Cephalometry in Orthodontics. Katherine Kula
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