Filtration is a critical process in X-ray generation that removes low-energy photons from the beam before it reaches the patient. By selectively absorbing these photons, the beam becomes more penetrating, a process known as beam hardening.

Filtration improves beam quality and reduces patient dose without significantly affecting diagnostic image quality.

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Purpose of Filtration

Remove photons that are too low in energy to penetrate tissue.
Reduce patient skin dose (especially superficial tissues).
Increase the mean photon energy (harder beam).
Produce a more uniform and efficient beam for imaging.

Filtration therefore serves a dual purpose: radiation protection and beam optimisation.

How do we describe filters?

Type	Description	Typical Contribution	Examples
Inherent filtration	Built into the tube assembly (envelope, oil, window).	~0.5–1.0 mm Al equivalent	Glass/metal envelope, oil, beryllium window
Added filtration	Thin sheets of metal placed in the beam path.	Adds 1–2 mm Al equivalent	Aluminium, copper filters
Total filtration	Combined inherent + added filtration.	≥ 2.5 mm Al equivalent (>70 kVp)	Required by IEC/ICRP standards
Compensating filtration see note at end for short summary	Shaped filters that equalise exposure in uneven anatomy.	Variable	Wedge, trough, bow-tie filters

These filters remove low energy photons preferentially and increase the mean energy of the beam. This process is called beam hardening. Let’s look at this process step-by-step.

Mechanism of beam hardening by filtration

Step 1:

The unfiltered X-ray beam emerging from the anode contains photons with a wide range of energies, from near 0 up to the maximum set by the tube voltage (E_max = kVp).
Low-energy photons make up a large portion of this beam, but they have insufficient energy to penetrate tissue, instead, they are absorbed in the patient’s skin or superficial tissues, increasing dose without contributing to image formation.

Step 2:

When the beam passes through a filter (typically aluminium or copper), photons interact with the filter material via the photoelectric effect and Compton scatter.

The probability of photoelectric absorption is inversely proportional to the energy (E) of the photon travelling through the filter.
Probability of photoelectric effect ∝ (1/E³).
Therefore, low-energy photons are far more likely to be absorbed than high-energy ones.
As a result, the low-energy portion of the spectrum is preferentially removed.

Step 3:

High-energy photons (which have a much lower interaction probability) are more likely to pass through the filter.
This selective removal of soft photons causes a shift in the average photon energy (E_mean) towards higher values. The beam becomes “harder”, meaning more penetrating.

Step 4:

The resulting beam after filtration has:

Fewer total photons (reduced intensity).
A higher mean energy (beam hardening).
More uniform penetration through tissue.

This process reduces skin dose and improves efficiency, as a greater proportion of the photons that reach the patient are capable of contributing to image formation.

Stage	Spectrum Characteristics	Mean Energy (Emean)	Clinical Effect
Before filtration	Broad range, many low-energy photons	Lower (~⅓ Emax)	Higher skin dose, poor efficiency
After filtration	Low-energy photons removed	Higher	Lower dose, more uniform penetration

Quantifying Beam Hardness: Half-Value Layer (HVL)

The half-value layer is the thickness of a specified material (usually aluminium) required to reduce beam intensity by 50%.

HVL = Thickness of material that halves intensity

Relationship with Beam Quality

Higher HVL → higher mean photon energy → harder beam.
HVL is measured as part of quality assurance to monitor filtration and tube output consistency.

Clinical Impact of Filtration and Beam Hardening

Parameter	Effect of Increased Filtration
Patient dose	↓ Decreases (fewer low-energy photons)
Beam penetration	↑ Increases
Image contrast	↓ Decreases slightly
Beam uniformity	↑ Improves
Tube load	↑ increase if detector exposure is kept constant (requires more output to maintain detectorexposure)

Key Takeaways and Exam Tips

Filtration removes low-energy photons → reduces dose and increases beam hardness.
Beam hardening = shift of mean photon energy to higher values.
Total filtration ≥ 2.5 mm Al equivalent for >70 kVp.
HVL is the standard measure of beam quality.
Added Cu filtration used in high-dose applications (fluoroscopy).
Mammography: uses low-Z filters (Mo, Rh) to preserve soft-tissue contrast.
Common exam question: “Explain how filtration affects the X-ray spectrum and patient dose.” → Removes low-energy photons, increases mean energy, reduces patient dose.

Up next:

End of Section 3 – Production of X-rays

You’ve now covered:

Target interactions – how photons are generated.
X-ray spectrum – their energy distribution.
Beam quality and quantity – how exposure factors control output.
Filtration and beam hardening – how the spectrum is refined and patient dose optimised.

Section 4 is all about how X-rays interact with matter.

Compensating Filters

Used when body thickness varies across the field, ensuring uniform receptor exposure.

Filter Type	Purpose / Example
Wedge filter	Evens out exposure in shoulder, lateral foot, chest (apex vs base).
Trough filter	Balances mediastinum and lung fields in chest radiography.
Bow-tie filter	Used in CT to reduce peripheral dose and equalise detector exposure.