Cone-beam Computed Tomography System Research Articles

Evaluation of scatter rejection and correction performance of 2D antiscatter grids in cone beam computed tomography.

We have been investigating two-dimensional (2D) antiscatter grids (2D ASGs) to reduce scatter fluence and improve image quality in cone beam computed tomography (CBCT). In this work, two different aspects of 2D ASGs, their scatter rejection and correction capability, were investigated in CBCT experiments. To correct residual scatter transmitted through the 2D ASG, it was used as a scatter measurement device with a novel method: grid-based scatter sampling. Three focused 2D ASG prototypes with grid ratios of 8, 12, and 16 were developed for linac-mounted offset detector CBCT geometry. In the first phase, 2D ASGs were used as a scatter rejection device, and the effect of grid ratio on CT number accuracy and contrast-to-noise ratio (CNR) evaluated in CBCT images. In the second phase, a grid-based scatter sampling method which exploits the signal modulation characteristics of the 2D ASG's septal shadows to measure and correct residual scatter transmitted through the grid was implemented. To evaluate CT number accuracy, the percent change in CT numbers was measured by changing the phantom from head to pelvis size and configuration. When 2D ASG was used as a scatter rejection device, CT number accuracy increased and the CT number variation due to change in phantom dimensions was reduced from 23% to 2-6%. A grid ratio of 16 yielded the lowest CT number variation. All three 2D ASGs yielded improvement in CNR, up to a factor of two in pelvis-sized phantoms. When 2D ASG prototypes were used for both scatter rejection and correction, CT number variations were reduced further, to 1.3-2.6%. In comparisons with a clinical CBCT system and a high-performance radiographic ASG, 2D ASG provided higher CT number accuracy under the same imaging conditions. When 2D ASG is used solely as a scatter rejection device, substantial improvement in CT number accuracy can be achieved by increasing the grid ratio. Two-dimensional ASGs also provided significant CNR improvement even at lower grid ratios. Two-dimensional ASGs used in conjunction with the grid-based scatter sampling method provided further improvement in CT number accuracy, irrespective of the grid ratio, while preserving 2D ASGs' capacity to improve CNR. The combined effect of scatter rejection and residual scatter correction by 2D ASG may accelerate implementation of new techniques in CBCT that require high quantitative accuracy, such as radiotherapy dose calculation and dual energy CBCT.

Deformable motion compensation for interventional cone-beam CT

Image-guided therapies in the abdomen and pelvis are often hindered by motion artifacts in cone-beam CT (CBCT) arising from complex, non-periodic, deformable organ motion during long scan times (5–30 s). We propose a deformable image-based motion compensation method to address these challenges and improve CBCT guidance. Motion compensation is achieved by selecting a set of small regions of interest in the uncompensated image to minimize a cost function consisting of an autofocus objective and spatiotemporal regularization penalties. Motion trajectories are estimated using an iterative optimization algorithm (CMA-ES) and used to interpolate a 4D spatiotemporal motion vector field. The motion-compensated image is reconstructed using a modified filtered backprojection approach. Being image-based, the method does not require additional input besides the raw CBCT projection data and system geometry that are used for image reconstruction. Experimental studies investigated: (1) various autofocus objective functions, analyzed using a digital phantom with a range of sinusoidal motion magnitude (4, 8, 12, 16, 20 mm); (2) spatiotemporal regularization, studied using a CT dataset from The Cancer Imaging Archive with deformable sinusoidal motion of variable magnitude (10, 15, 20, 25 mm); and (3) performance in complex anatomy, evaluated in cadavers undergoing simple and complex motion imaged on a CBCT-capable mobile C-arm system (Cios Spin 3D, Siemens Healthineers, Forchheim, Germany). Gradient entropy was found to be the best autofocus objective for soft-tissue CBCT, increasing structural similarity (SSIM) by 42%–92% over the range of motion magnitudes investigated. The optimal temporal regularization strength was found to vary widely (0.5–5 mm−2) over the range of motion magnitudes investigated, whereas optimal spatial regularization strength was relatively constant (0.1). In cadaver studies, deformable motion compensation was shown to improve local SSIM by ∼17% for simple motion and ∼21% for complex motion and provided strong visual improvement of motion artifacts (reduction of blurring and streaks and improved visibility of soft-tissue edges). The studies demonstrate the robustness of deformable motion compensation to a range of motion magnitudes, frequencies, and other factors (e.g. truncation and scatter).

