Clinical Cone Beam CT Research Articles

Reconstructing cone-beam CT with spatially varying qualities for adaptive radiotherapy: a proof-of-principle study

With the aim of maximally reducing imaging dose while meeting requirements for adaptive radiation therapy (ART), we propose in this paper a new cone beam CT (CBCT) acquisition and reconstruction method that delivers images with a low noise level inside a region of interest (ROI) and a relatively high noise level outside the ROI. The acquired projection images include two groups: densely sampled projections at a low exposure with a large field of view (FOV) and sparsely sampled projections at a high exposure with a small FOV corresponding to the ROI. A new algorithm combining the conventional filtered back-projection algorithm and the tight-frame iterative reconstruction algorithm is also designed to reconstruct the CBCT based on these projection data. We have validated our method on a simulated head-and-neck (HN) patient case, a semi-real experiment conducted on a HN cancer patient under a full-fan scan mode, as well as a Catphan phantom under a half-fan scan mode. Relative root-mean-square errors (RRMSEs) of less than 3% for the entire image and ~1% within the ROI compared to the ground truth have been observed. These numbers demonstrate the ability of our proposed method to reconstruct high-quality images inside the ROI. As for the part outside ROI, although the images are relatively noisy, it can still provide sufficient information for radiation dose calculations in ART. Dose distributions calculated on our CBCT image and on a standard CBCT image are in agreement, with a mean relative difference of 0.082% inside the ROI and 0.038% outside the ROI. Compared with the standard clinical CBCT scheme, an imaging dose reduction of approximately 3–6 times inside the ROI was achieved, as well as an 8 times outside the ROI. Regarding computational efficiency, it takes 1–3 min to reconstruct a CBCT image depending on the number of projections used. These results indicate that the proposed method has the potential for application in ART.

Assessment of contrast enhanced respiration managed cone-beam CT for image guided radiotherapy of intrahepatic tumors.

Contrast enhancement and respiration management are widely used during image acquisition for radiotherapy treatment planning of liver tumors along with respiration management at the treatment unit. However, neither respiration management nor intravenous contrast is commonly used during cone-beam CT (CBCT) image acquisition for alignment prior to radiotherapy. In this study, the authors investigate the potential gains of injecting an iodinated contrast agent in combination with respiration management during CBCT acquisition for liver tumor radiotherapy. Five rabbits with implanted liver tumors were subjected to CBCT with and without motion management and contrast injection. The acquired CBCT images were registered to the planning CT to determine alignment accuracy and dosimetric impact. The authors developed a simulation tool for simulating contrast-enhanced CBCT images from dynamic contrast enhanced CT imaging (DCE-CT) to determine optimal contrast injection protocols. The tool was validated against contrast-enhanced CBCT of the rabbit subjects and was used for five human patients diagnosed with hepatocellular carcinoma. In the rabbit experiment, when neither motion management nor contrast was used, tumor centroid misalignment between planning image and CBCT was 9.2 mm. This was reduced to 2.8 mm when both techniques were employed. Tumors were not visualized in clinical CBCT images of human subjects. Simulated contrast-enhanced CBCT was found to improve tumor contrast in all subjects. Different patients were found to require different contrast injections to maximize tumor contrast. Based on the authors' animal study, respiration managed contrast enhanced CBCT improves IGRT significantly. Contrast enhanced CBCT benefits from patient specific tracer kinetics determined from DCE-CT.

A quality assurance framework for the fully automated and objective evaluation of image quality in cone‐beam computed tomography

