Absorbed dose determination for tomographic implant site assessment techniques.

A set of data to compare the absorbed dose delivered by tomographic implant site assessment techniques was generated. Absorbed doses were measured in fourteen anatomic sites from (1) computed tomography scans and (2) a series of tomographic cuts performed on a linear tomography unit. The doses to the thyroid gland, the active bone marrow, the brain, the salivary glands, and the eyes were determined with the use of a tissue-equivalent phantom with lithium fluoride thermoluminescent dosimeters at the appropriate locations.     

Absorbed doses from spiral CT and conventional spiral tomography: a phantom vs. cadaver study

For several radiological examinations, a clinician can select between conventional and spiral computed tomography. Using both techniques, this study aimed at evaluating the difference in absorbed doses when examining a single lateral jaw segment in a human cadaver head and Rando phantom. The present study involved the placement of thermoluminescent dosimeter (TLD) chips (GR-200) in the thyroid gland, and bilaterally, in the parotid and submandibular glands and the lenses of the eyes in both a human cadaver and a Rando phantom at corresponding locations. Consecutive conventional spiral tomographic examinations were carried out in both the left upper and lower premolar area, using a Cranex TOME multifunctional unit. Each examination consisted of 4 slices with a 2 mm slice thickness and exposure parameters of 57 kV, 56 seconds and 1.6-2.0 mA. Regarding spiral computed tomography (CT), a Somatom Plus S scanner (Siemens, Erlangen, Germany), with a slice thickness of 1 mm with settings at 120 kV and 165 mA, was used on both phantoms and separately in the upper and lower jaw. With conventional tomography, the findings of the present study showed that the parotid and submandibular glands on the side near the X-ray tube received the highest dose, both for the cadaver head (doses ranging from 0.5 to 1.3 mGy) and the phantom (doses ranging from 0.6 to 2.6 mGy). For CT of the upper jaw, the highest doses were delivered to the parotid glands with an average absorbed dose of 9.2 and 10.6 mGy for the cadaver head and phantom, respectively. The submandibular glands received the highest doses during CT examination of the lower jaw with an average of 7.8 and 12.9 mGy for the cadaver head and phantom, respectively. It appears from the present investigation that if small edentulous regions are examined, radiation doses during conventional tomography remain much lower than during CT imaging. However, when multiple tomographic cuts are required, a spiral CT examination can replace a series of conventional examinations, especially in cases such as the rehabilitation of an edentulous upper jaw or a more complex surgery.     

Cross-sectional imaging of the jaws for dental implant treatment: accuracy of linear tomography using a panoramic machine in comparison with reformatted computed tomography

PURPOSE: Although various panoramic X-ray machines with linear tomographic functions are now frequently applied to diagnosis related to dental implant treatment, the angles of the tomographic objective planes are automatically determined and cannot be adjusted for individual patients. To resolve this problem, a direct laser positioning (DLP) system was developed. In this investigation, the measurement accuracy of images obtained by the DLP system in comparison with those from reformatted computed tomography (CT) was assessed. MATERIALS AND METHODS: A rectangular parallel piped phantom was scanned with the system and the height and width were measured on linear tomograms. Ten sites in 3 dried mandibles and 21 mandibular molar sites in 15 patients were examined both with the DLP system and the reformatted CT to compare the measured values on both images. RESULTS: The phantom experiment showed that the difference between the actual and measured heights and widths of the phantom were within 1 mm. DISCUSSION: The difference between the values obtained by the DLP system and CT was slightly larger in the patients than those in the dried mandibles. CONCLUSION: The DLP measurement accuracy was deemed sufficient for clinical use.     

Absorbed radiation by various tissues during simulated endodontic radiography

The amount of absorbed radiation by various organs was determined by placing lithium fluoride thermoluminescent chip dosimeters at selected anatomical sites in and on a human-like X-ray phantom and exposing them to radiation at 70- and 90-kV X-ray peaks during simulated endodontic radiography. The mean exposure dose was determined for each anatomical site. The results show that endodontic X-ray doses received by patients are low when compared with other radiographic procedures.     