Densely sampled spectral modulation for x-ray CT using a stationary modulator with flying focal spot: a conceptual and feasibility study of scatter and spectral correction.

Modulation of the x-ray source in computed tomography (CT) by a designated filter to achieve a desired distribution of photon flux has been greatly advanced in recent years. In this work, we present a densely sampled spectral modulation (DSSM) as a promising low-cost solution to quantitative CT imaging in the presence of scatter. By leveraging a special stationary filter (namely a spectral modulator) and a flying focal spot, DSSM features a strong correlation in the scatter distributions across focal spot positions and sees no substantial projection sparsity or misalignment in data sampling, making it possible to simultaneously correct for scatter and spectral effects in a unified framework. The concept of DSSM is first introduced, followed by an analysis of the design and benefits of using the stationary spectral modulator with a flying focal spot (SMFFS) that dramatically changes the data sampling and its associated data processing. With an assumption that the scatter distributions across focal spot positions have strong correlation, a scatter estimation and spectral correction algorithm from DSSM is then developed, where a dual-energy modulator along with two flying focal spot positions is of interest. Finally, a phantom study on a tabletop cone-beam CT system is conducted to understand the feasibility of DSSM by SMFFS, using a copper modulator and by moving the x-ray tube position in the X direction to mimic the flying focal spot. Based on our analytical analysis of the DSSM by SMFFS, the misalignment of low- and high-energy projection rays can be reduced by a factor of more than 10 when compared with a stationary modulator only. With respect to modulator design, metal materials such as copper, molybdenum, silver, and tin could be good candidates in terms of energy separation at a given attenuation of photon flux. Physical experiments using a Catphan phantom as well as an anthropomorphic chest phantom demonstrate the effectiveness of DSSM by SMFFS with much better CT number accuracy and less image artifacts. The root mean squared error was reduced from 297.9 to 6.5 Hounsfield units (HU) for the Catphan phantom and from 409.3 to 39.2HU for the chest phantom. The concept of DSSM using a SMFFS is proposed. Phantom results on its scatter estimation and spectral correction performance validate our main ideas and key assumptions, demonstrating its potential and feasibility for quantitative CT imaging.

High-resolution coded aperture optimization for super-resolved compressive x-ray cone-beam computed tomography.

Compressive x-ray cone-beam computed tomography (CBCT) approaches rely on coded apertures (CA) along multiple view angles to block a portion of the x-ray energy traveling towards the detectors. Previous work has shown that designing CA patterns yields improved images. Most designs, however, are focused on multi-shot fan-beam (FB) systems, handling a 1:1 ratio between CA features and detector elements. In consequence, image resolution is subject to the detector pixel size. Moreover, CA optimization for computed tomography involves strong binarization assumptions, impractical data rearrangements, or computationally expensive tasks such as singular value decomposition (SVD). Instead of using higher-resolution CA distributions in a multi-slice system with a more dense detector array, this work presents a method for designing the CA patterns in a compressive CBCT system under a super-resolution configuration, i.e., high-resolution CA patterns are designed to obtain high-resolution images from lower-resolution projections. The proposed method takes advantage of the Gershgorin theorem since its algebraic interpretation relates the circle radii with the eigenvalue bounds, whose minimization improves the condition of the system matrix. Simulations with medical data sets show that the proposed design attains high-resolution images from lower-resolution detectors in a single-shot CBCT scenario. Besides, image quality is improved in up to 5 dB of peak signal-to-noise compared to random CA patterns for different super-resolution factors. Moreover, reconstructions from Monte Carlo simulated projections show up to 3 dB improvements. Further, for the analyzed cases, the computational load of the proposed approach is up to three orders of magnitude lower than that of SVD-based methods.

Radiation dosimetry of a clinical prototype dedicated cone-beam breast CT system with offset detector.

