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Year : 2019  |  Volume : 7  |  Issue : 1  |  Page : 6-11

Entrance skin dose of the thyroid gland area following exposure with different protocols of two panoramic and cone-beam computed tomography devices

1 Department of Periodontics, School of Dentistry, Guilan University of Medical Sciences, Rasht, Iran
2 Department of Oral and Maxillofacial Radiology, School of Dentistry, Guilan University of Medical Sciences, Rasht, Iran

Date of Web Publication11-Jun-2019

Correspondence Address:
Negar Khosravifard
Department of Oral and Maxillofacial Radiology, School of Dentistry, Guilan University of Medical Sciences, Rasht, Gilan Province
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jomr.jomr_4_19

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Background: Cone-beam computed tomography (CBCT) has gained considerable use, while radiation burden remains an important concern. Aims: This study aimed at determining the entrance skin dose of the thyroid gland area through exposure with the normal, soft, and hard modes of Vatech (Pax-i) panoramic device as well as different field of views (FOVs) of Vatech (Pax-i 3D) CBCT system. Materials and Methods: Dose measurements were performed on a head-and-neck phantom by an ion chamber dosimeter. Panoramic imaging was done in three normal, soft, and hard modes, each entailing woman, man, and child options. CBCT examinations were performed with various FOVs, each having normal, soft, and hard modes as well as woman, man, and child choices. Doses obtained from different protocols of the two imaging modalities were compared by paired t-test. Results: All FOVs in CBCT resulted in greater radiation dose than panoramic in each of the normal, soft, and hard exposure modes (P < 0.05). Dose amounts for the child and adult modes differed statistically significant in panoramic radiography as well as 80 × 80 and 90 × 120 FOVs of CBCT while in 50 × 50 maxilla, 50 × 50 mandible, 50 × 80 and 150 × 150 FOVs of CBCT, The child and adult doses were almost similar. Conclusions: Even the smallest size of FOV in the soft exposure mode which is expected to have the lowest amount of radiation among CBCT protocols resulted in greater thyroid exposure compared to panoramic examination. Furthermore, care should be taken in the selection of FOV, particularly for CBCT examinations of the children.

Keywords: Cone-beam computed tomography, panoramic radiography, radiation dosage, thyroid gland

How to cite this article:
Saberi BV, Khosravifard N, Mohtavipour T, Khaksari F, Abbasi S, Shahmalakpoor A. Entrance skin dose of the thyroid gland area following exposure with different protocols of two panoramic and cone-beam computed tomography devices. J Oral Maxillofac Radiol 2019;7:6-11

How to cite this URL:
Saberi BV, Khosravifard N, Mohtavipour T, Khaksari F, Abbasi S, Shahmalakpoor A. Entrance skin dose of the thyroid gland area following exposure with different protocols of two panoramic and cone-beam computed tomography devices. J Oral Maxillofac Radiol [serial online] 2019 [cited 2023 Mar 28];7:6-11. Available from: https://www.joomr.org/text.asp?2019/7/1/6/259980

  Introduction Top

One of the main aspects of diagnostic imaging is weighing of potential benefits against risks of radiation exposure. In maxillofacial imaging, the concern is mainly focused on the radiation-sensitive tissues and organs involved, namely, bone marrow and thyroid and salivary glands.[1],[2] Although the amount of radiation used in dentistry is not typically high, the main threat lies in the stochastic effects which follow the linear, nonthreshold type of dose–response relationships.[3],[4],[5],[6] It has been claimed that 1.5%–2% of cancers in the United States are induced by X-ray exposure during diagnostic computed tomography (CT) imaging.[7]

