Connecting researchers with advanced imaging solutions

Welcome to the fifth issue of XCT Mastery Monthly! In our journey to master the art and science of X-ray Computed Tomography, we now turn our attention to a critical, yet often overlooked, component of any robust CT system: the X-ray filter. This month, we'll move beyond the basics to explore the vital role of filtration in enabling true quantitative analysis, mitigating artefacts in complex materials, and paving the way for advanced material characterisation. We will dissect sophisticated filtration strategies, including the ingenious Thoraeus filter, and chart the course towards the ultimate goal in analytical CT: the monochromatic beam.

The polychromatic problem

At the heart of every laboratory-based XCT scanner lies an X-ray tube that generates a polychromatic beam, a wide spectrum of photon energies. This is not just a technical footnote; it is the single greatest barrier to moving from qualitative imaging to quantitative measurement in material science. The reason is a phenomenon known as beam hardening (have a look at Issue #4 of XCT Mastery Monthly).

As this polychromatic beam traverses a material, the lower-energy ("softer") photons are preferentially absorbed over their high-energy ("harder") counterparts. This progressively increases the mean energy of the beam. Since higher-energy X-rays are less attenuating, the linear relationship between material thickness and measured attenuation—a fundamental assumption in CT reconstruction—breaks down.

This non-linearity gives rise to a host of artefacts that can render quantitative analysis impossible. For the materials scientist, these are not just visual imperfections; they are critical data corruptions. Below, I added a quick recap of some common artefacts discussed in the previous Issue to introduce the filtration topic:

  • Cupping Artefacts: Consider a scanof a solid, homogenous cylinder of an additively manufactured titaniumalloy (e.g., Ti-6Al-4V) intended for an aerospace application. Beamhardening will cause the reconstructed density to appear lowest at thecore and highest at the rim. This "cupping" of the densityprofile could completely mask subtle but critical density variationsarising from the manufacturing process, such as localised sinteringissues, or lead to incorrect inputs for finite element analysis (FEA)models.

  • Streaking Artefacts: Imaginescanning a complex, multi-material assembly like a carbon fibre reinforcedpolymer (CFRP) panel with metallic fasteners. The intense beam hardeningthat occurs as X-rays pass through the high-density fasteners createssevere dark streaks in the adjacent, low-density polymer matrix. Thesestreaks can obscure or mimic critical manufacturing defects likedelaminations, porosity, or fibre waviness, making a reliable qualityassessment impossible. Similarly, in geological core samples, streakingbetween dense pyrite nodules can prevent accurate segmentation andquantification of the surrounding porous rock matrix, crucial forpetrophysical analysis.

  • Photon Starvation: When scanninghighly attenuating materials like tungsten carbide or lead shielding, themajority of the beam (even the high-energy photons) can be blocked. Thisleads to "photon starvation" at the detector, resulting inextreme noise and streak artefacts that can completely obscure theinternal structure.

Mitigating these effects is the primary motivation for X-ray filtration, the first and most crucial step in taming the polychromatic beam.

Strategic filtration

The goal of filtration is to "pre-harden" the beam by removing low-energy photons before they reach the sample. This makes the beam's energy spectrum narrower and more stable as it passes through the material, restoring a more linear attenuation response. The selection of a filter is a strategic decision based on the interplay between the X-ray source, the filter itself, and the material under investigation.

The core toolkit: Aluminium (Al) Copper (Cu) and Tin (Sn)

  • Aluminium (Al): As a low-densitymetal, Al is the standard choice for filtering out the lowest-energyX-rays without drastically reducing photon flux. It is the workhorse forlower-density materials.

  • Copper (Cu): Being denser, copperprovides much stronger filtration. It aggressively removes low andmid-energy photons, producing a highly hardened beam.

  • Tin (Sn): Being even denser thancopper, tin offers the most aggressive filtration among the three. It isexceptionally effective at removing low and mid-energy photons, resultingin an extremely hardened beam. Tin is particularly useful when veryhigh-energy X-rays are required, or when filtering out a broad spectrum oflower-energy photons is critical for the application (e.g., to reducescatter or achieve specific penetration depths).