A clinical-prototype, dedicated, cone-beam breast computed tomography (CBBCT) system with offset detector is undergoing clinical evaluation at our institution. This study is to estimate the normalized glandular dose coefficients ( ) that provide air kerma-to-mean glandular dose conversion factors using Monte Carlo simulations. The clinical prototype CBBCT system uses 49kV x-ray spectrum with 1.39mm 1st half-value layer thickness. Monte Carlo simulations (GATE, version 8) were performed with semi-ellipsoidal, homogeneous breasts of various fibroglandular weight fractions ( , chest wall diameters ( cm), and chest wall to nipple length ( ), aligned with the axis of rotation (AOR) located at 65cm from the focal spot to determine the . Three geometries were considered - -cm detector with no offset that served as reference and corresponds to a clinical CBBCT system, -cm detector with 5cm offset, and a -cm detector with 10cm offset. For 5cm lateral offset, the ranged mGy/mGy and reduction in with respect to reference geometry was observed only for 18cm ( ) and 20cm ( ) diameter breasts. For the 10cm lateral offset, the ranged mGy/mGy and reduction in was observed for all breast diameters. The reduction in was , , , , and for 8, 10, 14, 18, and 20cm diameter breasts, respectively. For a given breast diameter, the reduction in with offset-detector geometries was not dependent on . Numerical fits of were generated for each geometry. The and the numerical fit, would be of benefit for current CBBCT systems using the reference geometry and for future generations using offset-detector geometry. There exists a potential for radiation dose reduction with offset-detector geometry, provided the same technique factors as the reference geometry are used, and the image quality is clinically acceptable.

Measurement of Peri-Implant Bone Width with and without Metal Artifact Reduction Algorithm Using Two Cone-Beam Computed Tomography Software Programs

Objective: To assess the effect of metal artifact reduction (MAR) algorithm of two cone-beam computed tomography (CBCT) software programs on the accuracy of peri-implant bone width measurements . Material and Methods: This in vitro study evaluated 35 bovine rib bone blocks measuring 8 x 8 x 11 mm. Titanium implants were inserted in bone blocks and placed in a wax model of mandible. CBCT scans were obtained with ProMax 3D and Cranex 3D CBCT systems with and without the MAR algorithm. The width of buccal and lingual bone plates surrounding the implant was measured on CBCT scans by two observers. The Cronbach’s alpha was calculated to assess inter-observer agreement. The area under the curve (AUC), sensitivity and specificity values were calculated and compared. Results: The two observers had an excellent agreement. The accuracy of Cranex 3D was higher than that of ProMax 3D (p<0.05). Both CBCT systems showed higher accuracy when the MAR algorithm was not used (p<0.05). Both CBCT systems showed higher accuracy for measuring the width of the lingual plate than buccal plate (p<0.05) . Conclusion: The application of MAR algorithm did not notably increase the measurement accuracy in any CBCT system. Cranex 3D showed generally higher measurement accuracy than ProMax 3D.

The proximity of mental foramen to mandibular first premolar teeth: Cone beam computed tomography analysis in Turkish population

Background/Aim: The aim of this study was to evaluate the proximity of the mental foramen (MF) to the roots of the mandibular first premolar teeth in the Turkish population according to gender and different age groups. Material and Methods: The study was conducted on 385 samples of 242 patients who presented at the Dicle University, Faculty of Dentistry, Department of Dentomaxillofacial Radiology. The proximity of the MF to mandibular first premolar teeth was examined on cone beam computed tomography (CBCT) images. Two-way Anova variance analysis was used in the statistical analyses of the data. Results: According to the analysis results, no statistically significant difference was determined between the age and gender groups (p&gt;0.05). In the evaluation of the gender groups, the mean value was 6.6997 mm for females and 6.5445 mm for males. Conclusions: With 3-dimensional imaging technology, the CBCT system provides detailed information about the MF localization. This method has shown that the localization of the MF could change associated with variables such as age and gender.

Comparison of limited- and large-volume cone-beam computed tomography using a small voxel size for detecting isthmuses in mandibular molars.

