there are three basic concepts associated with the radiation dose : the absorbed dose , equivalent dose , and effective dose . the absorbed dose is used to describe the amount of x - ray energy absorbed by a unit mass ( total weight ) of tissue . the equivalent dose is used to compare the biologic effect of different types of radiation on tissue or an organ . the si unit is sievert ( sv ) . for a diagnostic x - ray examination , the abosorbed dose is equal to the equivalent dose , that is , 1 gray equals 1 sievert . for the estimation of radiation risk , which is the possibility of biological consequences after radiation exposure to human beings , the effective dose is a measurement of the degree of harmful effects on the human body of one kind of radiation . the si unit for the effective dose is the sievert , but in practice , milli- or micro - sievert is often used . to determine the effective dose , the phantom can be made with a real human skull covered with soft tissue - equivalent materials or only made with bone- and soft tissue - equivalent materials . the phantom usually comprises nine sections and in each section there are holes in the places where the tissues are being measured . thermoluminescent dosimeters ( tlds ) , optically stimulated luminescent dosimeters ( osls ) , or radiophotoluminescence glass dosimeters can be positioned in the holes for the measurement of the absorbed dose of the corresponding tissue . the tissues measured can include the bone marrow , thyroid gland , esophagus , salivary glands , skin , bone surface , brain , pituitary , and eyes , and in most studies,1 - 14 tlds are used for the mesasurement of the radiation dose ( fig . it is worth noticing that at this stage only the absorbed dose is measured . in order to determine the effective dose , which is used to estimate the risk in human beings , the absorbed dose for individual organs must be translated to an equivelant dose and then multiplied with a weighting factor defined by the international commission on radiological protection ( icrp ) for the specific organ . later , the effective doses of these organs are summed up to obtain a total effective dose . the total dose is used to represent the potential risk of the whole body exposed to radiation . we have thus shown that the effective dose is a representation of the potential risk of radiation dose to the patient , and in most studies , the effective dose is drived from absorbed dose , which is measured with tlds by the use of an anthropomorphic phantom . thus , in the following discussion , only the reported effective dose derived from a phantom is addressed . many studies have been performed to estimate the effective doses of different cbct units . however , a simple comparison can not be made since , as one researcher has noted , " significant differences in dose for the same examination have been reported for different cbct units , and significant differences in dose have been reported for different examinations or techniques with the same unit"13 and another has observed that " the results are often difficult to compare when a number of different phantoms and dosimeters have been used together with different assumptions."15 to avoid these research limitations , ludlow et al5 and pauwels et al14 investigated the effective dose of 8 and 14 cbct units , respectively , by using the same phantom and tld dosimeters . although the phantoms and dosimeters employed in the two studies were different , from table 1 we can still see that the effective dose is quite different from one cbct unit to another , irrespective of the size of the field of view ( fov ) used . the highest effective dose is 1073 sv for cb mercuray with a large fov scanning for maxillofacial region , while the lowest effective dose is only 19 sv for the kodak 9000 3d with a scanning area of the front region of the upper jaw . this is a difference of almost 500 times between the highest and lowest effective doses . when we look futher into the data in table 1 , we can see that the effective dose is closely related to the protocol used for scanning . since a protocol is a combination of kvp , mas , and voxel sizes and other factors , the effective dose is in reality closely related to the chosen exposure parameters . in the study performed by ludlow et al , the effective dose for maximum quality cb mercuray ( 1073 sv ) is almost twice that of standard quality cb mercuray ( 569 sv ) , the effective dose for cbct unit galileos obtained at the default exposure ( 70 sv ) is almost half of that obtained at maximum exposure ( 128 sv ) , and the effective dose for standard and high resolution images from the prexion 3d were 189 sv and 388 sv , respectively . the later data also indicate that with an increase in the spatial resolution , the effective dose is increased as well . this is also confirmed by the study conducted by davies et al.16 field of view ( fov ) is another factor that plays an important role in the assessment of the effective dose of one cbct examination . when the exposure parameters such as the kvp and mas are maintained at the same level , the larger the fov used , the higher the effective dose obtained . this is substantiated by the effective doses for cb mercuay in table 1 , where the effective dose is 1073 sv for a large fov with maximum quality , 560 sv for a medium fov , and 407 sv for a small fov . the