Thousands of cone-beam computed tomography (CBCT) scanners for vascular, maxillofacial, neurological, and body imaging are in clinical use today, but there is no consensus on uniform acceptance and constancy testing for image quality (IQ) and dose yet. The authors developed a quality assurance (QA) framework for fully automated and time-efficient performance evaluation of these systems. In addition, the dependence of objective Fourier-based IQ metrics on direction and position in 3D volumes was investigated for CBCT. The authors designed a dedicated QA phantom 10 cm in length consisting of five compartments, each with a diameter of 10 cm, and an optional extension ring 16 cm in diameter. A homogeneous section of water-equivalent material allows measuring CT value accuracy, image noise and uniformity, and multidimensional global and local noise power spectra (NPS). For the quantitative determination of 3D high-contrast spatial resolution, the modulation transfer function (MTF) of centrally and peripherally positioned aluminum spheres was computed from edge profiles. Additional in-plane and axial resolution patterns were used to assess resolution qualitatively. The characterization of low-contrast detectability as well as CT value linearity and artifact behavior was tested by utilizing sections with soft-tissue-equivalent and metallic inserts. For an automated QA procedure, a phantom detection algorithm was implemented. All tests used in the dedicated QA program were initially verified in simulation studies and experimentally confirmed on a clinical dental CBCT system. The automated IQ evaluation of volume data sets of the dental CBCT system was achieved with the proposed phantom requiring only one scan for the determination of all desired parameters. Typically, less than 5 min were needed for phantom set-up, scanning, and data analysis. Quantitative evaluation of system performance over time by comparison to previous examinations was also verified. The maximum percentage interscan variation of repeated measurements was less than 4% and 1.7% on average for all investigated quality criteria. The NPS-based image noise differed by less than 5% from the conventional standard deviation approach and spatially selective 10% MTF values were well comparable to subjective results obtained with 3D resolution pattern. Determining only transverse spatial resolution and global noise behavior in the central field of measurement turned out to be insufficient. The proposed framework transfers QA routines employed in conventional CT in an advanced version to CBCT for fully automated and time-efficient evaluation of technical equipment. With the modular phantom design, a routine as well as an expert version for assessing IQ is provided. The QA program can be used for arbitrary CT units to evaluate 3D imaging characteristics automatically.

Can dental cone beam computed tomography assess bone mineral density?

Mineral density distribution of bone tissue is altered by active bone modeling and remodeling due to bone complications including bone disease and implantation surgery. Clinical cone beam computed tomography (CBCT) has been examined whether it can assess oral bone mineral density (BMD) in patient. It has been indicated that CBCT has disadvantages of higher noise and lower contrast than conventional medical computed tomography (CT) systems. On the other hand, it has advantages of a relatively lower cost and radiation dose but higher spatial resolution. However, the reliability of CBCT based mineral density measurement has not yet been fully validated. Thus, the objectives of this review are to discuss 1) why assessment of BMD distribution is important and 2) whether the clinical CBCT can be used as a potential tool to measure the BMD. Brief descriptions of image artefacts associated with assessment of gray value, which has been used to account for mineral density, in CBCT images are provided. Techniques to correct local and conversion errors in obtaining the gray values in CBCT images are also introduced. This review can be used as a quick reference for users who may encounter these errors during analysis of CBCT images.