PurposeThis study was performed to compare the ability of limited- and large-volume cone-beam computed tomography (CBCT) to display isthmuses in the apical root canals of mandibular molars.Materials and MethodsForty human mandibular first molars with isthmuses in the apical 3 mm of mesial roots were scanned by micro-computed tomography (micro-CT), and their thickness, area, and length were recorded. The samples were examined using 2 CBCT systems, using the smallest voxels and field of view available for each device. The Mann-Whitney, Friedman, and Dunn multiple comparison tests were performed (α=0.05).ResultsThe 3D Accuitomo 170 and i-Cat devices detected 77.5% and 75.0% of isthmuses, respectively (P>0.05). For length measurements, there were significant differences between micro-CT and both 3D Accuitomo 170 and i-Cat (P<0.05).ConclusionBoth CBCT systems performed similarly and did not detect isthmuses in the apical third in some cases. CBCT still does not equal the performance of micro-CT in isthmus detection, but it is nonetheless a valuable tool in endodontic practice.

Model-based dual-energy tomographic image reconstruction of objects containing known metal components

Dual-energy (DE) decomposition has been adopted in orthopedic imaging to measure bone composition and visualize intraarticular contrast enhancement. One of the potential applications involves monitoring of callus mineralization for longitudinal assessment of fracture healing. However, fracture repair usually involves internal fixation hardware that can generate significant artifacts in reconstructed images. To address this challenge, we develop a novel algorithm that combines simultaneous reconstruction-decomposition using a previously reported method for model-based material decomposition (MBMD) augmented by the known-component (KC) reconstruction framework to mitigate metal artifacts. We apply the proposed algorithm to simulated DE data representative of a dedicated extremity cone-beam CT (CBCT) employing an x-ray unit with three vertically arranged sources. The scanner generates DE data with non-coinciding high- and low-energy projection rays when the central source is operated at high tube potential and the peripheral sources at low potential. The proposed algorithm was validated using a digital extremity phantom containing varying concentrations of Ca-water mixtures and Ti implants. Decomposition accuracy was compared to MBMD without the KC model. The proposed method suppressed metal artifacts and yielded estimated Ca concentrations that approached the reconstructions of an implant-free phantom for most mixture regions. In the vicinity of simple components, the errors of Ca density estimates obtained by incorporating KC in MBMD were ∼1.5–5× lower than the errors of conventional MBMD; for cases with complex implants, the errors were ∼3–5× lower. In conclusion, the proposed method can achieve accurate bone mineral density measurements in the presence of metal implants using non-coinciding DE projections acquired on a multisource CBCT system.

A Novel Low-Dose Dual-Energy Imaging Method for a Fast-Rotating Gantry-Type CT Scanner

CT scan by use of a beam-filter placed between the x-ray source and the patient allows a single-scan low-dose dual-energy imaging with a minimal hardware modification to the existing CT systems. We have earlier demonstrated the feasibility of such imaging method with a multi-slit beam-filter reciprocating along the direction perpendicular to the CT rotation axis in a cone-beam CT system. However, such method would face mechanical challenges when the beam-filter is supposed to cooperate with a fast-rotating gantry in a diagnostic CT system. In this work, we propose a new scanning method and associated image reconstruction algorithm that can overcome these challenges. We propose to slide a beam-filter that has multi-slit structure with its slits being at a slanted angle with the CT gantry rotation axis during a scan. A streaky pattern would show up in the sinogram domain as a result. Using a notch filter in the Fourier domain of the sinogram, we removed the streaks and reconstructed an image by use of the filtered-backprojection algorithm. The remaining image artifacts were suppressed by applying l0 norm based smoothing. Using this image as a prior, we have reconstructed low- and high-energy CT images in the iterative reconstruction framework. An image-based material decomposition then followed. We conducted a simulation study to test its feasibility using the XCAT phantom and also an experimental study using the Catphan phantom, a head phantom, an iodine-solution phantom, and a monkey in anesthesia, and showed its successful performance in image reconstruction and in material decomposition.

Efficacy of Periapical Radiography and Three Cone-Beam Computed Tomography Systems for Detection of Peri-Implant Dehiscence Defects: An in- Vitro Study.

