Certain factors of aberration to be considered in clinical roentgenographic cephalometry

Abstract

1. 1. The type of x-ray machine is an important factor in roentgenographic cephalometry because of the long Focal-Film distance involved. A more powerful x-ray machine than a dental x-ray machine is desirable. 2. 2. The milliamperage is a factor in radiation for milliamperage determines the density in a roentgenogram. The kilovoltage is another factor in radiation. The higher the kilovoltage, the greater the contrast of an image on a film. An x-ray machine should be calibrated so that the operator can control the milliamperage and kilovoltage. Such control is usually not incorporated in a dental x-ray machine. 3. 3. The focal spot: The smaller the focal spot, the finer the detail sharpness and definition of the image on the film; and, contrariwise, the larger the focal spot the less sharpness of detail. However, this factor can be greatly minimized by a long Focal-Film distance. A tube equipped with a small focal spot has less capacity for the amount of radiation delivered per unit of time exposure than a tube with a larger focal spot. This is an important factor where the Focal-Film distance is long as it is in roentgenographic cephalometry. Radiation varies directly with the square of the distance. 4. 4. The Focal-Film distance is a factor in detail sharpness and magnification distortion in roentgenograms. The longer the Focal-Film distance, the sharper the detail and the less magnification distortion. At least, a Focal-Film distance of 60 inches is desirable in roentgenographic cephalometry. 5. 5. The Object-Film distance is also a factor in detail sharpness and magnification distortion. The shortest possible Object-Film distance will greatly improve definition and minimize, to a marked degree, magnification distortion. 6. 6. Secondary radiation is the causative factor in obscuring sharpness of detail in a roentgenogram. The size and volume of an anatomical part radiographed determines the amount of secondary radiation produced. Limiting devices such as cylinders, cones, and diaphragms attached to the x-ray head concentrate the primary beam of radiation to a particular area of the head to be radiographed. This, in effect, decreases the undesirable aberrant factor of secondary radiation. 7. 7. Cassettes, with intensifying screens, are very necessary in roentgenographic cephalometry. Although they reduce the definition and detail sharpness of a roentgenogram, the gain in the extreme shortness of exposure time more than compensates for loss of definition. Intensifying screens of various speeds determine the factor of time exposure. 8. 8. The kind of equipment set up is another important factor in roentgenographic cephalometry. A setup where the x-ray head and cephalostat are fixed introduces less factors of variability and human error of adjustment than one where the height of the x-ray head and cephalostat are adjusted to the patient. The adjustment of the patient into the cephalostat should be so organized as to produce the least amount of discomfort to the patient. An uncomfortable position of the patient may introduce the undesirable factor of movement.