PO-0892: Introduction of image lag correction on clinical cone-beam CT systems

PO-0892: Introduction of image lag correction on clinical cone-beam CT systems

청소년 측두하악관절 골관절염의 보존적 치료효과 및 관절면의 변화 비교

2009년부터 2012년까지 부산대학교치과병원 구강내과에 측두하악장애를 주소로 내원한 만 12세-19세(평균나이 : <TEX>$15.75{\pm}2.17$</TEX>세)의 청소년환자 중 임상검사, 방사선검사 및 전산화 단층촬영 (Cone Beam Computed Tomography, CBCT)을 통하여 측두하악관절 골관절염으로 진단되고 하나이상의 과두나 하악와의 관절하골에서 erosive change를 보이는 149명 167개의 관절을 대상으로 하였다. 치료 후 평균 9개월(<TEX>$9.03{\pm}4.64$</TEX> 개월) 이후 CBCT를 재촬영하고, 임상검사를 재실시하였다. 구강내과 전문의와 치과 방사선과 전문의가 영상 진단을 하였고, 임상 검사 결과와 CBCT의 영상 결과를 추적 연구 비교하여 다음과 같은 결과를 얻었다. 1. 약물치료, 물리치료, 행동조절치료, 교합안정장치치료를 병행한 환자군은 교합안정장치치료를 병행하지 않은 환자군에 비해 치료 후 통증, 개구제한 증상에서 유의한 증상 개선을 나타냈다. 2. 교합안정장치치료를 병행한 급성환자군은 교합안정장치치료를 병행하지 않은 급성환자군보다 치료 후 통증, 관절잡음, 개구제한이 더 많이 개선되었고, 그 중 통증과 개구제한은 유의한 증상 개선을 보였다. 3. 치료 후 침식을 보이는 하악과두의 골변화 양상은 치료방법에 상관없이 개선되는 방향(improved)으로 진행되었으나, 교합안정장치치료를 병행한 환자군에 비해서 교합압정장치치료를 병행하지 않은 환자군에서는 하악과두의 침식성 골 변화가 더 악화(worsen)되어 나타나는 경우가 많았으며, 반대쪽 하악과두에 새로운 침식이 나타난 경우도 더 많이 관찰되었다. This study is designed to evaluate the treatment outcome of occlusal stabilizing splint and to assess follow-up study of condylar bony changes using cone beam computed tomography(CBCT) in adolescents patients (12-19 years) with TMJ osteoarthritis(OA). 167 eroded condyles in 149 subjects were chosen among the patients who presented to the Department of Oral Medicine of Pasan National University Hospital, diagnosed as TMJ osteoarthritis by clinical exam, x-ray and CBCT from 2009 to 2012. They were treated conservatively with physical therapy, medication, behavioral therapy and occlusal stabilizing splint therapy. After average 9 months, CBCT was retaken and subjective symptoms and clinical findings were investigated. Condyle bony changes were classified by unchanged, less severe and more severe. The obtained results were as follow: 1. Pain, Noise, LOM(Limitation of motion) and MCO(Maximum comfortable opening) measurement of TMJ OA patients were markedly improved after conservative treatment. 2. In the occlusal stabilizing splint therapy group, Pain and LOM were statistically significant improved than non-occlusal stabilizing splint therapy group. 3. In the acute occlusal stabilizing splint therapy group, Pain and LOM were remarkably improved. 4. In comparison of CBCT1 and CBCT2 images, the transition of bone changes to lesser severe was most commonly in joint with erosive change. 5. In the non-occlusal stabilizing splint therapy group, the transition of condylar bone changes from erosion to more severe was many than occlusal stabilizing splint therapy group.

Radiation dose evaluation of dental cone beam computed tomography using an anthropomorphic adult head phantom

Dental cone beam computed tomography (CBCT) provides high-resolution tomographic images and has been gradually used in clinical practice. Thus, it is important to examine the amount of radiation dose resulting from dental CBCT examinations. In this study, we developed an in-house anthropomorphic adult head phantom to evaluate the level of effective dose. The anthropomorphic phantom was made of acrylic and filled with plaster to replace the bony tissue. The contour of the head was extracted from a set of adult computed tomography (CT) images. Different combinations of the scanning parameters of CBCT were applied. Thermoluminescent dosimeters (TLDs) were used to measure the absorbed doses at 19 locations in the head and neck regions. The effective doses measured using the proposed phantom at 65, 75, and 85kVp in the D-mode were 72.23, 100.31, and 134.29µSv, respectively. In the I-mode, the effective doses were 108.24, 190.99, and 246.48µSv, respectively. The maximum percent error between the doses measured by the proposed phantom and the Rando phantom was l4.90%. Therefore, the proposed anthropomorphic adult head phantom is applicable for assessing the radiation dose resulting from clinical dental CBCT.