Background: Early detection of peri-implant bone defects is highly important because these defects eventually lead to gingival recession, bone loss and implant failure.Objective: This study aimed to assess and compare the efficacy of periapical radiography and three CBCT systems for the detection of peri-implant dehiscence defects.Material and Methods: In this vitro study, 124 titanium implants were placed in bovine ribs. The bone pieces were then mounted in boxes in the form of mandible and red dental wax was used to simulate the soft tissue. Crestal bone defects with 2, 3, and 4 mm depth were created in the ribs using a round bur. Periapical and CBCT images were then obtained. Images were investigated by two oral and maxillofacial radiologists twice with a two-week interval. The results were analyzed using chi-square, Kappa coefficient, Cochrane’s Q and McNemar tests as well as the receiver operating characteristic (ROC) curve.Results: The two observers showed good agreement in detection of sound and defective samples on periapical radiographs and CBCT scans. The level of agreement was low in detection of two samples with 2 mm defects on CBCT scans taken with Planmeca and NewTom 3G systems at the time of second assessment. NewTom 3G had the highest sensitivity (68.9%, 74.2% and 86.3%, respectively) and specificity (100% for all three) compared to other systems for detection of 2, 3 and 4 mm crestal bone defects. Conclusion: The inter-observer agreement increased with increase in depth of defects. NewTom 3G had the highest accuracy for detection of crestal bone defects.

Application of a New Type of Natural Calcined Bone Repair Material Combined with Concentrated Growth Factors in Bone Regeneration in Rabbit Critical-Sized Calvarial Defect.

Purpose This study is aimed at investigating bone regeneration in critical-sized defects in rabbit calvarium using a novel nano- (n-) hydroxyapatite hybrid scaffold with concentrated growth factors (CGFs). Methods Twenty-four male adult rabbits were chosen to establish a critical-sized bone defect model and randomly divided into two groups. Two defects of 15 mm diameter each were created in the parietal bone of each animal. Group A had n-hydroxyapatite hybrid scaffold placed in the experimental defect on the right, and the left defect was unfilled as blank. Group B had hydroxyapatite hybrid scaffold mixed with CGF placed in the right defect and CGF on the left. Six animals in each group were sacrificed after 6 and 12 weeks. Cone-beam computed tomography system scanning and hematoxylin and eosin (HE) staining were used to detect osteogenesis within the defects. Results The treatment with n-hydroxyapatite hybrid scaffold along with CGF resulted in a significantly higher amount of new bone at 6 and 12 weeks compared to the treatment with CGF alone and the controls. No apparent inflammation and foreign body reaction were observed through HE staining. Conclusions The new synthesized n-hydroxyapatite hybrid scaffold and CGF can be applied for bone defect regeneration to promote the process to a certain extent.

Positioning accuracy of a single-isocenter multiple targets SRS treatment: A comparison between Varian TrueBeam CBCT and Brainlab ExacTrac.

This study compared the positioning accuracy between cone-beam CT (CBCT) and ExacTrac (ETX) for a single-isocenter multiple target stereotactic radiosurgery (SRS) on two TrueBeam STx systems. A single-isocenter treatment plan was simulated on an anthropomorphic head phantom with six spherical steel ball bearings (BBs). One of the BBs was chosen to be the isocenter. The five off-isocenter targets were located at various distances from the isocenter. MV portal images were generated to evaluate the deviations between the expected and the real center of the targets after CBCT and ETX positioning, respectively. The evaluation of the positioning accuracy for the isocenter target showed that CBCT and ETX positioning provided comparable, sub-millimetric results. Deviations in positioning accuracy were also calculated for all other targets, also showing comparable results for CBCT and ETX. Moreover, our study showed that the deviation between CBCT and ETX positioning were in better agreement for TBSTx1 and deviated slightly higher on TBSTx2 (maximum: 1.23mm at S/I direction), due to a less perfect alignment between the CBCT coordinate system and the ETX coordinate system on TBSTx2 compared to TBSTx1. This study also showed a correlation between the target positioning accuracy and the distance to the isocenter. The positioning accuracy of ETX and CBCT for targets located at isocenter and off-isocenter locations was compared on two treatment machines and found comparable. Our study highlights the importance of a proper calibration procedure, to ensure correct alignment between the CBCT, ETX and machine coordinate systems.

