Cone-beam Computed Tomography Simulation Research Articles

4D-AirNet: a temporally-resolved CBCT slice reconstruction method synergizing analytical and iterative method with deep learning

Four-dimensional (4D) cone-beam CT (CBCT) reconstructs temporally-resolved phases of 3D volumes often with the same amount of projection data that are meant for reconstructing a single 3D volume. 4D CBCT is a sparse-data problem that is very challenging for high-quality 4D CBCT image reconstruction. Here we develop a new method, namely 4D-AirNet, that synergizes analytical and iterative method with deep learning for high-quality temporally-resolved CBCT slice reconstruction. 4D-AirNet is an unrolling method using the optimization framework of fused analytical and iterative reconstruction (AIR), which is based on proximal forward-backward splitting (PFBS). Three different strategies are developed for 4D-AirNet: random-phase (RP), prior-guided (PG), and all-phase (AP). RP-AirNet and PG-AirNet utilize phase-by-phase training and reconstruction, while PG-AirNet also uses a prior image reconstructed with all-phase projection data. Dense connectivity is built into 4D-AirNet networks for improved reconstruction quality. In contrast, AP-AirNet trains and reconstructs all phases simultaneously. In addition, the joint regularization method of DL and conventional spatiotemporal total variation (TV) is investigated. 4D-AirNet methods were evaluated in comparison with conventional iterative (TV) and deep learning (LEARN) methods, using simulated 2D-t CBCT scans from a lung dataset with various sparse-data levels. The reconstruction results suggest 4D-AirNet methods outperform TV and LEARN, and AP-AirNet provides the best reconstruction quality overall.

Surgically Facilitated Orthodontic Therapy (SFOT): Diagnosis and Indications in Interdisciplinary Dentofacial Therapy Involving Tooth Movement.

What are the key considerations for the interdisciplinary dentofacial therapy (IDT) team in the diagnostic process to evaluate a patient for surgically facilitated orthodontic therapy (SFOT)? SFOT creates a demineralized bone matrix and augments dentoalveolar deficiencies in the management of dentofacial disharmony malocclusion. Numerous indications and contraindications exist which are essential to the periodontal surgeon for proper case selection. A collaboratively focused workup by an IDT team is equally essential. Cone beam computed tomography (CBCT) and orthodontic simulation software which identifies changes in the dentoalveolar complex influenced by tooth movement are critical in providing transparency to the patient and IDT team during the treatment planning process. Such transparency can aid the team in assessing goals and outcomes that consider and respect foundational dentoalveolar parameters while contributing to sustainable outcomes. The SFOT IDT decision making process is complex, yet full of opportunities. Embracing SFOT IDT with innovative and novel 3D technology can optimize conditions leading to long-term outcomes that align with periodontal stability as well as patient goals and preferences which permeate beyond the achievement of "straightening teeth". Transparency of complex IDT through 3D CBCT and orthodontic simulation software engenders "collaborative accountability" and can improve patient communication which is critical to developing meaningful and essential treatment plans.

VACUUM AND THERMOPLASTIC MOULD-BASED IMMOBILIZATION SYSTEMS USED IN PATIENT UNDERGOING PELVIC RADIATION THERAPY: A COMPARATIVE STUDY

