掌大像小道 wrote:
唉唉唉唉唉...看來...(恕刪)
版大 怎麼看的出來 專業的人不多呢?
Do yourself and everybody a favor do some research before you ask question like this.
It's easily found in any radiology text book.
Localized Radiation and Total Body Exposure
It is important to differentiate between localized radiation and total body radiation (Figure 5-3). When a dental radiograph is taken, the patient’s face is exposed to an x-ray beam that is 2 3/4 inches in diameter (with circular collimation). This is a localized exposure and represents less than 1% of the total area of the body. A rad of radiation to the localized area means that each gram of body tissue in that area absorbs 1 rad. A rad of total body exposure means that each gram of tissue in the entire body absorbs 1 rad. In dentistry, the x-ray machine delivers a localized exposure that results in a total body exposure much less than the facial exposure. In fact, the total body exposure from a dental radiograph is approximately 1/10,000 of the facial exposure.
When discussions of dental x-ray dosages appear in magazine articles or if patients quote such articles, it is important to determine whether localized exposure or total body exposure is being discussed. For instance, what is the facial exposure of radiation to the patient from a full-mouth survey of radiographs? An average full-mouth survey at 70 kVp and 10 mA, using American National Standards Institute (ANSI) group E film, produces a skin exposure to the patient’s face of approximately 2 to 3 R (150 mR per film × 20 films). This is not the total body exposure but a localized exposure. If it were a total body dose, it would far exceed the patient’s allowable maximum permissible yearly dose. The total body dose could be approximated by dividing the localized dose, 3 R, by 10,000. The most common misuse of such data occurs when people refer to the recommendation of the National Bureau of Standards that the total body dose for the general public for ionizing radiation should not exceed 500 mR in any 1 year. The misstatement is, “You are allowed 500 mR per year, and when your dentist x-rays your entire mouth, you are exposed to 3 R of radiation.” The mistakes here are equating the total body dose to the localized dose and using a nonoccupational exposure for a patient exposure. The truer figure to compare the dental facial dose with the 500 mR standard would be 0.0003 R (0.3 mR).
(Frommer, Herbert H.. Radiology for the Dental Professional, 8th Edition. C.V. Mosby, 062005. 5.5.2).
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