A phantom for testing Cone Beam CTs.

Cone Beam Computed Tomography (CBCT) scanners are becoming more common for dental and maxillofacial/head scanning, but performing image quality tests on these systems is difficult. There are quality assurance (QA) phantoms commercially available but they can be expensive, bulky and not optimised for CBCT imaging limits. Smaller phantoms often lack features that are recommended for testing CBCT systems. A custom made phantom can provide more useful test objects in a more convenient size and at a lower cost. The proposed phantom is called the "Karu" Cone Beam CT Phantom and is constructed with a 3D printed poly lactic acid (PLA) shell, with 3D printed inserts for holding the test details in place. Tests included are geometric accuracy (in three dimensions), Hounsfield Unit (HU) accuracy, low contrast detectability, spatial resolution (using line pairs), and uniformity/artifacts/noise. The phantom was scanned on a number of scanners and was clearly able to differentiate scanners producing poorer quality images from better quality ones. The phantom could be produced for under NZ $2000.

Early Experience of Head and Neck Positioning With a Commercial 3D Surface-Imaging System

We usually use full face and neck thermoplastic masks to immobilize for patients receiving radiation therapy for head and neck cancer. Positioning is confirmed by cone beam CT(CBCT). These masks increase skin dose and make hard to evaluate patient motion during irradiation. In our institute, we used a commercial 3D surface-imaging system (Catalyst-HD, C-RAD AB, Sweden) with open-face immobilization systems in head and neck IMRT. This surface image guided radiation therapy (S-IGRT) system can set up the patient’s position without radiation exposure and evaluate patient motion in real time. The purpose of this study was to evaluate the daily set up accuracy for head and neck cancer with Catalyst-HD at our institution and to evaluate patient motion during irradiation. We analyzed fifteen patients with head and neck cancer (11 male/ 4 female) treated with S-IGRT system. The median age was 67 years (range, 31-87 years). The median radiation dose was 70Gy (range, 54-70Gy) delivered in 1.8-2.0Gy per fractions. The patients were fixed by open-face immobilization systems for S-IGRT system. First, they were set up with S-IGRT system, next the position was corrected by CBCT with bone matching. We analyzed the set-up errors adjusted by CBCT system. We measured patient motion with Catalyst-HD and analyzed maximum motion distance of patient during irradiation. All cases were finished the planned treatment. The 309 daily set up data were analyzed. Setup error was measured as six-dimensional shifts (lateral/vertical/longitudinal/roll/pitch/rotation) with Catalyst-HD system. Median error values were 0.4 ± 1.4mm (range, -5.1 to 5.0mm), -0.3 ± 1.8mm (range, -5.7 to 5.0mm), -0.8 ± 1.5mm (range -6.3 to 4.2 mm), 0.2 ± 1.0 degree (range, -2.8 to 2.4 degree), 0.5 ± 0.9 degree (range, -2.8 to 2.8 degree), -0.4 ± 1.2 degree (range, -2.9 to 2.8 degree). Isocenter gap were 2.4 ± 1.2mm (range, 0.2 to 7.2mm). Median distance of maximum motion distance during irradiation (lateral/vertical/longitudinal/roll/pitch/rotation) were 1.6 ± 1.9mm (range, -4.3 to 7.5mm), -0.8 ± 1.3mm (range, -2.9 to 3.9mm), 0.0 ± 1.9mm(range, -6.9 to 7.2mm), -0.2 ± 0.8 degree (range, -2.9 to 4.1 degree), 0.0 ± 1.2 degree (range, -6.7 to 3.1 degree), 0.2 ± 0.8 degree (range, -1 to 4.1 degree). S-IGRT for Head and neck cancers fixed by open-face immobilization systems made set up accuracy acceptable to irradiation without radiation exposure. This system may be useful for dose-free set up as highly precise treatment.

