Effective dose range for dental cone beam computed tomography scanners

https://doi.org/10.1016/j.ejrad.2010.11.028Get rights and content

Abstract

Objective

To estimate the absorbed organ dose and effective dose for a wide range of cone beam computed tomography scanners, using different exposure protocols and geometries.

Materials and methods

Two Alderson Radiation Therapy anthropomorphic phantoms were loaded with LiF detectors (TLD-100 and TLD-100H) which were evenly distributed throughout the head and neck, covering all radiosensitive organs. Measurements were performed on 14 CBCT devices: 3D Accuitomo 170, Galileos Comfort, i-CAT Next Generation, Iluma Elite, Kodak 9000 3D, Kodak 9500, NewTom VG, NewTom VGi, Pax-Uni3D, Picasso Trio, ProMax 3D, Scanora 3D, SkyView, Veraviewepocs 3D. Effective dose was calculated using the ICRP 103 (2007) tissue weighting factors.

Results

Effective dose ranged between 19 and 368 μSv. The largest contributions to the effective dose were from the remainder tissues (37%), salivary glands (24%), and thyroid gland (21%). For all organs, there was a wide range of measured values apparent, due to differences in exposure factors, diameter and height of the primary beam, and positioning of the beam relative to the radiosensitive organs.

Conclusions

The effective dose for different CBCT devices showed a 20-fold range. The results show that a distinction is needed between small-, medium-, and large-field CBCT scanners and protocols, as they are applied to different indication groups, the dose received being strongly related to field size. Furthermore, the dose should always be considered relative to technical and diagnostic image quality, seeing that image quality requirements also differ for patient groups. The results from the current study indicate that the optimisation of dose should be performed by an appropriate selection of exposure parameters and field size, depending on the diagnostic requirements.

Introduction

In recent years, cone beam computed tomography (CBCT) has become a widely accepted radiographic tool for diagnosis, treatment planning and follow-up in dentistry. This modality is also known as digital volume tomography (DVT). CBCT allows the acquisition of three-dimensional volumes of the dental arches and surrounding tissues at a high spatial resolution and a low radiation dose. There are a number of different dental applications that benefit from the use of CBCT, each with specific requirements regarding the size of the acquired volume and the image quality in terms of spatial and contrast resolution [1].

The number of CBCT devices available on the market has increased substantially and new models are being developed and released on a continuous basis. These devices exhibit a wide variability in terms of crucial exposure parameters such as the X-ray spectrum (voltage peak and filtration), X-ray exposure (mA and number of projections) and volume of the exposed field. Also, many devices allow a degree of versatility regarding the exposure, allowing the operator to select certain exposure parameters. It is clear that the range of devices and imaging protocols that are available will result in different absorbed radiation doses for the patient with, to some extent, the amount of dose being reflected in the image quality of the scan. Radiation dose and image quality, together with the size of the field of view (FOV), determine whether or not a certain CBCT imaging protocol from a given device is suitable for a specific dental application by following the generally applied ALARA (As Low As Reasonably Achievable) principle of radiation exposure [2], [3].

To measure the radiation risk for patients from a radiographic modality, the effective dose is still accepted as the most suitable figure of merit, even though alternatives are under consideration [4], [5], [6], [7]. The effective dose is measured in practice using an anthropomorphic phantom, representing the shape and attenuation of an average human, most commonly an adult male [8]. There have been a number of studies measuring the effective dose on dental CBCT using thermoluminescent dosimeters (TLDs) in combination with a human phantom [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. These studies provide some estimation of the range of doses that are obtained from these devices, but are not comparable, seeing that different types of phantoms are used as well as different TLD positioning schemes, with the number of TLDs applied to the different organs often being too low for an accurate and reproducible estimation of the organ and effective doses [11], [12], [13], [14], [15], [16], [17], [18].

The aim of the current study was to perform a broad evaluation of the organ and effective doses obtained from CBCT, using a wide range of devices and imaging protocols.

Section snippets

Materials and methods

To estimate the effective dose for an average adult male, two similar types of anthropomorphic male Alderson Radiation Therapy (ART) phantoms (Radiology Support Devices Inc., CA, USA) were used. They represent an average man (175 cm tall, 73.5 kg) and consist of a polymer mould simulating the bone, embedded in soft tissue equivalent material. They are transected into 2.5 cm thick slices, each containing a grid for TLD placement. The upper 11 slices (i.e. head and neck region) were used for TLD

Results

Due to the large differences in acquired volume, which is one of the main determinants of the effective dose, the results were split up by dividing the CBCT devices into three categories: large FOV (maxillofacial), medium FOV (dentoalveolar) and small FOV (localised). This allows for a fairer comparison between protocols, as different FOV sizes are used for different subsets of patients. It should be noted that some devices allow for a range of field sizes, and can therefore be found in more

Discussion

In the present study, effective dose estimations were performed on a wide range of dental CBCT devices, investigating the difference in dose due to variability in FOV size, tube output and exposure factors.

A large number of TLDs was used to ensure that the measurement was as accurate as possible. The TLDs were positioned throughout the head and neck to correctly cover all radiosensitive organs. By performing measurements on a large number of CBCT devices, differences in dose between the

Acknowledgements

The research leading to these results has received funding from the European Atomic Energy Community's Seventh Framework programme FP7/2007-2011 under grant agreement no. 212246 (SEDENTEXCT: Safety and Efficacy of a New and Emerging Dental X-ray Modality).

The Manchester authors acknowledge the support of the NIHR Manchester Biomedical Research Centre.

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