Let's have a look at an example of an unfiltered X-ray beam (180 keV, 0.5 mm of Al inherent filtration, Fig.1) and a few examples of X-ray spectra filtered with different thicknesses of Al (Fig.2), Cu (Fig.3a and 3b) and Sn (Fig.4). The filters were generated with TASMICS profiles calculated by A.M. Hernandez, J.M. Boone, "Tungsten Anode Spectral Model using Interpolating Cubic Splines; Unfiltered x-ray spectra from 20 kV to 640 kV" Med Phys. 41, 042101 (2014)


Figure 1 - 180 keV, 0.5 mm Al inherent filtration - Average energy =62 keV


Figure 2 - 180 keV, 0.5 mm Al inherent filtration + 1.5 mm Al - Average energy = 67 keV


Figure 3a - 180 keV, 0.5 mm Al inherent filtration + 0.5 mm Cu - Average energy = 85 keV


Figure 3b - 180 keV, 0.5 mm Al inherent filtration + 1 mm Cu - Average energy = 95 keV


Figure 4 - 180 keV, 0.5 mm Al inherent filtration + 0.85 mm Sn - Average energy = 114 keV

As it's clear from the plots, increasing the Z of the filter and its thickness, the X-ray spectra (Bremsstrahlung) move to the 'right', showing an increase in the mean energy of the beam (Fig.5 below)

 


Figure 5 - Schematic of Beam Hardening explained

Developing a filtration strategy

Choosing the right filter is a balancing act between artefact reduction, image contrast, and scan time. Consider these scenarios:

  • Scenario 1: Porosity in additively manufactured AlSi10Mg. The goal is to detect fine pores (~10-50 µm) and check foruniform density. The material itself is not extremely dense.

  • Scenario 2: Phase analysis in a geological core. The sample contains a mix of low-density quartz, mid-densitycalcite, and high-density pyrite.

  • Scenario 3: Fibre orientation in acarbon fibre composite. The density difference between the carbonfibres and the polymer matrix is very low.

Advanced filtration: engineering the X-ray spectrum

For more demanding applications, especially in multi-material analysis, simple filters give way to more complex, engineered solutions.

The Thoraeus filter

The Thoraeus filter is one of my favourite ones. It is a prime example of spectral engineering. Its brilliance lies in the sequential use of K-edge absorption, a sharp discontinuity in a material's attenuation spectrum. A typical Thoraeus filter consists of three layers arranged in a specific order:

  1. Tin (Sn) (K-edge at 29.2 keV):Placed closest to the source, tin aggressively absorbs photons below itsK-edge and the continuous spectrum above it. However, this processgenerates unwanted fluorescent X-rays from the tin itself at around 25-29keV.

  2. Copper (Cu) (K-edge at 8.98 keV):The copper layer's primary job is to absorb the characteristic fluorescentphotons from the tin. In turn, it produces its own, much lower-energyfluorescence.

  3. Aluminium (Al) (K-edge at 1.56 keV): The final layer is a "cleanup" filter, easilyabsorbing the low-energy fluorescence from the copper, resulting in asignificantly hardened and cleaner X-ray spectrum.

Different types of Thoraeus filters exist depending on the thicknesses of the individual components:

  1. Thoraeus I 0.2 mm Sn + 0.25 mm Cu +1 mm Al

  2. Thoraeus II 0.4 mm Sn + 0.25 mm Cu+ 1 mm Al

  3. Thoraeus III 0.6 mm Sn + 0.25 mm Cu+ 1 mm Al

Other combinations can be made, and a common one, also used in cultural heritage recent studies, is 0.25 mm Sn + 0.5 mm Cu + 0.5 mm Al (Fig. 6)


Figure 6 - examples of filtration used in cultural heritage investigations (Kiss et al. 2023,

Material Science Utility: The highly hardened, narrow-band spectrum produced by a Thoraeus filter is extremely valuable for dual-energy CT (DECT). By performing two separate scans—one with low-energy filtration (e.g., Al) and one with strong, high-energy filtration (e.g., Thoraeus)—we can solve for two material-dependent unknowns. This allows us to create virtual maps of effective atomic number (Zeff) and density (ρ), enabling material decomposition. For instance, in a complex electronic component, DECT can be used to differentiate between tin-lead solder, copper traces, and silicon dies, even if their grayscale values overlap in a standard CT scan.

