Beam filtration in mammography selectively modifies the X-ray spectrum to produce photons within the energy range that maximises soft-tissue contrast while minimising patient dose.

Unlike general radiography, which uses broad aluminium filters, mammography employs K-edge filters made of materials such as molybdenum (Mo), rhodium (Rh), or silver (Ag). These filters transmit only a narrow band of photon energies that are most effective for imaging breast tissue.

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Purpose of filtration

An unfiltered X-ray spectrum contains:

Low-energy photons that are completely absorbed by the skin, increasing dose without improving image quality.
High-energy photons that penetrate the breast with little interaction, reducing subject contrast.

The filter’s role is to remove both extremes and transmit only those photons within the desired energy window (typically 17–23 keV).

Mechanism of K-edge filtration

K-edge filters rely on a sharp increase in attenuation that occurs when photon energy exceeds the K-shell binding energy of the filter material.

Filter material	K-edge energy (keV)	Common use
Molybdenum (Mo)	20.0	With Mo target (Mo/Mo) for thin or fatty breasts
Rhodium (Rh)	23.2	With Mo or Rh target for thicker or denser breasts
Silver (Ag)	25.5	With tungsten target in digital systems

How it works

Photons with energies below the K-edge are attenuated mainly by the photoelectric effect, so low-energy photons are removed.
Photons just below the K-edge are transmitted efficiently, forming the desired spectral window.
Photons above the K-edge are again strongly attenuated due to increased photoelectric absorption within the filter material, thereby suppressing unwanted high-energy photons.

Thus, the filter acts like a band-pass window, transmitting only photons within the optimal diagnostic energy range.

Interaction of target and filter

Each filter is paired with a target material to fine-tune the output spectrum:

Target–Filter Combination	Resulting Beam Characteristics	Application
Mo/Mo	Narrow spectrum centred at 17–19 keV	Standard for thin or fatty breasts
Mo/Rh	Slightly higher energy, broader spectrum	Thicker or denser breasts
Rh/Rh	Peak around 20–23 keV	Dense tissue requiring more penetration
W/Rh or W/Ag	Broad, higher-energy output	Digital mammography and tomosynthesis

By adjusting both target and filter, the system maintains optimal contrast-to-dose performance across varying breast thicknesses and densities.

Beam quality and Half-Value Layer (HVL)

Beam quality in mammography is expressed in terms of the Half-Value Layer (HVL), the thickness of a specified material (usually aluminium) that reduces beam intensity by half.

Typical mammography HVL values:

0.3–0.4 mm Al at 28 kVp (Mo/Mo)
0.4–0.5 mm Al at 30 kVp (Rh/Rh)
> 0.5 mm Al for tungsten targets (digital systems)

A higher HVL corresponds to a harder (more penetrating) beam.
Beam hardness must be carefully controlled: too low → high dose, too high → poor contrast.

Key points and exam tips:

K-edge filtration provides a narrow, optimised X-ray spectrum centred on 17–23 keV.
Both low- and high-energy photons are removed: low energies by general attenuation, high energies by K-edge absorption.
Target–filter pairing (Mo/Mo, Mo/Rh, Rh/Rh, W/Rh) tailors beam quality to breast composition.
HVL quantifies beam hardness and is closely monitored in QA testing.
Over-filtration increases noise; under-filtration increases dose.
Common exam question: “Explain how K-edge filtration in mammography optimises image contrast and patient dose.”

Up next

Next, we will move on to Mammography Geometry and Compression, describing how system geometry, breast positioning, and compression improve image quality, reduce scatter, and lower patient dose.