口腔颌面锥形束CT放射剂量与放射防护

提要：口腔颌面锥形束CT是口腔医学领域一种新型影像摄取技术。本文从以下5个方面对其放射剂量及如何进行有效防护进行了系统阐述：（1）放射量的测定;（2）口腔颌面锥形束CT与螺旋CT有效剂量比较;（3）口腔颌面锥形束CT与口腔常规X线平片检查有效剂量比较;（4）现有口腔颌面锥形束CT间有效剂量比较;（5）口腔颌面锥形束CT的放射防护。     

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口腔颌面锥形束CT是口腔医学领域一种新型影像摄取技术。本文从以下5个方面对其放射剂量及如何进行有效防护进行了系统阐述:(1)放射量的测定;(2)口腔颌面锥形束CT与螺旋CT有效剂量比较;(3)口腔颌面锥形束CT与口腔常规X线平片检查有效剂量比较;(4)现有口腔颌面锥形束CT间有效剂量比较;(5)口腔颌面锥形束CT的放射防护。     

Organ radiation dose assessment for conventional spiral tomography: a human cadaver study

The head of a human cadaver was positioned in a Cranex TOME multifunctional unit (Orion Corporation Soredex, Helsinki, Finland) to measure the organ radiation doses after tomographic examinations of the upper and lower edentulous jaw bone. Five consecutive examinations were carried out in the upper and lower anterior jaw regions to cover the entire frontal area, 2 in the upper and lower left premolar regions, and 3 and 4 in the upper and lower left molar regions, respectively. Each examination consisted of 4 slices with a 2 mm slice thickness. Thermoluminescent dosimeter chips were placed in the thyroid gland and bilaterally in the parotid and submandibular glands. Dosimetric measurements were repeated for the different tomographic examinations mentioned above. For spiral tomography in the maxilla, organ doses for both parotid glands were most elevated, while those for the thyroid glands were the lowest. Average doses per examination reached levels of 0.27 mGy for the right (OS) parotid gland with frontal tomography, and 3.89 mGy and 1.67 mGy for the parotid gland at tube-side (TS) for premolar and molar tomography. For the thyroid gland, a minimal dose of < or = 0.004 mGy was noticed for all examinations. For spiral tomography of the frontal area in the mandible, the OS parotid gland received the highest dose (0.77 mGy), while for an analysis of the premolar and molar areas, doses were more elevated for the TS parotid gland (1.22 mGy and 1.72 mGy, respectively). For the TS submandibular gland, organ doses were also raised, with values of 0.39 mGy for frontal, 1.31 mGy for premolar and 1.61 mGy for molar tomography. This study thus indicates that for conventional spiral tomographic examinations in the maxilla and the mandible, radiation doses for the TS submandibular and parotid glands were significantly more elevated than those to the thyroid gland. These values remain however below the organ doses previously reported for spiral CT involving both a full upper or lower jaw.     

Hypothetical mortality risk associated with spiral computed tomography of the maxilla and mandible

In the present study, dose measurements have been conducted following examination of the maxilla and mandible with spiral computed tomography (CT). The measurements were carried out with 2 phantoms, a head and neck phantom and a full body phantom. The analysis of applied thermoluminescent dosimeters yielded radiation doses for organs and tissues in the head and neck region between 0.6 and 16.7 mGy when 40 axial slices and 120 kV/165 mAs were used as exposure parameters. The effective dose was calculated as 0.58 and 0.48 mSv in the maxilla and mandible, respectively. Tested methods for dose reduction showed a significant decrease of radiation dose from 40 to 65%. Based on these results, the mortality risk was estimated according to calculation models recommended by the Committee on the Biological Effects of Ionizing Radiations and by the International Commission on Radiological Protection. Both models resulted in similar values. The mortality risk ranges from 46.2×10-6 for 20-year-old men to 11.2×10⁻⁶ for 65-year-old women. Using 2 methods of dose reduction, the mortality risk decreased by ∼ 50 to 60% to 19.1×10⁻⁶ for 20-year-old men and 5.5×10⁻⁶ for 65-year-old women. It can be concluded that a CT scan of the maxillofacial complex causes a considerable radiation dose when compared with conventional radiographic examinations. Therefore, a careful indication for this imaging technique and dose reduction methods should be considered in daily practice.     

Evaluation of skin dose in a low dose spiral CT protocol.

