X-ray Computed Tomography (CT)
What is X-ray Computed Tomography (CT)
The gray levels in a CT slice image correspond to X-ray attenuation, which reflects the proportion of X-rays scattered or absorbed as they pass through each voxel. X-ray attenuation is primarily a function of X-ray energy and the density and composition of the material being imaged.
Fundamental Principles of X-ray Computed Tomography (CT)
X-ray Computed Tomography (CT) Instrumentation - How Does It Work?
The great majority of CT systems use X-ray tubes, although tomography can also be done using a synchrotron or gamma-ray emitter as a monochromatic X-ray source. Important tube characteristics are the target material and peak X-ray energy, which determine the X-ray spectrum that is generated; current, which determines X-ray intensity; and the focal spot size, which impacts spatial resolution.
Most CT X-ray detectors utilize scintillators. Important parameters are scintillator material, size and geometry, and the means by which scintillation events are detected and counted. In general, smaller detectors provide better image resolution, but reduced count rates because of their reduced area compared to larger ones. To compensate, longer acquisition times are used to reduce noise levels. Common scintillation materials are cesium iodide, gadolinium oxysulfide, and sodium metatungstate.
- Measuring 3D size and spatial distribution of crystals, clasts, vesicles, etc.
- Nondestructive volumetric study of rare specimens (fossils, meteorites, etc.)
- 3D measurement of fluid flow fields, including porosity, microporosity, and fracture extent and roughness
- 3D fabric determination (foliations, shape preferred orientations, network properties)
- Inspection and measurement of morphology in fossils and Recent biological specimens
- Detection and examination of high-density economic trace phases
- Reconnaissance imaging of samples for optimal geochemical exploitation (for example, locating crystal central sections, spiral axes, solid and fluid inclusions).
Strengths and Limitations of X-ray Computed Tomography (CT)?
- Entirely non-destructive 3D imaging
- Little or no sample preparation required
- Reconstruction is generally attenuation-conservative, allowing sub-voxel level details to be extracted.
- Resolution limited to about 1000-2000x the object cross-section diameter; high resolution requires small objects
- Finite resolution causes some blurring of material boundaries
- Calibration of gray levels to attenuation coefficients complicated by polychromatic X-rays
- Large (dm-scale) geological specimens cannot be penetrated by low-energy X-rays, reducing resolving capability
- Not all features have sufficiently large attenuation contrasts for useful imaging (carbonate fossils in carbonate matrix; quartz vs. plagioclase)
- Image artifacts (beam hardening) can complicate data acquisition and interpretation
- Large data volumes (gigabytes+) can require considerable computer resources for visualization and analysis
User's Guide - Sample Collection and Preparation
Data Collection, Results and Presentation
The two standard modes of 3D visualization are volume rendering and isosurfacing. Volume rendering consists of mapping each CT value to a color and an opacity. Thus, some phases can be rendered transparent, allowing internal features to be revealed. Isosurfacing involves defining 3D contour surfaces that delineate boundaries between CT numbers, much as contour lines separate elevations values on a topo map.
Because CT data sets typically comprise hundreds of images and thousands of megabytes, they are not amenable to traditional publishing. However, CT data and visualizations are increasingly being served over the world wide web. An example is the Library of Digital Morphology (more info) website.
The following literature can be used to further explore X-ray Computed Tomography (CT)
- ASTM, 1992, Standard Guide for Computed Tomography (CT) Imaging, ASTM Designation E 1441 - 92a. In: 1992 Annual Book of ASTM Standards, Section 3 Metals Test Methods and Analytical Procedures. ASTM, Philadelphia, pp. 690-713.
- Ketcham, R.A. and Carlson, W.D., 2001, Acquisition, optimization and interpretation of X-ray computed tomographic imagery: Applications to the geosciences. Computers and Geosciences, 27, 381-400.
For more information about X-ray Computed Tomography (CT) follow the links below.
Teaching Activities and Resources
Teaching activities, labs, and resources pertaining to X-ray Computed Tomography (CT).