Tomography of Chondrites, Etc.
Overview
Volume Graphics Viewer software
Images
Technical details
Acknowledgments
Overview
X-ray Computed Tomography (CT) is a nearly nondestructive technique that allows us to obtain
images of the interiors of solid objects. Medical tomography (CAT-scanning) uses low-energy x-rays to image
the interior of the human body. The manipulation of these images (the Computer-assisted part of 'CAT') is called
'visualization'. From visualizations of the interiors of opaque solids (for example, meteorites), we can
understand how the objects are put-together, in 3 dimensions (3D). Higher energy x-rays are used for imaging of
rocks, bones (fossils), ceramics (e.g., archeological specimens), and metals (e.g., aircraft parts).
The quality of a CT-scan can be measured in several ways. The spatial resolution is measured by the size of each
'voxel' or 'volume element' in the data. In 2D images, a 'pixel', short for 'picture element', has a particular
size. A satellite may image the surface of Mars in square pixels 2 kilometers on a side, using a wide-angle
camera, and at 6 meters/side using a narrow angle camera. It won't cover as much area at the high resolution (6m), but it will pick up a lot of the detail. For meteorites, I have worked at fairly high resolution: 1-20 micron per voxel edge. Each voxel is a cube, and each voxel has a unique value associated with it. That value is the average
x-ray attenuation experienced by the x-rays that encounter that voxel. So a voxel filled with metal, which stops
(attenuates) x-rays very strongly, has a high value, but a voxel filled with air, which x-rays hardly even see,
has a very low value. In an 8-bit grayscale image, typical of most tiff files around the year 2003-7, there are
256 values (2 to the 8th power). The images are collected at higher bit depth, so they actually contain
'deeper' information, but are most easiyl visualized in 256 grayscales (8-bit) which is compatible with most
currently available hardware and software.
Two kinds of x-ray sources are commonly used: (1) adapted medical scanners, such as the facility at
U. Texas (see below), and (2) synchrotrons. I have used the Advanced Photon Source synchrotron at the
Argonne National Laboratory, of the United States Department of Energy, to obtain most of the images that
are on these pages. The synchrotron tomography setup we used allows spatial resolutions to as small
as 1 micron/voxel edge. But the objects must be small to allow x-rays to penetrate, because
spatial resolution decreases as x-ray energy increases. Larger objects, such as pieces of meteorite
10mm in cross section (about the thickness of a pencil), are imaged at ~17 micron/voxel edge. The term
high resolution has different meaning depending on what you want to image. Recently, Tsuchiyama and
colleagues at the SPring8 facility in Japan have imaged individual grains collected by the Stardust
mission with voxels less than 100 nanometers(nm)/edge! They use a newly developed technique (Fresnel zone
plate imaging). By contrast, the U.T. Austin facility has pages that describe high resolution as
'tens of microns'. It depends on the size of the objects you want to look at.
These techniques are described in detail in this paper:
Meteorite 3-Dimensional Synchrotron Microtomography: Methods and Applications
Ebel D.S. and Rivers M.L. (2007) Meteoritics and Planetary Science 42: 1627-1646.
Volume Graphics Viewer (software)
The software package from Volume Graphics GMBH is used by many tomography
labs for visulization and image processing of CT scan stacks. A free viewer is available for download, enabling
users to view data, but not to manipulate it in the full VG system. The links below are for myVGL Release 2.0
myvgl20_win32_setup.exe (setup file: 42 MB)
myvgl_install.pdf (instructions for setup)
myvgl20.pdf (manual for myVGL release 2.0)
Images
A DVD of tomography results was recently published with this paper:
Meteorite 3-Dimensional Synchrotron Microtomography: Methods and Applications
Ebel D.S. and Rivers M.L. (2007) Meteoritics and Planetary Science 42: 1627-1646.
Link to DVD with Tomography results: DVD
These images are provided as a public service for interested colleagues and web surfers.
Click on Meteorites in 2D and 3D
for more meteorite images.
CMBexps-1
Technical Details
What is tomography?
Link to U.T. Austin CT lab's excellent overview of tomography.
Ring artifacts: Ring artifacts result from defects in the CCD camera which obtained the images.
They propagate through the math which produces the final images shown here.
Useful links:
University of Texas at Austin, Center for X-ray Computer-aided Tomography
Tutorial on Synchrotron Tomography
(by M.L. Rivers, GSECARS, U. Chicago)
Software for Image Processing
(GSECARS, U. Chicago)
Acknowledgments
Use of the Advanced Photon Source was supported by the U.S. Department
of Energy, Office of Science, Office of Basic Energy Sciences, under Contract
No. W-31-109-ENG-38. Use of the GSE-CARS beamline, University of Chicago, is gratefully
acknowledged. This work is supported by the American Museum of Natural History.
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Last revised: 14-Sept-2010 (DSE)