Introduction of cone-beam CT (CBCT) to the field of dentistry has brought about many advantages and improvements owing to its three-dimensional, distortion-free images, relatively low cost, and low radiation dose compared to CT.[8],[9],[10] In recent years, great attempts have been made by CBCT manufacturers to develop devices with larger field of view (FOV) which, although add to the applicability of CBCT, could eventually result in greater amounts of radiation exposure to the patients.[9],[11],[12] In case CBCT is substituted for CT, the patients benefit from lower amounts of radiation; however, when this imaging modality comes in place of conventional radiographic techniques such as the widely used panoramic imaging, higher amounts of radiation hazard imposed on the patients become a challenging issue. Similar to any X-ray imaging procedure, in CBCT devices, the key exposure parameters such as kVp, mA, and exposure time have certain role in determining the amount of radiation exposure and therefore patient dose. Thus, devices that provide operator-controlled exposure parameters could be considered to have superiority over those with fixed exposure settings.[13],[14],[15] Considering the widespread use of CBCT imaging as well as the introduction of devices with variable imaging modes and FOVs, studies on the radiation dose produced by different FOVs of these machines, particularly in comparison with conventional techniques, seem to be necessary. In the present research, we intended to determine the entrance skin dose (ESD) of the thyroid gland area through exposure with two panoramic (Pax-i) and CBCT (Pax-i 3D) systems of Vatech (Gyeonggi, Korea) in different imaging modes provided by the manufacturer.

  Materials and Methods Top

In the present study, a head-and-neck phantom (Parsa radiology head phantom, Mana Dental, Tehran, Iran) was used for dose measurements in the thyroid gland area. All the measurements were performed by means of a calibrated ion chamber dosimeter (Arrow-Tech, USA, Model W 138-S, range 0–2 mSv) and dosimeter charger (Arrow-Tech, USA, Model 909B). Prior to the study, dosimeter calibration was performed by the Atomic Energy Organization (Tehran, Iran).

The phantom was placed in the panoramic machine (Pax-i, Vatech, Gyeonggi, Korea) in the standard upright position according to patients' comfort [Figure 1]a. The ion chamber dosimeter was fixed on the phantom's neck in the vicinity of the thyroid gland [Figure 1]b. The position of the dosimeter remained exactly the same for all the exposures. The the focal trough indicator light was adjusted in a fixed position as well. Panoramic exposures were performed using different protocols provided by the manufacturer. [Table 1] illustrates the imaging modes and their associated exposure parameters. Noteworthy to mention that, due to the very low amount of radiation resulting from a single panoramic exposure, each exposure protocol was repeated five times consecutively and the cumulative dose displayed by the dosimeter was divided by five.
Figure 1: (a) Position of the phantom within the panoramic machine is illustrated from the front view (b) position of the phantom within the panoramic machine is illustrated from the side view (arrow points to the dosimeter fixed in the location of the thyroid gland)

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Table 1: Predetermined panoramic imaging modes and their exposure parameters

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Similar to the panoramic examinations, the phantom with the attached dosimeter was positioned within the CBCT unit (Pax-i 3D, Vatech, Gyeonggi, Korea) in the standard upright position just as adjusted for the patients. The exposures were made in various FOVs offered by the system. For each FOV, different exposure protocols were applied, and the dose was measured. [Table 2] presents different imaging modes of CBCT and their features.
Table 2: Different field of views provided by the cone-beam computed tomography device and associated exposure protocols and parameters

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ESD of the thyroid gland area obtained from each of the abovementioned panoramic and CBCT imaging modes was transferred to the SPSS software (version 20, SPSS Inc., Chicago, IL, USA) for the statistical analyses. Comparison of the results between panoramic and CBCT examinations was performed using paired t-test with confidence interval of 95%.

  Results Top

In order to compare the three normal, soft, and hard imaging modes between panoramic and CBCT examinations, initially, the mean value of the doses obtained from woman, man, and child patients was calculated for each modality. Subsequently, comparisons among panoramic and different FOVs of CBCT were made using the obtained mean values. [Table 3], [Table 4], [Table 5] demonstrate comparison of the doses for panoramic and CBCT examinations in the soft, normal, and hard modes, respectively.
Table 3: Comparison of entrance skin dose of the thyroid gland area among panoramic and different field of views of cone-beam computed tomography in the soft mode

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Table 4: Comparison of entrance skin dose of the thyroid gland area among panoramic and different field of views of cone-beam computed tomography in the normal mode

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Table 5: Comparison of entrance skin dose of the thyroid gland area among panoramic and different field of views of cone-beam computed tomography in the hard mode

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As perceived from the results presented in [Table 3], [Table 4], [Table 5], in each of the three normal, soft, and hard modes, ESD of the thyroid gland area was significantly greater in all FOVs of CBCT compared to panoramic radiography (P < 0.05).

