X-ray microtomography

Microtomography (commonly known as Industrial CT Scanning), like tomography, uses x-rays to create cross-sections of a 3D-object that later can be used to recreate a virtual model without destroying the original model. The term micro is used to indicate that the pixel sizes of the cross-sections are in the micrometer range.^[1] These pixel sizes have also resulted in the terminology micro-computed tomography, micro-ct, micro-computer tomography, high resolution x-ray tomography, and similar terminologies. All of these names generally represent the same class of instruments.

This also means that the machine is much smaller in design compared to the human version and is used to model smaller objects. In general, there are two types of scanner setups. In one setup, the X-ray source and detector are typically stationary during the scan while the sample/animal rotates. The second setup, much more like a clinical CT scanner, is gantry based where the animal/specimen is stationary in space while the X-ray tube and detector rotate around. These scanners are typically used for small animals (in-vivo scanners), biomedical samples, foods, microfossils, and other studies for which minute detail is desired.

The first X-ray microtomography system was conceived and built by Jim Elliott in the early 1980s. The first published X-ray microtomographic images were reconstructed slices of a small tropical snail, with pixel size about 50 micrometers.^[2] Many believe that the technology did not really take off until the advent of the cone beam reconstruction algorithm originally authored by Lee Feldkamp.

Working principle

• Imaging system

Fan beam reconstruction

The fan-beam system is based on a 1-dimensional x-ray detector and an electronic x-ray source, creating 2-dimensional cross-sections of the object. Typically used in human Computed tomography systems.

Cone beam reconstruction

The cone-beam system is based on a 2-dimensional x-ray detector (camera) and an electronic x-ray source, creating projection images that later will be used to reconstruct the image cross-sections.

• Open/Closed systems

Open x-ray system

In an open system, x-rays may escape or leak out, thus the operator must stay behind a shield, have special protective clothing, or operate the scanner from a distance or a different room. Typical examples of these scanners are the human versions, or designed for big objects.

Closed x-ray system

In a closed system, x-ray shielding is put around the scanner so the operator can put the scanner on a desk or special table. Although the scanner is shielded, care must be taken and the operator usually carries a dose meter, since x-rays have a tendency to be absorbed by metal and then re-emitted like an antenna. Although a typical scanner will produce a relatively harmless volume of x-rays, repeated scannings in a short timeframe could pose a danger.

Closed systems tend to become very heavy because lead is used to shield the x-rays. Therefore, the smaller scanners only have a small space for samples.

Three-dimensional (3D) image reconstruction

The principle

Because microtomography scanners offer isotropic, or near isotropic, resolution, display of images does not need to be restricted to the conventional axial images. Instead, it is possible for a software program to build a volume by 'stacking' the individual slices one on top of the other. The program may then display the volume in an alternative manner.

Volume rendering

Volume rendering is a technique used to display a 2D projection of a 3D discretely sampled data set, as produced by a microtomography scanner. Usually these are acquired in a regular pattern (e.g., one slice every millimeter) and usually have a regular number of image pixels in a regular pattern. This is an example of a regular volumetric grid, with each volume element, or voxel represented by a single value that is obtained by sampling the immediate area surrounding the voxel.

Image segmentation

Where different structures have similar threshold density, it can become impossible to separate them simply by adjusting volume rendering parameters. The solution is called segmentation, a manual or automatic procedure that can remove the unwanted structures from the image.

Typical use

• Biomedical

• Both in vitro and in vivo small animal imaging
• Human skin samples
• Bone samples, ranging in size from rodents to human biopsies
• Lung imaging using respiratory gating
• Cardiovascular imaging using cardiac gating
• Tumor imaging (may require contrast agents)
• Soft tissue imaging (may require contrast agents)

• Electronics

Small electronic components. E.g. DRAM IC in plastic case.

• Microdevices

E.g. spray nozzle

• Composite materials and metallic foams

E.g. composite material with glass fibers 10 to 12 micrometres in diameter

• Polymers, plastics

E.g. plastic foam

• Diamonds

E.g. detecting defects in a diamond and finding the best way to cut it.

• Food and seeds

• E.g. piece of chocolate cake, cookies
• 3-D Imaging of Foods Using X-Ray Microtomography^[3]

• Wood and paper

E.g. piece of wood to visualize year periodicity and cell structure

• Building materials

E.g. concrete after loading.

• Geology

• E.g. sandstone
• porosity and flow studies

• Microfossils

E.g. bentonic foraminifers

• Space

E.g. Locating Stardust-like particles in aerogel using x-ray techniques^[4]

• Others

E.g. cigarettes

• Stereo images

Visualizing with blue and green or blue filters to see depth

Publications

• MicroComputed Tomography: Methodology and Applications
• Synchrotron and non synchrotron X-ray microtomography threedimensional representation of bone ingrowth in calcium phosphate biomaterials
• Microfocus X-ray Computer Tomography in Materials Research
• Locating Stardust-like particles in aerogel using x-ray techniques
• 3-D Imaging of Foods Using X-Ray Microtomography
• Use of micro CT to study kidney stones
• Application of the Gatan X-ray Ultramicroscope (XuM) to the Investigation of Material and Biological Samples
• 3D Synchrotron X-ray microtomography of paint samples

References

1. ^ MeSH X-Ray+Microtomography
2. ^ JC Elliott and SD Dover. X-ray microtomography. J. Microscopy 126, 211-213, 1982.
3. ^ http://www.cb.uu.se/~cris/Documents/GIT2003.pdf
4. ^ http://www.lpi.usra.edu/meetings/lpsc2003/pdf/1228.pdf