CBCT)imaging technology into a small animal conformal radiation therapy device (SACRTD) using a six degrees of freedom (6DOF) high accuracy/precision robotic arm. Method and Materials: A 225kV dual focus (0.4mm and 3mm) X‐ray tube (GE Seifert Isovolt Titan) is used for both radiation and imaging. An industrial robot (Adept Viper S650) is used for positioning and motion of a small animal for targeted radiation delivery and CBCTimaging with positional repeatability of ±0.020 mm in XYZ direction and angular precision of ±0.2°. A series of 2D‐radiographic projection images were recorded in cone beam mode with a flat panel amorphous silicon detector (Perking Elmer), which captures 1024 by 1024 pixels images (200μm pixel size) at a frame rate of 7.5Hz. An open source CBCTreconstruction tool (OSCAR‐2, University of Toronto) using Feldkamp‐Davis‐Kress (FDK) filtered back projection algorithm was employed for CBCTimage reconstruction. Results: Initial testing of the CBCT system using phantoms and mice was successful. Radiographs were cropped and downsampled to a spatial resolution of 0.4mm. Reconstructed matrices had dimensions of 181×267×97 voxels with 0.3mm×0.3mm×0.3mm voxel spacing. With the current computer the entire acquisition and reconstruction procedure takes > 7 minutes. The OSCAR‐2 algorithm offers various reconstruction filters, but more sophisticated tools specific for small animal imaging are needed to improve the quality of CBCTimages. Conclusion: This work demonstrated that a 6DOF robot is an excellent device for moving/rotating a small laboratory animal for acquiring high spatial resolutionradiographs with the necessary accuracy and precision for successful CBCTreconstruction. It also demonstrated that an open source CBCT software tool could be adapted to work with the SACRTD. The integration of CBCTimaging into the SACRTD opens new research paths requiring image guided radiation delivery to specific structures/volumes in small animals.

Scatter correction for cone‐beam CT in radiation therapy

Cone-beam CT (CBCT) is being increasingly used in modern radiation therapy for patient setup and adaptive replanning. However, due to the large volume of x-ray illumination, scatter becomes a rather serious problem and is considered as one of the fundamental limitations of CBCT image quality. Many scatter correction algorithms have been proposed in literature, while a standard practical solution still remains elusive. In radiation therapy, the same patient is scanned repetitively during a course of treatment, a natural question to ask is whether one can obtain the scatter distribution on the first day of treatment and then use the data for scatter correction in the subsequent scans on different days. To realize this scatter removal scheme, two technical pieces must be in place: (i) A strategy to obtain the scatter distribution in on-board CBCT imaging and (ii) a method to spatially match a prior scatter distribution with the on-treatment CBCT projection data for scatter subtraction. In this work, simple solutions to the two problems are provided. A partially blocked CBCT is used to extract the scatter distribution. The x-ray beam blocker has a strip pattern, such that partial volume can still be accurately reconstructed and the whole-field scatter distribution can be estimated from the detected signals in the shadow regions using interpolation/extrapolation. In the subsequent scans, the patient transformation is determined using a rigid registration of the conventional CBCT and the prior partial CBCT. From the derived patient transformation, the measured scatter is then modified to adapt the new on-treatment patient geometry for scatter correction. The proposed method is evaluated using physical experiments on a clinical CBCT system. On the Catphan 600 phantom, the errors in Hounsfield unit (HU) in the selected regions of interest are reduced from about 350 to below 50 HU; on an anthropomorphic phantom, the error is reduced from 15.7% to 5.4%. The proposed method is attractive in applications where a high CBCT image quality is critical, for example, dose calculation in adaptive radiation therapy.

Quantitative analysis of the image quality in megavoltage cone-beam computed tomography

Objective To quantitatively analyze the image quality of megavoltage cone-beam CT (MVCBCT) under different scanning conditions to provide reference in clinical applications. Methods Si-emens ONCOR linear accelerator with MVCBCT was used to scan the phantom under different conditions. The image quality was evaluated in terms of image noise, uniformity, spatial resolution, contrast resolution, the number of Monitor Units(MUs) used in imaging,and the size of the reconstruction matrix. The comparison of the image quality between MVCBCT and conventional simulator CT was also analyzed. Results The image noise was decreased with the increase of the number of MUs. The uniformity index showed that the system u-niformity was weakly dependent on MU numbers or the size of the reconstruction matrix. Except for the ima-ges with 5 MUs,all other images had the spatial resolution of 0.4 lp/mm with a reconstruction matrix of 256 ×256. Better low contrast resolution was achieved by using more MUs. For typical pelvis and head-and-neck patients,the imaging dose at the center was 0.8 cGy/MU and 0.7 cGy/MU, respectively,and the maxi-mum dose was about 1.2 cGy/MU. For typical abdomen patients,the image maximum dose and center dose was 1.3 cGy/MU and 0.7 cGy/MU,respectively. Conclusions The image quality of MVCBCT is inferior to the conventional kilo-voltage CT. However,with the optimization of the parameters in imaging,we can a-chieve sufficient image contrast in the bone,air and some soft-tissue structures with low imaging dose to pa-tients. Such images can be used for IGRT. Key words: Tomograph,X-ray computed; Megavoltage cone-beam; Image quality; Quantita-tive analysis

Tetrahedron beam computed tomography (TBCT): a new design of volumetric CT system

Volumetric CT imaging systems usually comprise a point x-ray source and a 2D detector. Flat panel imager (FPI)-based cone beam CT (CBCT) has become an important online imaging modality for image-guided radiotherapy and intervention. However, due to excessive scatter photons and inferior detector performance, the image quality of current CBCT is significantly inferior to diagnostic fan-beam CT. We propose a novel tetrahedron beam computed tomography (TBCT) imaging system which consists of a linear scan x-ray source and a linear x-ray detector array. The linear x-ray tube and detector array are aligned perpendicular and parallel to the rotation plane, respectively. The x-ray beams are narrowly collimated into fan beams and focused on the linear detector array. The linear detector and linear x-ray source form a ‘tetrahedron’ volume instead of a ‘cone’ volume. TBCT is similar to CBCT in image reconstruction geometry; however, its image quality will be significantly superior to that of CBCT due to its scatter rejection mechanism and the use of high-performance discrete x-ray detectors. In this paper, we describe the design of the TBCT system for image-guided radiotherapy and some results of preliminary studies.

