Читать книгу Cephalometry in Orthodontics - Katherine Kula - Страница 9
ОглавлениеTwo-dimensional (2D) cephalometry has been an integral component of orthodontic patient assessment since Broadbent described the technique in 1931.1 For years the only image receptor for cephalometry was radiographic film, which limited the clinician to a 2D patient assessment. Today there are multiple receptor options such as photostimulable phosphor (PSP), charge-coupled device (CCD), and derived 2D data from cone beam computed tomography (CBCT). While CBCT allows for clinicians to evaluate the patient in three dimensions, most patient assessment is still performed on 2D data. This chapter discusses the various techniques for generating traditional 2D cephalograms, cephalograms derived from three-dimensional (3D) data, radiation exposures, and advantages/disadvantages of the various techniques and image receptors.
Patient Positioning
Regardless of image receptor, proper patient positioning is essential to producing an acceptable cephalometric image.
Lateral cephalogram
The patient’s head is positioned with the left side of the face next to the image receptor, with the midsagittal plane parallel to the image receptor and the Frankfort plane parallel to the floor2 (Fig 2-1). The patient’s head should be stabilized in a cephalostat. The cephalostat is a device with two ear rods that are placed in the external auditory meatuses and a nasion guide. Head stabilization serves two purposes: (1) It diminishes patient movement, and (2) it ensures reproducibility to allow for sequential evaluation over time.
Fig 2-1 Proper patient positioning for a lateral cephalogram showing the cephalostat around the patient’s head.
The radiation source is positioned so that the distance between the source and the midsagittal plane is 60 inches. The receptor (or film) should be placed 15 cm (approximately 6 inches) from the midsagittal plane. The x-ray beam should be collimated to the size of the receptor, and the center of the beam should be directed through the external auditory meatus (Fig 2-2). Because of the projection geometry, structures away from the receptor will be magnified more than the structures close to the receptor.
Fig 2-2 Graphic representation of patient positioning for a lateral cephalogram (viewed from above).
Posteroanterior cephalogram
Patient positioning for the posteroanterior (PA) cephalogram uses the same armamentarium as the lateral cephalogram. The patient is positioned with the midsagittal plane perpendicular to the receptor (nose toward the receptor) and the Frankfort plane parallel to the floor (Fig 2-3). The source should be 60 inches from the ear rods, and the receptor should be positioned 15 cm from the ear rods.2
Fig 2-3 Proper patient positioning for a PA cephalogram.
Patient Protection
There has been a great deal of discussion regarding the need for shielding of the patient from the primary beam. The American Dental Association,3 National Council on Radiation Protection and Measurements,4 and US Food and Drug Administration3 have created fairly specific recommendations for patient shielding for intraoral imaging but not for extraoral imaging. Nevertheless, many of the factors involved in the recommendations are applicable to extraoral imaging. Use of fast image receptors and collimation of the primary beam to the size of the receptor significantly reduce the dose to the patient. However, these factors do not reduce the dose to zero. Consequently, there is slight potential for risk to the patient.
The effects of high doses of x-radiation are well documented, but the effects of low doses of radiation have only been inferred or derived from a model. The accepted model for determining risk from x-radiation is the linear, nonthreshold model. This model suggests that risk is directly related to radiation dose. The concept of threshold is that there is a level of exposure below which there is no risk. The nonthreshold model indicates that there is no safe dose of radiation to the patient. Ludlow et al calculated effective doses for commonly used dental radiographic examinations and reported effective doses of 5.6 microsieverts (μSv) for a lateral cephalogram (using PSP) and 5.1 μSv for the PA cephalogram (using PSP).5 For comparison, they reported an effective dose for panoramic imaging (CCD) ranging from 14.2 to 24.3 μSv and 170.7 μSv for a full-mouth series using F-speed film and round collimation.5 In a different study, Ludlow et al evaluated the effective dose from several CBCT systems and reported effective doses ranging from 58.9 to 557.6 μSv.6 For a comparison, the paper also reported an effective dose for conventional CT of 2,100 μSv for a maxillomandibular scan.6
There is a huge range of effective doses for these imaging modalities for many reasons. First, all of these systems use different exposure factors (eg, kilovoltage peak [kVp], milliamperage [mA], exposure time) and cover many different critical organs. The critical organs most commonly included in dose calculation for the maxillofacial complex are the thyroid gland, salivary glands, and bone marrow. This gets complicated pretty quickly. Now imagine what it must be like for the patient and parent when you start to describe effective dose. A better way to talk to the patient and parent is the concept of benefit versus risk. Explain to the patient and/or parent the reason you need the radiographic image (eg, asymmetry, impacted teeth) and that the risk to the patient is minimal. There is even some research that indicates that the lap apron provides no added protection from scatter radiation.7 This study, while not directly applicable, looks at the imaging modality that most closely approximates the field of view for orthodontic evaluation (panoramic). Still, it is important to realize that patients and parents are concerned about any radiation exposure. It probably takes less time to shield the patient with a lap apron than to explain why you do not need it. The thyroid collar should not be used for either 2D cephalometry or 3D CBCT.