With regard to comparison of the absorbed dose between child and adult (man + woman) patients in the summation of the normal, soft, and hard modes of panoramic and each FOV of CBCT, it was shown that in the panoramic, CBCT 80 × 80 and CBCT 90 × 120 examinations, the adult dose was remarkably greater (P < 0.05) than the child dose; however, the rest of CBCTs' FOVs were not accompanied by such a difference. [Table 6] displays the results for the adult and child doses in each imaging modality.
Table 6: Comparison of entrance skin dose of the thyroid gland area between child and adult patients in panoramic and different field of views of cone-beam computed tomography

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  Discussion Top

Determining patient dose is of utmost importance in maxillofacial imaging procedures mainly due to estimating the risk of stochastic effects.[3],[16] Many studies by far have investigated the radiation dose received by different organs, particularly the more radiosensitive ones during exposure by various radiographic techniques.[1],[2],[8],[9],[17] In the present study, we evaluated the ESD of the thyroid gland area following exposure by panoramic and CBCT examinations. Although ESD measurement is not as applicable as the equivalent dose, it could be considered an approximation of the radiation dose imparted to the organs through a specific task. The reason for selection of these two imaging modalities was the vast application of panoramic images in dentistry and at the same time increasing tendency toward three-dimensional imaging. The dose evaluations were performed on Pax-i and Pax-i 3D units of Vatech. The devices provide normal, soft, and hard exposure modes for panoramic and various FOVs in addition to the normal, soft, and hard modes for CBCT imaging. In each of the three normal, soft, and hard modes, ESD of the thyroid gland was higher in all FOVs of CBCT compared to the panoramic examination, bringing to attention that even the smallest size of FOV for CBCT, that is 50 × 50 in our study, puts the patient in a greater radiation burden compared to panoramic imaging. Similarly, Sezgin et al.[18] stated that the dose of CBCT is not low enough for being routinely used instead of panoramic radiographs. In their study, the effective dose in CBCT, multislice computed tomography (MSCT), and panoramic radiography was compared with the conclusion that MSCT dose is considerably higher than the two other techniques and CBCT is also associated with greater effective dose compared to the panoramic examination. Several researches have arrived at the same conclusion regarding the general comparison of panoramic and CBCT techniques;[17],[18],[19],[20],[21],[22] however, the number of those making comparison on the basis of different FOVs and different exposure modes as our study did is not that great.[15],[19]

Ludlow et al.[19] compared NewTom 9000™ CBCT and Orthophos Plus DS panoramic units in terms of effective dose. They used thermoluminescent dosimeters (TLDs) placed throughout the layers of a tissue-equivalent RANDO phantom. Three different examination techniques were created for CBCT by alteration in the orientation of the phantom and collimation of the beam. The techniques included maxilla, mandible, and a combination of the two. Effective doses were calculated with and without summation of salivary gland exposures and considered as ESAL and EICRP60, respectively. They concluded that, depending on the examination technique, ESAL and EICRP60 for CBCT examination are 2–4 and 3–7 times greater than that of panoramic examination, respectively.

Palomo et al.[15] performed a dose evaluation research on CB MercuRay CBCT scanner. They evaluated how different FOVs and exposure parameters including kVp and mA influence the radiation dose; therefore, their study could be considered very similar to ours. They concluded that, with reducing kVp from 120 to 100, dose was lowered by 0.62 times and by using smaller FOV, 5%–10% dose reduction was achieved and even greater reduction seem to be associated with the organs that were obviously excluded from the path of direct beam. Similarly, in our study, it was revealed that reducing kVp and mA as shifting from hard toward soft mode resulted in significant dose reduction. Moreover, reducing FOV in CBCT was accompanied by remarkable reduction in dose; nevertheless, in all situations, the dose for panoramic examination remained constantly lower than CBCT.

