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

Development and validation of a 3D anthropomorphic phantom for dental CBCT imaging research

BACKGROUND

Optimization of dental cone beam computed tomography (CBCT) imaging is still in a preliminary stage and should be addressed using task-based methods. Dedicated models containing relevant clinical tasks for image quality studies have yet to be developed.

PURPOSE

To present a methodology to develop and validate a virtual adult anthropomorphic voxel phantom for use in task-based image quality optimization studies in dental CBCT imaging research, focusing on root fracture (RF) detection tasks in the presence of metal artefacts.

METHODS

The phantom was developed from a CBCT scan with an isotropic voxel size of 0.2 mm, from which the main dental structures, mandible and maxilla were segmented. The missing large anatomical structures, including the spine, skull and remaining soft tissues, were segmented from a lower resolution full skull scan. Anatomical abnormalities were absent in the areas of interest. Fine detailed dental structures, that could not be segmented due to the limited resolution and noise in the clinical data, were modelled using a-priori anatomical knowledge. Model resolution of the teeth was therefore increased to 0.05 mm. Models of RFs as well as dental restorations to create the artefacts, were developed, and could be inserted in the phantom in any desired configuration. Simulated CBCT images of the models were generated using a newly developed multi-resolution simulation framework that incorporated the geometry, beam quality, noise and spatial resolution characteristics of a real dental CBCT scanner. Ray-tracing and Monte Carlo techniques were used to create the projection images, which were reconstructed using the classical FDK algorithm. Validation of the models was assessed by measurements of different tooth lengths, the pulp volume and the mandible, and comparison with reference values. Additionally, the simulated images were used in a reader study in which two oral radiologists had to score the realism level of the model's normal anatomy, as well as the modelled RFs and restorations.

RESULTS

A model of an adult head, as well as models of RFs and different types of dental restorations were created. Anatomical measurements were consistent with ranges reported in literature. For the tooth length measurements, the deviations from the mean reference values were less than 20%. In 77% of all the measurements, the deviations were within 10.1%. The pulp volumes, and mandible measurements were within one standard deviation of the reference values. Regarding the normal anatomy, both readers considered the realism level of the dental structures to be good. Background structures received a lower realism score due to the lack of detailed enough trabecular bone structure, which was expected but not the focus of this study. All modelled RFs were scored at least adequate by at least one of the readers, both in appearance and position. The realism level of the modelled restorations was considered to be good.

CONCLUSIONS

A methodology was proposed to develop and validate an anthropomorphic voxel phantom for image quality optimization studies in dental CBCT imaging, with a main focus on RF detection tasks. The methodology can be extended further to create more models representative of the clinical population.

A pilot study of half-value layer measurements using a semiconductor dosimeter for intraoral radiography

Purpose

This pilot study was conducted to evaluate half-value layer (HVL) measurements obtained using a semiconductor dosimeter for intraoral radiography.

Materials and Methods

This study included 8 aluminum plates, 4 of which were low-purity (less than 99.9%) and 4 high-purity (greater than 99.9%). Intraoral radiography was performed using an intraoral X-ray unit in accordance with the dental protocol at the authors' affiliated hospital: tube voltage, 60 kVp and 70 kVp; tube current, 7 mA; and exposure time, 0.10 s. The accuracy of HVL measurements for intraoral radiography was assessed using a semiconductor dosimeter. A simple regression analysis was performed to compare the aluminum plate thickness and HVL in relation to the tube voltage (60 kVp and 70 kVp) and aluminum purity (low and high).

Results

For the low-purity aluminum plates, the HVL at 60 kVp (Y) and 70 kVp (Y) was significantly correlated with the thickness of the aluminum plate (X), with Y = 1.708 + 0.415X (r=0.999, P<0.05) and Y = 1.980 + 0.484X (r=0.999, P<0.05), respectively. Similarly, for the high-purity aluminum plates, the HVL at 60 kVp (Y) and 70 kVp (Y) was significantly correlated with the plate thickness (X), with Y = 1.696 + 0.454X (r=0.999, P<0.05) and Y = 1.968 + 0.515X (r=0.998, P<0.05), respectively.

