The UT scanner was built by Bio-Imaging
Research, Inc. of Lincolnshire, Illinois, in accordance with
design specifications developed by the lab in consultation with
the manufacturer's engineering staff. Our objective was to produce
an instrument that would complement existing facilities employing
modified medical scanners (capable of penetrating specimens of
moderate density, decimeters to meters in size, with spatial resolution
on the order of a millimeter) and highly specialized facilities
employing synchrotron radiation (capable of penetrating low- to
moderate-density specimens, up to several millimeters in size,
with spatial resolution on the order of a few micrometers).
The scanner is comprised of two subsystems within a single
radiation-safety enclosure: one yields ultra-high-resolution data
on small objects that can be penetrated by relatively low-energy
X-rays; the other yields high-resolution data on larger or denser
objects that can be penetrated only by higher-energy X-rays. Figure 1 provides
examples of the range of imaging tasks successfully accommodated
by this modular design.
ULTRA-HIGH-RESOLUTION SUBSYSTEM. Ultra-high-resolution tomography of specimens up to a few cm in diameter employs a 200-kV microfocal X-ray source. The system's magnification, which increases with the specimen's proximity to the X-ray source, combines with the fixed pixel size of the video image to determine the limits of spatial resolution. The great flexibility of this system allows imaging of specimens from several cm to a few mm in diameter with spatial resolution from ~250 µm to ~5 µm (e.g., Figs. 1a, 1b). In addition, the ability
of the microfocal source to provide a stable X-ray output, even
at mean energies of 30-50 keV, permits excellent discrimination
among materials of closely similar attenuation, for specimens
that can be penetrated by relatively low-energy radiation (e.g.,
Fig. 1c).
HIGH-ENERGY SUBSYSTEM. Tomography of large specimens
(e.g., Fig. 1d) employs a 420-kV tungsten X-ray source, a rotating turntable that can accommodate samples up to 50 kg in weight, and either of two available high-energy detectors. One detector is a 512-channel cadmium-tungstate solid-state linear array, which provides superior sensitivity because of its high absorption efficiency. Its vertical aperture (slice thickness) ranges from 5 mm down to 0.25 mm, with a horizontal channel pitch of 0.31 mm. The other high-energy detector is a 2048-channel gadolinium oxysulfide radiographic line scanner; this detector, although less sensitive, provides higher in-plane spatial resolution, with a channel pitch of 0.025 mm. It can be used either in a high-resolution mode, with a vertical aperture of 0.25 mm, or (by sacrificing some sensitivity) using a vertical aperture that can vary from 0.5 to 5 mm. Operating in translate-rotate mode, the high-energy subsystem can image specimens up to 500 mm in diameter. The source and detectors are capable of 750 mm of vertical motion, but the unobstructed sample volume is 1500 mm tall, so specimens up to a meter and a half in maximum dimension (e.g., segments of drill cores) can be scanned by first imaging one half, then inverting the specimen to scan the other half. At the opposite end of the size spectrum, this subsystem can also be used to scan any smaller specimen for which ~250-µm spatial resolution is sufficient.
SCANNING MODES. Adding to the flexibility of this scanner is its ability to acquire data in several different modes, to further optimize performance tradeoffs. Both subsystems can collect data in third-generation geometry (rotate-only; centered and variably offset) and the high-energy subsystem can also be operated in second-generation geometry (translate-rotate), which allows for increased resolution within subvolumes of the specimen by selective reconstruction of the raw absorption data. On both subsystems, complete control over the translational positioning of the specimen ensures that the maximum magnification (hence, maximum resolution) can always be achieved. A "multi-slice" mode on the high-resolution subsystem permits simultaneous acquisition of data for several slices simultaneously, markedly reducing scan times at a small cost in data quality owing to slight off-axis distortion. Both of the scanner's subsystems are capable of digital radiography (standard 2-D X-ray projection), and the high-resolution subsystem is also capable of real-time radiography (projection done continuously with immediate display).
DIGITAL IMAGE ANALYSIS LABORATORY
Special hardware and software -- and a repository of specialized technical expertise -- are required in order to extract full scientific value from the raw CT images. Because data volumes for 3-D reconstruction are generally very large (up to several hundreds of megabytes), their manipulation requires high-end computational resources capable of managing and rapidly processing immense amounts of data. Visualization often relies upon 3-D renderings of specimens, and usually profits from the ability to render as transparent some portions of the specimen (those that occupy a specified range of densities) to enhance other features of scientific value (e.g., Fig. 2 (top),(middle)). Also frequently
essential is a means of interactively viewing these renderings,
enabling a user to examine the digital version of the specimen
at any arbitrary viewing angle, or cut a virtual section through
it at any point in any orientation (e.g., Fig. 2 (bottom)). Animations of sequential views at successive equally spaced angles, simulating rotation of the specimen about an arbitrary axis, are often the best means of examining relationships among features (various examples may be viewed on on our exaples page for geological applications). For large CT datasets, all of these aids to scientific interpretation require very fast hardware running specialized software, and many of them demand a surprising degree of proficiency with the software to accomplish one's desired aims. Because systems capable of meeting these exacting requirements, and the expertise to use them effectively, are often not available to potential users at their home institutions, an adjacent computer laboratory dedicated to the processing of the digital images acquired from the CT scanner is a vital adjunct to the tomography laboratory. The digital image-analysis laboratory is a multi-platform facility (Windows, Macintosh, Linux) able to provide services to outside investigators in a computing environment familiar to them, and able to convert data into formats easily integrated into their existing research programs. The same experienced research scientists responsible for day-to-day operation of the scanner also staff the image-analysis laboratory, ensuring proper interpretation of the data and maintenance of its integrity throughout processing. Our image-analysis capability is amplified by access to the UT Visualization Lab.
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