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Resolution: What does it mean in X-ray CT?

by Paul on 01 May 2023 at 13h56
The definition of resolution in X-ray CT is quite complex and many different terms are used to define this parameter. These different terms need to be clearly defined and understood in order to be able to compare one system to another. Several non-equivalent terms are used to define the resolution performance of X-ray CT systems. This article explains how to objectively compare different CT systems and technologies.
X-ray CT is now increasingly used in many academic and industrial fields, as an exact technology for 3D imaging at high-level resolutions. X-ray CT can be used at different scales, from micro-CT to nano-CT. The difference between these two terms is the achievable resolution.
Here is the point: the definition of the word « resolution » may seem straightforward, but in fact, several terms are used to represent the resolution capabilities of an X-ray CT scanner. These terms need to be clearly understood because usually, the first question when discussing X-ray CT is the following: « What is the best achievable resolution for this equipment? »
This question can have several answers, depending on the part that needs to be scanned, the scanning setup and so on.

Two levels in terms of resolution: Micro & Nano

A distinction is made between micro and nanotomography. Nanotomography is a high-precision technology for 3D imaging at sub-micron resolution. The technical concept is based on the further development of micro-CT technology. It should be noted that there is no generally accepted standard for industrial CT systems.
The only way to test the quality of the scan is to image a small feature of known dimensions and ensure that the feature is visible on the CT slice image. This is why, at RX Solutions, we offer resolution charts on which small features of small dimensions are engraved. Two different charts can be used, depending on whether you want to check the 2D or 3D resolutions.

Resolution: Different terms, different meanings 

In fact, the word « resolution » can be related to numerous terms, and it is essential to understand what each of these terms means to make an accurate and precise comparison between different CT scanners.
These terms include:
• Spatial resolution
• Voxel size
• Focal Spot size
• Nominal resolution
• Detail detectability

SPatial resolution

Spatial resolution is the most common term to define X-ray CT resolution. It establishes the measure of how closely lines can be resolved in an image. It’s the minimum resolution at which a pair feature can be resolved by the imaging system.

The spatial resolution of an x-ray or CT system is a measure of the ability of a system to differentiate small structures. The spatial resolution value is multi-factors dependent, from the X-ray source to the detector, source blur, source and system stability, imaging artefacts and other key aspects of a system that can degrade an image.

voxel Size

A voxel is a 2D pixel in 3D. This term is the contraction of VOlumetric piXEL, it’s a simple 2D pixel but seen in 3D. The voxel size depends on the magnification and is related to the distance of the sample from the x-ray source and detector.
Voxel size and spatial resolution are often confused. The voxel size is the size of a pixel in 3D. Voxel size isn’t related to the achievable spatial resolution of the CT system. For example, if the x-ray spot size (focused x-ray spot from the source) becomes larger than the chosen voxel size, the spatial resolution of the system becomes poorer.

Minimum voxel size

Minimum voxel size The minimum voxel size is calculated by assuming a tiny sample placed as close to the source and as far from the detector as possible. This is the limit set by the maximum SDD offered by the system coupled to the sample size.

Focal spot size

The X-ray focal spot size is the focused x-ray spot from the source. The X-ray focal spot size is the limitation factor of the achievable resolution of an X-ray system.
As an example, if your CT system is equipped with an X-ray source that have a minimum focal spot size of 4 µm, even if you reach voxel size below 4 µm, you will not be able to resolve the details in the image as the spatial resolution is physically limited by the X-ray source.

Nominal resolution

This value refers only to the theoretical value of the resolution of the X-ray scanner, not to a real value. It does not provide any evidence of the actual performance of a system. It is usually the same value as the minimum voxel size.

Detail detectability

As previously mentioned, the voxel size can on some CT systems be far better than the achievable spatial resolution of a CT system. This means that the image will be blurred, but tiny details can nevertheless be imaged. The detail detectability is usually 3 to 5 times the voxel size. Several voxels are necessary to visualize a detail.

RX SOlutions: A ct portfolio dedicated to high-resolution

- RX Solutions' range of CT systems covers the full resolution scale, with a range of micro & nano tomographs that also combine both levels of resolution in one piece of equipment.
- Specialising in the manufacture of tomographic systems that achieve true 3D resolution down to 0.4µm.
- Dual tube CT systems that achieve 0.4 µm resolution.

Our innovative X-ray CT solutions are suitable for product development, turning ideas into reality from brainstorming to the actual product ready for manufacturing. User-driven improvements are the core of our business, offering you optimal solutions with the right and powerful interface & components.
For a better understanding of X-ray CT, check our blog or contact an expert at www.rx-solutions.com