Conclusion

This pilot study examined the relationship between aluminum plate thickness and HVL measurements using a semiconductor dosimeter for intraoral radiography. Semiconductor dosimeters may prove useful in HVL measurement for purposes such as quality assurance in dental X-ray imaging.

Monitoring cone-beam CT radiation dose levels in a University Hospital

OBJECTIVE

To present patient dose levels for different CBCT scanners, acquired by a dose monitoring tool in a University Hospital, as a function of field of view (FOV), operation mode, and patient age.

METHODS

An integrated dose monitoring tool was used to collect radiation exposure data [type of CBCT unit, dose-area product (DAP), FOV size, and operation mode] and patient demographic information (age, referral department) performed on a 3D Accuitomo 170 and a Newtom VGI EVO unit. Effective dose conversion factors were calculated and implemented into the dose monitoring system. For each CBCT unit, the frequency of examinations, clinical indications, and effective dose levels were obtained for different age and FOV groups, and operation modes.

RESULTS

A total of 5163 CBCT examinations were analyzed. Surgical planning and follow-up were the most frequent clinical indications. For the standard operation mode, effective doses ranged from 35.1 to 300 µSv and 9.26-117 µSv using 3D Accuitomo 170 and Newtom VGI EVO, respectively. In general, effective doses decreased with increasing age and FOV size reduction.

CONCLUSIONS

Effective dose levels varied notably between systems and operation modes.Operation mode selection and FOV size were indication-oriented, with larger FOV sizes election serving surgical planning and follow-up. Seeing the influence of FOV size on effective dose levels, manufacturers could be advised to move toward patient-specific collimation and dynamic FOV selection. Systematically monitoring patient doses could be recommended for steering future CBCT optimization.

Outcomes of different radioprotective precautions in children undergoing dental radiography: a systematic review

PURPOSE

To evaluate the effectiveness of all radioprotective measures in underage patients who undergo a dental radiodiagnostic examination.

METHODS

A systematic review was performed including randomised controlled trials (RCTs), or cluster trials, cohort studies, cross-sectional studies, case-control studies and comparative in vitro research. These studies examined the healthy underage human population (below 18 years) undergoing a dental radiodiagnostic examination. All radioprotective measures were included except for justification as an intervention. The primary outcomes were in vivo mortality and morbidity. Some surrogate or indirect outcomes such as in vitro effective dose and organ absorbed doses were also accepted. Secondary outcomes with regards to image quality and therapeutic value were also analysed.

RESULTS

Eighteen papers were eligible for implementation. Fifteen studies underwent narrative synthesis. Regression analysis was performed on three studies.

CONCLUSION

The following radioprotective measures can reduce the exposure dose. For lateral cephalometry: collimation, filtration, the fastest receptor type and circumstantial thyroid shielding. For oblique lateral radiographs: the shortest exposure time, a smaller horizontal angulation, a longer focus to skin distance. For intraoral radiography: rectangular collimation, the fastest image receptor speed and thyroid shielding when the thyroid gland is in line of or very close to the primary beam. For panoramic radiographs: collimation, the fastest receptor type and the use of automatic exposure control (AEC) or manual adjustment of intensity. For cone-beam computed tomography: collimation, the largest voxels size in relation to the treatment need, change in image settings such as ultra-low dose settings, shorter exposure time, a lower amount of projections, lower beam intensity, reduction of the potential, use of a thyroid shield except in two situations and the use of AEC. All of the changes in exposure parameters should be performed while maintaining a sufficient therapeutic value on an individual and indication-based level.