Patient-specific scatter correction in clinical cone beam computed tomography imaging made possible by the combination of Monte Carlo simulations and a ray tracing algorithm

Purpose. Cone beam computed tomography (CBCT) image quality is limited by scattered photons. Monte Carlo (MC) simulations provide the ability of predicting the patient-specific scatter contamination in clinical CBCT imaging. Lengthy simulations prevent MC-based scatter correction from being fully implemented in a clinical setting. This study investigates the combination of using fast MC simulations to predict scatter distributions with a ray tracing algorithm to allow calibration between simulated and clinical CBCT images. Material and methods. An EGSnrc-based user code (egs_cbct), was used to perform MC simulations of an Elekta XVI CBCT imaging system. A 60keV x-ray source was used, and air kerma scored at the detector plane. Several variance reduction techniques (VRTs) were used to increase the scatter calculation efficiency. Three patient phantoms based on CT scans were simulated, namely a brain, a thorax and a pelvis scan. A ray tracing algorithm was used to calculate the detector signal due to primary photons. A total of 288 projections were simulated, one for each thread on the computer cluster used for the investigation. Results. Scatter distributions for the brain, thorax and pelvis scan were simulated within 2% statistical uncertainty in two hours per scan. Within the same time, the ray tracing algorithm provided the primary signal for each of the projections. Thus, all the data needed for MC-based scatter correction in clinical CBCT imaging was obtained within two hours per patient, using a full simulation of the clinical CBCT geometry. Conclusions. This study shows that use of MC-based scatter corrections in CBCT imaging has a great potential to improve CBCT image quality. By use of powerful VRTs to predict scatter distributions and a ray tracing algorithm to calculate the primary signal, it is possible to obtain the necessary data for patient specific MC scatter correction within two hours per patient.

Relaxed Statistical Shape Models for 3D Image Segmentation – Application to Mandible Bone in Cone-beam CT Data

Patient specific surface models of the jaw are beneficial for pre-operative planning and manufacturing of customized prosthesis. Such models can be generated on the basis of dental cone-beam CT images, but those suffer from a comparatively bad image quality with regard to the signal-to-noise ratio. Therefore, in this work, a statistical shape model (SSM) is used for robust segmentation of the mandible bone. While previous works with that application require manual interaction during SSM construction, we establish correspondence fully automatic by minimizing the description length of the model. Subsequently, the mandible bone is automatically localized and segmented using the SSM as shape constraint. The standard SSM constraint is known to be inherently limited insofar as patient specific anatomical details can often not be represented. To overcome this limitation, a new, mathematically sound, computationally fast, and intuitively interpretable, relaxed SSM constraint is derived, which can be applied without any user-provided parameter. Evaluation on clinical cone beam CT images yields an improvement of the Jaccard coefficient up to 45% compared to the standard SSM constraint. Our results are similar to that of alternative methods in the literature, indicating the general potential of the proposed relaxed SSM constraint for medical image segmentation. Keywords: Active shape model, Cone beam CT, Dental imaging, Image segmentation, Mandible bone, Optimization, Shape constraint, Statistical shape model.

Extracting respiratory signals from thoracic cone beam CT projections

The patient respiratory signal associated with the cone beam CT (CBCT) projections is important for lung cancer radiotherapy. In contrast to monitoring an external surrogate of respiration, such a signal can be extracted directly from the CBCT projections. In this paper, we propose a novel local principal component analysis (LPCA) method to extract the respiratory signal by distinguishing the respiration motion-induced content change from the gantry rotation-induced content change in the CBCT projections. The LPCA method is evaluated by comparing with three state-of-the-art projection-based methods, namely the Amsterdam Shroud method, the intensity analysis method and the Fourier-transform-based phase analysis method. The clinical CBCT projection data of eight patients, acquired under various clinical scenarios, were used to investigate the performance of each method. We found that the proposed LPCA method has demonstrated the best overall performance for cases tested and thus is a promising technique for extracting a respiratory signal. We also identified the applicability of each existing method.

Identification of occlusal prematurity by clinical examination and cone-beam computed tomography

The aim of this study was to evaluate the ability to identify occlusal prematurity by images from paraxial slices of cone-beam computed tomography (CBCT). For such purpose, a pilot study was performed in which 16 asymptomatic young patients were subjected to a clinical examination, including a careful occlusal analysis and then individual deprogrammer devices ("Lucia's JIG") were fabricated. Premature contacts were clinically identified in centric relation (CR) for each patient by jaw manipulation and interocclusal marking with articulating paper (Accufilm). Subsequently, these devices were adjusted in CR and used during the tomographic exams in such a way that CBCT in CR could be obtained. After routine processing, the images were analyzed in order to identify occlusal prematurity on the displayed images by 30 professionals divided according to areas of activity (occlusion specialist, general practitioner and radiologist; n=10 per area) and time of professional experience (less than 5, between 5 and 10, and over 10 years). By comparing the premature contacts identified in the clinical analysis and CBCT images, an agreement index between these two variables was calculated. Data were analyzed statistically by ANOVA and Scott-Knott test (α=0.05). The results showed that the identification of occlusal prematurity by paraxial CBCT slices was proven to be a method of average reliability.