exposure parameters for all of the three fov examinations were kept at 120 kvp and 150 mas . a study by qu et al13 further discloses the positive relationship between the fov and effective dose . in this study , 12 protocols that combined different patient size , fov , kvp , ma , and exposure times were employed for the estimation of effective doses of the promax 3d cbct unit . while holding all of the other exposure parameters constant , the researchers found that for a scanning area of full volume height with a full volume diameter ( 8 cm8 cm ) , the effective dose ( 298 sv ) is much higher than the effective doses obtained from used other scanning fovs , specifically , half the volume height ( upper jaw ) with a full volume diameter ( 4 cm8 cm , 131 sv ) , half the volume height ( lower jaw ) with a full volume diameter ( 4 cm8 cm , 171 sv ) , a full volume height with half the volume diameter ( anterior region , 8 cm4 cm , 127 sv ) , and a full volume height with half the volume diameter ( posterior region , 8 cm4 cm , 197 sv ) . the above demonstrates that the effective dose is different from one cbct unit to another and closely related to the exposure parameters used for scanning ; for a given model of a cbct unit , the larger the fov used for scannning , the higher the effective dose derived when all the other exposure parameters are kept at the same level . similarly , the higher the spatial resolution chosen for scanning , the higher the effective dose is . there are few studies focusing on the direct comparison of the effective doses obtained from cbct and conventional dental radiography . the results from the direct comparison studies were summarized in table 2 , where the effective dose for panoramic radiography is about 22.0 sv , for lateral cephalometric examination about 4.5 sv and for cbct examnation the effective dose is 61 - 134 sv . no study has performed a direct comparison of the effective dose from intraoral and cbct examinations . in the guidelines19 provided by the european academy of dento - maxillofacial radiology , other studies20 - 26 that exclusively estimated the effective dose of conventional dental radiography have demonstrated that the range of the effective dose for a panoramic radiograph is 3.85 - 38.0 sv ( table 3 ) , for a lateral cephalometric examination is 1.1 - 5.6 sv ( table 4 ) , for posteroanterior cephalometric radiograph , 5.1sv , and for one introal examination , 0.65 - 9.5 sv ( table 5 ) . these data indicate that the effective dose of cbct is several to hundreds of times higher than the effective dose from a conventional dental radiographic examination . more attention is paid to the effective dose of cbct and multislice ct ( msct ) since both techniques provide three dimensional images . the effective doses from the literature on cbct and msct are shown in table 6 . however , in some of the studies , the scanning area , i.e. the fov was not well defined . to avoid the effect of the fov on the assessment of effective dose , qu et al13 strictly defined the scanning area for both msct and cbct examinations in their study . the results showed that the effective doses of mscts are about several to ten times higher than those of cbcts . for example , when scaning both the maxilla and mandible , the effective dose is about 94.9 sv for cbct newtom 9000 , 249.1 sv for cbct dct - pro , and 1066.1 for ge 8-slice msct . however , it should be borne in mind that although the effective dose of msct is much higher than that of cbct , the image qualities for the two techniques are quite different . for hard tissue , such as bone and tooth , the image quality of cbct is equal to or better than the image quality of msct , but for soft tissues , the image from cbct is not satisfactory due to the inherent drawbacks of the technique . many studies have been performed to estimate the effective doses of different cbct units . however , a simple comparison can not be made since , as one researcher has noted , " significant differences in dose for the same examination have been reported for different cbct units , and significant differences in dose have been reported for different examinations or techniques with the same unit"13 and another has observed that " the results are often difficult to compare when a number of different phantoms and dosimeters have been used together with different assumptions."15 to avoid these research limitations , ludlow et al5 and pauwels et al14 investigated the effective dose of 8 and 14 cbct units , respectively , by using the same phantom and tld dosimeters . although the phantoms and dosimeters employed in the two studies were different , from table 1 we can still see that the effective dose is quite different from one cbct unit to another , irrespective of the size of the field of view ( fov ) used . the highest effective dose is 1073 sv for cb mercuray with a large fov scanning for maxillofacial region , while the lowest effective dose is only 19 sv for the kodak 9000 3d with a scanning area of the front region of the upper jaw . this is a difference of almost 500 times between the highest and lowest effective doses . when we look futher into the data in table 1 , we can see that the effective dose is closely related to the protocol used for scanning . since a protocol is a combination of kvp , mas , and voxel sizes and other factors , the effective dose is