Cone Beam CT of the Head and Neck: An Anatomical Atlas
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As before, the book introduces readers to the different ways of viewing CBCT data sets and guides clinicians in identifying familiar and unfamiliar anatomical landmarks in the three planes of section axial, sagittal, and coronal.
Cone Beam CT of the Head and Neck An Anatomical Atlas - Ghent University Library
New to this edition are chapters presenting endodontic applications of CBCT, selected cases from radiology practice, and issues of risk and liability associated with capturing CBCT data. In addition, the anatomy chapter has been updated with many new illustrations and a new section on small-volume anatomy. Comprehensive case presentations demonstrate the diagnostic and treatment-planning capabilities of CBCT in its full range of applications while at the same time highlighting situations in which traditional radiographic imaging will suffice. The pages and pages of various CBCT images and coloured renderings are amazing in both quality and detail.
My knowledge of anatomy has never had such a boost! I found them all so clear and easy to look at and absorb. An extremely valuable book and a great source of reference. It is concluded that as long as the image quality is acceptable for diagnostic purposes, the mAs of the CBCT should be kept in a low range to minimize the absorbed dose. Currently, cone beam computed tomography CBCT is an acceptable dental imaging modality used for every field in dentistry including implant planning, orthodontics, and maxillofacial surgery 1.
Compared to other dental imaging techniques, CBCT has great advantages such as excellent spatial resolution, overlap of teeth, and acquisitioning three dimensional 3D volumes of dental arches and surrounding tissues 2 , 3. In addition, images obtained from CBCT have excellent tissue contrast due to eliminating blurring and provide orthogonal views by reducing projection effects 4. CBCT has further advantages such as cost beneficience and considerably reduced effective radiation dose compared to regular CT 5. While CBCT has the capability of producing 3D images with significantly less radiation than conventional CT, settings of the device plays a key role in the resulting radiation dose.
The setting parameters include kilovoltage peak kVp and milliamperage mA 6 , 7. Previous studies have shown that both the lower and the higher exposure settings in medical CT units result in acceptable image quality 6 - Although conventional dental imaging modalities still deliver lower radiation doses to patients, for some special cases in orthodontic treatment planning, undoubtedly a CBCT is preferred over a CT image 8.
Ludlow and co-workers found that as a dose sparing technique, dental CBCT is recommended compared to standard clinical scans for dental and maxillofacial radiographic imaging. Pauwels and co-workers showed that the dose for different organs varies in a wide range of value because of several factors such as radiation exposure setting, characteristics of primary beam and beam positioning respect to sensitive organs 9. They found that with the highest available kVp setting, the most optimal contrast is achievable at a fixed dose.
Cone Beam Ct of the Head and Neck: An Anatomical Atlas by Kenneth Abramovitch (E
There was a great potential for dose reduction through mA with a minimal loss in image quality Palomo and co-workers investigated various exposure settings, filters, and different collimation, and they found that lower settings and using the available collimation options a result in reduction in radiation dose In recent years, the number of accessible CBCT units has increased and new models have continuously been established. These devices cover a wide-ranging variability in terms of essential setting parameters including kVp, mA, filtration, and field of view.
In addition, there is a degree of possibility in many devices for selecting certain exposure factors. The amount of absorbed dose in organs and the image quality for each scan depends on the type of device and imaging protocols. To determine the risk of radiation for patients from X-ray imaging modalities, the effective dose is the preferable parameter over other alternatives 15 - In dosimetry, it is impossible to directly place any kind of dosimeter inside an internal organ of body. Therefore, in order to measure the effective dose, an anthropomorphic phantom representing of an average human is frequently used In the present study, we aimed to evaluate the effect of mA variation on the absorbed dose of the mandible and salivary glands according to various values of mA routinely used in CBCT.
In this study, a two dimensional film dosimetry method is represented by using radiographic film, and using the head phantom the effect of different milliamparages has been investigated on the distribution and amount of the received dose. In this study, the measurements were performed by using a head phantom made of soft tissue-equivalent materials. The tissue-equivalent substitutes used for the phantom should meet two goals: similar physical properties to human tissue, such as density and attenuation coefficients, and simplicity of integration into the phantom construction process A urethane-based resin was used to simulate the X-ray attenuation and density of human soft tissue.