Enhancement of four-dimensional cone-beam computed tomography by compressed sensing with Bregman iteration

In four-dimensional (4D) cone-beam computed tomography (CBCT), there is a spatio-temporal tradeoff that currently limits the accuracy. The aim of this study is to develop a Bregman iteration based formalism for high quality 4D CBCT image reconstruction from a limited number of low-dose projections. The 4D CBCT problem is first divided into multiple 3D CBCT subproblems by grouping the projection images corresponding to the phases. To maximally utilize the information from the under-sampled projection data, a compressed sensing (CS) method with Bregman iterations is employed for solving each subproblem. We formulate an unconstrained optimization problem based on least-square criterion regularized by total-variation. The least-square criterion reflects the inconsistency between the measured and the estimated line integrals. Furthermore, the unconstrained problem is updated and solved repeatedly by Bregman iterations. The performance of the proposed algorithm is demonstrated through a series of simulation studies and phantom experiments, and the results are compared to those of previously implemented compressed sensing technique using other gradient-based methods as well as conventional filtered back-projection (FBP) results. The simulation and experimental studies have shown that artifact suppressed images can be obtained with as small as 41 projections per phase, which is adequate for clinical 4D CBCT reconstruction. With such small number of projections, the conventional FDK failed to yield meaningful 4D CBCT images, and CS technique using conjugate gradient was not able to recover sharp edges. The proposed method significantly reduces the radiation dose and scanning time to achieve the high quality images compared to the 4D CBCT imaging based on the conventional FDK technique and the existing CS techniques.

Effects of orthodontic treatment on human alveolar bone density distribution

The objective of this study was to examine if non-invasive clinical cone beam computed tomography (CBCT)-based degree of bone mineralization (DBM) measurement can be used to detect the different results from orthodontic treatment between the maxilla and mandible in human patients. CBCT images were taken before and after orthodontic treatment from 43 patients (19 males and 24 females, 14.36 ± 1.50 years). A histogram of computed tomography (CT) attenuation value, which is equivalent to the DBM, was obtained from the alveolar cortical (AC), trabecular (AT), and enamel (E) regions of each image. Mean, standard deviation (SD), and coefficient of variation (COV) of the CT attenuation values were computed. The regional variations and percentage (%) differences between the E and alveolar regions of the CT attenuation parameters at the maxilla and mandible were analyzed before and after orthodontic treatment. The AC had higher mean and variability (SD and COV) than the AT before and after treatment (p < 0.001). The variability was higher in the mandibular AC than in the maxillar AC (p < 0.01) independent of orthodontic treatment. The percentage (%) difference of variability of CT attenuation values changed for both AT and AC in the maxilla after orthodontic treatment, while that changed for only the AT (p < 0.02), but not for AC, in the mandible (p > 0.16). The alveolar cortical region of the mandible responded differently to orthodontic treatment compared with other alveolar regions. The CBCT-based DBM analysis can be used clinically to assess alveolar bone quality changes induced by orthodontic treatment to improve treatment planning and result evaluation.