Exposure Factors
All radiographic imaging is predicated by differential absorption of the x-ray beam by the region of interest. Multiple exposure factors need to be adjusted depending on the patient’s size and bone density. These exposure factors—kVp, mA, and exposure time—are discussed below.
Kilovoltage peak (kVp)
Kilovoltage refers to the energy or penetrating power of the x-ray beam. Peak simply refers to the highest energy in a polyenergetic beam. The mean beam energy is generally considered to be one-third of the peak. As kilovoltage increases, the beam energy and penetrating power also increase. Conversely, lower kilovoltage produces lower beam energy and generates photons that are more likely to be absorbed by the region of interest. Kilovoltage should be increased for patients with large or dense facial bones and decreased for patients with small or less dense facial bones. Most cephalometric units function in a range of 70 to 90 kVp. CBCT units function between 90 and 120 kVp.
Milliamperage (mA) and exposure time
Milliamperage is the determinant of the tube current and controls the number of photons of x-radiation that are produced in the tube head. It is often adjusted because of the density of the soft tissues of the head and neck, and it is often reported together with exposure time (seconds). Both mA and exposure time have a direct relationship with output. It is important to remember that mAs = mAs. This simply means that as long as the product of mA and exposure time remains constant, the output of the machine will also remain constant. For example, if mA is 5 and the exposure time is 0.5 seconds, the mAs is 2.5. If the milliamperage is 10, the exposure time would need to be decreased to 0.25 seconds to maintain output.
Collimation/Soft Tissue Filtration
The shape and size of the primary beam of x-radiation is controlled by collimation of the beam. The radiation beam should be collimated to the size of the image receptor. Collimating the beam to the size of the receptor decreases the exposure and dose received by the patient. The cephalometric unit should have a mechanism to filter the soft tissues of the nose and lips. Generally, the x-ray beam is generated to penetrate bony structures and will burn out soft tissue structures. The soft tissue filter attenuates the beam prior to it contacting the patient, providing some radiation protection to the patient and decreasing the energy of the beam so that the soft tissues will be enhanced in the cephalogram.
Image Distortion/Magnification
A 2D cephalogram will contain some image distortion in the form of differential magnification because a 3D object is being imaged using diverging radiation rays. Structures away from the image receptor will be magnified much more than objects that are positioned close to the image receptor. Magnification is calculated by dividing the distance from the source of radiation to the image receptor (SID) by the distance from the source to the object of interest (SOD). Based on this calculation, it is easy to see that the right and left sides of the skull will be different sizes in a lateral cephalogram. Because there is a potential for distortion just from projection geometry, it is essential to either record the distance from the center of the cephalostat to the image receptor or to establish a standard distance when evaluating sequential cephalograms.
Image Receptors
Film-based systems
Film-based cephalometry employs indirect-exposure radiographic film positioned between two intensifying screens. Intensifying screens convert x-radiation into light. Indirect-exposure film is more sensitive to light than it is to x-radiation. As a consequence, the use of intensifying screens and indirect-exposure radiographic film allows for very low exposure times. Different types of intensifying screens emit different wavelengths of light. Care must be taken to match the spectral sensitivity of the film to the light emitted from the intensifying screens. Rare-earth intensifying screens emit either green or blue light. Traditional or par screens emit purple light. If intensifying screens emit a green light, you must use green-sensitive film to produce an acceptable image. Figure 2-4 shows examples of film-based lateral and PA cephalograms.
