640.e2 Silva et al American Journal of Orthodontics and Dentofacial Orthopedics May 2008Table I. Sites in which TLD 100H chips were placed Table II.Technical parameters and FOV exposure of(all TLDs were in the primary radiation beam) the phantomOrgan TLD location Tube Tube energy currentBone marrow Third cervical vertebra FOV (kV) (mA) Mandibular ramusSpine Cervical NewTom 9000 23 cm 110 5.4Brain Hypophysis i-CAT 13 cm 120 23.87Eye Lens Orthophos DS Program 1 70 10 per 14 s Maxillary and mandibular premolars Orthophos DS Lateral cephalometric 76 9 per 0.64 s Maxillary sinus ﬂoor Somaton Sensation 10 cm 120 90Thyroid gland ThyroidSalivary glands Submandibular ParotidSkin Thyroid be used during the experiments. Prior to every expo- Neck (back) sure, the dosimeters were annealed at 240°C for 10 Philtrum minutes and cooled to 35°C. All TLDs were read Parotid immediately after each exposure, using a Harshaw Nasion 5500 Series Automatic TLD Reader (Harshaw/Bicron, Solon, Ohio). The devices used in this study were the NewTomalometric radiography or the collimated lateral beam 9000 (QR, Verona, Italy), the i-CAT (Imaging Sciencesfor cephalometric images.17 Radiation doses between International, Hatﬁeld, Pa), the Panoramic Orthophosconventional and CBCT images for orthodontic prac- Plus DS (Sirona Dental Systems, Bernsheim, Ger-tices have not been compared. The aim of this study many), and the multi-slice CT (Somatom Sensation 64;was to compare the absorbed radiation and effective Siemens Medical Solutions, Erlangen, Germany). Ta-doses for conventional panoramic and cephalometric ble II shows the technical parameters and the ﬁeld ofimaging, 2 CBCT units, and a multi-slice CT unit in view (FOV) for each unit. Considering the smallorthodontic practice. amount of radiation and the exposure latitude of the TLDs, the phantom, after loading with the TLD, was exposed 5 times, to provide a reliable measure ofMATERIAL AND METHODS radiation in the dosimeters. Later, the values were Dose measurements were carried out on an anthro- divided by 5 to obtain a value for each region. Thyroidpomorphic phantom, especially designed for dosimetry shields were not used during the exposures. To com-studies in dental radiography. The phantom was devel- pare the data, we used a large FOV to obtain imagesoped and built at the University of Göttingen (Germa- from the whole maxilla and mandible parts. The x-rayny)18 and consisted of 48 transverse sections, each 6 parameters were for a young male adult. Automaticmm thick with small holes positioned perpendicular to parameters were used for the NewTom and the i-CAT.the axial axis of the phantom. Lithium ﬂuoride ther- Since the phantom is composed of multiple slabs,moluminescent dosimeter chips (Harshaw TLD-100H, several tapes were used to align them and maintain theirThermo Electron, Oakwood Village, Ohio) were used. positions when the phantom was irradiated. The phan-One advantage of lithium ﬂuoride-based thermolumi- tom was positioned according to the manufacturer’snescent materials is their tissue-equivalent properties. speciﬁcations for each machine, following the refer-The absorbed dose from selected locations, correspond- ence lines and head rests. The dosimetry was performeding to the radiosensitive organs of interest, was mea- 3 times for each technique to ensure reliability.sured by using a set of 48 TLD chips, which were After reading, a sensitivity value was applied forindividually packed in thin polyethylene bags to pre- each TLD. The exposure doses were recorded invent contamination by dirt and humidity. The TLD nanocoulombs (nC), and, after applying energy calibra-chips were placed in each phantom site inside and on tion factors (reader calibration factor and elementthe surface of the phantom, as shown in Table I. Three correction coefﬁcient), the dosimetry data were con-dosimeters were placed in each anatomic site to calcu- verted into milligrays (mGy) and recorded. The stan-late the mean value of each location, while retaining the dard deviation of readings from TLD-100H was lesssame dosimeters in the same positions for each expo- than Ϯ 5%. Doses from the 3 TLDs, located in the samesure. Before the study, all dosimeters were calibrated tissue or organ, were averaged, representing the organusing the same type and range of radiation that would dose. The weighted dose for bone marrow was calcu-
