X-ray Filtration and Grids Overview

Filters

• 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

• 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

• 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 Filtering

• 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

• 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.

Contrast Agents

• 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.

Bucky Factor

• 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.

Air Gap Technique

• 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.