Halve the dose while maintaining image quality in paediatric Cone Beam CT

Cone beam CT (CBCT) for dentomaxillofacial paediatric assessment has been widely used despite the uncertainties of the risks of the low-dose radiation exposures. The aim of this work was to investigate the clinical performance of different CBCT acquisition protocols towards the optimization of paediatric exposures. Custom-made anthropomorphic phantoms were scanned using a CBCT unit in six protocols. CT slices were blinded, randomized and presented to three observers, who scored the image quality using a 4-point scale along with their level of confidence. Sharpness level was also measured using a test object containing an air/PMMA e,dge. The effective dose was calculated by means of a customized Monte Carlo (MC) framework using previously validated paediatric voxels models. The results have shown that the protocols set with smaller voxel size (180 µm), even when decreasing exposure parameters (kVp and mAs), showed high image quality scores and increased sharpness. The MC analysis showed a gradual decrease in effective dose when exposures parameters were reduced, with an emphasis on an average reduction of 45% for the protocol that combined 70 kVp, 16 mAs and 180 µm voxel size. In contrast, both "ultra-low dose" protocols that combined a larger voxel size (400 µm) with lower mAs (7.4 mAs) demonstrated the lowest scores with high levels of confidence unsuitable for an anatomical approach. In conclusion, a significant decrease in the effective dose can be achieved while maintaining the image quality required for paediatric CBCT.

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.

Estimation of the radiation dose for pediatric CBCT indications: a prospective study on ProMax3D

BACKGROUND

An increasing number of CBCT units and a wide variability of radiation doses have been reported in dentistry lately.

AIM

To estimate the effective, cumulative, and organ absorbed doses in children exposed to CBCT over 2 years.

DESIGN

A prospective study was conducted in children who underwent CBCT diagnostic imaging with the ProMax3D machine. Organ and effective doses were calculated by Monte Carlo simulation using 5- and 8-year-old pediatric voxel phantoms. Extrapolation procedures were applied to estimate doses for other ages and CBCT protocols used in clinical conditions.

RESULTS

The median effective dose was 137.9 μSv, and the median cumulative dose was 231.4 μSv. Statistically significant differences in the effective doses and cumulative doses were found for various indications of CBCT in children (P < 0.001). The median absorbed organ dose for brain and thyroid was significantly higher for the clinical condition that required large FOVs (2.5 mGy and 1.05 mGy, respectively) compared to medium (0.19 and 0.51 mGy) and small FOVs (0.07 and 0.24 mGy; P < 0.05). The radiation dose of salivary glands did not vary significantly with FOV.

CONCLUSION

The results revealed the variation of CBCT doses and the influence of FOV size in pediatric exposure.

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.

Development of a paediatric head voxel model database for dosimetric applications

OBJECTIVE

To develop a database of paediatric head voxel models intended for Monte Carlo (MC) dosimetric applications.

METHODS

Seventeen head and neck CT image data sets were retrieved from the picture archiving and communicating system of our hospital and were reformed into voxel models. 22 organs were segmented at each data set. The segmented organ masses were compared to the respective age- and gender-specific ICRP reference mass value. Adjustments were made such that segmented and reference mass values coincide within a tolerance of 10%. A dental cone beam CT cleft palate simulation study was set up to demonstrate the applicability of our database to MC frameworks and to investigate the need for age- and gender-specific paediatric models.

RESULTS

The designed database covers the age range from 2 months to 14 years old. Each model represents a reference head voxel phantom for its corresponding age and gender category. The simulation study revealed absorbed organ dose differences larger than 50% among the 5, 8 and 12 years old models when exposed to identical conditions.

CONCLUSION

Children cannot be represented by one average phantom covering the entire age range like adults due to the fact that their organs change rapidly in size and shape. A database of paediatric head voxel models was designed to enable dose calculations via MC simulations. Advances in knowledge: The application of each model of the database to MC frameworks provides age- and gender-specific organ dose estimations from medical exposures in the head and neck region.

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.