Beam hardening phenomenon

Beam hardening phenomenon refers to the effect of changing the energy of the x-ray beam has on imaging. 

Principle

An x-ray beam is composed of individual photons with a range of energies. As the beam passes through an object, it becomes "harder" (mean energy increases) because the lower-energy photons are absorbed more rapidly, leaving behind only the high energy photons. 

Artifacts

Two types of artifact can result from this effect :

1. cupping artifacts
2. dark bands or streaks between dense objects in the image.

Cupping artifacts

X rays passing through the middle portion of a uniform cylindrical phantom are hardened more than those passing though the edges because they are passing though more material.

As the beam becomes harder, the rate at which it is attenuated decreases, so the beam is more intense when it reaches the detectors than would be expected if it had not been hardened.

Streaks and dark bands

In very heterogeneous cross sections, dark bands or streaks can appear between two dense objects in an image.

They occur because the portion of the beam that passes through one of the objects at certain tube positions is hardened less than when it passes through both objects at other tube positions.

This type of artifact can occur both in bony regions of the body and in scans where a contrast medium has been used.

Minimising beam hardening

A number of techniques can be used to minimize this effect, including:

1. filtration
2. calibration correction
3. beam hardening correction software

Filtration

A flat piece of attenuating, usually metallic material is used to "pre-harden" the beam by filtering out the lower-energy components before it passes through the patient.

An additional "bowtie" filter further hardens the edges of the beam, which will pass through the thinner parts of the patient.

Calibration correction

Manufacturers calibrate their scanners using phantoms in a range of sizes. This allows the detectors to be calibrated with compensation tailored for the beam hardening effects of different parts of the patient.

Beam hardening correction software

An iterative correction algorithm may be applied when images of bony regions are being reconstructe (i.e. minimize blurring of the bone–soft tissue interface in brain scans)^2,3.