AIM: The authors evaluated radiation and skin doses absorbed by the eye lens, as well as the parotid and thyroid glands, during a low dose maxillary spiral computed tomography (CT). METHODS: Three spiral CTs were carried out, one after the other, changing from time to time MAS, pitch factor, Kv and consequently, CTDI/Vol, until the minimum values detectable by the equipment were reached (MAS 28, Kv 80, CTDI/Vol 2.5, Pitch 1). The quality of the images was evaluated on the grounds of being able to visualise the anatomic structures considered. The amount of radiation effectively absorbed by the soft tissue (skin doses) can be measured with the help of dosimeters placed at the level of eye lens, parotid and thyroid glands. The spiral CTs were performed with a Siemens 16 CT sensation machine, using a tissue equivalent Head-Neck RT Humanoid phantom. RESULTS: From the results of this study, it appears possible to obtain high quality images, useful for diagnosing numerous skeletal pathologies in orthodontics, reducing of about 90% the radiation dose per unit of volume and, at the same time, reducing the soft tissue (skin) dose of about 85% when compared to pre-established settings. CONCLUSION: The authors conclude their research emphasizing the possibility of applying low dose protocols, not only in orthodontics but in any dental field, whenever a spiral CT is needed for a more detailed diagnosis.     

Optimized 3-D CT scan protocol for longitudinal morphological estimation in craniofacial surgery

Frequent three-dimensional computed tomography scanning may cause deterioration of the lenses of the eye, which are susceptible to x-rays. The authors performed an experimental study using a phantom head to establish an optimized three-dimensional computed tomography scan protocol for longitudinal morphological estimation in craniofacial surgery. Volume computed tomography scans were performed using a Hi-Speed Advantage SG CT scanner (GE Medical Systems, Milwaukee, WI) in the axial plane with a combination of scan parameters of varied values. The radiation doses induced by each scanning were measured using thermoluminescent dosimeter chips attached to the position of the lenses in the phantom. Two-dimensional images in the coronal plane and three-dimensional images of the osseous surface were generated from each accumulated data set. For each scan parameter, the images generated from data accumulated using different values were compared. The study showed that lens radiation dose increased with tube potential and was almost directly proportional to tube current and 1/pitch. The slice thickness did not affect lens radiation doses significantly. Images with good contrast resolution and low artifact level sufficient for estimating morphological changes were obtained using a low tube potential of 100 kVp and a tube current of 100 mA. In regard to z-axis spatial resolution, a slice thickness of less than 3 mm was required for precisely pointing out bony edges in the two-dimensional reformation images. These results led us to conclude that volume computed tomography for longitudinal examination in craniofacial surgery should be kept to a minimum frequency and performed using a low-dose technique, small slice thickness, and large pitch.     

The role of objective plane angulation on the mandibular image using cross-sectional tomography

Cross-sectional jaw images in the buccolingual direction obtained by conventional or computerized tomography are used in the image diagnosis of dental implant treatment. This study was performed to clarify the subjective image quality of the mandibular depiction by shifting the angles of the tomographic objective plane. A panoramic machine with a linear tomographic function was used to obtain cross-sectional tomographic images on bilateral first molar regions of 10 dried human mandibles. The angles of tomographic objective planes were shifted horizontally within a range of +/- 20 degrees at intervals of 5 degrees from the tomographic objective plane, which was automatically determined. The image qualities of 4 anatomical structures-alveolar crest, buccal and lingual cortical bone, and mandibular canal-were subjectively scored on a 5-point scale method. As a result, the permitted tomographic objective angles were from -1.7 degrees to 2.5 degrees, a range of 4.2 degrees for all 4 anatomical structures. When this result was compared with a previous geometric result, the permitted range of the angles was quite narrow. The tomographic objective angles should be manually set in accordance with an optimal tomographic plane for individual patients by using the positioning technique in linear tomography.     

