Density Equalization Filters

a. Density equalization filters are special accessory devices used when it is desirable to cause a variation of x-ray intensity across a given beam. When the filter is introduced into the path of the x-ray beam, its intensity is modified in differing degrees over given portions of the file being irradiated (figure 1-7). This is achieved by selective absorption.

Figure 1-7. Density equalization filters.

b. These devices are especially effective where the part or areas to be examined present widely varying densities, all of which must be satisfactorily demonstrated on one film. For example, in making the dorsoplanter projection of the foot (figure 1-8), dorsoplanter or plantodorsal projections of the os calcis, lateral placentograms, and examinations of the thoracolumbar region are needed. When normal radiographic techniques are used to demonstrate such parts, it usually happens that if the thin parts of the subject are properly exposed, the thick parts may be considerably underexposed. Conversely, gross overexposure of thin parts may occur in the exposure factors or are adjusted for satisfactory demonstration of the thick parts. This problem can be corrected by the proper use of the density equalization filters. Also, the absorbed radiation dosage to the patient is decreased.

c. Filters of this type are sometimes referred to as compensating filters, wedge filters, differential-absorption filters, supplementary filters, or balancing filters.

DENSITY EQUALIZATION FILTER

PORTION OF X-RAY BEAM OF PROPORTIONALLY DECREASED INTENSITY DIRECTED OVER THE THIN PARTS

NORMAL FILTER

PORTION OF X-RAY BEAM OF RELATIVELY GREATER INTENSITY IS DIRECTED OVER THE THICK PARTS

NORMAL FILTER

PORTION OF X-RAY BEAM OF RELATIVELY GREATER INTENSITY IS DIRECTED OVER THE THICK PARTS

DENSITY EQUALIZATION FILTER

PORTION OF X-RAY BEAM OF PROPORTIONALLY DECREASED INTENSITY DIRECTED OVER THE THIN PARTS

Figure 1-8. Relationships of the density equalization filter, the x-ray cone (or bean), and the thin and thick parts of the anatomical structure being radiographed.

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