Proper use of radiographic filters

X-Ray Compensating Filters

A simple or complex system of placing a discretionary metal filter (usually aluminum, usually wedge-shaped) in front of the collimator in order to attenuate (reduce) a portion of the primary beam so as to compensate for varying body part thicknesses in the same field of view. The classic example would be an AP full spine 14 x 36" projection, but filters are also needed for other commonly-produced views.

The thick part of such a wedge filter is designed to limit the amount of x-ray reaching a thinner (or less dense) body part, while the full intensity of the x-ray beam is allowed to reach the thicker body part. The filter is placed in front of the collimator (via a special adjustable filter holder, magnets, or even velcro) after collimating appropriately, because in most cases the filter is opaque and obliterates the light beam from the collimator. The gradual wedge design of the filter blends the difference between thick and thinner attenuation so that no horizontal filter line shows on the finished radiograph.

Attempts to compensate for varying thicknesses in body parts should never be made by utilizing split or gradient intensifying screens or by blocking a portion of a screen, as these improper procedures would produce the compensation after the full amount of radiation had already passed through the patient's body. Wedge compensation occurs before the radiation reaches the patient, thus providing the dual benefit of protecting the patient while obtaining a higher quality film.

    Measure and compute technique for the thickest part of the abdomen and
    place the thick part of the filter in front of the thinner thorax. This avoids full
    spine films that are too dark at the top and too light through the pelvis at the
    bottom. (Lateral full spine projections should be avoided; rather, three
    separate sectional views should be produced to accompany the
    APFS projection [plus one additional AP cervical film; either APOM or
    15° tilt-up].)

    Procedure and rationale same as above. (The only exception to using a filter
    on an AP thoracic projection would be for a male patient with unusually
    developed pectoral muscles (as a body builder) and a very thin waist.)

    Measure side-to-side across the shoulders, on the outside of the arms, and
    compute technique sufficient to penetrate that thick and dense region. Then
    place the wedge filter inverted so that the thick part of the wedge attenuates
    the x-ray beam through the lower air-filled thorax, and the full amount of the
    beam is available to penetrate the dense upper thorax. This usually facilitates
    visualization of the difficult-to-see upper thoracic vertebrae without over
    penetrating the lower thoracics.

    Measure side-to-side at widest part of hips and compute technique sufficient
    to penetrate that. Then place the wedge filter so that the thick part of the
    wedge attenuates the x-ray beam through the thinner waist, and the full
    amount of the beam is available to penetrate the dense pelvis. This avoids
    lateral lumbar films that are too dark at the top while being underpenetrated at
    the lumbosacral junction.