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 14x36" 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.
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FILTER
on COLLIMATOR
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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.
Compensating
filtration should be used in the following circumstances:
1.
for all AP full spine projections
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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].)
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2.
for virtually all AP thoracic projections
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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.)
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3.
for most lateral thoracic projections
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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 overpenetrating the lower thoracics.
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4.
for most lateral lumbar projections on females
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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.
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5.
for all AP and oblique foot projections
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Measure
the thickest part of the foot, up near the ankle.
Place wedge in front of the forefoot, thus
avoiding overpenetration of
the toes, while allowing visualization of the tarsals.
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6.
for all swimmer's projections
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Place
filter to attenuate the beam through the cervical spine above the
clavicle,
while allowing visualization of the cervicothoracic junction.
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7.
for neutral AP shoulder projections
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Avoids
overpenetrating the acromioclavicular joint while allowing
visualization of the
glenohumeral joint.
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