Introduction: The aim of the present study was to compare two immobilization systems for comparison of setup errors in targeted radiotherapy.
 Methods: Retrospective analysis was done for the patients undergoing radiotherapy from May 2012 to December 2018 at our institution. Immobilization was performed on 30 patients sessions (Vacuum cushion i.e., Vac-Lok™ = 15; Thermoplastic mould i.e., Pelvicast pelvic masks = 15). A total of 763 cone-beams were analysed. The target lesion location was verified by cone-beam computed tomography (CBCT) prior to each session, with displacements assessed by CBCT simulation prior to each treatment session. Systematic setup errors, random setup errors, isocenter deviations in the Medio-lateral (ML), Supero-inferior (SI), Antero-posterior (AP), Rotation (yaw) directions of the patient position was calculated.
 Results: On comparing the Vac-Lok™ and Pelvicast pelvic masks group with respect to Systematic and random error in the lateral, longitudinal, vertical and YAW direction, no statistically significant difference was seen except the random error in YAW direction (P=0.037, Unpaired t-test). There was no difference observed in comparing the isocentric deviation.
 Conclusion: It was inferred and concluded that using a vacuum cushion for pelvic radiotherapy provides no added benefit compared to using a thermoplastic mould. Thermoplastic mould is recommended for patients receiving pelvic radiotherapy to improve overall reproducibility.Keywords: Rotational therapy; Radiotherapy; Systematic, random error; Thermoplastic mould; Vacuum cushion.

"Dose of the day" based on cone beam computed tomography and deformable image registration for lung cancer radiotherapy.

PurposeAdaptive radiotherapy (ART) has potential to reduce toxicity and facilitate safe dose escalation. Dose calculations with the planning CT deformed to cone beam CT (CBCT) have shown promise for estimating the “dose of the day”. The purpose of this study is to investigate the “dose of the day” calculation accuracy based on CBCT and deformable image registration (DIR) for lung cancer radiotherapy.MethodsA total of 12 lung cancer patients were identified, for which daily CBCT imaging was performed for treatment positioning. A re‐planning CT (rCT) was acquired after 20 Gy for all patients. A virtual CT (vCT) was created by deforming initial planning CT (pCT) to the simulated CBCT that was generated from deforming CBCT to rCT acquired on the same day. Treatment beams from the initial plan were copied to the vCT and rCT for dose calculation. Dosimetric agreement between vCT‐based and rCT‐based accumulated doses was evaluated using the Bland‐Altman analysis.ResultsMean differences in dose‐volume metrics between vCT and rCT were smaller than 1.5%, and most discrepancies fell within the range of ± 5% for the target volume, lung, esophagus, and heart. For spinal cord Dmax, a large mean difference of −5.55% was observed, which was largely attributed to very limited CBCT image quality (e.g., truncation artifacts).ConclusionThis study demonstrated a reasonable agreement in dose‐volume metrics between dose accumulation based on vCT and rCT, with the exception for cases with poor CBCT image quality. These findings suggest potential utility of vCT for providing a reasonable estimate of the “dose of the day”, and thus facilitating the process of ART for lung cancer.

Automatic pedicle screw planning using atlas-based registration of anatomy and reference trajectories

An algorithm for automatic spinal pedicle screw planning is reported and evaluated in simulation and first clinical studies. A statistical atlas of the lumbar spine (N = 40 members) was constructed for active shape model (ASM) registration of target vertebrae to an unsegmented patient CT. The atlas was augmented to include ‘reference’ trajectories through the pedicles as defined by a spinal neurosurgeon. Following ASM registration, the trajectories are transformed to the patient CT and accumulated to define a patient-specific screw trajectory, diameter, and length. The algorithm was evaluated in leave-one-out analysis (N = 40 members) and for the first time in a clinical study (N = 5 patients undergoing cone-beam CT (CBCT) guided spine surgery), and in simulated low-dose CBCT images. ASM registration achieved (2.0 ± 0.5) mm root-mean-square-error (RMSE) in surface registration in 96% of cases, with outliers owing to limitations in CT image quality (high noise/slice thickness). Trajectory centerlines were conformant to the pedicle in 95% of cases. For all non-breaching trajectories, automatically defined screw diameter and length were similarly conformant to the pedicle and vertebral body (98.7%, Grade A/B). The algorithm performed similarly in CBCT clinical studies (93% centerline and screw conformance) and was consistent at the lowest dose levels tested. Average runtime in planning five-level (lumbar) bilateral screws (ten trajectories) was (312.1 ± 104.0) s. The runtime per level for ASM registration was (41.2 ± 39.9) s, and the runtime per trajectory was (4.1 ± 0.8) s, suggesting a runtime of ~(45.3 ± 39.9) s with a more fully parallelized implementation. The algorithm demonstrated accurate, automatic definition of pedicle screw trajectories, diameter, and length in CT images of the spine without segmentation. The studies support translation to clinical studies in free-hand or robot-assisted spine surgery, quality assurance, and data analytics in which fast trajectory definition is a benefit to workflow.