口腔医学X线检查人体受照剂量的研究

目的 测量口腔医学X线检查时人体接受的辐射剂量。方法 测量成人尸体头部在口腔X线检查时的辐射剂量。采用32片氟化锂热释光放射性测量仪贴敷在头部的不同位置以代表辐射接受部,进行口内牙片、咬合片、头部测量片、曲面断层和面颅部CT扫描的X线检查。结果 上述检查方法人体受照剂量分别为:口内牙片,0.006 mSv;咬合片,0.008 mSv;头部测量片,0.008 mSv;曲面断层,0.010 mSv和面颅部CT扫描(常规,2.020 mSv;"三维,"2.571 mSv)。结论 CT扫描较常规X线摄影具有辐射量大的显著特点,是控制医疗辐射剂量的重点。为降低患者的辐射损伤,应严格掌握X线检查(尤其是CT扫描)的适应证。     

DNA double-strand breaks of human peripheral blood lymphocyte induced by CT examination of oral and maxillofacial region

Objectives

To explore whether a computed tomography (CT) examination of the head and neck region induces biological damage and whether the damage was correlated with the radiation dose.

Materials and methods

Peripheral blood was taken from 33 individuals who received head and neck CT examinations. Blood samples were divided into three groups: the control group and the in vivo and in vitro irradiation groups. The number of DNA double-strand breaks was estimated by comparing the changes in the rates of γ-H2AX foci formation in the peripheral blood before and after CT examination. The absorbed dose and effective dose were calculated with the software VirtualDose based on the Monte Carlo method, and the absorbed doses in blood were estimated accordingly.

Results

The γ-H2AX foci rates were increased in the in vivo (p < 0.001) and in vitro irradiation groups (p < 0.001) after CT examination when compared with those in the control group. The rate of γ-H2AX foci formation showed linear dose–responses for the CT dose index volume (CTDI_vol), dose–length product (DLP), and blood dose after CT examination.

Conclusions

A CT examination of the head and neck region provides a high enough radiation dose to induce DNA double-strand breaks in cells in the peripheral blood. There was a linear correlation between the formation of DNA double-strand breaks and radiation doses after CT examination.

Clinical relevance

In addition to ensuring image quality, in a real clinical situation, the scanning area should be strictly administered, and repeated operations should be avoided to minimise the patient’s radiation dose.

     

口腔颌面锥形束CT与螺旋CT辐射剂量的比较研究

目的 比较口腔颌面锥形束CT与螺旋CT在扫描口腔颌面部相同部位时的辐射剂量,为临床安全有效应用提供实验数据.方法 使用热释光剂量芯片测量两种口腔颌面锥形束CT和一种螺旋CT在扫描头颈部体模上颌、下颌、上颌+下颌时的吸收剂量.按照国际放射防护委员会2007年推荐的组织权重因子,计算各个扫描程序的有效剂量.使用单因素方差分析对所有扫描程序得到的有效剂量进行比较分析,P＜0.05为差异有统计学意义.结果 两种口腔颌面锥形束CT的辐射剂量范围41.8 ～249.1 μSv.螺旋CT对上颌、下颌及上颌+下颌进行扫描时的有效剂量分别为506.7、829.9和1066.1 μSv,螺旋CT辐射剂量显著高于两种口腔颌面锥形束CT(P ＜0.001).同一机型仅进行上颌或下颌扫描的辐射剂量显著低于同时扫描上颌+下颌时的辐射剂量(P值分别为0.003和0.001).结论 在对口腔颌面部相同区域进行扫描时,以上两种口腔颌面锥形束CT的辐射剂量均比螺旋CT低;而两种口腔颌面锥形束CT的辐射剂量会因曝光参数的不同而有所差异.     

A comparison of a new limited cone beam computed tomography machine for dental use with a multidetector row helical CT machine

OBJECTIVE: We sought to compare a new limited cone beam computed tomography (CT) machine for dental use (3DX) with the multidetector CT machine for image quality and skin doses. STUDY DESIGN: Images of the right maxillary central incisor and the left mandibular first molar of an anthropomorphic phantom were taken by both the 3DX and the multidetector CT. A 5-point method was used to evaluate the depiction of cortical and cancellous bone, enamel, dentin, pulp cavity, periodontal ligament space, lamina dura, and overall impressions. Furthermore, the skin doses for both modalities were compared. RESULTS: The image quality of the 3DX was better than the multidetector CT for all items (P < .01). Moreover, the mean skin doses with the multidetector CT were 458 mSv per examination, whereas the doses with the 3DX were 1.19 mSv per examination. CONCLUSIONS: These results clearly indicate the superiority of the 3DX in the display of hard tissues in the dental area while substantially decreasing the dose to the patient.     

