This presentation discusses x-ray filtration and beam restriction. It describes how filters absorb low energy x-rays to harden the beam and reduce patient exposure. Various types of filters are discussed including inherent, added, and compensating filters. Beam restrictors like aperture diaphragms, cones, cylinders, and collimators are also summarized. Collimators provide rectangular fields and allow visualization of the beam's edge and center. Automatic collimators precisely match the beam size to the cassette. In summary, filters and restrictors improve image quality and reduce scatter while limiting exposure to relevant anatomy.
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X-ray Filtration
• Filtration is the use of soft metal (Al ) to absorb
low energy x-rays that reach the patient and are
absorbed superficially.
• This affects both the quality and quantity of the
beam.
• Beam hardening: quality of the beam is
increased because the average energy of the
beam is increased.
• This reduces the patient exposure.
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Types of Filtration
• Inherent Filtration (built- in) :
– Glass envelope enclosing anode and
cathode + insulating oil + window.
Usually varies between .5mm aluminum
equivalent
• Added Filtration:
– metal added between the collimators and
the tube for extra filtration
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Added Filtration
• Filter thickness: An aluminum filter >3mm thickness
offers no advantage .
• National council on radiation protection and measurement
recommendation for filtration.
operating KVP Total filtration
< 50kVp 0.5 mm aluminum
50-70kVp 1.5 mm aluminum
>70kVp 2.5 mm aluminum
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Added Filtration
• Effect on exposure factors: The major
disadvantage of filtration is reduction in the intensity of
X-ray beam .
• We must compensate it by increasing mAs.
• Filter absorbs many photons ,hence the total
number reaching the patient actually decreases.
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COMPENSATING FILTERS
• Some structures contain areas of significantly
varied tissue thickness and density that must be
shown on one image.
A.(AP) projection of the thoracic spine.
B.Axiolateral projection of the hip.
C.Lateral cervicothoracic region .
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Wedge filters
A. Thick portion of wedge partially
attenuates x-ray beam over upper thoracic
area while no filter area receives full
exposure to penetrate thick portion of spine.
An even image density results.
B, Trough filter in position for AP projection
of chest. Note how two side wedges partially
attenuate x-ray beam over lung areas while
mediastinum receives full exposure. A
better-quality image of chest and
mediastinal structures results.
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A, Supertech wedge:AP
projection of hips, knees, and
ankles on long (51-inch) film
B,Trough,collimator-mounted
aluminum filter with double
wedge used for AP projections
of thoracic spine.
C, Boomerang contact
filter:used for AP projections of
shoulder and facial bones.
compensating filters in use today
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compensating filters in use today
D, Ferlic collimator-mounted
filter used for AP and PA oblique
(scapular Y) projections of
shoulder.
E, Ferlic collimator-mounted
filter used for lateral projections
of cervicothoracic region
(swimmer’s technique).
F, Ferlic collimator-mounted
filter for AP axial projections of
foot.
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• Filtration only affects patient dose and the
beam. It has no effect on the image because
those low energy photons wouldn't reach the
IR anyway.
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BEAM RESTRICTORS
.
• X Ray beam restrictors is a device that is attached to
the X-ray tube housing, to regulating the size and
shape of an X-ray beam.
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3 Types of beam-restricting
devices
• Aperture Diaphragm
• Cones and Cylinders
• collimators
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Aperture Diaphragm
• Diaphragm consists of a
sheet of lead with a hole in
the center.
• The size of the hole
determines the size and
shape of the X-ray beam.
• The aperture can be altered
to any size and shape.
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• This partially exposed periphery
of the x ray field is called the
PENUMBRA.
• TECHNIQUE TO REDUCE
THE WIDTH OF THE
PENUMBRA:
- By positioning the
aperture diaphragm as far away
from the x ray target as possible.
DISADVANTAGE:-large
penumbra at the periphery of
the x ray field
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CONES AND CYLINDERS
• CONES: flare shaped.
• CYLINDERS: beam restriction at the far
end of the barrel, so less penumbra.
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Radiograph of frontal and
maxillary sinuses
Without using cylinder Using cylinder
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Beam restriction with a
cylinder takes place at far end
of the barrel, so there is less
penumbra.
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Collimators
• Best all round x ray beam restrictor device.
TWO ADVANTAGES:
1. Provides an infinite variety of rectangular
x ray fields.
2.Light beam shows the centre and the exact
configuration of the x ray field.
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STRUCTURE OF A COLLIMATOR
• Two sets of shutters to control
the dimension.
- each shutter contains 4
or more lead plates , which
move in independent pairs.
- when the shutters are
closed, they meet at the centre.
• Light beam from a light bulb
in the collimator.
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• The light beam is deflected
by a mirror mounted in the
path of the x ray beam at an
angle of 45 degree.
•The target of the x ray tube
and the light bulb should be
the exactly same distance
from the centre of the mirror.
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• A collimator can also identify the
center of the x ray field which is
accomplished by painting a cross line
on a thin sheet of Plexiglas mounted
on the end of the collimator.
• A backup system is available in case
if the light burns out with the help of a
calibrated scale which will determine
the X ray field size for various target-
film distances.
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AUTOMATIC COLLIMATORS
• POSITIVE BEAM LIGHTING DEVICES
• Shutters are motor driven.
• When a cassette is loaded into the film holder,
sensors will identify the alignment of the cassette.
• Then the information is relayed to the collimator
motors, which will position the shutters to exactly
match the size of the film being used.
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• These devices must be accurate to within 2%
of the source to image distance (SID).
• A perfectly aligned collimator will leave an
unexposed border on all sides of the
developed film
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References
• Christensen's Physics of Diagnostic Radiology.
• Radiographic Imaging and Exposure4th Edition
Terri Fauber.