in reality closely related to the chosen exposure parameters . in the study performed by ludlow et al , the effective dose for maximum quality cb mercuray ( 1073 sv ) is almost twice that of standard quality cb mercuray ( 569 sv ) , the effective dose for cbct unit galileos obtained at the default exposure ( 70 sv ) is almost half of that obtained at maximum exposure ( 128 sv ) , and the effective dose for standard and high resolution images from the prexion 3d were 189 sv and 388 sv , respectively . the later data also indicate that with an increase in the spatial resolution , the effective dose is increased as well . this is also confirmed by the study conducted by davies et al.16 field of view ( fov ) is another factor that plays an important role in the assessment of the effective dose of one cbct examination . when the exposure parameters such as the kvp and mas are maintained at the same level , the larger the fov used , the higher the effective dose obtained . this is substantiated by the effective doses for cb mercuay in table 1 , where the effective dose is 1073 sv for a large fov with maximum quality , 560 sv for a medium fov , and 407 sv for a small fov . the exposure parameters for all of the three fov examinations were kept at 120 kvp and 150 mas . a study by qu et al13 further discloses the positive relationship between the fov and effective dose . in this study , 12 protocols that combined different patient size , fov , kvp , ma , and exposure times were employed for the estimation of effective doses of the promax 3d cbct unit . while holding all of the other exposure parameters constant , the researchers found that for a scanning area of full volume height with a full volume diameter ( 8 cm8 cm ) , the effective dose ( 298 sv ) is much higher than the effective doses obtained from used other scanning fovs , specifically , half the volume height ( upper jaw ) with a full volume diameter ( 4 cm8 cm , 131 sv ) , half the volume height ( lower jaw ) with a full volume diameter ( 4 cm8 cm , 171 sv ) , a full volume height with half the volume diameter ( anterior region , 8 cm4 cm , 127 sv ) , and a full volume height with half the volume diameter ( posterior region , 8 cm4 cm , 197 sv ) . the above demonstrates that the effective dose is different from one cbct unit to another and closely related to the exposure parameters used for scanning ; for a given model of a cbct unit , the larger the fov used for scannning , the higher the effective dose derived when all the other exposure parameters are kept at the same level . similarly , the higher the spatial resolution chosen for scanning , the higher the effective dose is . there are few studies focusing on the direct comparison of the effective doses obtained from cbct and conventional dental radiography . the results from the direct comparison studies were summarized in table 2 , where the effective dose for panoramic radiography is about 22.0 sv , for lateral cephalometric examination about 4.5 sv and for cbct examnation the effective dose is 61 - 134 sv . no study has performed a direct comparison of the effective dose from intraoral and cbct examinations . in the guidelines19 provided by the european academy of dento - maxillofacial radiology other studies20 - 26 that exclusively estimated the effective dose of conventional dental radiography have demonstrated that the range of the effective dose for a panoramic radiograph is 3.85 - 38.0 sv ( table 3 ) , for a lateral cephalometric examination is 1.1 - 5.6 sv ( table 4 ) , for posteroanterior cephalometric radiograph , 5.1sv , and for one introal examination , 0.65 - 9.5 sv ( table 5 ) . these data indicate that the effective dose of cbct is several to hundreds of times higher than the effective dose from a conventional dental radiographic examination . more attention is paid to the effective dose of cbct and multislice ct ( msct ) since both techniques provide three dimensional images . the effective doses from the literature on cbct and msct are shown in table 6 . however , in some of the studies , the scanning area , i.e. the fov was not well defined . to avoid the effect of the fov on the assessment of effective dose , qu et al13 strictly defined the scanning area for both msct and cbct examinations in their study . the results showed that the effective doses of mscts are about several to ten times higher than those of cbcts . for example , when scaning both the maxilla and mandible , the effective dose is about 94.9 sv for cbct newtom 9000 , 249.1 sv for cbct dct - pro , and 1066.1 for ge 8-slice msct . however , it should be borne in mind that although the effective dose of msct is much higher than that of cbct , the image qualities for the two techniques are quite different . for hard tissue , such as bone and tooth , the image quality of cbct is equal to or better than the image quality of msct , but for soft tissues , the image from cbct is not satisfactory due to the inherent drawbacks of the technique . to perform one medical x - ray examination , three main factors must be taken into account : the x - ray unit , patient for examination , and receptor used for capturing the image of the patient . therefore , when an x - ray examination is indicated for a patient , the patient dose can be reduced by the reduction of the x - ray intensity emitted from the employed x - ray unit , increasing of the imaging