Assessment of Intra-fraction Motion during Frameless Stereotactic Radiosurgery

Frameless stereotactic radiosurgery (SRS) is facilitated by cone-beam CT (CBCT) imaging, online re-planning, and intra-fraction motion monitoring (IFMM). The purpose of this study was to evaluate the intra-fraction motion detected using frameless immobilization for SRS through the CBCT and IFMM systems. Patients treated with frameless SRS over a 2-month study period were included for analysis. For SRS, patients are immobilized in a personalized thermoplastic mask and headrest followed by attachment of a reflective marker on their nose. The patient’s treatment position is verified using a localization CBCT against the reference CBCT acquired at simulation, while the IFMM system continuously monitors their nose marker. Repeat localization CBCTs were acquired during treatment if IFMM thresholds (1.5mm) were exceeded or for voluntary patient breaks. Where feasible, a post-treatment CBCT was acquired after treatment delivery. Using bony anatomy for image registration, the difference between localization and post CBCTs, and localization and repeat localization CBCTs (in situations where IFMM thresholds were exceeded) were used to quantify intra-fraction motion. The IFMM data associated with the time points of the CBCTs were also extracted to assess intra-fraction motion. Thirty plans were reviewed from 26 patients (19 single fraction, 11 multi-fraction, 4 patients treated concurrently with single and multi-fractionation). Over 52 treatment fractions, 113 localization, repeat localization and post-treatment CBCTs were acquired. 28 sets of localization-post CBCTs and 25 sets of localization-repeat localization CBCTs were included for analysis. The average ± standard deviation intra-fraction motion quantified on CBCTs was -0.11±0.37mm, -0.13±0.20mm, 0.18±0.74mm in the left/right (L/R), anterior/posterior (A/P) and superior/inferior (S/I) axes. The average ± standard deviation rotational displacement measured on CBCTs was 0.07±0.56⁰, -0.14±0.64⁰, 0.02±0.54⁰ in pitch, yaw and roll. The average ± standard deviation intra-fraction motion quantified on IFMM was -0.09±0.68mm, -0.11±0.5mm, 0.25±0.76mm in the L/R, A/P and S/I axes. There was a positive correlation between the measured vector CBCT and IFMM displacements (r = 0.49). Preliminary analysis indicate the largest intra-fraction motion, as measured by both CBCT and IFMM systems for frameless GK-SRS, is in the S/I axis. Positional deviations noted by the IFMM system during SRS provides a good indication of intra-fraction patient movement.

Optimization for customized trajectories in cone beam computed tomography

PurposeWe developed a target‐based cone beam computed tomography (CBCT) imaging framework for optimizing an unconstrained three dimensional (3D) source‐detector trajectory by incorporating prior image information. Our main aim is to enable a CBCT system to provide topical information about the target using a limited angle noncircular scan orbit with a minimal number of projections. Such a customized trajectory should include enough information to sufficiently reconstruct a particular volume of interest (VOI) under kinematic constraints, which may result from the patient size or additional surgical or radiation therapy‐related equipment.MethodsA patient‐specific model from a prior diagnostic computed tomography (CT) volume is used as a digital phantom for CBCT trajectory simulations. Selection of the best projection views is accomplished through maximizing an objective function fed by the imaging quality provided by different x‐ray positions on the digital phantom data. The final optimized trajectory includes a limited angular range and a minimal number of projections which can be applied to a C‐arm device capable of general source‐detector positioning. The performance of the proposed framework is investigated in experiments involving an in‐house‐built box phantom including spherical targets as well as an Alderson‐Rando head phantom. In order to quantify the image quality of the reconstructed image, we use the average full‐width‐half‐maximum (FWHMavg) for the spherical target and feature similarity index (FSIM), universal quality index (UQI), and contrast‐to‐noise ratio (CNR) for an anatomical target.ResultsOur experiments based on both the box and head phantom showed that optimized trajectories could achieve a comparable image quality in the VOI with respect to the standard C‐arm circular CBCT while using approximately one quarter of projections. We achieved a relative deviation <7% for FWHMavg between the reconstructed images from the optimized trajectories and the standard C‐arm CBCT for all spherical targets. Furthermore, for the anatomical target, the relative deviation of FSIM, UQI, and CNR between the reconstructed image related to the proposed trajectory and the standard C‐arm circular CBCT was found to be 5.06%, 6.89%, and 8.64%, respectively. We also compared our proposed trajectories to circular trajectories with equivalent angular sampling as the optimized trajectories. Our results show that optimized trajectories can outperform simple partial circular trajectories in the VOI in term of image quality. Typically, an angular range between 116° and 152° was used for the optimized trajectories.ConclusionWe demonstrated that applying limited angle noncircular trajectories with optimized orientations in 3D space can provide a suitable image quality for particular image targets and has a potential for limited angle and low‐dose CBCT‐based interventions under strong spatial constraints.

