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Beam Hardening: Definition, Causes and How to Reduce it

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What comes to mind when you hear of beam hardening? 

Challenges often occur during X-ray inspection processes that hinder the correct representation of the test samples for an accurate result. One of these challenges involves the reconstruction of an X-ray image where the edge looks brighter than other parts of the image despite the material being homogenous. This challenge is often caused by an assumption that X-ray beam attenuation is linear rather than being treated as a spectrum range of energies. Consequently, the artifact produced in this situation is known as beam hardening and is the focus of this post. 

So, if you want to know what causes it and how to reduce it then let’s dive in together. 

 

I. What is Beam Hardening?

Beam hardening is a type of physics-based artifact that emanates from manual procedures demanded in obtaining information about the internal structure of a test sample or material.

Beam hardening is the situation that occurs when an X-ray beam composed of a range of energies permeates an object and ends with specific weakening lower energy photons. The outcome is theoretically related to a high-pass filter, that is, only higher energy photons are left to supply the beam. Therefore, the mean beam energy is heightened, a concept also known as beam hardening.

Furthermore, Beam Hardening is a type of feature where the X-ray spectrum shifts to higher powerful energy which is a result of the withdrawal of a lower x-ray electromagnetic wave. 

 

Beam Hardening- Definition, Causes and How to Reduce it

 

II. Why is Beam Hardening Important?

Beam hardening is crucial when it comes to obtaining useful data about the internal structure of an object under assessment because accurate data is to be obtained from material inspection and analysis.

Another noteworthy importance is that Beam hardening increases the possibility of acquiring a better quality image as a result of the increase in the photons application on the object of interest. With better image quality, accurate assessment becomes a possibility. 

 

III. What Affects Beam Hardening?

Multicolored spectrum x-ray beams are what most industrial x-ray sources generate. Since the X-ray depletion depends solely on energy, when the beam permeates the object of inspection, little energy X-rays are sharply depleted than high energy X-rays. Therefore, the quantity of photons striking an X-ray detector is not entirely proportionally related to the permeated material density. 

Conversely, many re-established methods speculate that linear depletion causes the external part to have a different color from its internal structure (also known as the cupping artifacts). Likewise, thicker objects have streaks on its outer part (also known as streaking artifacts) which form some of the effects of beam hardening. 

The beam-hardening effect emanates from favorable integrations of low-energy multicolored electromagnetic waves, and X-ray beams that infiltrate an object. The precipitating refined beam has an elevated apportioning of “harder,” or higher photons which elevate the mean energy of the beam and culminate in low-declining artifacts.

 

IV. What is Beam Hardening Artifact?

Beam Hardening is the most repeatedly experienced artifact in Computerized Tomography scanning, it precipitates the sides of the object of interest to seem brighter than its middle or other parts, even when the material looks the same all over.

Depending on its location in an image Beam hardening artifact has the ability to transform the CT value of material and makes it detrimental to getting accurate results. Therefore, the efforts made to use a single CT value to establish and measure the scope of a definite object are usually controversial. 

 

V. What Causes Beam Hardening Artifacts?

Beam Hardening Artifacts are often the result of polychromatic energies passing through an object thereby ending with systematic attenuation, i.e., not uniformly attenuated. 

Beam Hardening Artifacts are also caused by the increase in mean X-ray energy at different angles and hardening of the X-ray beam occurs as it passes through the object of interest. Consequently, lower-energy X-rays diminishes more readily than higher-energy X-rays.

Sometimes there are measures taken to neglect these exterior parts of the image and analyze just the center of the image. Although this method separates the unfavorable part of the problem, the artifact remains unending and so, portions of the image are affected.

For dense materials, when the volume of photons striking the object of interest is too rapid and intense due to its thickness and density, it begins to form streaks and dark bands around the object. Despite these causative factors in material inspection, there are also readily available remedies to these challenges and that’s what we’ll be considering next.

 

VI. How to Reduce Beam Hardening Artifacts? 

You would agree with us that for every problem, there is usually a solution to it, and the challenges posed by Beam Hardening Artifacts are no different. Numerous methods of correcting this phenomenon have been made available and we’ll be considering a few of them.

In Beam Hardening Reduction, most contemporary CT scanners use filters in a move to subdue the challenges posed by beam hardening. Usually, an attenuating substance (often metallic in mold) is deployed to harden the beam before it reaches the object of interest. It’s also very much important to position the uniform attenuating material/bow tie filter within the x-ray collimator to pre-harden the beam, this technique is typically referred to as filtration.

 Computerized Tomography scanners frequently need to be calibrated with phantoms from specific vendors to control or correct unavoidable beam hardening artifacts like cupping. For this reason, choosing the appropriate scan for the object of interest is quite crucial for the correction adopted to be effective in the corresponding calibration correction in the reconstructed image.

Another Beam Hardening correction method is the beam hardening correction software, which is a repetitive correction algorithm software that can be used to limit the artifacts.

A different corrective method entails increasing the tube voltage for better permeation of high-density objects to effectively eliminate any form of streak artifacts, or by utilizing a double energy imaging approach.

Other types of corrective measures are adopted depending on the manufacturer’s choice of beam hardening correction to be deployed. 

 

Conclusion

In conclusion, Beam hardening artifacts stem from the irregular fluctuations of x-ray beams composed of multicolored photons that permeate an object which results in selective reduction of energy.

Also, beam hardening artifacts manifest in two forms which are streak or dark bands artifacts or cupping artifacts. Cupping artifact is when the exterior parts of the object of interest develop a different color from its interior and streak artifact occurs when the outer part of an object exhibits streak patterns, usually on materials with high density.

And finally, some corrective methods of curbing beam hardening are filtration which utilizes a uniform attenuating material to pre harden the beam, calibration correction which utilizes a range of uniform phantoms to correct especially cupping artifacts, and beam hardening correction software which uses a repetitive software algorithm to limit beam hardening artifacts.

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