Comparison of micro-CT and cone beam CT-based assessments for relative difference of grey level distribution in a human mandible

The purpose of this study was to examine the ability of CT to assess the relative difference of degree of bone mineralization (grey level) parameters in a human mandible. Ten mandibular sections from cadavers (81.5 ± 12.1 years) were scanned using micro-CT with 27.2 μm voxel size and cone beam CT (CBCT) with 200 μm, 300 μm, and 400 μm voxel sizes. In addition, 15 clinical CBCT images from young patients (mean age 18.9 ± 3.3 years) were identified. After segmentation of bone voxels, alveolar bone and basal cortical bone regions were digitally isolated. A histogram of grey level, which is equivalent to degree of bone mineralization, was obtained from each region of the CT images. Mean, standard deviation (SD), coefficient of variation (COV), fifth percentile low (Low(5)) and high (High(5)) of alveolar bone and basal cortical bone regions were obtained. Percentage differences of grey level parameters between alveolar and basal cortical bones were computed. The alveolar bone region had significantly lower Mean, Low(5) and High(5) values but significantly higher SD and COV than the basal cortical bone region for all CT images (p < 0.05). All parameters were significantly lower for the old cadaver group than for the young patient group (p < 0.05). CBCT and micro-CT provide comparable results in the assessment of relative difference in grey level distribution between alveolar and basal cortical bone regions in the human mandible. The percentage difference relative to an internal reference (basal cortical bone) can be a reliable method when assessing the degree of bone mineralization using CBCT images for both cross-sectional and longitudinal comparisons.

Relationship between x‐ray illumination field size and flat field intensity and its impacts on x‐ray imaging

X-ray cone-beam CT (CBCT) is being increasingly used for various clinical applications, while its performance is still hindered by image artifacts. This work investigates a new source of reconstruction error, which is often overlooked in the current CBCT imaging. The authors find that the x-ray flat field intensity (I(0)) varies significantly as the illumination volume size changes at different collimator settings. A wrong I(0) value leads to inaccurate CT numbers of reconstructed images as well as wrong scatter measurements in the CBCT research. The authors argue that the finite size of x-ray focal spot together with the detector glare effect cause the I(0) variation at different illumination sizes. Although the focal spot of commercial x-ray tubes typically has a nominal size of less than 1 mm, the off-focal-spot radiation covers an area of several millimeters on the tungsten target. Due to the large magnification factor from the field collimator to the detector, the penumbra effects of the collimator blades result in different I(0) values for different illumination field sizes. Detector glare further increases the variation, since one pencil beam of incident x-ray is scattered into an area of several centimeters on the detector. In this paper, the authors study these two effects by measuring the focal spot distribution with a pinhole assembly and the detector point spread function (PSF) with an edge-spread function method. The authors then derive a formula to estimate the I(0) value for different illumination field sizes, using the measured focal spot distribution and the detector PSF. Phantom studies are carried out to investigate the accuracy of scatter measurements and CT images with and without considering the I(0) variation effects. On our tabletop system with a Varian Paxscan 4030CB flat-panel detector and a Varian RAD-94 x-ray tube as used on a clinical CBCT system, the focal spot distribution has a measured full-width-at-half-maximum (FWHM) of around 0.4 mm, while non-negligible off-focal-spot radiation is observed at a distance of over 2 mm from the center. The measured detector PSF has an FWHM of 0.510 mm, with a shape close to Gaussian. From these two distributions, the author calculate the estimated I(0) values at different collimator settings. The I(0) variation mainly comes from the focal spot effect. The estimation matches well with the measurements at different collimator widths in both horizontal and vertical directions, with an average error of less than 3%. Our method improves the accuracy of conventional scatter measurements, where the scatter is measured as the difference between fan-beam and cone-beam projections. On a uniform water cylinder phantom, more accurate I(0) suppresses the unfaithful high-frequency signals at the object boundaries of the measured scatter, and the SPR estimation error is reduced from 0.158 to 0.014. The proposed I(0) estimation also reduces the reconstruction error from about 20 HU on the Catphan©600 phantom in the selected regions of interest to less than 4 HU. The I(0) variation is identified as one additional error source in x-ray imaging. By measuring the focal-spot distribution and detector PSF, the authors propose an accurate method of estimating the I(0) value for different illumination field sizes. The method obtains more accurate scatter measurements and therefore facilitates scatter correction algorithm designs. As correction methods for other CBCT artifacts become more successful, our research is significant in further improving the CBCT imaging accuracy.

