Contrast

Understanding XRM Technology
Contrast
Contrast

Overview


Optimal Contrast for Your Most Challenging Samples

Contrast in X-ray imaging is typically achieved through the absorption of X-rays within a sample. Since measuring the attenuation of an X-ray beam through a sample is relatively fast and efficient in high-absorbing materials, conventional micro-CT systems use a single large area detector to optimize capture of higher X-ray energies.

This approach works for dense and high-Z materials like metal parts and bones that provide sufficient contrast but not as well for others, especially at the nanoscale. ZEISS 3D X-ray microscopes (XRM) feature a unique contrast-enhancing architecture and suite of software tools that give you unprecedented resolution and contrast to reveal details in difficult-to-image materials. ZEISS XRM offer phase and absorption contrast to optimize visualization of features of interest in a wide range of samples. Phase and absorption contrast optimize imaging for a diverse range of materials such as polymers, oxides, composites, fuel cells, geological samples and biological materials.

Gain the superior contrast you need to visualize and quantify a wide range of materials including:

Low atomic number (low Z) materials such as soft tissue, polymers, fluids and gels, carbon and glass fibers, and silicon.
Materials of similar Z indices like ceramic compounds sintered from multiple elements of similar atomic number and insects and fossils trapped in amber

Contrast_Fig1a
Unstained Soft Tissue – Phase contrast was employed to image unstained cartilage in a mouse knee


Contrast_Fig1b
Plants – Pear imaged with absorption contrast – no visibility of cell walls (left), and pear imaged with phase contrast, showing details of cell walls in normal cells and stone cells (right)


Architecture


Absorption and Phase Contrast: now you don’t have to settle for less

Absorption contrast imaging, essentially shadow or projection imaging, uses the varying attenuation power of different materials to generate contrast. It is best suited to specimens containing properties of varying density—for example, material and pore space. Phase contrast imaging uses the refraction of X-rays rather than absorption. It is very sensitive to interfaces between materials of similar density or low absorption (edge enhancement).

ZEISS Xradia Versa XRM use absorption contrast and add phase contrast with a unique detector system—a rotating detector turret with selectable magnification and field-of-view pairings. Each detector objective features a matched scintillator that optimizes contrast to preferentially image within its contrast-forming useful energy band.

ZEISS Xradia Ultra XRM offer both absorption and phase contrast to optimize visualization of features of interest in a wide range of samples. Integrated phase contrast technology employing the Zernike method enhances the visibility of grain boundaries and material interfaces when absorption contrast is low, enabling visibility of ultra- and nano-structures without staining.


Applications and Techniques


Flexible Tools to Optimize Your Discovery

ZEISS Xradia 520 Versa offers breakthrough DSCoVer Advanced Compositional Contrast to extend the detail captured in a single energy absorption image. The tool combines information from tomographies taken at two different X-ray energies to highlight distinctions in materials such as silicon and aluminum that have similar X-ray attenuation characteristics.

In standard single absorption tomography, the resulting grayscale signal depends on the material’s effective atomic number and electron density. The contrast weighting of properties depends on the X-ray energy used to image the sample: at lower X-ray energies, the photoelectric effect dominates (related to a material’s effective atomic number) while Compton scattering dominates at higher energies. Compositional contrast is achieved flexibly by using two tomographies, one captured using photoelectric effects and one featuring Compton.

Xradia Versa architecture is uniquely suited for this due to superior low kV imaging and flexible high kV imaging. Xradia 520 Versa can access X-ray energies up to 160 kV while non-flat panel micro-CTs generally only access 50 or 100 kV.

For more information, go to Downloads and select Product Info: Xradia 520 Versa.

Contrast_Fig2
A single energy scan shows that aluminum and silicon are virtually identical (left), with very similar grayscale contrast. Using the DSCoVer interface to “tune” and highlight aluminum (middle) enables separation of the particles. 3D rendering shows Aluminum/green; Silicates/red (right)

At the nanoscale, choose the X-ray energy that optimizes contrast in your research samples 5.4 keV or 8.0 keV

Ultimately, in XRM, contrast depends on the material being imaged and the X-ray energy used. The Xradia Ultra family comprises Xradia 800 Ultra, operating at 8 keV photon energy, and Xradia 810 Ultra, operating at 5.4 keV. In general, lower energy X-rays are absorbed more strongly and therefore provide higher contrast. Thus, as long as transmission remains sufficient, the resulting image quality and/or throughput are greatly improved with Xradia 810 Ultra. For materials of higher density, or thick specimens, the higher X-ray energy of Xradia 800 Ultra may be needed to provide sufficient transmission.

Contrast_Fig3
Dentin imaged at 5.4 keV, left, and 8.0 keV, right. At 5.4 keV, image quality is equivalent while acquisition is 10 times faster due to optimized contrast



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Additional information on contrast can be found in the ZEISS Xradia Technology Note Contrast with a 3D X-ray Microscope For Difficult-to-Image Materials and the product info ZEISS Xradia 810 Ultra. Both are accessible from Downloads.


Downloads