Fig 2-4 Examples of film-based lateral (a) and PA (b) cephalograms.
Darkroom procedures
As with any film-based imaging, chemical processing must be performed to convert the latent or chemical image into a visible image. All film processors go through the same steps: development, fixation rinse, and drying. The function of the developer is to convert the silver ions on the film into metallic silver. The process of fixation stops the development process and renders an archival image. The quality of correctly processed film images will not change over time; unfortunately, however, most images are not correctly processed. Quality assurance in the darkroom is essential for quality film-based imaging. Quality assurance pertains to many components of the darkroom—lighting as well as the activity of the processing chemistry. Processing chemistry must be replenished every day. Developer and fixer activity diminish due to workload rather than time, so it is essential to have an ongoing program of assessing the activity of the chemistry. Finally, because direct- and indirect-exposure films require different safelight filters, make sure that the safelight in the darkroom does not fog the film prior to processing.
Digital systems
Digital receptors are divided into two groups: indirect digital and direct digital systems. PSP plates are considered to be indirect digital sensors, whereas CCD and complementary metal oxide semiconductors (CMOS) are considered to be direct digital sensors. There are many advantages to the use of digital receptors (Box 2-1). In addition to reduced exposure, a huge advantage is the ability to enhance images once they are captured. Electronic image storage and image transmission are also advantages of digital receptors compared with film-based systems. While automated analysis can be performed on a film-based image that is converted into a digital image through a process called analog to digital conversion, data is lost in the process, whereas with digital images automated analysis can be performed without any lost data. Staff efficiency is also increased with the use of digital receptors: There is no downtime spent waiting for the image to be processed.
Box 2-1 Advantages and disadvantages of digital cephalometric imaging
Advantages
•Exposure reduction
•Image enhancement
•Digital image storage
•Automated analysis
•Image transmission
•Increased staff efficiency
Disadvantages
•High initial cost
•Differences in projection geometry
There are two potential disadvantages to the use of digital receptors: (1) cost and (2) differences in projection geometry (see Box 2-1). There is no doubt that the initial cost of a digital cephalometric unit is higher than the cost of a film-based system. However, when one factors in the costs of film, processing chemistry, and lost staff efficiency, the difference in initial cost is recouped rather quickly. The issue of differences in projection geometry is covered in the section entitled “Digital Versus Conventional Cephalometry.”
PSP plates
PSP plates are image receptors that convert x-radiation into an electrical charge contained within the imaging plate. PSP plates come in all sizes (from 0 to an 8 × 10-inch plate) for cephalometry. The PSP plate is placed in the 8 × 10-inch cassette with the intensifying screens removed. The imaging plate is coated with europium-activated barium fluorohalide. The electronic information is converted into a visible image by subjecting the phosphor plate to a helium-neon laser. The PSP plate in turn emits a blue-violet light at 400 nm that is captured by the scanner and converted into a digital image. As a final step, the plate must be exposed to white light to remove the latent image; this step is performed in most scanners automatically. PSP plates are considered to be indirect digital images because the x-ray data is captured as analog or continuous data and converted into digital data in the scanner. This is the same reason that film-based images that are scanned as digital images are considered to be indirect digital images.
CCD/CMOS receptors
Direct digital cephalometric x-ray machines use either a CCD or CMOS receptor for image capture. While these two types of digital receptor differ with regard to image capture and data transfer, both generate comparable images. Some panoramic cephalometric combination machines use only one sensor that has to be moved depending on the type of image captured. Other combination machines use two sensors, which is much more efficient and decreases the risk of damaging the sensor by dropping it. The majority of these units capture an image in a scanning motion either horizontally or vertically (Figs 2-5 and 2-6). This type of image capture differs from film-based and PSP imaging, which capture the image in a single exposure. Image capture with the scanning motion requires the patient to remain motionless for up to 10 seconds. The possibility for motion artifact increases as the exposure (or in this case scanning) time increases. At least two companies (Carestream and Vatech) have produced a “one-shot” image capture system that potentially can create the same projection geometry as conventional cephalometry while significantly decreasing the time the patient must remain motionless.
Fig 2-5 Graphic representation of scanning motion for direct digital cephalometric units.
Fig 2-6 Examples of direct digital (scanned) lateral (a) and PA (b) cephalograms.
