X-ray microtomography

X-ray microtomography, like tomography and x-ray computed tomography, uses x-rays to create cross-sections of a physical object that can be used to recreate a virtual model (3D model) without destroying the original object. The prefix micro- (symbol: µ) is used to indicate that the pixel sizes of the cross-sections are in the micrometre range.^[1] These pixel sizes have also resulted in the terms high-resolution x-ray tomography, micro–computed tomography (micro-CT or µCT), and similar terms. Sometimes the terms high-resolution CT (HRCT) and micro-CT are differentiated,^[2] but in other cases the term high-resolution micro-CT is used.^[3] Virtually all tomography today is computed tomography.

Micro-CT has applications both in medical imaging and in industrial computed tomography. 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.^[4]

Working principle

Imaging system

Fan beam reconstruction

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

Cone beam reconstruction

The cone-beam system is based on a 2D 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.

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^[5]
• Insects^[6]

Electronics

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

Microdevices

• Spray nozzle

Composite materials and metallic foams

• Composite material with glass fibers 10 to 12 micrometres in diameter

Polymers, plastics

• Plastic foam

Diamonds

• Detecting defects in a diamond and finding the best way to cut it.

Food and seeds

• Piece of chocolate cake, cookies
• 3-D imaging of foods using X-ray microtomography^[7]

Wood and paper

• Piece of wood to visualize year periodicity and cell structure

Building materials

• Concrete after loading

Geology

• Sandstone
• Porosity and flow studies

Microfossils

• Bentonic foraminifers

Space

• Locating stardust-like particles in aerogel using X-ray techniques^[8]
• samples returned from asteroid 25143 Itokawa by the Hayabusa mission^[9]

Stereo images

• Visualizing with blue and green or blue filters to see depth

Others

• Cigarettes

References