A data-efficient method for local noise power spectrum (NPS) estimation in FDK-reconstructed 3D cone-beam CT.

For computed tomography (CT) systems in which noise is nonstationary, a local noise power spectrum (NPS) is often needed to characterize its noise property. We have previously developed a data-efficient radial NPS method to estimate the two-dimensional (2D) local NPS for filtered back projection (FBP)-reconstructed fan-beam CT utilizing the polar separability of CT NPS. In this work, we extend this method to estimate three-dimensional (3D) local NPS for feldkamp-davis-kress (FDK)-reconstructed cone-beam CT (CBCT) volumes. Starting from the 2D polar separability, we analyze the CBCT geometry and FDK image reconstruction process to derive the 3D expression of the polar separability for CBCT local NPS. With the polar separability, the 3D local NPS of CBCT can be decomposed into a 2D radial NPS shape function and a one-dimensional (1D) angular amplitude function with certain geometrical transforms. The 2D radial NPS shape function is a global function characterizing the noise correlation structure, while the 1D angular amplitude function is a local function reflecting the varying local noise amplitudes. The 3D radial local NPS method is constructed from the polar separability. We evaluate the accuracy of the 3D radial local NPS method using simulated and real CBCT data by comparing the radial local NPS estimates to a reference local NPS in terms of normalized mean squared error (NMSE) and a task-based performance metric (lesion detectability). In both simulated and physical CBCT examples, a very small NMSE (<5%) was achieved by the radial local NPS method from as few as two scans, while for the traditional local NPS method, about 20 scans were needed to reach this accuracy. The results also showed that the detectability-based system performances computed using the local NPS estimated with the NPS method developed in this work from two scans closely reflected the actual system performance. The polar separability greatly reduces the data dimensionality of the 3D CBCT local NPS. The radial local NPS method developed based on this property is shown to be capable of estimating the 3D local NPS from only two CBCT scans with acceptable accuracy. The minimum data requirement indicates the potential utility of local NPS in CBCT applications even for clinical situations.

Dose area product in estimation of effective dose of the patients undergoing dental cone beam computed tomography examinations

Objective: To investigate the relationship of the effective dose and dose area product (DAP) in dental cone beam computed tomography (CBCT) examinations and to propose conversion factors for estimation of effective doses of the patients using DAP. Dependence of organ doses on DAP was also investigated. Materials and Methods: Different exposure geometries in Cranex3Dx CBCT device were simulated using Monte Carlo simulation and computational anthropomorphic phantom. Then organ doses and effective dose for every exposure geometry was compared to DAP and analysed. Results: The effective dose in all simulated CBCT protocols and positions with 180° tube rotation ranged from 5 μSv for 50 × 50 mm2 field of view (FOV) localised on one tooth using lowest resolution to 265 μSv for the largest FOV and highest resolution. In case of 360° tube rotation the effective dose ranges from 6 to 332 μSv for the same FOV sizes and positions as well as resolutions as in 180° tube rotation. Conclusions: Though the DAP introduces a large uncertainty in the risk measure in dental CBCT, it represents the dose and FOV size which are the most important scanning parameters affecting the dose. To decrease uncertainty in the risk measure, the effective dose has to be estimated for usual clinical positions of the FOV.