Bowtie filters

Commonly used in medical CT, the bowtie filter—thicker at the edges, thinner in the centre—is immensely useful for the cylindrical or rod-like samples common in material science (e.g., Cylindrical Li-ion cells, metal billets, ceramic rods, geological cores). Its function is to pre-attenuate the beam more on the periphery, where it passes through less material, and less in the centre. This evens out the photon flux reaching the detector, which:

  • Prevents detector saturation:Avoids "blooming" artefacts at the edges of the detector.

  • Improves Signal-to-Noise (SNR):Reduces noise in the highly attenuated central region of the sample.

  • Increases dynamic range: Allows thedetector to operate in its optimal sensitivity range for all parts of thesample.

The result is a reconstruction with uniform noise characteristics, which is critical for detecting subtle flaws within the sample volume.

The ultimate goal: a monochromatic X-ray beam

While filtration significantly improves beam quality, the ultimate solution to eliminating beam hardening artefacts is to use a monochromatic X-ray beam—a beam consisting of photons of a single energy. Achieving this in a lab setting is challenging, but several techniques bring us closer to this ideal.

Crystal monochromators

One of the most effective ways to produce a monochromatic beam is by using a crystal monochromator. These devices utilise the principle of Bragg diffraction. When a polychromatic X-ray beam strikes a single crystal (like silicon or germanium) at a specific angle, only a very narrow band of energies is diffracted towards the sample. While this produces a beautifully clean beam, the trade-off is a significant reduction in intensity, often requiring longer scan times or more powerful sources.

Advanced X-ray sources and optics

  • MetalJet Sources: These sources usea liquid metal jet as the anode target, allowing for much higher electronbeam power without melting the target. This results in a significantlybrighter X-ray source with a more prominent characteristic peak that canbe used for quasi-monochromatic imaging. A very established companyproducing metalJet sources is the Swedish Excillium (https://www.excillum.com/)

  • Polycapillary Optics: These arebundles of thousands of tiny, curved glass tubes that can collect, focus,or parallelise X-rays from a source. While not a monochromating techniqueon their own, they efficiently guide and shape the beam for use with otherfiltering or monochromating elements. This technique is employed in one ofour nano-CT systems, the ZEISS Xradia 810 Ultra X-ray microscope (https://www.zeiss.com/microscopy/en/products/x-ray-microscopy/xradia-ultra.html)

The software correction

Hardware filtration and advanced sources should always be paired with powerful software correction. Modern iterative reconstruction algorithms (e.g., MBIR - Model-Based Iterative Reconstruction) can incorporate a physical model of the X-ray spectrum and the beam hardening phenomenon. For complex multi-material samples, where a simple polynomial correction fails, these advanced algorithms are essential for producing an accurate and artefact-free reconstruction.

Conclusion

X-ray filtration is the unsung hero in the search for quantitative material science. It is the critical first step that transforms a CT scanner from a simple 3D imaging device into a precise metrological instrument. By moving beyond a "one-size-fits-all" approach and strategically selecting filters—from simple copper plates to engineered Thoraeus stacks—we can modify the polychromatic beam. When combined with emerging technologies like advanced sources and reconstruction algorithms, these techniques allow us to eliminate artefacts and extract true, quantitative data on density, composition, and internal structure.

As we drive towards Industry 4.0 and the age of digital twins, the ability to generate a precise, accurate, and reliable digital copy of a material or component is no longer a luxury; it is a necessity. A masterful understanding and application of X-ray filtration is fundamental to this future, ensuring the XCT data we produce is not just a picture, but a measurement you can trust.

Hope you enjoyed this issue of XCT Mastery Monthly!

Please stay tuned and don't miss the next issue at the end of July 2025!