Panjnoush et al.[20] compared the absorbed dose among a number of different organs while imaged with panoramic examination, linear tomography, CBCT, and CT. They used TLDs inserted within the layers of a RANDO phantom. The target organs that they investigated were the eye lens, buccal skin, and thyroid, parotid, submandibular, and sublingual glands. Organs receiving the highest and lowest radiation doses, respectively, for each imaging modality were as follows: panoramic – submandibular gland and buccal skin; linear tomography – submandibular gland and lens; CBCT – parotid gland and thyroid gland; CT – submandibular gland and sublingual gland. A more recent study by Heiden et al.[17] also claimed that, in both panoramic and CBCT examinations, organs receiving the highest amount of radiation exposure are the thyroid and salivary glands.

One of the major concerns in maxillofacial imaging is directed toward orthodontic patients as they are often children and adolescents and therefore more prone to the hazardous outcomes of exposure with ionizing radiation.[3] In that regard, multiple studies have been carried out; two more recent to mention are the studies of Kadesjo et al.[21] and Signorelli et al.[22] The former study compared the dose in CBCT and conventional (panoramic and periapical) radiographs taken for the evaluation of impacted maxillary canines in orthodontic patients. The obtained values for radiation dose in descending order belonged to the following situations: CBCT (NewTom 5G), CBCT (ProMa × 3D), panoramic, maxillary central incisor periapical projection, and maxillary lateral incisor periapical projection. In the latter study, radiation dose was compared between CBCT and a set of conventional radiographs including panoramic, lateral cephalomtery, and posteroanterior cephalometry. It was proved that a single scan of CBCT has 2–4 times greater radiation dose compared to the entirety of the mentioned set of conventional radiographs. One of the calculations of our study was related to ESD in the child and adult patient modes of the examined devices. For panoramic imaging as well as each FOV of CBCT, child and adult (man and woman) conditions existed for making the exposures. The results showed that, in panoramic radiography, CBCT 80 × 80 and 90 × 120, adult dose was statistically greater than child dose, implying that care should be taken in the rest of conditions, i.e., CBCT 50 × 50 maxilla, 50 × 50 mandible, 50 × 80, and 150 × 150 as child dose is almost as much as adult dose.

Considering the type of dosimeter, many studies have utilized TLDs;[5],[8],[9],[15],[17],[19],[21] however, we performed dose measurements by means of a calibrated pencil ion chamber. Ion chamber dosimeters are sufficiently accurate to be adapted for dosimetric studies, and their ease of use as well as immediacy makes them very popular among the various kinds of dosimetric tools.[23],[24] It is worth mentioning that Isoradi and Ropolo[25] compared pencil ion chamber and TLDs for their accuracy of dose measurement in panoramic radiography and found the two methods to have 92% agreement in their results.

A wide variety of target organs by far have been investigated regarding the dose they receive through different maxillofacial radiographic examinations. It is well known that certain organs are more susceptible to the harmful effects of radiation, some of which to mention are the salivary gland, thyroid gland, bone marrow, and eye lens.[1],[2],[8],[9],[26] The thyroid gland is of great concern particularly in women and children.[17] Studies have shown a link between maternal thyroid exposure through dental radiography and risk of low-birth weight infants.[5] Moreover, the risk of thyroid carcinoma either in adults or children always remains a certain threat.[1],[3] Therefore, our study focused on ESD of the thyroid gland area following exposure by panoramic and CBCT examinations and as formerly discussed, it was revealed that CBCT was associated with definitely greater amounts of radiation in comparison with panoramic examination.

  Conclusions Top

All the three normal, soft, and hard exposure modes followed the same pattern: all sizes of FOV for CBCT were accompanied by greater thyroid exposure in comparison with panoramic radiography. Adult and child doses were not statistically different in CBCT FOVs of 50 × 50 maxilla, 50 × 50 mandible, 50 × 80, and 150 × 150. Therefore, in case panoramic examination provides adequate diagnostic information, CBCT is by no means an appropriate substitute. Furthermore, whenever CBCT examinations are necessary, proper FOV should be carefully chosen according to the imaging procedure's purpose.

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There are no conflicts of interest.

  References Top

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  [Figure 1]

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]

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