Weight-Bearing Cone-Beam CT Scan Assessment of Stability of Supination External Rotation Ankle Fractures in a Cadaver Model

The utility of computed tomography (CT) for measuring medial clear space (MCS) for determination of the stability of supination external rotation (SER) ankle fractures and in comparison to standard radiographs is unknown. We compared MCS on gravity stress (GS) radiographs to GS and weight bearing (WB) cone-beam CT (CBCT). An AO SER 44B3.1 ankle fracture was simulated in 10 human cadavers, also serving as controls. MCS was measured on GS radiographs, GS CBCT, and a simulated WB CBCT scan. Specimens were stable if MCS was <5 mm and unstable if MCS was ≥5 mm. Paired t tests were used to compare MCS from each imaging modality for controls versus SER injuries and stable versus unstable specimens. Compared with controls assessed by GS radiographs, MCS was greater for an SER injury when assessed by GS radiograph and GS CBCT scan within the stable group. Compared with controls assessed by GS radiographs, MCS was greater for SER injuries when assessed by GS radiograph, GS CBCT scan, and WB CBCT within the unstable group. MCS was reduced for stable versus unstable SER injuries assessed by WB CBCT. In a cadaveric model of SER ankle fracture, the medial clear space was statistically significantly greater for the experimental condition when assessed by gravity stress radiograph and gravity stress CBCT scan. Under weight-bearing conditions, the cone-beam CT scanner distinguished between stable and unstable ankles in the experimental condition. This study suggests that a WB cone-beam CT scan may be able to distinguish between stable and unstable SER ankle fractures and influence operative decision making.

Abstract ID: 85 Investigating the physics of a CBCT projection shading correction based on a prior CT

The adoption of cone beam computed tomography (CBCT) image guidance in proton therapy has spurred research on CBCT image correction for dose calculation. Initially, methods based on deformable image registration gained attention [1] , however projection correction approaches based on prior CT information [2] have been shown to perform well for several body sites [3] . The shading correction algorithm used in [2] , [3] relies on image processing of the subtraction of digitally reconstructed radiographs (DRR) of a DIR-registered prior CT, and CBCT projections. In this study we have performed Monte Carlo (MC) simulation of CBCT imaging to disentangle the different sources of projection degradation, and to evaluate the physicality of the shading correction. The GATE MC toolkit’s fixed-forced-detection actor was employed, along with source and detector models optimized for an Elekta XVI CBCT system, to simulate CBCT imaging of an electron density phantom. The shading correction algorithm was applied to the measured projections and the so-called scatter component (SCA) was compared to the MC simulated scatter. Measured and simulated CBCT projections (scatter + primary) agreed well, with the largest discrepancy found for a bone insert (3% of log transformed projections). Important disagreement was observed with the MC scatter signal when the contribution from beam hardening was kept in the SCA. Undoing the beam hardening correction from the SCA using functions derived from MC primary projections and DRRs greatly improved agreement of scatter signals from MC and SCA (3% on average), with the largest discrepancy found for the bone insert (12%). Residual discrepancies were shown to stem from the intrinsic limitations of the SCA. Proton therapy dose calculations on corrected CBCT will be presented in addition to the results above.

Deep-Inspiration Breath-Hold Intensity Modulated Radiation Therapy to the Mediastinum for Lymphoma Patients: Setup Uncertainties and Margins

Patient setup for treating large target volumes can be challenging. In the present study, we measured the local uncertainties in the treatment of mediastinal lymphoma and investigated the need for region-specific planning target volume (PTV) margins. The data from 30 patients who had undergone radiation therapy for mediastinal lymphoma were retrospectively analyzed. A computed tomography (CT)-on-rails (CTOR) system in the treatment room was used for daily image guidance. The total PTV was split into 6 regions: neck, supraclavicular fossa, axilla, mediastinum, upper heart, and lower heart. The total PTV and the 6 local regions were separately aligned to the planning CT scan using automatic rigid registration. The residual local errors using 3 setup strategies were investigated: no image guidance, CTOR setup to total PTV, and simulated cone beam CT setup to the mediastinum. Errors were recorded in the anteroposterior, superoinferior, and right-left directions separately. Using the residual error calculations, the margins required to cover 95% of the clinical target volume for 90% of the patients was estimated. For each patient, 12 to 21 days of daily CTOR data were available for analysis. The residual errors for the total PTV and mediastinum setups were both smaller than those with no image guidance. The lower heart region had more uncertainty with all 3 setup strategies. Margin analysis revealed that the magnitude of the margin is dependent on the imaging strategy, direction, and local region inside the PTV. Margins >7 mm are necessary to account for uncertainty in the neck, lower heart, and axilla regions even under daily CT guidance. Setup uncertainties in the mediastinum are not uniform and are dependent on target location and imaging strategy. However, with the appropriate margin, we can target regions that might not be visualized with the available on-board imager system.

