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Oral Med + Oral Patho + Radio

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Effects

Direct

Indirect

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If kVp is lowered, the mean energy of the beam decreases. This results in: (1) an image with greater contrast

(assuming that exposure time is increased)

(2) a beam with more low-energy photons that carry the potential for risk but are not useful in making an image

(3) reduced beam intensity requiring increased exposure time, thus increasing the risk of the patient moving and blurring the image.

Although image diagnosis may be improved slightly with increased image contrast (low kVp) images, the patient dose is somewhat reduced with higher kVp exposures. The best balance is to use 60 to 80 kVp. The availability of constant-potential (fully rectified), high-frequency or direct current (DC) dental x-ray units has made possible the production of radiographs with lower kilovoltage and at reduced levels of radiation. The surface exposure required to produce a comparable radiographic density using a constant-potential unit is approximately 25% less than that of a conventional self-rectified unit operating at the same kilovoltage. Currently several manufacturers produce DC units.

? FIG. 5-15 Radiographs of a step wedge made at 40 to 100 kVp. As the kVp increases, the mA is reduced to maintain the uniform middle-step density. Note the long gray scale (low contrast) with high kVp. (Courtesy Carestream Health, Inc., exclusive manufacturer of Kodakdental systems.)

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glands

GRANULAR CELL TUMOR: A submucosal mass consisting of diffuse sheets of large cells of either nerve or muscle origin, with a cytoplasm of densely packed eosinophilic granules (lysosomal bodies) and commonly found on the dorsal surface of the tongue.

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The OKC exhibits a recurrence rate of 25% to 60%, similar to that of an ameloblastoma.

Also known as the nasoalveolar cyst and the Klestadt cyst, this rare condition occurs entirely in the soft tissues of the anterior maxillary vestibule, below the ala of the nose and deep in the nasolabial crease . Most of these cysts occur in the fourth and fifth decades of life and have a female predilection of approximately 3 to 1.

NASOPALATINE DUCT CYST (INCISIVECANAL CYST)The nasopalatine duct cyst is the most common non-odontogenic cyst of the oral cavity. occurring in about I% of the population.

MEDIAN PALATAL (PALATINE) CYSTThe media n palatal cyst is a rare fissural cyst that theoretically develops from epithelium entrapped along the embryonic line of fusion of the lat eral palatal shelves ofthe maxilla . This cyst may be difficult to distinguish from a nasopalatine duct cyst. In fact . most "median palatal cysts" may represent posteriorly positioned nasopalatine duct cysts. Because the nasopalatine ducts courseposteriorl y and superiorly as they extend from the inci-

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d

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e

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vital

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>mA >penetration>density <penetration>time >penetration>KVp >penetration

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The radiosensitivity of a tissue or organ is measured by its response to irradiation. Loss of moderate numbers of cells does not affect the function of most organs. However, with the loss of large numbers of cells, all affected organisms display an observable result. The severity of this change depends on the dose and thus the amount of cell loss. The following discussion pertains to the effect of irradiation of tissues and organs when the exposure is restricted to a small area. Moderate

The response of cells, tissues, and organs to irradiation depends on exposure conditions and the cell environment

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Three methods exist for minimizing this loss of image clarity and improving the quality of radiographs:1. Use as small an effective focal spot as

practical.2. Increase the distance between the

focal spot and the object by using a long, open-ended cylinder.

3. Minimize the distance between the object and the film.

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The purpose of filtration is to remove the low-energy x-ray photons selectively from the x-ray beam. This results in decreased patient exposure with no loss of radiographic information. When an x-ray beam is filtered with 3 mm of aluminum, the surface exposure is reduced to about 20% of that with no filtration. In light of this and other information, the federal government has designated the specific amount of filtration, expressed as minimum half-value layer, required for dental x-ray machines operating at various kilovoltages. Practically, these requirements can be met by having 1.5 mm Al total filtration when operation from 50 to 70 kVp and with 2.5 mm Al total filtration when operating above 70 kVp

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Occasionally it is desirable to obtain radiographs of a woman who is pregnant. The x-ray beam is confined to the head and neck region in dental x-ray examinations; thus, fetal exposure is only about 1 microgray (μGy) for a full mouth examination. This exposure is quite small compared with that received normally from natural background sources. However, concerns have been raised about a possible relationship between maternal radiation dose to the thyroid gland from dental radiographs and low birth-weight babies, prompting the ADA to recommend the use of protective thyroid collars and aprons during dental radiography, especially of children, women of childbearing age, and pregnant women. Because the use of radiographs in all patients is predicated on there being a diagnostic need for them, the guidelines apply to patients who are pregnant as well as those who are not.

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