A comparative evaluation of Cone Beam Computed Tomography (CBCT) and Multi-Slice CT (MSCT): Part I. On subjective image quality

Aims To compare image quality and visibility of anatomical structures in the mandible between five Cone Beam Computed Tomography (CBCT) scanners and one Multi-Slice CT (MSCT) system. Materials and methods One dry mandible was scanned with five CBCT scanners (Accuitomo 3D, i-CAT, NewTom 3G, Galileos, Scanora 3D) and one MSCT system (Somatom Sensation 16) using 13 different scan protocols. Visibility of 11 anatomical structures and overall image noise were compared between CBCT and MSCT. Five independent observers reviewed the CBCT and the MSCT images in the three orthographic planes (axial, sagittal and coronal) and assessed image quality on a five-point scale. Results Significant differences were found in the visibility of the different anatomical structures and image noise level between MSCT and CBCT and among the five CBCT systems ( p = 0.0001). Delicate structures such as trabecular bone and periodontal ligament were significantly less visible and more variable among the systems in comparison with other anatomical structures ( p = 0.0001). Visibility of relatively large structures such as mandibular canal and mental foramen was satisfactory for all devices. The Accuitomo system was superior to MSCT and all other CBCT systems in depicting anatomical structures while MSCT was superior to all other CBCT systems in terms of reduced image noise. Conclusions CBCT image quality is comparable or even superior to MSCT even though some variability exists among the different CBCT systems in depicting delicate structures. Considering the low radiation dose and high-resolution imaging, CBCT could be beneficial for dentomaxillofacial radiology.

Scatter kernel estimation with an edge-spread function method for cone-beam computed tomography imaging

The clinical applications of kilovoltage x-ray cone-beam computed tomography (CBCT) have been compromised by the limited quality of CBCT images, which typically is due to a substantial scatter component in the projection data. In this paper, we describe an experimental method of deriving the scatter kernel of a CBCT imaging system. The estimated scatter kernel can be used to remove the scatter component from the CBCT projection images, thus improving the quality of the reconstructed image. The scattered radiation was approximated as depth-dependent, pencil-beam kernels, which were derived using an edge-spread function (ESF) method. The ESF geometry was achieved with a half-beam block created by a 3 mm thick lead sheet placed on a stack of slab solid-water phantoms. Measurements for ten water-equivalent thicknesses (WET) ranging from 0 cm to 41 cm were taken with (half-blocked) and without (unblocked) the lead sheet, and corresponding pencil-beam scatter kernels or point-spread functions (PSFs) were then derived without assuming any empirical trial function. The derived scatter kernels were verified with phantom studies. Scatter correction was then incorporated into the reconstruction process to improve image quality. For a 32 cm diameter cylinder phantom, the flatness of the reconstructed image was improved from 22% to 5%. When the method was applied to CBCT images for patients undergoing image-guided therapy of the pelvis and lung, the variation in selected regions of interest (ROIs) was reduced from >300 HU to <100 HU. We conclude that the scatter reduction technique utilizing the scatter kernel effectively suppresses the artifact caused by scatter in CBCT.

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.

Respiratory Motional Effect on Cone-Beam CT in Lung Radiation Surgery

The cone-beam CT (CBCT), which is acquired using an on-board imager (OBI) attached to a linear accelerator, is used effectively in the verification of setup accuracy for lung radiation surgery. In this study, the respiratory organ motional effect on the CBCT was evaluated with a properly devised phantom system, and the level of possible error in conditions of a real clinical process was assessed. In a comparison study between the CBCT in static status and CBCT images acquired in 20 different motional cases, we confirmed that the image quality and information of CBCT were degraded, with an increase of motional ranges in the region of inhomogeneous structures. The 4D-CT MIP (50∼55%) for the planning of lung radiation surgery and the 4D-CT MIP (full phase) were compared with CBCT in the various motional cases for the evaluation of the influence of the motional effect on CBCT in the process of the setup error correction. The average ratio of relative difference between plan CT: 4D-CT MIP (50%∼55%) and CBCT was 5.79% and between plan CT: 4D-CT MIP (50%∼55%) and 4D-CT MIP (full phase) was 42.95% in the phantom study. In the analysis of clinical cases of lung radiation surgery, the gross tumor volumes were compared in each CT image. The average ratio of relative difference between plan CT: 4D-CT MIP (50∼55%) and CBCT was 10.72% and between plan CT: 4D-CT MIP (50∼55%) and 4D-CT MIP (full phase) was 28.19%. These results showed that, although a respiratory organ motional effect on CBCT introduced variation in image quality, the error as a result of this variation could be estimated relatively low in the setup error correction for a gated-lung radiation surgery when the planning was performed in 4D-CT MIP (50∼55%), which already included a related signal of motional effect.