Control of Scatter Radiation
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Image Production
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Effect of Scatter Radiation on Image Contrast
One of the most important characteristics of image quality is contrast, the visible difference between the light and dark areas of an image. Contrast is the degree of difference in OD between areas of a radiographic image. Contrast resolution is the ability to image and distinguish soft tissues.
Even under the most favorable conditions, most remnant x-rays are scattered.
When primary x-rays interact with the patient, x-rays are scattered from the patient in all directions.
Two types of devices reduce the amount of scatter radiation that reaches the image receptor: beam restrictors and grids.
Beam Restrictors
Basically, three types of beam-restricting devices are used: the aperture diaphragm, cones or cylinders, and the variable-aperture collimator (Figure 14-11).
Aperture Diaphragm
An aperture is the simplest of all beam-restricting devices. It is basically a lead or lead-lined metal diaphragm that is attached to the x-ray tube head. The opening in the diaphragm usually is designed to cover just less than the size of the image receptor used. Figure 14-12 shows how the x-ray tube, the aperture diaphragm, and the image receptor are related.
FIGURE 14-11 Three types of beam-restricting devices.
Dental radiography represents another application of aperture diaphragms. Dental radiographs are customarily obtained at 20 or 40 cm SID. Most dental imaging systems are supplied with rectangular collimation, which requires that the dental radiologic technologist precisely align and position the x-ray tube head, the patient, and the image receptor.
Cones and Cylinders
Radiographic extension cones and cylinders are considered modifications of the aperture diaphragm.In both, an extended metal structure restricts the useful beam to the required size. The position and size of the distal end act as an aperture and determine field size.
Radiographic cones and cylinders produce restricted useful x-ray beams of circular shape.
In contrast to the beam produced by an aperture diaphragm, the useful beam produced by an extension cone or cylinder is usually circular. Both of these beam restrictors are routinely called cones, even though the most commonly used type is actually a cylinder.
One difficulty with using cones is alignment. If the x-ray source, cone, and image receptor are not aligned on the same axis, one side of the radiograph may not be exposed because the edge of the cone may interfere with the x-ray beam. Such interference is called cone cutting.
Variable Aperture Collimator
The light-localizing variable-aperture collimator is the most commonly used beam-restricting device in radiography.
Automatic variable-aperture collimator. (Courtesy Huestis Medical.)
Simplified schematic of a variable-aperture light-localizing collimator.
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1 comment:
Appreciation is probably incalculable. Thanks for sharing. I’ll come back to read more of your works.
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