radiation filters

1. Filters
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
composed of photons that have a whole spectrum of energies; this is
they are polychromatic.
As polychromatic radiation passes through a patient, most of the
lower energy photons are absorbed in the first few centimeters of
tissue and only the higher energy photons penetrate through the
patient to form the radiographic image. Because the patient's radiation
dose depends on the number of absorbed photons, the first few
centimeters of tissue receive much more radiation than the rest of the
patient. This 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.
Filters are usually sheets of metal, and their main function in
diagnostic radiology is to reduce the patient's radiation dose.
 In a radiologic examination the x-ray beam is filtered by
absorbers at three different levels. Beginning at the x-ray source,
these are as follows:
1. The x-ray tube and its housing (inherent filtration)
2. Sheets of metal placed in the path of the beam (added filtration)

2. 3. The patient.
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. The
materials responsible for inherent filtration are the glass envelope en-
closing the anode and cathode, the insulating oil surrounding the tube,
and the window in the tube housing. Inherent filtration is measured in
aluminum equivalents,
Inherent filtration usually varies between 0.5 and 1.0 mm aluminum
equivalent, and the glass envelope is responsible for most of it.
Because filtration in- creases the mean energy of an x-ray Beam it
decreases tissue contrast. The decrease is insignificant in the higher
energy range but with lower energy radiation, under 30 kVp, this loss
of contrast may be detrimental to image quality.
ADDED FILTRATION
Added filtration results from absorbers placed in the path of the x-ray
beam. Ideally, a filter material should absorb. All low energy photons
and transmit all high energy photons. Unfortunately, no such material
exists. A material can be selected, however, to absorb principally low
energy radiation by utilizing the proclivity of photoelectric attenuation
for low energy photons.

3. The energy of the radiation filtered from the beam can be regulated by
selecting a material which an appropriate atomic number. Aluminium
and copper are the materials usual.ly selected for diagnostic radiology.
Aluminium, with an atomic number of 13 is an excellent filter
material for low energy radiation and a good general purpose filter.
Copper with an atomic number of 29 is a better filter for high energy
radiation .most radiologists prefer to use a single filter material,
usually aluminum. Copper is never used by itself as filter material. It
is always used in combination with aluminum as a compound filter.
The only reason for going to copper is to cut down on the thickness of
the filter. A com- pound filter consists of two or more layers of
different metals. The layers are arranged so that the higher atomic
number element, copper, faces the x-ray tube, and the lower atomic
number element, aluminum, faces the patient. Most filtration occurs
in the copper, and the purpose of the aluminum is to absorb the
characteristic radiation from copper. Photoelectric attenuation in
copper produces characteristic radiation with an energy of about 8
keV, which is energetic enough to reach the patient and significantly
increase skin doses. The aluminum layer absorbs this characteristic
radiation. Its own characteristic radiation has so little energy (1.5
keV) that it is absorbed in the air gap between the patient and filter.