American Journal of Orthodontics and Dentofacial Orthopedics Silva et al 640.e3Volume 133, Number 5lated by using the sum of the radiation from the third Table III. Mean absorbed doses (Gy) to various tissuescervical vertebra and the mandibular ramus, as previ- for each unitously described.19 The submandibular and parotid Panoramic/salivary gland doses were used for calculating the NewTom lateral Multi-sliceweighted dose for the salivary glands. The thyroid 9000 i-CAT cephalometric CTgland was individually calculated considering its spe- Bone marrowciﬁc weighted factor. For the skin surface area, 5 points Third cervicalwere measured: thyroid skin, neck (back), philtrum, vertebra 648.9 731.3 62.8 7525.6parotid, and nasion skin. These values were used to Mandibular ramus 1244.7 1282.9 360.4 9930.4 Braincalculate the equivalent dose (HT) with the equation Hypophysis 316.1 745.0 30.2 1488.9HT ϭ ⌺ WR ϫ DT, where the HT for a tissue or an Eyeorgan is the product of the radiation-weighting factor Lens 472.8 1229.2 45.8 892.8(WR) and the average absorbed dose (DT) measured for Thyroid glandthat organ.20 The equivalent dose was used to compare Thyroid 232.4 124.3 13.1 1417.7 Salivary glandsthe effects of different types of radiation on tissues or Submandibular 1426.7 1364.1 566.8 11815.0organs in sieverts (Sv). Parotid 1678.7 1502.2 324.4 14204.4 The effective dose (E) is a calculation proposed by Skinthe International Commission on Radiological Protec- Thyroid 663.8 157.5 25.9 1889.0 Neck (back) 1257.1 651.1 270.8 15837.2tion (ICRP).20 It is calculated by multiplying actual Philtrum 3273.6 1434.9 25.3 12791.8organ doses by risk-weighting factors (related to an Parotid 1489.4 1510.9 608.7 14734.4organ’s sensitivity) and represents the dose that the Nasion 451.2 1060.9 19.9 1008.2total body can receive and that would provide the samecancer risk as different doses to various organs.20 Thiseffective dose was calculated as follows: E ϭ ⌺ WT ϫ HT, Table IV.Mean equivalent dose (Sv) and effectivewhere E is the product of the ICRP’s tissue-weighting dose (Sv) for each unitfactor (WT) for the type or tissue or body and the Panoramic/human-equivalent dose for tissue (HT). The tissue- NewTom lateral Multi-slice 9000 i-CAT cephalometric CTweighting factor represents the contribution of eachtissue or organ to the overall risk. This dose was Bone marrow* 946.8 1007.1 211.6 872.8expressed in Sv. In this study, we used the weighting Brain 316.1 745.0 30.2 1488.9factors proposed by the ICRP in 2005 and approved by Eye 472.8 1229.2 45.8 892.8 Thyroid gland 232.4 124.3 13.1 1417.7the Main Commission of ICRP at its meeting in March Salivary glands† 1552.7 1433.15 445.5 13009.72007; they include the salivary tissue in the risk Skin‡ 1427.0 963.0 190.1 9252.1estimation.21 Effective dose 56.2 61.1 10.4 429.7 *Mean of mandibular ramus and cervical spine. † Mean of submandibular and parotid glands.RESULTS ‡ Mean of thyroid skin, neck, philtrum, parotid, and nasion skin. The average doses absorbed by the organs areshown in Table III. The mean value was obtained from9 measurements from each technique and each site (3 DISCUSSIONdosimeters in each site, performed 3 times). The lowest Most orthodontic patients are children in activeorgan dose and equivalent dose (13.1 Sv) was re- growth, who are more sensitive to the effects ofceived by the thyroid gland, during the panoramic and radiation. Images are usually required for planning,lateral cephalometric examination. The highest mean treatment evaluation, and follow-up. Many questions inorgan dose (15,837.2 Sv) was obtained for the neck orthodontic practice can be answered by conventionalskin with the multi-slice CT. Table IV gives the radiographic images alone, although a 3D view is oftenequivalent and effective doses. Again, the highest value required.6-11 However, the selection criteria for anwas associated with the multi-slice CT (429.7 Sv) image at any treatment phase should follow theduring brain exposure. The highest effective doses were ALARA principle. We found a higher effective doseobserved, in decreasing order, for multi-slice CT, i- related to the CBCT, compared with the conventionalCAT, NewTom 9000, and panoramic and lateral ceph- images usually required for orthodontic treatment. Thealometric images. choice of CBCT should be related to the patient’s