Physical phantom studies of helical cone‐beam CT with exact reconstruction

Onboard cone-beam computed tomography (CBCT) connected to radiotherapy linear accelerators suffers CT number consistency and uniformity problems in addition to limited longitudinal coverage. Such problems have prevented CBCT from being fully utilized in many quantitative applications including tumor response evaluation and daily radiation dose computation. This paper presents a feasibility study on the helical CBCT scan with exact reconstruction that could be a potential solution. A Varian TrueBeam treatment machine was programmed in the research mode to accomplish helical scans that required synchronized gantry circular rotation and couch table linear motion. Two physical phantoms were scanned in both 360° and 720° helical trajectories. A Katsevich exact reconstruction algorithm was implemented and tested with digital phantom simulations. It was further optimized to account for mechanical instabilities of both gantry rotation and couch table motion from the physical phantom measurements. Preprocessing was employed to correct photon scattering, beam hardening, and bowtie filtration. The reconstructed images were compared to those reconstructed from the FDK approximate reconstruction algorithm using the same phantom projections. Comparisons have also been made with the clinical circular CBCT images and the diagnostic helical CT images of the same physical phantoms. Satisfactory reconstruction results were obtained for the Katsevich algorithm in digital phantom study. Physical phantom results demonstrated that a 360° helical scan could provide up to 19 cm longitudinal coverage, which could be increased to 54 cm with a 720° helical scan. Image spatial resolution and soft tissue contrast were sufficient. The Q-value, which combined the spatial frequency response (modulation transfer function) and the image noise, was calculated, and suggested that the Katsevich algorithm was superior to the FDK algorithm. A helical CBCT scan is useful to extend the longitudinal coverage. The Katsevich exact reconstruction algorithm could provide additional advantages in image qualities over the traditional FDK approximate algorithm. The combination of helical CBCT scan with exact reconstruction was proved feasible and would render CBCT more useful in image-guided radiation therapy.

Cone beam computed tomography imaging as a diagnostic tool in determining root fracture in endodontically treated teeth

Introduction: Vertical root fractures (VRFs) of endodontically treated teeth are serious complications that are associated with a poor survival prognosis. The diagnosis of VRFs is difficult, because the clinical signs and symptoms are often obscure and periapical radiographs (PAs) provide limited diagnostic information. Recently, Cone Beam Computed Tomography (CBCT) has been used to address complicated cases of VRFs with an inconclusive diagnosis. Aim: To determine the clinical diagnostic accuracy of CBCT in detecting VRFs in complex failing cases of endodontically treated teeth, as an adjunct to PAs. Materials and Methods: Evaluating the diagnostic accuracy of CBCT in a series of clinical cases of suspected VRFs, presented via clinical photographs, PAs, and CBCT images. Results: CBCT imaging significantly aided the differential diagnosis in most cases. Conclusions: CBCT imaging provides a 3D image and information in many cases of VRF that 2-dimensional PAs fail to generate. The slice thickness and interslice interval of the CBCT image influence the perception of the normal anatomy and pathoses.

Evaluation of image quality for different kV cone-beam CT acquisition and reconstruction methods in the head and neck region

Purpose. To evaluate the image quality obtained in a standard QA phantom with both clinical and non-clinical cone-beam computed tomography (CBCT) acquisition modes for the head and neck (HN) region as a step towards CBCT-based treatment planning. The impact of deteriorated Hounsfield unit (HU) accuracy was investigated by comparing results from clinical CBCT image reconstructions to those obtained from a pre-clinical scatter correction algorithm. Methods. Five different CBCT acquisition modes on a clinical system for kV CBCT-guided radiotherapy were investigated. Image reconstruction was performed in both standard clinical software and with an experimental reconstruction algorithm with improved beam hardening and scatter correction. Using the Catphan 504 phantom, quantitative measures of HU uniformity, HU verification and linearity, contrast-to-noise ratio (CNR), and spatial resolution using modulation transfer function (MTF) estimation were assessed. To benchmark the CBCT image properties, comparison to standard HN protocols on conventional CT scanners was performed by similar measures. Results. The HU uniformity within a water-equivalent homogeneous region was considerably improved using experimental vs. standard reconstruction, by factors of two for partial scans and four for full scans. Similarly, the amount of capping/cupping artifact was reduced by more than 1.5%. With mode and reconstruction specific HU calibration using seven inhomogeneity inserts comparable HU linearity was observed. CNR was on average 5% higher for experimental reconstruction (scaled with the square-root of dose between modes for both reconstruction methods). Conclusions. Judged on parameters affecting the common diagnostic image properties, improved beam hardening and scatter correction diminishes the difference between CBCT and CT image quality considerably. In the pursuit of CBCT-based treatment adaptation, dedicated imaging protocols may be required.

