An evaluation of radiation dose from dental cone-beam computed tomography

Resumo

Dental cone beam computed tomography (CBCT) has enormous features including better image quality, acceptable size, and a lower radiation dose than those of helical computerized tomography (CT) scanning. Moreover, CBCT is more suitable for dentists to obtain and analyse images and it is more comfortable for patients due to technological enhancements. CBCT produces three-dimensional (3D) images of the head and neck and is being used in the various fields of dentistry such as dental surgery, endodontic, trauma, implant dentistry, lesions and diseases in the head and neck and orthodontics. In this work, four protocols were examined. Three voxel size settings were evaluated: 420 μm, 380 μm and 320 μm. Field of view and voltage tube were constant at (13cmx15cm) and 90 kV. The absorbed doses and effective doses were calculated for each CBCT scan protocols. A direct relationship was found between effective dose and the resolution options with lower resolution yielding lower effective dose. Modification of resolution options leads to changes in effective doses. This study emphasized the importance of selecting exposure parameters in terms of voxel size settings or resolution options.

Texto integral

Introduction

  Numerous studies have been carried out about cone beam computerized tomography (CBCT) scanners to calculate the dose of radiation that subjects received [1-8]. Most of these studies have used thermoluminescence dosemeters (TLDs) that are placed within phantoms made with tissues equivalent to human tissues to evaluate standard patient doses [1,9,10]. In 1998, CBCT was announced in dentistry by Mozzo et al. [4]. CBCT produces three-dimensional (3D) images on the head and neck and is being used in the various fields of dentistry such as dental surgery, endodontics, trauma, implant dentistry, lesions and diseases of soft tissue in the head and neck and orthodontics [2,4,11-13]. CBCT has been developed to be the most useful and important device in oral and maxillofacial diagnosis, oral treatment planning and radiation therapy [14-17]. CBCT examination has higher radiation dose than conventional panoramic imaging and it has lower radiation dose than conventional CT scanning [1,12]. Dental radiologists prefer cone beam computerized tomography (CBCT) to helical computerized tomography (CT) scanning because CBCT has enormous features including better image quality, inexpensive, acceptable size, its availability and a lower radiation dose. Moreover, CBCT is more suitable for dentists to obtain and analyse images and it is more comfortable for patients due to technological enhancements [3,4]. On the other hand, the examination dose of CBCT depends on its configurations and radiation protocols. CBCT has various FOVs options and VOX size settings to be convenient for dental examinations. Large, medium and small FOVs produce volumes satisfactory for covering the maxillofacial region, dentoalveolar and localized imaging, respectively. The radiation field of dentistry is the head and neck area including eyes, the thyroid gland and salivary glands [3,18-20]. Hence, It is significant to minimize the dose of CBCT that patients obtained to the lowest value because of radiological hazards [5,9,21]. However, the image quality should be produced as good as possible while the radiation dose is still low [19,20]. The purpose of this vitro study was (a) to evaluate the seven tissues doses for four scan protocols using CBCT (KaVo OP 3D Pro) scanner and (b) to investigate changes in resolution options with effective dose.

Methods

The measurement of this study was conducted with TLD-100 (LiF) dosimeters put in sealed plastic beaker and fixed in human phantom as shown in Figure 1. In total, 84 dosimeters (21 chips for each CBCT scan protocol) were placed in seven anatomical locations as illustrated in Figure 2 and reported in Table 1.

Figure 1. TLD-100

Figure 2. The image of phantom

Before radiation exposure, lithium fluoride dosimeters (TLD-100) were calibrated as described in Handbook of Thermoluminescence [22]. Hence, the relation between TLD data and X-ray doses was applied for the assessment of absorbed doses in the anatomical regions of phantom. In each radiation exposure, 21 dosimeters were applied. Three TLDs were used to estimate background radiation, which was measured as 0.013 mGy. The lowest TLD reading was three times greater than background values which were subtracted from TLD readings.

Cone beam computed tomography (CBCT) (KaVo ORTHOPANTOMOGRAPH (OP) 3D Pro CBCT scanner, Germany) was used in this study. The parameters of CBCT scan protocols are shown in Table 2. Table 2 indicates that protocol¹, protocol² and protocol³ have the same field of view (13cmx15cm) which covers the most maxillofacial region. The resolutions of protocol¹, protocol² and protocol³ are low resolution (420 μm voxel size), standard resolution (380 μm voxel size) and high resolution (320μm voxel size), respectively. Hence, the lower the voxel size the higher the resolution with low tube current.

The pre-irradiation annealing of all TLDs were carried out in Muffle furnace-Gallenkamp oven, for 1 h at 400^oC followed by a low temperature thermal processing for 2 hours at 100^°C . The post-irradiation annealing of TLDs were achieved for 10 min at 100^◦C [22]. The thermoluminescent reader was a Harshaw model 2000 B/C and it is manufactured by Harshaw Filtrol Partnership.