receptor capturing speed and collimation , or shielding of the x - ray beam to the patient . this section will only focus on the shielding devices for the reduction of the radiation dose . the shielding devices include a leaded thyroid collar for the protection of the thyroid gland , leaded glasses for the protection of the eye lens , a leaded hat for the protection of the brain , and a leaded apron for the protection of the body trunk . it is well known that a thyroid collar is effective for the protection of the thyroid gland in an intraoral examination . however , for a cbct examination , is it still effective when the x - ray unit rotates around the patient ? with this question in mind , two studies were conducted . one study was mainly aimed to identify the effectiveness of a thyroid collar on the dose redution of the thyroid gland.28 in this study , five conditions were tested as follows : 1 ) without a collar around the neck ; 2 ) with one collar loosely on the front of the neck ; 3 ) with two collars loosely on the front and back of the neck ; 4 ) with one collar tightly on the front of the neck ; and 5 ) with two collars tightly on the front and back of the neck . the results showed that when the thyroid collars were used loosely around the neck , no effective organ dose reduction was observed . when one thyroid collar was used tightly on the frontof the neck , the effective organ dose to the thyroid gland and esophagus were reduced to 15.9 sv ( 48.7% reduction ) and 1.4 sv ( 41.7% reduction ) , respectively . a similar organ dose reduction ( 46.5% and 41.7% ) was achieved when cbct scanning was performed with two collars tightly affixed to the front and back of the neck . the study supported the use of a thyroid collar during a cbct scan . in a subsequent study , different oral and maxillofacial regions were scanned with the phantom tightly wearing one or two thyroid collars.29 the results also supported the use of thyroid collars ( 61% thyroid dose reduction for a large view examination , 72% thyroid dose reduction for a medium fov , and 70% thyroid dose reduction for a small fov ) and further disclosed that the total effective dose for medium and small fov examinations were also significantly reduced by the use of a thyroid collar . the use of leaded glasses during a cbct examinaiton was also investigated.30 in the study peformed by prins et al , three phantoms representing an adult male , an adult female , and a child were employed . the results showed that the radiation dose to the eye lens could be reduced by over 60% without having a deleterious effect on the image quality in the area of clinical significance for dental imaging . considering the above , one conclusion that could be drawn was that a thyroid collar and leaded glasses should be used during a cbct examination , given that diagnostic information and image quality are not reduced . the effective dose of cbct , conventional dental radiography , and multislice ct and the effect of a thyroid collar and leaded glasses on the dose reduction was presented in this paper . based on the above analysis , we can conclude the following : 
 the patient radiation dose is much lower for cbct than for helical ct;the patient radiation dose is closely related to the fov and exposure parameters used for a cbct examination . without alteration of any other exposure parameters , the larger the fov used for scanning , the higher the radiation dose is;compared with conventional dental radiography , the effective dose of cbct is several to hundreds of times higher;to reduce the patient dose to the greatest possible extent , the chosen cbct scanning protocol should be in accordance with the dignostic task at hand;a thyroid collar should be used for cbct scanning ; wearing leaded glasses is recommended when it does not detract from imaging quality . the patient radiation dose is much lower for cbct than for helical ct ; the patient radiation dose is closely related to the fov and exposure parameters used for a cbct examination . without alteration of any other exposure parameters , the larger the fov used for scanning , the higher the radiation dose is ; compared with conventional dental radiography , the effective dose of cbct is several to hundreds of times higher ; to reduce the patient dose to the greatest possible extent , the chosen cbct scanning protocol should be in accordance with the dignostic task at hand ; a thyroid collar should be used for cbct scanning ; wearing leaded glasses is recommended when it does not detract from imaging quality .
after over one decade development , cone beam computed tomography ( cbct ) has been widely accepted for clinical application in almost every field of dentistry . meanwhile , the radiation dose of cbct to patient has also caused broad concern . according to the literature , the effective radiation doses of cbcts in nowadays market fall into a considerably wide range that is from 19 sv to 1073 sv and closely related to the imaging detector , field of view , and voxel sizes used for scanning . 
 to deeply understand the potential risk from cbct , this report also reviewed the effective doses from literatures on intra - oral radiograph , panoramic radiograph , lateral and posteroanterior cephalometric radiograph , multi - slice ct , and so on . 
 the protection effect of thyroid collar and leaded glasses were also reviewed .