Development and validation of a hybrid simulation technique for cone beam CT: application to an oral imaging system

Monte Carlo methods for device simulations in radiation therapy

Monte Carlo (MC) simulations play an important role in radiotherapy, especially as a method to evaluate physical properties that are either impossible or difficult to measure. For example, MC simulations (MCSs) are used to aid in the design of radiotherapy devices or to understand their properties. The aim of this article is to review the MC method for device simulations in radiation therapy. After a brief history of the MC method and popular codes in medical physics, we review applications of the MC method to model treatment heads for neutral and charged particle radiation therapy as well as specific in-room devices for imaging and therapy purposes. We conclude by discussing the impact that MCSs had in this field and the role of MC in future device design.

A GPU-accelerated framework for rapid estimation of scanner-specific scatter in CT for virtual imaging trials

Virtual imaging trials (VITs), defined as the process of conducting clinical imaging trials using computer simulations, offer a time- and cost-effective alternative to traditional imaging trials for CT. The clinical potential of VITs hinges on the realism of simulations modeling the image acquisition process, where the accurate scanner-specific simulation of scatter in a time-feasible manner poses a particular challenge. To meet this need, this study proposes, develops, and validates a rapid scatter estimation framework, based on GPU-accelerated Monte Carlo (MC) simulations and denoising methods, for estimating scatter in single source, dual-source, and photon-counting CT. A CT simulator was developed to incorporate parametric models for an anti-scatter grid and a curved energy integrating detector with an energy-dependent response. The scatter estimates from the simulator were validated using physical measurements acquired on a clinical CT system using the standard single-blocker method. The MC simulator was further extended to incorporate a pre-validated model for a PCD and an additional source-detector pair to model cross scatter in dual-source configurations. To estimate scatter with desirable levels of statistical noise using a manageable computational load, two denoising methods using a (1) convolutional neural network and an (2) optimized Gaussian filter were further deployed. The viability of this framework for clinical VITs was assessed by integrating it with a scanner-specific ray-tracer program to simulate images for an image quality (Mercury) and an anthropomorphic phantom (XCAT). The simulated scatter-to-primary ratios agreed with physical measurements within 4.4% ± 10.8% across all projection angles and kVs. The differences of ∼121 HU between images with and without scatter, signifying the importance of scatter for simulating clinical images. The denoising methods preserved the magnitudes and trends observed in the reference scatter distributions, with an averaged rRMSE value of 0.91 and 0.97 for the two methods, respectively. The execution time of ∼30 s for simulating scatter in a single projection with a desirable level of statistical noise indicates a major improvement in performance, making our tool an eligible candidate for conducting extensive VITs spanning multiple patients and scan protocols.

Two-step validation of a Monte Carlo dosimetry framework for general radiology

The Monte Carlo technique is considered gold standard when it comes to patient-specific dosimetry. Any newly developed Monte Carlo simulation framework, however, has to be carefully calibrated and validated prior to its use. For many researchers this is a tedious work. We propose a two-step validation procedure for our newly built Monte Carlo framework and provide all input data to make it feasible for future related application by the wider community. The validation was at first performed by benchmarking against simulation data available in literature. The American Association of Physicists in Medicine (AAPM) report of task group 195 (case 2) was considered most appropriate for our application. Secondly, the framework was calibrated and validated against experimental measurements for trunk X-ray imaging protocols using a water phantom. The dose results obtained from all simulations and measurements were compared. Our Monte Carlo framework proved to agree with literature data, by showing a maximal difference below 4% to the AAPM report. The mean difference with the water phantom measurements was around 7%. The statistical uncertainty for clinical applications of the dosimetry model is expected to be within 10%. This makes it reliable for clinical dose calculations in general radiology. Input data and the described procedure allow for the validation of other Monte Carlo frameworks.

Use of cone beam computed tomography in implant dentistry: current concepts, indications and limitations for clinical practice and research

Diagnostic radiology is an essential component of treatment planning in the field of implant dentistry. This narrative review will present current concepts for the use of cone beam computed tomography imaging, before and after implant placement, in daily clinical practice and research. Guidelines for the selection of three-dimensional imaging will be discussed, and limitations will be highlighted. Current concepts of radiation dose optimization, including novel imaging modalities using low-dose protocols, will be presented. For preoperative cross-sectional imaging, data are still not available which demonstrate that cone beam computed tomography results in fewer intraoperative complications such as nerve damage or bleeding incidents, or that implants inserted using preoperative cone beam computed tomography data sets for planning purposes will exhibit higher survival or success rates. The use of cone beam computed tomography following the insertion of dental implants should be restricted to specific postoperative complications, such as damage of neurovascular structures or postoperative infections in relation to the maxillary sinus. Regarding peri-implantitis, the diagnosis and severity of the disease should be evaluated primarily based on clinical parameters and on radiological findings based on periapical radiographs (two dimensional). The use of cone beam computed tomography scans in clinical research might not yield any evident beneficial effect for the patient included. As many of the cone beam computed tomography scans performed for research have no direct therapeutic consequence, dose optimization measures should be implemented by using appropriate exposure parameters and by reducing the field of view to the actual region of interest.