Diagnostic paths for a mouth-breathing patient

The clinical choice of diagnostic tests or treatment options is determined by the probability that the value of their execution (called the warrant for the test) exceeds their cost, and by their usefulness. The purpose of this study was to determine the warrant and usefulness of STOP-Bang, an obstructive sleep apnea screening questionnaire, and cone-beam computed tomography (CBCT) information about the minimal cross-sectional area for referring a mouth-breathing patient to a sleep specialist and for modifying planned orthodontic treatment. A branching survey was used to identify the prominence of paths between the presenting situation, 2 diagnostic tests, and 2 referral and/or treatment options. A description was given of a hypothetical patient: an overweight, mouth-breathing female teenager. Path analysis was used as a method for quantifying diagnostic warrant and usefulness. There was a wide variation among the 125 orthodontists who responded to the survey. All paths were chosen. The use of tests altered the referral (χ2=8.039; P=0.03) and/or treatment decisions (χ2=12.636; P=0.005). Ownership of a CBCT system significantly influenced the use of this diagnostic test, with owning a CBCT system resulting in greater use in-office (χ2=50.416; P<0.001) and greater use in the study (χ2=22.959; P<0.001). The usefulness of the diagnostic tests could not be determined directly because common values were used for each test, but the variation in the use of this standard stimulus was very large, indicating personal differences in the interpretation of actual data. Wide variation in the choice and interpretation of diagnostic tests for referral and orthodontic treatment modification relative to airway condition exists among orthodontists. Diagnostic path analysis is a potentially useful model for studying how practitioners make decisions independent of research evidence.

Short Scan Source-detector Trajectories for Target-based CBCT.

We proposed a target-based cone beam computed tomography (CBCT) imaging framework in order to optimize a free three dimensional (3D) source-detector trajectory by incorporating prior 3D image data. We aim to enable CBCT systems to provide topical information about a region of interest (ROI) using a short-scan trajectory with a reduced number of projections. The best projection views are selected by maximizing an objective function fed by the image quality by means of applying different x-ray positions on the digital phantom data. Finally, an optimized trajectory is selected which is applied to a C-arm device able to perform general source-detector positioning. An Alderson-Rando head phantom is used in order to investigate the performance of the proposed framework. Our experiments showed that the optimized trajectory could achieve a comparable image quality in the ROI with respect to the reference C-arm CBCT while using approximately one-quarter of projections. An angular range of 156° was used for the optimized trajectory.

A low-cost geometry calibration procedure for a modular cone-beam X-ray CT system

ABSTRACT Accurate knowledge of the acquisition geometry of a CT scanning system is key for high quality tomographic imaging. Unfortunately, in modular X-ray CT setups, geometry misalignment occurs each time the setup is changed, which calls for an efficient calibration procedure to correct for geometric inaccuracies. Although many studies have been dealing with the calibration of X-ray CT systems, these are often specifically designed for one setup and/or expensive. In this work, we explore the possibilities of a low-cost, easy-to-build, and modular phantom, constructed from LEGO bricks, which serves as a structure to hold small metal beads, for geometric calibration of a tomographic X-ray system. By estimating the bead coordinates using deep learning, and minimizing center-to-center distances of the metal beads between measured and reference projection data, geometry parameters are derived. With simulated as well as real experiments, it is shown that the LEGO phantom can be used to accurately estimate the geometry of a modular X-ray CT system.