640.e4 Silva et al American Journal of Orthodontics and Dentofacial Orthopedics May 2008clinical needs. We measured the doses associated with nations. However, some orthodontic patients alsoconventional panoramic and cephalometric images. By require a temporomandibular series, posteroanteriorusing digital images, the dose could be half as much.15 cephalograms, periapical views of the anterior teeth,These doses were calculated for a young man. Due to occlusal, or bite-wing radiographs. Sometimes, a com-differences in size and susceptibility, the actual values plete-mouth survey is needed.27 Taking into accountfor children would be greater. that the effective doses related to a full-mouth radio- Among the CBCT units evaluated, the highest dose graphic survey, as reported by Gibbs,28 are 13 to 14was related to the i-CAT (61 Sv). Ludlow et al,14 Sv (with rectangular collimation), 64 to 73 Sv (withcomparing CBCTs with a larger FOV, showed higher round collimation), and 83 to 100 Sv (with short-conedoses for CB Mercuray and i-CAT, and lower dose for bisecting), the sum of the effective doses for panoramicthe NewTom, as also seen in this study. and lateral cephalometric and periapical images would Considering only the radiation dose, the use of a be in the same range or even higher than that of CBCT,CBCT image is not recommended routinely in orth- and still without 3D evaluation. Other studies shouldodontic practice. Therefore, the decision making in oral address the clinical diagnostic value of 3D imaging toradiology is a balance between the risk assessment and evaluate the risk-beneﬁt balance in using CBCT rou-the diagnostic information needed. When additionalinformation is necessary, such as for patients with tinely for orthodontic patients.impacted teeth, dental resorption, ankylosis, temporo- Comparing imaging quality, Holberg et al29 foundmandibular joint evaluation, or surgical planning, better visualization for the periodontal ligament spaceCBCT should be the method of choice, since the CT using CT compared to CBCT, although no statisticaldelivers a much higher dose, as shown in this study and tests were applied to their results. Swennen and Schu-by others.12,22,23 Farman and Scarfe24 showed that 3D tyser12 highlighted the limitations of CBCT for orth-cephalometric assessments can be made from existing odontic purposes, such as scanning volume and re-database projections. They suggested using a CBCT stricted FOV, when compared with multi-slice CT.scanner to provide a very low dose traditional cepha- However, Hashimoto et al30 demonstrated that CBCTlometric image and then 3D images from speciﬁc imaging performance was better than 4-row multi-regions. This could lead to a 3D assessment when detector helical CT. It is also expected that CBCTneeded, without unnecessary patient exposure. How- manufacturers will improve reconstruction algorithmsever, European guidelines indicate no lateral cephalo- and postprocessing imaging, providing higher resolu-gram for Class I malocclusions. In these situations, the tion to the images, while keeping the radiation exposureincreased radiation with CBCT scans at the end of to the patient as low as possible. Personal clinicaltreatment is not justiﬁable. Three-dimensional images experience should also be considered when evaluatingare probably unnecessary for all patients in orthodontic these relatively new images. Practitioners should bepractices, but clinical studies should conﬁrm this. The aware that 3D data present new challenges and aALARA principle can also be followed by using skull different approach from traditional viewing of staticcaps, eye covers, and thyroid shields during conven- images. For this, some training is necessary to prescribetional lateral cephalogram to reduce radiation exposure. the image properly and interpret the 3D images. Nev- Now, there is a tendency to use 3D imaging for ertheless, CT (not CBCT) for orthodontic purposesorthodontic planning and for ﬁnding unexpected ana- should be restricted to patients who have a real neces-tomic variations that can expand and change the treat-ment plan. However, a question remains. Are we really sity for larger FOV visualization (greater than 30 cm)missing some aspects in 2-dimensional (2D) images, or soft-tissue assessment.even without clear clinical reasons for 3D images? Thelimitations of 2D images have been well studied in the CONCLUSIONSliterature. According to McKee et al,25 mesiodistaltooth angulations in both jaws when measuring pan- From the dose to the patient point of view, theoramic radiographs are not accurate. Adams et al26 routine use of CBCT is not recommended in orthodon-showed the limitations of standard 2D cephalometry. tic procedures, because conventional images deliverEvaluating distances in 3D space with 2D images lower doses to patients. However, when 3D imaging isexaggerates the true measure and gives a distorted view required in orthodontic practice, CBCT should beof craniofacial growth. preferred over multi-slice CT. Further studies should We intended to compare CBCT images with con- address whether the diagnostic value of CBCT imagingventional panoramic and lateral cephalometric exami- justiﬁes the higher dose.
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