Quantitative Performance Characterization of Radiation Dose for the Carestream CS9600 Cone-Beam Computed Tomography Machine

IntroductionCone-beam computed tomography (CBCT) machines produce relatively low levels of harmful ionizing radiation, as compared with the computed tomography devices used in medical practices. The Carestream CS9600 CBCT imaging device has been recently introduced into the marketplace, and the manufacturer reports the use of an increased x-ray tube voltage (120 kVp) for the device, along with a reduced patient dose that is achieved using added filtration. Independent dosimetry studies are performed to ensure appropriate radiation exposure dose levels are within recommended safety guidelines.The purpose of this study is to independently evaluate and measure the radiation exposure dose performance parameters of the CS9600 CBCT, including its multiple field of view, exposure settings, and filtration options.MethodsA thimble ionization chamber and PMMA phantom were used to characterize dose index using the established SEDENTEXTCT evaluation method.ResultsThe phantom-obtained radiation dose index measures ranged from 0.128782–13.848 milligrays (mGy) for the various scanning options evaluated. The field of view, type of filter used, and phantom size all had a direct impact on the relationship between the experimentally obtained dose index measures and the dose area product values reported by the manufacturer.ConclusionsA strong linear correlation was observed between the experimentally obtained dose index measures and the manufacturer-reported dose area product values. The 0.7 mm Cu filter that has been added to the CS9600 reduced the exposure dose index measures even with the x-ray tube kilovoltage peak (kVp) being increased to 120 kVp, as compared with the 0.15 mm Cu filter at 90 kVp.     

Reproducibility of three-dimensional cephalometric landmarks in cone-beam and low-dose computed tomography

Objectives

The purpose of this study is to compare the reproducibility of three-dimensional cephalometric landmarks on three-dimensional computed tomography (3D-CT) surface rendering using clinical protocols based on low-dose (35-mAs) spiral CT and cone-beam CT (I-CAT). The absorbed dose levels for radiosensitive organs in the maxillofacial region during exposure in both 3D-CT protocols were also assessed.

Materials and methods

The study population consisted of ten human dry skulls examined with low-dose CT and cone-beam CT. Two independent observers identified 24 cephalometric anatomic landmarks at 13 sites on the 3D-CT surface renderings using both protocols, with each observer repeating the identification 1 month later. A total of 1,920 imaging measurements were performed. Thermoluminescent dosimeters were placed at six sites around the thyroid gland, the submandibular glands, and the eyes in an Alderson phantom to measure the absorbed dose levels.

Results

When comparing low-dose CT and cone-beam CT protocols, the cone-beam CT protocol proved to be significantly more reproducible for four of the 13 anatomical sites. There was no significant difference between the protocols for the other nine anatomical sites. Both low-dose and cone-beam CT protocols were equivalent in dose absorption to the eyes and submandibular glands. However, thyroid glands were more irradiated with low-dose CT.

Conclusions

Cone-beam CT was more reproducible and procured less irradiation to the thyroid gland than low-dose CT.

Clinical relevance

Cone-beam CT should be preferred over low-dose CT for developing three-dimensional bony cephalometric analyses.     

Reference doses for dental radiography.

OBJECTIVE: To establish reference doses for use within dental radiography. DESIGN: Retrospective analysis, single centre. SETTING: UK General Dental Practice, 1995-1998. METHOD: A statistical analysis was performed on the results from NRPB evaluations of dental x-ray equipment within general practice. The third quartile patient entrance dose was determined from 6,344 assessments of intra-oral x-ray equipment. The third quartile dose-width product was determined from 387 assessments of panoramic x-ray equipment. RESULTS: The third quartile patient entrance dose for an adult mandibular molar intra-oral radiograph is 3.9 mGy. The third quartile dose-width product for a standard adult panoramic radiograph is 66.7 mGy mm. CONCLUSION: NRPB recommends the adoption of reference doses of 4 mGy for an adult mandibular molar intra-oral radiograph and 65 mGy mm for a standard adult panoramic radiograph. These reference values can be used as a guide to accepted clinical practice. Where radiography is carried out using doses above these reference values, a thorough review of radiographic practice should be made to either improve techniques, or justify keeping the current techniques. However, attainment of doses at or below the reference values cannot be construed as achievement of optimum performance; further dose reductions below the reference value are still practicable.     

