Beam collimation and filtration are fundamental mechanisms for shaping and optimising the X-ray beam.
Together, they determine how much of the beam reaches the patient, how much scatter is produced, and how effectively image quality is maintained while minimising dose.

Proper beam control ensures that only the region of clinical interest is irradiated and that the beam contains useful, penetrating photons capable of image formation.

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Beam collimation

Collimation is the process of restricting the size and shape of the X-ray field to the required area of anatomy. By confining the beam, collimation reduces patient dose whilst also reducing scatter radiation reaching the detector which improves image quality.

Light Beam Diaphragm (LBD) / Variable-Aperture Collimator

Most modern systems use a variable rectangular collimator with two pairs of lead shutters:

One pair controls the length of the field (longitudinal shutters).
The other controls the width (lateral shutters).

A light source and angled mirror project a visible light field corresponding to the X-ray field. Accurate alignment between the light field and X-ray field is critical for patient safety and image accuracy. Misalignment leads to cut-off or unnecessary irradiation.

Quality assurance standard: The light and radiation fields must coincide within ±2% of the source–image distance (SID).

Functions:

Limits exposure to the region of interest.
Reduces patient dose and scatter production.
Improves image contrast and positioning accuracy.

Beam filtration

Filtration selectively removes low-energy photons from the beam that would be absorbed by the patient without contributing to image formation. This process “hardens” the beam, increasing its mean energy and penetration. As we’ll see later, low energy photons are more likely to be attenuated via the photoelectric effect, whereas higher energy photons are more likely to be transmitted. This is how filters “selectively remove” low energy photons that contribute to dose but would never reach the detector.

Types of filtration

Type	Description	Typical Value / Example
Inherent filtration	Filtration built into the X-ray tube: glass/metal envelope, insulating oil, and exit window.	~0.5–1.0 mm Al equivalent
Added filtration	External aluminium or copper plates inserted into the beam.	Typically adds 1–2 mm Al equivalent
Total filtration	Combined effect of inherent + added filtration.	Must be ≥ 2.5 mm Al (for >70 kVp, IEC standard)
Compensating filtration	Shaped filters that balance exposure across tissues of varying thickness.	Wedge, trough, bow-tie filters

Compensating filters

Used when the anatomy has uneven thickness, preventing over- or underexposure.

Common filter types	Application
Wedge filter	Equalises exposure for lateral foot, shoulder, or chest.
Trough filter	Balances exposure between mediastinum and lungs in chest radiography.
Bow-tie filter	Used in CT; reduces peripheral dose while maintaining central beam uniformity.

Effects of Collimation and Filtration

Parameter	Effect of Increased Collimation / Filtration
Patient dose	↓ Decreases (less tissue irradiated, fewer low energy photons, more beam penertration)
Scatter radiation	↓ Decreases (tissue outside of the irradiated area no longer a source of scatter)
Image contrast	↑ Improves (less scatter reaching detector, if filtration is too high image contrast can decrease)
Beam energy	↑ Increases (beam hardening)
Exposure time (mAs)	↑ slight increase (may need increase to compensate for removed low-energy photons)

Off-Focus (Extra-Focal) Radiation

Not all electrons strike the focal spot precisely.
Some scatter off the anode and strike other parts of the target or tube housing, producing off-focus radiation.

Consequences:

Produces unwanted exposure outside the intended field (fuzzy image edges).

Controlled by:

Lead collimator shutters (primary barrier).
Lead-lined housing to absorb stray radiation.
Grids at the detector to absorb secondary scatter.

Key takeaways and exam tips

Collimation defines beam size → reduces patient dose and scatter.
Filtration removes low-energy photons → increases mean energy (beam hardening).
Total filtration ≥ 2.5 mm Al for diagnostic tubes above 70 kVp.
Compensating filters equalise exposure across uneven anatomy.
Light–radiation field congruence must be within 2% of SID.
Tighter collimation = higher contrast, lower dose.
Common exam questions: “What is the purpose of filtration in diagnostic X-ray imaging?” → to remove low-energy photons that contribute to dose but not image formation. “Explain how collimation reduces patient dose but improves image quality.” → Less tissue exposed to ionising radiation, tissue adjacent to exposed area no longer a source of scatter. “What are the two types of filtration?”→ Inherent, added.