Simultaneous calibration phantom commission and geometry calibration in cone beam CT

Geometry calibration is a vital step for describing the geometry of a cone beam computed tomography (CBCT) system and is a prerequisite for CBCT reconstruction. In current methods, calibration phantom commission and geometry calibration are divided into two independent tasks. Small errors in ball-bearing (BB) positioning in the phantom-making step will severely degrade the quality of phantom calibration. To solve this problem, we propose an integrated method to simultaneously realize geometry phantom commission and geometry calibration. Instead of assuming the accuracy of the geometry phantom, the integrated method considers BB centers in the phantom as an optimized parameter in the workflow. Specifically, an evaluation phantom and the corresponding evaluation contrast index are used to evaluate geometry artifacts for optimizing the BB coordinates in the geometry phantom. After utilizing particle swarm optimization, the CBCT geometry and BB coordinates in the geometry phantom are calibrated accurately and are then directly used for the next geometry calibration task in other CBCT systems. To evaluate the proposed method, both qualitative and quantitative studies were performed on simulated and realistic CBCT data. The spatial resolution of reconstructed images using dental CBCT can reach up to 15 line pair cm−1. The proposed method is also superior to the Wiesent method in experiments. This paper shows that the proposed method is attractive for simultaneous and accurate geometry phantom commission and geometry calibration.

Motion compensation in extremity cone-beam CT using a penalized image sharpness criterion

Cone-beam CT (CBCT) for musculoskeletal imaging would benefit from a method to reduce the effects of involuntary patient motion. In particular, the continuing improvement in spatial resolution of CBCT may enable tasks such as quantitative assessment of bone microarchitecture (0.1 mm–0.2 mm detail size), where even subtle, sub-mm motion blur might be detrimental. We propose a purely image based motion compensation method that requires no fiducials, tracking hardware or prior images. A statistical optimization algorithm (CMA-ES) is used to estimate a motion trajectory that optimizes an objective function consisting of an image sharpness criterion augmented by a regularization term that encourages smooth motion trajectories. The objective function is evaluated using a volume of interest (VOI, e.g. a single bone and surrounding area) where the motion can be assumed to be rigid. More complex motions can be addressed by using multiple VOIs. Gradient variance was found to be a suitable sharpness metric for this application. The performance of the compensation algorithm was evaluated in simulated and experimental CBCT data, and in a clinical dataset. Motion-induced artifacts and blurring were significantly reduced across a broad range of motion amplitudes, from 0.5 mm to 10 mm. Structure similarity index (SSIM) against a static volume was used in the simulation studies to quantify the performance of the motion compensation. In studies with translational motion, the SSIM improved from 0.86 before compensation to 0.97 after compensation for 0.5 mm motion, from 0.8 to 0.94 for 2 mm motion and from 0.52 to 0.87 for 10 mm motion (~70% increase). Similar reduction of artifacts was observed in a benchtop experiment with controlled translational motion of an anthropomorphic hand phantom, where SSIM (against a reconstruction of a static phantom) improved from 0.3 to 0.8 for 10 mm motion. Application to a clinical dataset of a lower extremity showed dramatic reduction of streaks and improvement in delineation of tissue boundaries and trabecular structures throughout the whole volume. The proposed method will support new applications of extremity CBCT in areas where patient motion may not be sufficiently managed by immobilization, such as imaging under load and quantitative assessment of subchondral bone architecture.

