Filtration in Radiography

Filtration is the process of eliminating undesirable low-energy x-ray photons by the insertion of absorbing materials into the primary x-ray beam.
Proper insertion of filtration allows radiographers to shape the photon emission spectrum into a more useful beam, also referred to as hardening the beam by removing low-energy (soft) photons.
The primary reason for filtration is to eliminate photons that increase radiation dose to the patient but do not enhance the radiographic image.

At 20 keV, about 45 percent of the incident photons penetrate 1 cm of soft tissue, while only 0.0006 percent penetrate 15 cm.
At 50 keV, 3.5 percent of the incident photons penetrate 15 cm, indicating significant absorption.
Significant soft tissue penetration occurs at energy levels between 30 and 40 keV, emphasizing the need to filter out low-energy photons which contribute to patient dose without benefiting image quality.

Any material designed to selectively absorb photons from the x-ray beam is referred to as a filter.
Common filter materials used in diagnostic radiology include:
- Aluminum (standard filter material)
- Glass, oil, copper, and tin (used in certain situations).
Filtration is often expressed in terms of aluminum equivalency (Al/Eq). For instance, certain components of a collimator may be equivalent to 0.5 mm Al/Eq.
Half-Value Layer (HVL): The amount of absorbing material that reduces the intensity of the primary beam to one-half its original value. It is expressed in aluminum filtration equivalency (e.g., HVL = 2.0 mm Al/Eq).
Federal regulations specify minimum HVLs for diagnostic x-ray tubes as outlined in Title 21 of the Code of Federal Regulations (21 CFR 1020.30).

Resulting from the composition of the x-ray tube and housing—parts of the structures such as the glass envelope contribute to inherent filtration. A typical x-ray tube's inherent filtration ranges from 0.5 to 1 mm Al/Eq.
Mammography tubes utilize beryllium windows, which can reduce inherent filtration to 0.1 mm Al/Eq.
Age-related degradation in tubes can lead to increased inherent filtration, affecting efficiency; hence, HVL testing is recommended as quality control.

Added filtration occurs outside the tube and housing before reaching the image receptor.
Materials selected for added filtration (e.g., aluminum) aim to absorb low-energy photons while transmitting high-energy ones.
A collimator typically adds an average of 1 mm Al/Eq of filtration, primarily due to the silver mirror in the device.

Compound filters utilize multiple materials that complement each other's photon-absorbing abilities—designed to sequentially absorb characteristic photons created by each layer. It is also known as a K-edge filter.
The highest atomic number material is placed closest to the tube, with the final layer typically being aluminum.
Copper filters alongside aluminum are used especially in higher energy applications, requiring precise thickness to avoid absorbing desirable energies.

Compensating filters address unequal subject density, utilizing absorbers to help equalize the primary beam's absorption.
Common compensating filters include wedge and trough filters, effective for various dental and thoracic imaging procedures.
Custom designs may be implemented yielding unique configurations that suit specific patient body parts and imaging needs, using materials like aluminum, leaded plastic, or simple saline bags.

Filtration is crucial for reducing patient exposure by eliminating low-energy photons while ensuring sufficient beam intensity for quality imaging.
The total filtration is the sum of inherent and added filtration; it does not include compensating filters.
The thickness of added filtration can vary based on equipment use, influencing patient dose and image quality.
The behavior of photon attenuation varies with thickness and material type; therefore, proper selection is vital for effective filtration.

Presents comparison of Entrance Skin Exposure (ESE) with different levels of aluminum filtration.

Aluminum Filtration (mm)	ESE (mR) at 60 kVp	Decrease in Exposure Dose (%)	ESE (mR) at 85 kVp	Decrease in Exposure Dose (%)
None	2,380	-	1,225	-
0.5	1,850	22%	860	30%
1.0	1,270	47%	684	44%
3.0	465	80%	287	77%