Filtration

Filtration is the process of shaping the X-ray beam to increase the ratio of photons useful for imaging. It is the removal of parts of the X-ray spectrum using absorbing materials in the X-ray beam.

Types of Filtration:

Filtration is in principal divided into 2 parts

• Inherent filtration

• Added filtration

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.

Added filtration:

Added filtration results from absorbers placed in the path of the X-ray beam. The purpose of added filtration is to remove the low-energy photons. Added filters are wedge filters and heavy metal or k-edge filters.

Filters

Filters are sheets of metal placed in the path of the X-ray beam near the X-ray tube housing to absorb low energy radiation before it reaches the patient. Their main function is to protect the patient from useless radiation and they perform their function remarkably well, frequently reducing skin exposures by as much as 80%. Aluminium is usually selected as the filter material for diagnostic radiology.

An X-ray filter is a device to block or filter out some or all wavelengths in the X-ray spectrum.

X-ray filters are used to block low-energy X-rays during medical x-ray imaging. Low energy X-rays are more likely to be absorbed by the patient's soft tissues. This causes non-stochasic radioactive effects, and does not contribute to image quality.

X-ray filters are used in X-ray crystallography, where crystalline lattice spacings can be determined using Bragg diffraction. The filters allow only a single X-ray wavelength to penetrate through to a target crystal, allowing the resulting scattering to determine the diffraction distance.

Wedge Filters:

Wedge filters are occasionally used in diagnostic radiology to abtain films of more uniform density when the part being examined diminishes greatly in thickness from one side of the field to the other. The filter is shaped like a wedge. Wedge filters are often used in lower-limb angiography.

Heavy metal or k-edge filters:

These filters make use of the k-absorption edge of elements with atomic numbers greater than 60, and may offer advantage when imaging barium or iodine. The heavy metal filter transmits a significantly narrower spectrum of energies than aluminium with decreased numbers of both low and high energy photons.

Molybdenum filters:

A special appilication of k-edge filters is use of molybdenum filters with molybdenum target. X-ray tubes used for mammography to reduce the amount of higher energy radiation in the molybdenum tube spectrum, a molybdenum filter of 0.030 mm thickness is commonly used.