Another material combined from urethane-based and calcium carbonate CaCO 3 was used to match human bone tissue within the diagnostic energy range 80 - kVp Various factors were considered for designing and constructing the phantom. Regarding jaw dimension, teeth positioning and other segments of the dentomaxilla area, phantom designing was performed considering capability of dosimetry within mentioned situations in two parts including the head and jaw.
The phantom was designed based on actual axial sections of a patient CT scan. Therefore, the geometry of each structure was matched accurately to the real head and radiological image of the phantom had great precision In the first step, we created a 3D file from CT scan images of a normal patient using 3D-Doctor software. Then, we extracted some parts of the maxillofacial area in Rhino software and a 3D file compatible to carbon dioxide laser Co2-laser machine was created and cut from perspex as soft tissue material. Bone equivalent material was a combination of a polyurethane based resin and CaCo 3 that was prepared using a laboratory mixer.
At last we used some simple tools to embed this material in the desired segments Figure 1 shows the slices of the phantoms in which equivalent bone material was embedded and the completed head phantom. The selected areas for dosimetry were the sections in which the parotid gland, submandibular gland, and mandibular bone were placed. To achieve this goal, the precise locations of bones were determined so that the difference in the absorbed radiation dose between soft tissue and bone was clear. It should be noted that all slices of the phantom were prepared using automatic laser cutter.
This laser cutter was moved along the border of the organ which was imported to the software. The imported data to software were derived from the CT scan of the patient directly. Finally, the constructed phantom was imaged by a Siemens slices CT scanner in order to obtain Hounsfield unit measurements of the equivalent soft and bone tissues Figure 2.
The obtained Hounsfield units were 38 and respectively which was in good agreement with real soft and bone tissue. This film is suitable for measurements of dose in the energy ranges of 20 - kVp X-rays Optical density and dose radiation of this film has a linear relationship. Each film was placed in a specific packet composed of three different layers for protection against light and humidity. In the first step, the film calibration curve was obtained by using the analogue radiology machine Varian-A tube.
Then film response relative to radiation exposure is measured and calculated by the given specific dose with 0. In order to produce an equal condition for calibration setting, the film was placed at cm distance of the tube under a 2 cm Plexiglass sheet. Regarding the capability of manual setting, the mA was considered as a varying factor and then scans were performed in the usual conditions of dental imaging of adults at 90kVp. We specified the situation of the mandible, parotid and submandibular glands on two adjacent slices on the phantom based on the atlas of anatomy.
Then, the films were placed between the two slices anatomically compatible with the desired organs Figure 3. For all these measurements as a routine procedure, gantry was rotated around the phantom in degree arc in the counter clockwise CCW direction. For every value of mA, measurements were repeated three times. The films were placed between the two slices anatomically compatible with the desired organs Figure 3.
These values of 6, 8 and 10 milliamperage are commonly used for adult dental scans. After processing and developing of the films, each one was scanned by a Microtek XL film scanner. This scanner has the capability of film scans with large sizes. All the images were saved in TIF format to keep the maximum information of the films.
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Finally, distribution of absorbed doses and isodose curves were obtained. The average values of three values of absorbed dose for each organ and the standard deviation were also calculated with MATLAB software. Table 1 illustrates the dose for each part selected on the phantom and Table 2 shows the ratio of average doses at 2, 4, 6, 8 and 10 mA settings to dose at 12 mA setting and 90 kVp.
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In addition, P value was calculated between absorbed dose related to every paired mAs in each organ. Also, Pearson coefficient was calculated for any of organs between for each value of mAs and its related absorbed dose. As shown in Table 1 , increasing mA has resulted in an increase in the average absorbed dose delivered to each part.
Correlation coefficient between mA and absorbed dose for each value of mA related to any of organs is positive and it shows that there is a positive correlation between mA and absorbed dose for any organ in each value of mA. The maximum and the minimum absorbed dose is placed in the area related to the right mandible and the left submandibular gland, respectively.
By changing the value of mA from 2 to 12, absorbed dose varied significantly, with a maximum 5. As mentioned in the previous sections, in order to meet ALARA principle, it is necessary to exist a balance between dose and image quality. Previous studies show that low exposure settings of radiation factors in medical CT scan units might result in similar image quality to higher exposure settings 5 - This study shows that increase in milliamprege of the device from 2 to 12 mA in routine condition in which kVp is set on 90, shows a meaningful increase in the absorbed dose.
This result is in conformity with a study conducted by Palomo et al.