Filtration

Filtration

The process of eliminating undesirable low-energy x-ray photons by inserting absorbing materials into the primary beam.

Reason for Filtration

To eliminate photons that increase radiation dose to the patient without enhancing the radiographic image.

Common Filter Material

Aluminum is the most common filter material used in x-ray filtration.

Aluminum equivalency (Al/Eq)

A measure expressing various types of filtration in terms of the thickness of aluminum.

Half-value Layer (HVL)

The amount of absorbing material that reduces the intensity of the primary beam to one-half its original value.

Inherent Filtration

Filtration resulting from the composition of the x-ray tube and housing.

Source of Inherent Filtration

Most of the inherent filtration comes from the window of the glass envelope of the x-ray tube.

Effects of Aging on Filtration

Aging x-ray tubes may become gassy, the anode may pit, and the glass envelope may gain a coating of vaporized metal.

Added Filtration

Filtration contributed by devices such as collimators in addition to the inherent filtration.

Compound Filter Example

The Thoraeus filter used in radiation therapy, which combines tin, copper, and aluminum.

Compensation in Filtration

Thicker portions of the filter are matched to less-dense patient body parts.

Total Filtration

The sum of inherent and added filtration, excluding compound or compensation filters.

Filtration Requirement Above 70 kVp

A minimum total filtration of 2.5 mm aluminum is required.

Scatter Radiation

Radiation produced during a Compton interaction that impairs image quality.

Methods to Control Scatter

Restricting the x-ray beam and using a grid to absorb scatter radiation.

Purpose of Grids

Devices that absorb scatter radiation placed between the patient and the image receptor.

Effect of Kilovoltage (kVp) on Scatter

As kVp increases, the percentage of primary photons that undergo scattering also increases.

Volume of Irradiated Tissue and Scatter

As the volume increases, the amount of scatter radiation also increases.

Atomic Number and Scatter

Higher atomic numbers result in more photoelectric absorption and less scatter.

Collimator

The foremost method for restricting the primary beam field size.

Devices Historically Used to Restrict Beam

Aperture diaphragms and cones/cylinders, although not commonly used today.

Bottom Shutters of Collimator

Reduce penumbra along the periphery of the beam due to their distance from the focal spot.

Upper Shutter of Collimator

Helps reduce off-focus radiation reaching the image receptor by absorbing it before it exits the tube.