Filters are used in x-ray imaging to improve image quality by removing low-energy photons that decrease contrast and increase patient dose. There are three types of filtration: inherent, added, and patient filtration. Inherent filtration occurs as x-rays pass through the tube housing and glass envelope. Added filtration, such as aluminum and copper filters, can be placed in the beam to preferentially remove low-energy photons. Different filter thicknesses and materials, such as wedge-shaped filters, are used depending on the application and patient thickness.

1. FILTERS

2. • Filtration is the process of shaping the x-ray beam to
increase the ratio of photons useful for imaging to those
photons that increase patient dose or decrease image
contrast.
• Diagnostic X-ray beams are polychromatic radiations which
are composed of photons having whole spectrum of energy
• Low energy photons are absorbed and high energy photons
penetrate the patient forming image.

3. • Tissue can be protected by absorbing the lower energy photons from the beam
before they reach the patient by interposing a filter material between the
patient and the x-ray tube.
• Filtration of Xray beam occur at three levels:
• 1)Inherent filtration
• 2)Added filtration
• 3)The patient

4. INHERENT FILTRATION
• Filtration resulting from the absorption of x rays as
they pass through the x-ray tube and its housing is
called inherent filtration.
• Parts of the tube responsible for this filtration are:
• 1)Glass envelope
• 2)Insulating oil
• 3)Window in the housing tube

5. • Aluminium equivalents: thickness of aluminum
that would produce the same degree of
attenuation as the thickness of the material.
•
• Inherent filtration usually varies between 0.5 and
1.0 mm alu minum equivalent, and the glass
envelope is responsible for most of it.

6. • Filtration decreases the tissue contrast, which affects the
tissue quality.
• The Beryllium window tubes in which the exit portal of the
glass envelope is replaced with beryllium, which is more
transparent to low energy radiation than glass.

7. ADDED FILTRATION
• When we place absorber in the pathway of xray beam,that is
called Added filtration
• Ideally, a filter material should absorb all low energy photons
and transmit all high en ergy photons.
• Aluminium and copper are usually taken for diagnostic
radiology.
• Aluminium is good filter for low energy radiation & Copper for
high energy radiation,but never used alone.
(characteristics radiation production)

8. COMPOUND FILTERS
Consist of 2 or more layers of two different materials usually
Cu & Al
Arranged in such a manner that Cu faces Xray tube and Al
faces patient, because Al absorbs all the characteristics
radiation produced by Cu.
Characteristics radiation of Al has low energy so that it get
absorbed in the air gap

9. FILTER THICKNESS
• Two millimeters of aluminum absorb nearly all photons with
energies less than 20 keV, so most of the advantages of fil
tration are achieved by this thickness.
• The effect of using filters with more thickness is an overall
attenuation of the beam, primarily by the absorption of high
energy photons.
• Overall intensity of beam is decreased,which increases the
time required to make an exposure.

11. • Use of filters will decrease the exposure of patients to the
radiation and giving them a considerable degree of
protection.
• The major disadvantage of filtration is a reduction in the
intensity of the x-ray beam. Filters absorb some photons at
all energy levels, and we must compensate for the loss of
higher energy photons by increasing exposure.

12. WEDGE FILTER
• Special type of wedge shaped filter is used in diagnostic
radiology when the density of the part to be examined is
not uniform in thickness.
• Less radiation is absorbed by the thinner part of the filter so
more radiation is available to penetrate the thicker part of
the patient.
• Wedge filters are often used in lower-limb angiography
when one images from the lower abdomen to the ankles
with a single exposure.

13. Wedge filter

14. HEAVY METAL FILTERS
• Contrast is greatest when contrast agent absorbs Xrays most
effectively.
• Maximum absorption by the contrast occurs when photon
energy of the x-ray beam is close to, but slightly above, the
K absorption edge of the absorber so that mass absorption
co-efficient is increased.
• The purpose of heavy metal filters is to produce an x-ray
beam that has a high number of photons in the specific
energy range that will be most useful in diagnostic imaging.

15. • The heavy metal filter transmits a significantly narrower
spectrum of energies than aluminum, with decreased
numbers of both low and high energy photons.
• The reduction in low energy photons will decrease the
patient's absorbed dose. Image contrast will be improved by
the reduction in higher energy photons(production of
characteristics radiation & decreased production of scatter
radiation.)

16. MOLYBDENUM FILTERS
• Molybdenum target Xrays have molybdenum filters which
are used in mammography.
• Mammography at 30-40 keV,molybdenum tube will produce
high energy radiation that will reduce the contrast of the
image.
• So we use molybdenum filters of 0.03 mm thickness which
will attenuate the Xrays above 20keV and subsequently
increase the contrast.