Table 1. Location of thermoluminescent dosimetry (TLD) chips in the phantom

Phantom location	TLD ID for each CBCT scan protocol
Throat	Protocol1:1,2,3, Protocol2: 22,23,24, Protocol3: 43,44,45, Protocol4: 64,65,66
Teeth	Protocol1: 4,5,6, Protocol2: 25, 26,27, Protocol3:46, 47,48, Protocol4: 67,68,69
Cheek	Protocol1: 7,8,9, Protocol2:28,29,30, Protocol3:49,50,51, Protocol4: 70,71,72
Eyes	Protocol1: 10,11,12, Protocol2:31,32,33, Protocol3:52,53,54, Protocol4: 73,74,75
Frontal (forehead)	Protocol1: 13,14,15, Protocol2:34,35,36, Protocol3:55,56,57, Protocol4: 76,77,78
Mid skull	Protocol1:16,17,18, Protocol2:37,38,39, Protocol3:58,59,60, Protocol4: 79,80,81
Occipital (back skull)	Protocol1:19,20,21, Protocol2:40,41,42, Protocol3:61,62,63, Protocol4: 82,83,84

Table 2. The parameters of CBCT scan protocols

CBCT scan protocol	VOX Size (μm)	FOV (cm)	Tube voltage (kV)	Tube current (mA)	Exposure time (second)
Protocol1	420 (low resolution)	13x15	90	3.2	4.5
Protocol2	380 (standard resolution)	13x15	90	5	8.1
Protocol3	320 (high resolution)	13x15	90	8	8.1
Protocol4	….	PAN	66	10	16

Effective dose (E) has been measured in Sievert (Sv), according to the ICRP60 as shown in the following equation [23]:

                                                                                                                        (1)

where W_T is the tissue weighting factors for each tissue (T) or organ and H is equivalent dose (H (SV) = D (Gy) x W_R) where, w_R is a radiation weighting factor and D is absorbed dose. The value of w_R is one for x-ray [24]. Thus, the effective dose depends on absorbed dose (D) and tissue weighting factors (W_T). The tissue weighting factor values of different tissues or organs are summarized in Table 3.

Table 3. Tissue weighting factors values of different tissues or organs [24]

Tissue or organ	Tissue weighting factor,WT
Gonads	0.20
Bone marrow (red)	0.12
Colon	0.12
Lung	0.12
Stomach	0.12
Bladder	0.05
Breast	0.05
Liver	0.05
Oesophagus	0.05
Thyroid	0.05
Skin	0.01
Bone surface	0.01
Remainder	0.05

Results

The measured absorbed doses (D) and effective doses (E) of several tissues including throat, teeth, cheek, eyes, frontal (forehead), mid skull and occipital (back skull) for CBCT scan protocol¹ (low resolution), protocol² (standard resolution) and protocol³ (high resolution) are summarized in Table 4. The variations of findings are illustrated in Figure 3 and Figure 4.

Table 4: The absorbed (D) dose and effective (E) dose for the various tissues based on each CBCT protocols

Tissue or Organ	Protocol1		Protocol2		Protocol3
	D (mGy)	E (mSv)	D (mGy)	E (mSv)	D (mGy)	E (mSv)
Throat	0.473	0.023	7.719	0.385	1.565	0.078
Teeth	4.95	0.049	13.215	0.132	16.326	0.163
Cheek	5.378	0.053	12.684	0.126	25.053	0.250
Eyes	2.099	0.020	7.577	0.075	12.962	0.129
Frontal (forehead)	3.46	0.034	5.104	0.051	8.465	0.084
Mid skull	4.894	0.048	13.013	0.130	20.904	0.209
Occipital (back skull)	2.61	0.026	7.8	0.078	4.188	0.041

Figure 3: The absorbed (D) doses for the various tissues based on each CBCT protocols

Figure 4: The effective (E) doses for the various tissues based on each CBCT protocols

Discussion

The highest effective dose was determined for the highest resolution due to the direct proportionality between resolution options and effective dose. The results of this investigation are in line with those of an earlier study that was published and shown that voxel size settings have an inverse relationship with the effective dose [20]. Therefore, it is universally acknowledged that changes in the CBCT exposure parameters affect the effective dose.

In CBCT scan protocols, the cheek and teeth received the largest absorbed dose because these tissues are directly exposed to the primary beam, as can be shown in Figure 3. As a result of these tissues being away from the X-ray beam, the throat and frontal (forehead) have the lowest absorbed dose during CBCT scan protocols. The X-rays scattered inside the human body are primarily to blame for the irradiation of tissues outside the X-ray beam. The findings of this study concur with earlier research that determined the highest and lowest absorbed dosages in these tissues [17]. Figure 4 displays the effective (E) dosages for the various tissues depending on each CBCT methodology.

This study compared the effects of low, standard, and high resolution types on the effective radiation doses. As anticipated, the effective dose was inversely related to the voxel size values and directly proportional to the resolution. This is primarily due to the longer exposure times required for photos with higher resolution. This outcome is consistent with a number of published research, [20,25,26] and indicates the essential of the requirement for high-resolution images against the increase radiation hazard associated with these images. Numerous studies have attempted to identify recommendations for high accompanied increases in voxel settings to prevent image noise [27]. Therefore, finding a balance between radiation dose and image resolution is crucial.

Conclusion    

It has been shown that variations in the CBCT exposure parameters impact on effective dose. Adjustment of resolution selections leads to variations in effective doses. This emphasizes the significance of choosing exposure factors in terms of voxel size settings or resolution options. Dentists should be pay attention in their choice of imaging parameters because this essentially impact on the patient.

Acknowledgment

The authors would like to knowledge all those contributed in declaring this issue.

Conflict of Interest

The authors declare that they have no conflict of interest.

Funding:     No funding was obtained for this study