Rotating and translating anthropomorphic head voxel models to establish an horizontal Frankfort plane for dental CBCT Monte Carlo simulations: a dose comparison study

In order to carry out Monte Carlo (MC) dosimetry studies, voxel phantoms, modeling human anatomy, and organ-based segmentation of CT image data sets are applied to simulation frameworks. The resulting voxel phantoms preserve patient CT acquisition geometry; in the case of head voxel models built upon head CT images, the head support with which CT scanners are equipped introduces an inclination to the head, and hence to the head voxel model. In dental cone beam CT (CBCT) imaging, patients are always positioned in such a way that the Frankfort line is horizontal, implying that there is no head inclination. The orientation of the head is important, as it influences the distance of critical radiosensitive organs like the thyroid and the esophagus from the x-ray tube. This work aims to propose a procedure to adjust head voxel phantom orientation, and to investigate the impact of head inclination on organ doses in dental CBCT MC dosimetry studies. The female adult ICRP, and three in-house-built paediatric voxel phantoms were in this study. An EGSnrc MC framework was employed to simulate two commonly used protocols; a Morita Accuitomo 170 dental CBCT scanner (FOVs: 60  ×  60 mm² and 80  ×  80 mm², standard resolution), and a 3D Teeth protocol (FOV: 100  ×  90 mm²) in a Planmeca Promax 3D MAX scanner. Result analysis revealed large absorbed organ dose differences in radiosensitive organs between the original and the geometrically corrected voxel models of this study, ranging from  -45.6% to 39.3%. Therefore, accurate dental CBCT MC dose calculations require geometrical adjustments to be applied to head voxel models.

CUSTOMISATION OF A MONTE CARLO DOSIMETRY TOOL FOR DENTAL CONE-BEAM CT SYSTEMS

A versatile EGSnrc Monte Carlo (MC) framework, initially designed to explicitly simulate X-ray tubes and record the output data into phase space data files, was modified towards dental cone-beam computed tomography (CBCT) dosimetric applications by introducing equivalent sources. Half value layer (HVL) measurements were conducted to specify protocol-specific energy spectra. Air kerma measurements were carried out with an ionisation chamber positioned against the X-ray tube to obtain the total filtration attenuation characteristics. The framework is applicable to bowtie and non-bowtie inherent filtrations, and it accounts for the anode heel effect and the total filtration of the tube housing. The code was adjusted to the Promax 3D Max (Planmeca, Helsinki, Finland) dental CBCT scanner. For each clinical protocol, calibration factors were produced to allow absolute MC dose calculations. The framework was validated by comparing MC calculated doses and measured doses in a cylindrical water phantom. Validation results demonstrate the reliability of the framework for dental CBCT dosimetry purposes.

Effective radiation dose and eye lens dose in dental cone beam CT: effect of field of view and angle of rotation

OBJECTIVE

To quantify the effect of field of view (FOV) and angle of rotation on radiation dose in dental cone beam CT (CBCT) and to define a preliminary volume-dose model.

METHODS

Organ and effective doses were estimated using 148 thermoluminescent dosemeters placed in an anthropomorphic phantom. Dose measurements were undertaken on a 3D Accuitomo 170 dental CBCT unit (J. Morita, Kyoto, Japan) using six FOVs as well as full-rotation (360°) and half-rotation (180°) protocols.

RESULTS

For the 360° rotation protocols, effective dose ranged between 54 µSv (4 × 4 cm, upper canine) and 303 µSv (17 × 12 cm, maxillofacial). An empirical relationship between FOV dimension and effective dose was derived. The use of a 180° rotation resulted in an average dose reduction of 45% compared with a 360° rotation. Eye lens doses ranged between 95 and 6861 µGy.

CONCLUSION

Significant dose reduction can be achieved by reducing the FOV size, particularly the FOV height, of CBCT examinations to the actual region of interest. In some cases, a 180° rotation can be preferred, as it has the added value of reducing the scan time. Eye lens doses should be reduced by decreasing the height of the FOV rather than using inferior FOV positioning, as the latter would increase the effective dose considerably.

ADVANCES IN KNOWLEDGE

The effect of the FOV and rotation angle on the effective dose in dental CBCT was quantified. The dominant effect of FOV height was demonstrated. A preliminary model has been proposed, which could be used to predict effective dose as a function of FOV size and position.

