X-ray Filtration and Grids Overview

Study Notes

Filtration is measured using the half-value layer (HVL). HVL measures the material needed to attenuate the X-ray to half its original output. This is measured in millimeters of aluminum (mm Al).
Filters are metal sheets placed in the X-ray beam between the X-ray tube and the patient. They are used to attenuate low-energy X-ray photons from the spectrum.
These low-energy X-rays do not contribute to image quality but increase the entrance skin dose (ESD) for the patient.
Types of filtration include inherent and added filtration.
Inherent filtration is part of the X-ray machine, including the tube and housing components (glass envelope, insulating oil, and housing window)
Added filtration is usually made of interchangeable metal sheets like aluminum (Al), copper (Cu), etc., and is added to the X-ray beam to further reduce patient dose.

Grids are devices containing multiple thin metallic strips with radiolucent spaces. They are placed behind the patient and in front of the image receptor to reduce scattered radiation.
Grid ratio is the height of the lead strips divided by the distance between the strips. A higher grid ratio is better at removing scatter.
Grid frequency is the number of grid lines per unit distance (e.g., inches or centimeters).
Grids improve contrast by reducing scattered radiation, primarily from Compton interaction. However they increase dose to the patient.
Grid types include parallel, linear, crossed, or focused, based on strips orientation and movement.

Collimators are X-ray beam restriction devices attached to the X-ray tube housing.
Collimators are usually made of metal (e.g., lead).
They reduce the size of the X-ray beam.
More collimation reduces the field of view (FOV) and irradiates a smaller volume of tissue. This leads to less scatter and higher contrast.
Collimating impacts image quality by decreasing scatter and increasing contrast.

K-edge filtration uses materials with a specific k-edge energy to selectively remove low energy X-rays that do not contribute to the image, and some higher energy X-rays just above the k-edge.
This improves image quality by reducing patient dose, and allows for higher contrast.
Materials such as Molybdenum (Mo) at 20 keV are frequently used in mammography.

Compensating filters adjust the radiation beam to even out the density variations in anatomy.
They reduce the radiation reaching thinner, less dense areas of the anatomy.
Common types include wedge-shaped or trough-shaped filters.

Iodine and Barium agents are used to improve contrast in images. Introduction of these agents changes the interaction probability of X-rays with the target tissue, producing higher contrast.

The ratio of incident X-rays to transmitted X-rays through a grid.
It represents the dose penalty for using a grid.
A higher Bucky factor means more scatter is removed, but more radiation reaches the patient.

The air gap technique reduces scatter by increasing the distance between the patient and the imaging detector. A larger air gap allows the scatters to deposit energy further from the detector and allows for the reduction of scatter reaching the detector.