23% compared to craniofacial (extra‐large) FOV.
Table 4.1 is based on typical exposure protocols and is calculated from data collated from multiple published studies. The levels of standard 2D dental radiography, CT and CBCT patients’ median effective dose are compared and are shown as equivalent to daily background radiation. An average small FOV effective dose is 50 μSv, while that of a dental panoramic is about 20 μSv. A complete mouth series done with a round collimator ranges from 100 (CCD) to 200 (photostimulable phosphor plate, PSP) μSv.
What do these figures mean to the lay public? With our responsibility as dentists to convey information in a manner understandable to those seeking our treatment and in order to obtain informed consent, it is imperative to present the issue in its context, without blinding the patient with scientific data. Thus, it may be more pertinent to use the comparison that (a) the average person receives a dose of about 8 μSv per day or 2700 μSv per year from the environment [28]; and (b) flying from New York to Tokyo by the transpolar route exposes the passenger to ionizing (cosmic) X‐rays of approximately 150 μSv and from New York to Seattle approximately 60 μSv [29].CBCT represents state‐of‐the‐art technology, with direct relevance to the determination of macroscopic anatomy and accurate positional diagnosis of impacted teeth. The machinery is not beyond the financial means of most hospitals, radiology institutes, imaging centres, dental clinics and dental school radiology departments. Its advantages to the orthodontist and surgeon are manifest. Its level of emitted ionizing radiation is low and the cost to the patient affordable. It is a recommended procedure for many of the cases discussed within the context of this book.
Table 4.1 Typical effective dose from radiographic examination.
Source: Reproduced by kind permission of Dr S.M. Mallya and Elsevier Publishers.
| Examination | Median Effective Dose | Equivalent Background Exposurea |
|---|---|---|
| Intra‐oral b | ||
| Rectangular collimation | ||
| Posterior bite‐wings: PSP or F‐speed film | 5 μSv | 0.6 day |
| Full‐mouth: PSP or F‐speed film | 40 μSv | 5 days |
| Full‐mouth: CCD sensor (estimated) | 20 μSv | 2.5 days |
| Round collimation | ||
| Full‐mouth: D‐speed film | 400 μSv | 48 days |
| Full‐mouth: PSP or F‐speed film | 200 μSv | 24 days |
| Full‐mouth: CCD sensor (estimated) | 100 μSv | 12 days |
| Extra‐oral | ||
| Panoramicb | 20 μSv | 2.5 days |
| Cephalometricb | 5 μSv | 0.6 day |
| Chestc | 100 μSv | 12 days |
| Cone beam CTb | ||
| Small field of view (<6 cm) | 50 μSv | 6 days |
| Medium field of view (dentoalveolar, full arch) | 100 μSv | 12 days |
| Large field of view (craniofacial) | 120 μSv | 15 days |
| Multidetector CT | ||
| Maxillofacialb | 650 μSv | 2 months |
| Headc | 2 mSv | 8 months |
| Chestc | 7 mSv | 2 years |
| Abdomen and pelvis, with and without contrastc | 20 mSv | 7 years |
a Approximate equivalent background exposure is calculated based on an estimated background radiation dose of 3.1 mSv/year. Exposures more than the equivalent of 3 days are rounded off to the nearest day, month or year.
b Median dose from dento‐maxillofacial radiography with typical exposure protocols is calculated from data collated from multiple published studies. Doses in the range of 10–1000 μSv are rounded off to the nearest multiple of 10.
c American College of Radiology, https://www.acr.org/Clinical‐Resources/Radiology‐Safety/Radiation‐Safety.
CCD, charge‐coupled device; CT, computed tomography; PSP, photostimulable phosphor.
Having said that, however, there is an inherent danger with this type of comprehensive imaging. The means of presentation of the results of the CBCT scan are very attractive to the layperson and several of the animations may be undertaken to impress the orthodontic patient, who may request a copy of the ‘before and after’ portfolio as a souvenir of the orthodontic treatment and outcome. In today’s world, this can easily become part of the ‘hard sell’ and a means of attracting new patients. Consequently, the danger is that the stage may be set for the production of animations for the sake of ‘completeness’, much of which may be superfluous to the clinical needs of the patient, resulting in excess exposure of the patient to a large overdose of ionizing radiation.
ALARA
This leads us to explain the term ALARA – and what it means in practice [30].
It is only many years after treatment with any form of radiation that we see the stochastic effects, which include a higher susceptibility of the individual to various forms of cancer. It is known that these effects are amplified with increased exposure and that children are more susceptible than adults. Yet it is children and young adults who are the main patient population for orthodontic treatment. It is therefore incumbent on the practitioner to reduce this exposure to the minimum, while deriving a maximum of information adequate to the problem in hand. This is what ALARA signifies – As Low As Reasonably Achievable.
As we have already noted, there is a selection of scanning protocols and a low‐dose feature in every machine. It stands to reason, therefore, that when a CBCT scan is justified, it is because plane 2D radiography cannot maximize the information