WE‐AB‐207A‐12: HLCC Based Quantitative Evaluation Method of Image Artifact in Dental CBCT

Purpose:Image artifacts are usually evaluated qualitatively via visual observation of the reconstructed images, which is susceptible to subjective factors due to the lack of an objective evaluation criterion. In this work, we propose a Helgason‐Ludwig consistency condition (HLCC) based evaluation method to quantify the severity level of different image artifacts in dental CBCT.Methods:Our evaluation method consists of four step: 1) Acquire Cone beam CT(CBCT) projection; 2) Convert 3D CBCT projection to fan‐beam projection by extracting its central plane projection; 3) Convert fan‐beam projection to parallel‐beam projection utilizing sinogram‐based rebinning algorithm or detail‐based rebinning algorithm; 4) Obtain HLCC profile by integrating parallel‐beam projection per view and calculate wave percentage and variance of the HLCC profile, which can be used to describe the severity level of image artifacts.Results:Several sets of dental CBCT projections containing only one type of artifact (i.e. geometry, scatter, beam hardening, lag and noise artifact), were simulated using gDRR, a GPU tool developed for efficient, accurate, and realistic simulation of CBCT Projections. These simulated CBCT projections were used to test our proposed method. HLCC profile wave percentage and variance induced by geometry distortion are about 3∼21 times and 16∼393 times as large as that of the artifact‐free projection, respectively. The increase factor of wave percentage and variance are 6 and133 times for beam hardening, 19 and 1184 times for scatter, and 4 and16 times for lag artifacts, respectively. In contrast, for noisy projection the wave percentage, variance and inconsistency level are almost the same with those of the noise‐free one.Conclusion:We have proposed a quantitative evaluation method of image artifact based on HLCC theory. According to our simulation results, the severity of different artifact types is found to be in a following order: Scatter&gt;Geometry&gt;Beam hardening&gt;Lag&gt;Noise&gt;Artifact‐free in dental CBCT

Effects of bladder and rectum management on dose distributions of target and organs at risk in intensity-modulated radiotherapy for prostate cancer

Objective To observe the motions of the rectum and bladder by image-guided radiotherapy (IGRT) and to analyze their impact on treatment. Methods Eighteen patients with prostate cancer undergoing intensity-modulated radiotherapy (IMRT) were enrolled in the study and 247 cone-beam computed tomography (CBCT) images were obtained from this study. The clinical target volume, bladder, and rectum were contoured on all simulated CT and CBCT to examine their volume and position changes. The dose distributions were recalculated based on the data of the x-, y-, and z-axis setup errors. The doses to planning target volume (PTV) and organs at risk were calculated in the replanning, and their impact on treatment was analyzed. Comparison of the planning and replanning results was made by paired t-test. The effects of displacements and volumes of the bladder and rectum on target doses were analyzed by Pearson correlation method. Results Great changes in the volumes of the bladder and rectum were observed during the treatment. For the planning and replanning results, PTVD95% was 7777.37 cGy vs. 7628.56 cGy (P=0.027), PTVDmin was 87.91 cGy vs. 83.35 cGy (P=0.000), and RVP was 5.89% vs. 8.31%(P=0.000). There were correlations between PTVD95% and the motions of the bladder and rectum, with correlation coefficients of 0.296 and 0.177, respectively. The correlation coefficient between rectal volume and PTVD95% was 0.115, indicating a certain correlation. There is a certain correlation between and PTVDmin and bladder volume, with a correlation coefficient of-0.128. Conclusions The recovery of the state during localization for the bladder and rectum, especially the latter, has great significance to ensure the target dose and reduce exposure of the rectum in the IMRT for prostate cancer. Key words: Prostate neoplasms/intensity modulated radiotherapy; Image-guided radiotherapy; Alignment; Dosemetry

Geant4 simulation of the Elekta XVI kV CBCT unit for accurate description of potential late toxicity effects of image-guided radiotherapy

This paper describes the modelisation of the Elekta XVI Cone Beam Computed Tomography (CBCT) machine components with Geant4 and its validation against calibration data taken for two commonly used machine setups. Preliminary dose maps of simulated CBCTs coming from this modelisation work are presented. This study is the first step of a research project, GHOST, aiming to improve the understanding of late toxicity risk in external beam radiotherapy patients by simulating dose depositions integrated from different sources (imaging, treatment beam) over the entire treatment plan. The second cancer risk will then be derived from different models relating irradiation dose and second cancer risk.