Monte Carlo modeling for dose assessment in cone beam CT for oral and maxillofacial applications

PURPOSE

To adjust Monte Carlo modeling for dose assessment in dedicated cone beam computed tomography (CBCT) of the oral and maxillofacial region.

METHODS

Two different CBCT systems with different fields of view (FOVs), beam qualities, and scan geometries were modeled using Monte Carlo simulation. Dose calculations for typical CBCT examinations were performed with the head and neck part of four computational anatomical phantoms.

RESULTS

Simulation results compared favorably to values acquired experimentally using physical phantoms (in the literature). For a given phantom scanned with 90 kV and 60 × 60 mm FOV, effective dose per mAs was on average the same for the two different systems. Exposing the four phantoms under identical settings for the same CBCT system resulted in variations in organ doses of greater than 100%, leading to differences in effective dose of 30%. For one system, the dose dependence on the operating tube potential can be described with a quadratic polynomial function. Dose distributions over the axial plane were presented as contour plots.

CONCLUSIONS

Monte Carlo modeling is an efficient and accurate means of evaluating dose distributions for dedicated cone beam oral and maxillofacial CT. Results suggest large dose differences among patients undergoing the same examination on a given system, supporting approaches toward patient specific dosimetry. A dedicated and standardized computational phantom for head and neck dosimetry should be established.

Bowtie filtration for dedicated cone beam CT of the head and neck: a simulation study

OBJECTIVE

To investigate the influence of bowtie filtration on dedicated cone beam CT (CBCT) of the head and neck.

METHODS

A validated hybrid simulation technique was used to model a commercial CBCT system with offset scanning geometry, 90 kV tube potential and 145×75 mm imaging field of view. Three bowtie filters were formulated to produce uniform flux intensity in the projection image of cylindrical objects of diameter 14, 16 and 18 cm. The influence of these simulated filters was compared with the original flat filtration in terms of the output radiation field, the dose delivered to the object, the scatter distribution in projections and the quality of the reconstructed image.

RESULTS

Compared against flat filtration, dose reduction for the bowtie case, examined as a function of radial distance within a 16-cm-diameter water cylinder, varied from 8.7% at the centre to 53.8% at the periphery. Scatter reduction, quantified using scatter-to-primary ratio in projection images, was up to 37.6% for a 14-cm-diameter cylindrical contrast phantom. Using the supplied routine image reconstruction, bowtie filtration resulted in comparable noise appearance, contrast resolution and artefact pattern for computational anatomical phantoms, with <5% difference in contrast-to-noise ratio.

CONCLUSION

Bowtie filtration can effectively reduce the dose and scatter in CBCT of the head and neck. For better image quality, corresponding modification to the image pre-processing and reconstruction is needed.

ADVANCES IN KNOWLEDGE

The hybrid simulation approach can usefully explore the impact of proposed system component and design changes.

Dosimetry of a cone beam CT device for oral and maxillofacial radiology using Monte Carlo techniques and ICRP adult reference computational phantoms

OBJECTIVES

The aim of this study was to calculate organ and effective doses for a range of available protocols in a particular cone beam CT (CBCT) scanner dedicated to dentistry and to derive effective dose conversion factors.

METHODS

Monte Carlo simulations were used to calculate organ and effective doses using the International Commission on Radiological Protection voxel adult male and female reference phantoms (AM and AF) in an i-CAT CBCT. Nine different fields of view (FOVs) were simulated considering full- and half-rotation modes, and also a high-resolution acquisition for a particular protocol. Dose-area product (DAP) was measured.

RESULTS

Dose to organs varied for the different FOVs, usually being higher in the AF phantom. For 360°, effective doses were in the range of 25-66 μSv, and 46 μSv for full head. Higher contributions to the effective dose corresponded to the remainder (31%; 27-36 range), salivary glands (23%; 20-29%), thyroid (13%; 8-17%), red bone marrow (10%; 9-11%) and oesophagus (7%; 4-10%). The high-resolution protocol doubled the standard resolution doses. DAP values were between 181 mGy cm(2) and 556 mGy cm(2) for 360°. For 180° protocols, dose to organs, effective dose and DAP were approximately 40% lower. A conversion factor (DAP to effective dose) of 0.130 ± 0.006 μSv mGy(-1) cm(-2) was derived for all the protocols, excluding full head. A wide variation in dose to eye lens and thyroid was found when shifting the FOV in the AF phantom.

CONCLUSIONS

Organ and effective doses varied according to field size, acquisition angle and positioning of the beam relative to radiosensitive organs. Good positive correlation between calculated effective dose and measured DAP was found.