Diagnostic x-ray beams are polychromatic( with
spectrum of many different energies).
High energy photons transmitted to form the
Low energy photons get absorbed/ scattered.
Contributes to the increase in patient radiation dose.
Or if scattered then degrade the image quality.
Thin sheets of metals(Al , Cu)
placed in the path of x-ray
To attenuate the low energy
(soft) x-ray photons from the
spectrum before reaching to
the patient body are filters.
And the process is known as
Unit is mm Al equivalent.
Absorption of x-rays as they pass
The insulating oil
Thickness 0.5-1mm Al equivalent
Results from the absorbers
(filters) placed in the path of x-
Outside the x-ray tube and
Silver on collimator mirror,
Al/Cu between the collimator
and protective housing.
Thickness 1-1.5mm Al
Can be customized(filter
thickness, type of metal)
Total filtration =
Below 50kvp 0.5 mm
50-70 kVp 1.5 mm
Copper(Z=29):- for high energy radiation
Aluminum(Z=13):- for low energy radiation
- most commonly preferred in
- low atomic number therefore excellent material
for absorbing low energy x-ray photons.
- low in weight therefore make the x-ray tube lighter
in weight and easy handling.
. 2-3 Examples of compensating filters in use today. A, Supertech wedge, collimator-
mounted Clear Pb filter used for 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. 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) and axiolateral projections
(Danelius-Miller method) of hip. F, Ferlic collimator-mounted filter for AP axial
projections of foot.
Consists of two or more layers of different materials.
Contains 3 layers(tin, copper, aluminum)
Each layer absorbs characteristics photons created in previous
Harden and smooth the spectrum of higher energy
Tin(K-edge = 29.2keV) absorbs characteristic radiation
produced by tungsten. Unfortunately tin produces its own
Copper(K-edge=9keV) compensate for the characteristic x-
ray(9-30keV) produced by tin.
Aluminum(K-edge=1.6keV) filter beyond copper absorbs the
very low energy characteristics x-ray produced by the copper.
Make use of principal of K-edge
Elements with atomic no. >60 (except Mo)
Principal of k-edge
Attenuation when x-ray energy below and
above K-edge but has a relative maximum
attenuation immediately above the k-edge.
Maximum contrast when the x-ray
energy is slightly above K-edge of the absorber.
K-edge of iodine= 33.17keV
K-edge of holmium= 55.6 keV
K-edge/heavy metal filters
Transmits a significantly narrower spectrum of
energies (hard x-rays) than aluminum, with
decreased no. of both low and high energies
in low energy photons decreases patient’s absorbed
in high energy photons improves the image
Increased x-ray tube loading due to the use of more
mAs so as to compensate for increased beam
k-edge filters V/S Aluminum filters
Applications of filters
• Diagnostic x-ray energy range Primary aluminum filter (mm Al)
• Orthovoltage range Compound filter (1-4mm Cu)
• Cesium & cobalt teletherapy
No filter (monoenergetic)
• Megavoltage x-ray beam 1. Inherent filtration of transmission
2. Flattening filter
• Pediatric applications K-edge filters as they use low kVp
• Mammography machines Molybdenum filters (k-edge filter)
reduce the amount of high energies x-
rays and improve the contrast in breast
On exposure factors
responsible for increased
=good image contrast
Thickness of absorber that
attenuate the intensity of the x-
ray beam to half its original
Indirect measure of quality of
The greater the HVL of the x-
ray beams, the better is the
quality of the x-ray photons.
Unit is mm of Al.
Half Value Layer (HVL)
Reduction in the intensity of x-ray photons,
Lengthen the time required to make an exposure,
May absorb primary x-ray beam during excessive
Increase tube loading (due to more mAs).