Digital Versus Conventional Cephalometry
Not all digital cephalometric images are the same. Cephalometric images captured on a PSP plate have the same projection geometry used to capture a film-based image. The majority of digital receptors, however, capture the image with a scanning motion and therefore have different magnification factors than in film-based cephalometry. Chadwick et al reported differences among several different systems that appear to be system dependent and recommended that the magnification factor be experimentally determined prior to any cephalometric analysis.8 McClure et al compared digital cephalometry with film-based cephalometry and found no differences in linear measurements; however, in their study, pretreatment cephalograms were compared with posttreatment cephalograms.9 The time frame between pre- and posttreatment images may introduce the confounder of active growth during the orthodontic treatment.
CBCT
CBCT began to appear in the late 1990s. CBCT machines consist of a radiation source shaped like a cone and a solid-state detector that rotates around the patient’s head and captures all of the scan data in a single rotation.10 This raw data is then reconstructed in the coronal, axial, and sagittal planes (also known as multiplanar reformation) (Fig 2-7). The data can be further reconstructed to produce either 2D images such as panoramic or cephalometric images (Fig 2-8) or 3D data sets11 (Fig 2-9). The images produced with CBCT are not magnified, so standard cephalometric analysis must be altered to address this difference in projection geometry.
Fig 2-7 Multiplanar reformation.
Fig 2-8 Examples of CBCT-derived lateral (a) and PA (b) cephalograms.
Fig 2-9 Examples of 3D reconstructions. (a) Lateral cephalometric rendering. (b) PA cephalometric rendering. (c) Submentovertex rendering.
While the name implies similarity with conventional CT, the two technologies differ in a number of ways. The most important difference for the patient is the difference in dose. Conventional CT produces a four- to tenfold higher dose than CBCT when imaging the maxillofacial region.6 There are several reasons for this difference in dose, but the fundamental difference is that CBCT captures the entire data set in one rotation, whereas conventional CT requires multiple rotations to capture the data. This single rotation decreases the dose but also is more susceptible to patient motion. If the patient moves during conventional CT imaging, only that slice of data is impacted. However, patient movement affects every voxel during CBCT image capture. Another difference has to do with the imaging of soft tissue. Conventional CT uses a high mA, which contributes to the soft tissue contrast; CBCT uses a fairly low mA, with minimal soft tissue contrast. CBCT will capture soft tissue, but the soft tissue is displayed as a fairly homogenous image.
Selection criteria
CBCT can provide a wealth of information regarding the maxillofacial regions. The ability to generate 3D images greatly enhances the treatment-planning process for many patients but may be unnecessary for some patients. The decision to scan or not to scan will be dependent on the patient’s condition. The delineation of whom to scan is called selection criteria. In 2013, the American Academy of Oral and Maxillofacial Radiology published clinical recommendations for the use of CBCT in orthodontics.12 The first recommendation is to use the appropriate imaging modality based on the patient’s clinical presentation and history.12 The second recommendation pertains to radiation risk assessment, and the third recommendation addresses ways to keep the dose to the patient as low as reasonably achievable (ALARA),12 such as focusing on resolution, scan time, and field of view (FOV).
Scanning protocol
Because the goal in orthodontic imaging is to get the best data with the lowest dose to the patient, several factors should be included in the scanning protocol. The first consideration is the volume of the scan. Volume will be dictated by the choice of the FOV. The larger the FOV, the higher the dose to the patient. Keeping the FOV as small as possible will minimize the dose to the patient. Determining resolution requirements is another way to diminish dose to the patient. Generally, the higher the resolution, the higher the dose. Using the lowest resolution that still provides adequate diagnostic information is a good way to decrease the dose to the patient. Finally, the last consideration is exposure time. As with any radiographic imaging, the longer the exposure time, the higher the dose. Therefore, the shortest scan time that provides adequate diagnostic information should be used. The imaging protocol should address all of these parameters.
Patient positioning and preparation
The patient should be draped with a lap apron for image acquisition. Patient positioning differs with every commercially available scanner. Some systems capture the image with the patient standing, while others capture the image with the patient seated. Regardless of manufacturer, all units provide some form of head stabilization. It is important to position the patient’s head with the Frankfort plane parallel to the floor and the midsagittal plane perpendicular to the floor. While the position of the head can be altered during the image-reconstruction process, the same cannot be said for the cervical spine. Many CBCT scanners provide a bite stick for patient positioning. The bite stick produces an end-to-end occlusion that can alter the width of the airway and the condyle/fossa relationship. The use of the bite stick should therefore be avoided.