Depth determination of artificial periodontal pockets using cone-beam tomography and radio-opaque material: an in vitro feasibility study.

In general, periodontal tissues are clinically assessed using calibrated periodontal probes and radiographs. Due to technical developments and the availability of cone-beam computed tomography (CBCT), 3-D imaging has become feasible and offers some advantages and potential for the evaluation of complex anatomical structures. The present pilot study illustrates and validates the possibility of radiographically visualizing and metrically assessing hard and soft tissue. Artificial periodontal pockets were created in porcine mandibles and measured by clinical (i.e. pocket probing) and radiographic means (CBCT). For the latter method, pockets were filled with a radiopaque material allowing visualization by CBCT. Clinically simulated pocket depth probing and CBCT measurements were compared. The results showed no statistically significant differences between the two methods. Thus, the CBCT visualization approach points towards the development of a new and promising radiographic all- in-one evaluation system of the periodontal status. However, more research and development is required.

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.

Comparison of simulated cone beam computed tomography to conventional helical computed tomography for imaging of rhinosinusitis

Cone beam computed tomography (CBCT) has emerged as a low radiation dose alternative to traditional computed tomography (CT) to evaluate the paranasal sinuses. The purpose of our study was to determine how often clinically important findings would be missed if CBCT was used routinely for sinus imaging. Retrospective review. We evaluated all maxillofacial CT scans performed for sinusitis over a 1-year period. The original multidetector CT (MDCT) images were reviewed retrospectively. A theoretical CBCT was then created from the original study utilizing only the bone algorithm images and separately reviewed. We calculated the proportion of abnormal findings that were identified on the MDCT but would have been missed by the theoretical CBCT, and reviewed the medical record to determine which potentially missed findings would have changed management. Radiation dose from the MDCT scanners was calculated and compared to published dose estimates for the paranasal sinuses on CBCT. Maxillofacial CTs from 361 consecutive patients were included, of which 12 (3.3%) demonstrated findings that would have been missed on the theoretical CBCT. Of those, four (1.1%) would have resulted in a change in management. Effective radiation dose for our scanners ranged from 0.67 mSv to 2.15 mSv, compared to a published estimated dose of 0.2 mSV for CBCT. In the majority of patients undergoing simulated CBCT for evaluation of sinusitis, incidental findings of soft-tissue disease were rare. With appropriate selection of patients, CBCT can offer substantial radiation dose reduction and may provide a viable alternative to standard MDCT sinus imaging protocols.

Depth determination of artificial periodontal pockets using cone-beam tomography and radio-opaque material: an in vitro feasibility study.

In general, periodontal tissues are clinically assessed using calibrated periodontal probes and radiographs. Due to technical developments and the availability of cone-beam computed tomography (CBCT), 3-D imaging has become feasible and offers some advantages and potential for the evaluation of complex anatomical structures. The present pilot study illustrates and validates the possibility of radiographically visualizing and metrically assessing hard and soft tissue. Artificial periodontal pockets were created in porcine mandibles and measured by clinical (i.e. pocket probing) and radiographic means (CBCT). For the latter method, pockets were filled with a radiopaque material allowing visualization by CBCT. Clinically simulated pocket depth probing and CBCT measurements were compared. The results showed no statistically significant differences between the two methods. Thus, the CBCT visualization approach points towards the development of a new and promising radiographic all- in-one evaluation system of the periodontal status. However, more research and development is required.

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.