Evaluation of referential dosages obtained by Cone-Beam Computed Tomography examinations acquired with different voxel sizes

Dental Press J Orthod 42 2010 Sept-Oct;15(5):42-3 * Access www.dentalpress.com.br/journal to read the full article. Editor’s summary The voxel size, the smallest unit of a ConeBeam Computed Tomography (CBCT) image, is related to the definition of tomographic image. The question raised by the authors of this study is whether voxel size can affect radiation dose during CT scanning. Measurement of dose-area product (DAP) and entrance skin dose (ESD) when Evaluation of referential dosages obtained by Cone-Beam Computed Tomography examinations acquired with different voxel sizes

obtaining CBCT images with an i-CAT (Imaging Sciences International, Hatfield, PA, USA) was performed according to the protocols specified in Table 1.In all protocols, the field of view (collimation) of the scan was equivalent to 6 cm.The tests were repeated four times for each protocol.
The median DAP and ESD values found for the four protocols are shown in Table 2.A significant difference (p <0.001) was found among the four protocols for the two radiation dose assessment methods.The size of the voxel by itself did not influence the exposed radiation dose.When the exposure factors (TE, kVp and mAs) are maintained, simply changing the voxel size does not influence the radiation dose significantly.However, the protocols correlate the use of smaller voxels with greater milliamperage exposure times, which invariably increases the exposure dose.
Questions to the authors 1) Which of the image acquisition protocols you tested is the most cost-effective?Why?Not only this but other studies have shown that the protocol using a 0.3 mm voxel offers a combination of good resolution and reduced radiation dose.It is therefore the most costeffective.
2) Does the size of the field of view (FOV) used in Cone-Beam CT examinations influence the radiation dose?
Yes. Especially when it comes to kerma area product (KAP), which increases the probability of stochastic effects.However, in our study, no influence was observed because we used the same FOV in all incidences and measurements.But, for example, in CBCT scans with a reduced FOV or restricted to measurement levels by sextants, the dose received is significantly reduced, implying very specific indications.
3) Do studies of radiation dose with Cone-Beam CT pose any difficulties or limitations?
Yes, researchers are still seeking a dosimetric quantity and/or a methodology that allows CBCT exposures to be assessed in order to estimate stochastic effects and compare exposures with other technologies.This is only made possible thanks to the volumetric acquisition and advanced technology of CBCT equipment.puted Tomography (CBCT), has recently become available.0][11][12] The voxel size is lower on CBCT compared with conventional CT.On the i-CAT device, for example, the voxel size can vary from 0.12 to 0.4 mm for the acquisition of images from the mandible, whereas on conventional CT the voxel size is normally 0.5-1 mm. 6,13Generally, the smaller the voxel size and the longer the scanning time, the better the resolution and the details.However, a smaller voxel size is associated with a longer scanning time, which has some disadvantages such as greater possibility of patient movement during the examination, elevated radiation doses and longer reconstruction time. 14,15he aim of this study was to evaluate the dosage area product (DAP) and entrance skin dose (ESD), using protocols with different voxel sizes, using the i-CAT CBCT device, to determine better parameters based on radioprotection principles.

MAteriAls And Methods
The DAP and ESD measurements using CBCT images from the i-CAT device (Imaging Sciences International, Hatfield, PA) were performed according to the protocols in Table 1.The scan height (collimation) was 6 cm for all protocols.The examinations were repeated four times for each protocol.
The RADCAL 9095 dose meter (Radcal.Corp., Monrovia, CA, USA) and the PTW DAP meter (PTW, Freiburg, Germany) were used.All equipment was calibrated in laboratories within the Brazilian Metrology Network (Rede Brasileira de Metrologia-RBM).A pencil-type ioniza-tion chamber (100 mm) was fixed on one end of the tomograph, coupled to an eletrometer, so that it was possible to measure the doses given while the images were obtained (ESD).A multiplicative factor calculation was performed based on the distance between the x-ray beam output and the sensor, to compensate for the distance from the center of the device to the position of the ionization chamber.For the DAP measurement, a PTW device was coupled to the other end of the device.
The Kruskal-Wallis and Dunn tests were used to assess the data; p<0.001 was considered statistically significant.

results
The median values for ESD and DAP for the four protocols are shown in Table 2. Statistically significant differences (p<0.001) were found among the four protocols for both radiation dose evaluation methods.
Dunn's test showed that in the DAP evaluation, protocols 2 and 3 showed a statistically significant difference, and it was not possible to detect which of the protocols were significantly difference in the ESD evaluation.

discussion
CBCT is a new technology and adequate knowledge is necessary to measure the radiation dose.We believe that the proposed method, using the ESD and DAP, can be considered for dose measurements in this type of examination.3 2010 Sept-Oct;15(5):42.e1-4 Protocols 1 and 2 showed very similar ESD and DAP values, and even though the voxel sizes were different, the exposure time (ET), the kilovoltage (kVp) and the milliamperage x exposure time (mAs) remained constant.The same applies to protocols 3 and 4 (Tables 1 and  2).This shows that the voxel size does not influence the radiation dose; that is, when the exposure factors (ET, kVp and mAs) are the same, a single alteration of the voxel size does not alter the radiation dose significantly.However, the protocols couple the use of smaller voxels with greater exposure time and milliamperage, which invariably cause an increase in the exposure dose.Completely pre-established protocols are provided by the i-CAT manufacturer. 15 greater voxel size, associated with a low mAs and reduced ET, is able to reduce the dose by as much as 50%.16 In our study, whereas the ET and mAs practically doubled from protocols 3 and 4 to protocols 1 and 2, the radiation doses (ESD and DAP) behaved similarly for all protocols, being approximately doubled in protocols 1 and 2 compared with protocols 3 and 4 (Tables 1 and 2).The limitation of the Dunn test in presenting significant difference among the protocols and in evaluating ESD occurred because of the small sample.But, despite the small sample, protocols 2 and 3 showed a significant difference between (p=0.0065) for the DAP; this was only possible because of the extremely relevant difference that exists between these protocols.
In conclusion, DAP and ESD are presented as evaluation methods for radiation doses in CBCT, and more studies are necessary to further elucidate such findings.The voxel size alone does not affect the radiation dose in CBCT (i-CAT) examinations.The radiation dose for CBCT (i-CAT) examinations is directly related to the exposure time and milliamperage.

AcKnowledgMents
The authors express sincere gratitude to CAPES (Coordination of Improvement of Higher Education), IFBA (Federal Institute of Technological of Bahia) and Clinica Odontobioimagem, for supporting our projects.

TablE 2 -
Mean values of radiation doses (ESD and DaP) for the four protocols.

TablE 1 -
Protocols for image acquisition for the i-CaT device.

TaBlE 1 -
Protocols for image acquisition for the i-CaT device.

TaBlE 2 -
Mean values of radiation doses (ESD and DaP) for the four protocols.