WE‐A‐301‐03: Enhancement of Four‐Dimensional Cone‐Beam Computed Tomography by Compressed Sensing with Bregman Iteration

Purpose: In four‐dimensional (4D) cone‐beam computed tomography (CBCT), there is a spatio‐temporal tradeoff. The aim of this study is to develop a Bregman iteration based formalism for high quality 4D CBCT image reconstruction from low‐dose projections. Methods: The 4D CBCT problem is first divided into multiple 3D CBCT subproblems by grouping the projection images corresponding to the phases. To maximally utilize the information from the under‐sampled projection data, a compressed sensing method is employed for solving each subproblem. We formulate an unconstrained lasso (least absolute shrinkage and selection operator) problem based on least‐square criterion regularized by total‐variation. The least‐square criterion reflects the inconsistency between the measured and the estimated line integrals. Furthermore, the unconstrained lasso problem is updated and solved repeatedly by Bregman iterations. Results: The performance of the proposed algorithm is demonstrated through a series of phantom experiments, and the results are compared to those of conventional filtered back‐projection (FBP). The simulation studies have shown that artifact suppressed images can be obtained with as small as 41 projections per phase, which is adequate for clinical 4D CBCT reconstruction without slowing down the gantry rotation. With such small number of projections, the conventional FBP failed to yield meaningful 4D CBCT images. Conclusions: The proposed method significantly reduces the radiation dose and scanning time to achieve the high quality images compared to the conventional 4D CBCT imaging based on FBP technique.

TH‐A‐214‐11: Dose to Implantable Cardiac Devices from Cone Beam CT

Purpose: To quantify the cumulative dose contribution to implantable cardiac rhythm management devices (ICRMDs) from cone beam CT (CBCT) for patients undergoing radiotherapy and to investigate dose reduction strategies. Methods: An implantable pacemaker (IP) and implantable cardioverter‐defibrillator (ICD) with internal MOSFET dosimeters were used to measure the cumulative CBCT dose to the IP and ICD. Specifically, the MOSFET threshold voltage shifts resulting from standard clinical CBCT imaging protocols were recorded and compared to shifts caused by peripheral megavoltage dose associated with 6 MV IMRT treatment delivery. The dose assessments were performed on an anthropomorphic phantom with the IP or ICD placed in the left clavicular area both inside and outside of the CBCT field of view (FOV). Two dose reduction strategies were evaluated: the use of a 2 mm copper plate to shield the device during imaging and the use of partial‐angle scanning. Results: The average internal IP and ICD doses due to standard clinical CBCT imaging protocols with the device inside the FOV were in the 2.2 – 4.3 cGy and 0.8 – 1.9 cGy range, respectively, but could reach as high as 11.2 cGy per scan. Five centimeters outside the FOV, the dose can reach up to 10% of the in‐field dose in the cranio‐caudal direction and up to 70% in the medio‐ lateral direction. Copper shielding resulted in a 3‐fold reduction in total CBCT dose and a posterior 180 degree scan reduced the dose by a factor of 8 in comparison to a full 360 degree scan. Conclusions: The CBCT imaging dose to ICRMDs can be substantial and needs to be accounted for. Use of a MOSFET dosimeter during the first treatment fraction is an effective way of obtaining a reasonable upper limit of the average cumulative ICRMD dose. Partial posterior scans can greatly reduce this dose.