Cone beam computed tomography radiation dose and image quality assessments

Diagnostic radiology has undergone profound changes in the last 30 years. New technologies are available to the dental field, cone beam computed tomography (CBCT) as one of the most important. CBCT is a catch-all term for a technology comprising a variety of machines differing in many respects: patient positioning, volume size (FOV), radiation quality, image capturing and reconstruction, image resolution and radiation dose. When new technology is introduced one must make sure that diagnostic accuracy is better or at least as good as the one it can be expected to replace. The CBCT brand tested was two versions of Accuitomo (Morita, Japan): 3D Accuitomo with an image intensifier as detector, FOV 3 cm x 4 cm and 3D Accuitomo FPD with a flat panel detector, FOVs 4 cm x 4 cm and 6 cm x 6 cm. The 3D Accuitomo was compared with intra-oral radiography for endodontic diagnosis in 35 patients with 46 teeth analyzed, of which 41 were endodontically treated. Three observers assessed the images by consensus. The result showed that CBCT imaging was superior with a higher number of teeth diagnosed with periapical lesions (42 vs 32 teeth). When evaluating 3D Accuitomo examinations in the posterior mandible in 30 patients, visibility of marginal bone crest and mandibular canal, important anatomic structures for implant planning, was high with good observer agreement among seven observers. Radiographic techniques have to be evaluated concerning radiation dose, which requires well-defined and easy-to-use methods. Two methods: CT dose index (CTDI), prevailing method for CT units, and dose-area product (DAP) were evaluated for calculating effective dose (E) for both units. An asymmetric dose distribution was revealed when a clinical situation was simulated. Hence, the CTDI method was not applicable for these units with small FOVs. Based on DAP values from 90 patient examinations effective dose was estimated for three diagnostic tasks: implant planning in posterior mandible and examinations of impacted lower third molars and retained upper cuspids. It varied between 11-77 microSv. Radiation dose should be evaluated together with image quality. Images of a skull phantom were obtained with both units varying tube voltage, tube current, degree of rotation and FOVs. Seven observers assessed subjective image quality using a six-point rating scale for two diagnostic tasks: periapical diagnosis and implant planning in the posterior part of the jaws. Intra-observer agreement was good and inter-observer agreement moderate. Periapical diagnosis was found to, regardless of jaw, require higher exposure parameters compared to implant planning. Implant planning in the lower jaw required higher exposure parameters compared to upper jaw. Substantial dose reduction could be made without loss of diagnostic information by using a rotation of 180 degrees, in particular implant planning in upper jaw. CBCT with small FOVs was found to be well-suited for periapical diagnosis and implant planning. The CTDI method is not applicable estimating effective dose for these units. Based on DAP values effective dose varied between 11-77 microSv (ICRP 60, 1991) in a retrospectively selected patient material. Adaptation of exposure parameters to diagnostic task can give substantial dose reduction.     

Reduction of body doses in rotational panoramic radiography by means of reduced beam width in combination with rare earth intensifying screens

The absorbed doses in the body of a Rando-Alderson phantom were measured in rotational panoramic radiography (OP-5) with LiF-700 crystals using conventional intensifying screens and standard collimator, as well as rare earth intensifying screens and a collimator slit of reduced width. The reduction of the absorbed doses obtained by using the latter combination was calculated. The reduction range in different regions was 0-50%. The greatest reduction was obtained in the cranial part of the phantom. In the caudal part little or no reduction was observed, probably because leakage radiation contributes substantially to the absorbed doses in this area. The average dose equivalent was calculated for organs in which measurements were made in several locations. The reduction was about 50% for these organs except for the gonads (8%). Compared to the estimated average annual dose equivalent originating from the natural background radiation sources, the dose equivalent resulting from this X-ray examination was very small except in the thyroid gland. In this organ the dose equivalent from one exposure corresponded to 36 days of natural background radiation, but it could be reduced to 18 days by using rare earth intensifying screens in combination with reduced beam width.