Many people first encounter medical imaging through routine dental checkups, which often involve an x-ray “screening” exam. In such exams, x-ray images are taken of the teeth in order to catch problems early, before symptoms arise. Dental imaging is the most common type of medical imaging, with roughly 100 million exams performed per year in the US.
Types of dental imaging
There are three main types of dental imaging:
- Intraoral radiography
- Panoramic radiography
- Cone-beam computed tomography
Intraoral radiography is the most common form of dental imaging. It involves placing a small image receptor inside the mouth and behind the teeth of interest and then briefly irradiating the receptor with a narrow x-ray beam from outside the cheek. The image receptor may be either an x-ray film or a digital device, and four exposures are typically obtained during a screening exam.
(Left) An x-ray machine for intraoral dental radiography.
(Middle) Digital electronic image receptors.
(Right) A bitewing intraoral radiograph.
(Images courtesy of Wikipedia Commons / Public Domain)
In panoramic radiography the x-ray source and receptor rotate together around the patient’s head to produce an image which shows all the teeth and surrounding jaw. This is considered a form of extraoral imaging because the image receptor is located outside the patient’s head.
A panoramic dental x-ray image. (Image courtesy of Wikipedia Commons / Public Domain)
In cone-beam computed tomography (CBCT) an x-ray source and image receptor rotate around the patient’s head as in panoramic radiography, but in CBCT the digital image receptor captures a series of projections at many different angles and a computer analyzes the data to create a 3-dimensional model of the patient’s anatomy. This model can be used to produce 2D and 3D images of various types and perspectives according to the needs of the physician. Such images are especially valuable for implant or surgical planning.
Radiation doses in dental imaging
Less radiation is used for dental imaging than for most other types of medical imaging, so shielding of the patient is not necessary for routine checkup x-rays. The effective radiation dose from intraoral radiography is about 0.005 mSv/exam, while panoramic radiography delivers about 0.025 mSv/exam and cone-beam computed tomography typically delivers 0.18 mSv/exam. For comparison, the radiation dose a person receives in the US from naturally occurring sources is roughly 0.008 mSv/day, which means the radiation dose from a typical dental checkup exam is roughly the same as that received just from being on earth for a day! See this website for more information about radiation dose from medical imaging.
Role of the medical physicist in dental imaging
As with other types of medical imaging, the medical physicist serves as an expert in image quality as well as in radiation safety for patients and staff. A medical physics survey includes an assessment of equipment performance plus measurements of radiation dose levels using specialized, calibrated instruments. Depending on state regulations, a medical physics survey may not be required for routine intraoral radiography systems because of the low radiation doses involved. For cone-beam computed tomography (CBCT) systems, however, many states require that a medical physicist perform an initial evaluation and then subsequent annual or biannual surveys. This is because CBCT involves higher radiation doses and is considered a form of advanced imaging that cannot be adequately evaluated through inspections by state regulators.