Image reconstruction
Once the appropriate scan is captured, the acquisition computer will generate data as a multiplanar reformation (MPR) providing images in the sagittal, coronal, and axial planes. While this data is captured in a 3D matrix, the MPR images are sequential 2D images. Further reconstruction is needed to generate useful 3D data. Additionally, conventional 2D images—cephalograms and panoramic radiographs—can be derived from the 3D data. As previously stated, there is no magnification in the CBCT-derived cephalograms as opposed to conventionally acquired cephalograms. Numerous studies have quantified the differences between landmark identification in conventional versus CBCT-derived cephalograms.13–15 For the most part, the identification of landmarks has been comparable between the two. Some of the outcomes of these studies suggest that some “landmarks” visualized in two dimensions are not necessarily point landmarks in 3D data. Research is ongoing to better elucidate cephalometric landmarks in 3D data sets.
References
1.Broadbent BH. A new x-ray technique and its application to orthodontia. Angle Orthod 1931;1:45–66.
2.Jacobson A, Jacobson RL (eds). Radiographic Cephalometry: From Basics to 3-D Imaging, ed 2. Chicago: Quintessence, 2006.
3.American Dental Association. Dental Radiographic Examinations: Recommendations For Patient Selection And Limiting Radiation Exposure [PDF]. http://www.ada.org/en/∼/media/ADA/Member%20Center/FIles/Dental_Radiographic_Examinations_2012. Accessed 22 May 2017.
4.Brand JW, Gibbs SJ, Edwards M, et al. Radiation Protection in Dentistry [Report 145]. Bethesda, MD: National Council on Radiation Protection and Measurements, 2003.
5.Ludlow JB, Davies-Ludlow LE, White SC. Patient risk related to common dental radiographic examinations: The impact of 2007 International Commission on Radiological Protection recommendations regarding dose calculation. J Am Dent Assoc 2008;139:1237–1243.
6.Ludlow JB, Davies-Ludlow LE, Brooks SL, Howerton WB. Dosimetry of 3 CBCT devices for oral and maxillofacial radiology: CB Mercuray, NewTom 3G and i-CAT. Dentomaxillofac Radiol 2006;35:219–226.
7.Rottke D, Grossekettler L, Sawada K, Poxleitner P, Schulze D. Influence of lead apron shielding on absorbed doses from panoramic radiography. Dentomaxillofac Radiol 2013;42:20130302.
8.Chadwick J, Prentice RN, Major PW, Lam EW. Image distortion and magnification of 3 digital CCD cephalometric systems. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2009;107:105–112.
9.McClure SR, Sadowsky PL, Ferreira A, Jacobson A. Reliability of digital versus conventional cephalometric radiology: A comparative evaluation of landmark identification error. Semin Orthod 2005;11:98–110.
10.White SC, Pharoah MJ. Oral Radiology Principles and Interpretation, ed 7. St Louis: Mosby/Elsevier, 2014.
11.Swennen G, Schutyser F, Hausamen J. Three-Dimensional Cephalometry: A Color Atlas and Manual. Berlin: Springer-Verlag, 2006.
12.Clinical recommendations regarding use of cone beam computed tomography in orthodontics. Position statement by the American Academy of Oral and Maxillofacial Radiology. Oral Surg Oral Med Oral Pathol Oral Radiol 2013;116:238–257 [erratum 2013;116:661].
13.Park JW, Kim N, Chang YI. Comparison of landmark position between conventional cephalometric radiography and CT scans projected to midsagittal plane. Korean J Orthod 2008;38:426–436.
14.Chien PC, Parks ET, Eraso F, Hartsfield JK, Roberts WE, Ofner S. Comparison of reliability in anatomical landmark identification using two-dimensional digital cephalometrics and three-dimensional cone beam computed tomography in vivo. Dentomaxillofac Radiol 2009;38:262–273.
15.Zamora N, Llamas JM, Cibrián R, Gandia JL, Paredes V. Cephalometric measurements from 3D reconstructed images compared with conventional 2D images. Angle Orthod 2011;81:856–864.
