X-ray microtomography, often called micro-CT, is a non-destructive technique that generates three-dimensional images of an object’s internal structure. It functions like a medical CT scanner but on a much smaller scale, creating detailed 3D models from 2D X-ray images to achieve resolutions at the micrometer level. This technology has become an important tool across many fields of science and industry, allowing for the detailed examination of a wide variety of specimens.
Principles of X-ray Microtomography
The process involves passing an X-ray beam through a sample as it rotates. In one common setup, the X-ray source and detector remain stationary while the sample is turned on a rotation stage. An alternative configuration, similar to clinical CT scanners, uses a gantry system where the source and detector rotate around a stationary specimen.
As X-rays travel through the object, different materials absorb the radiation at different rates. This phenomenon, known as attenuation, depends on the material’s density and the atomic numbers of its constituent elements. These variations in absorption create contrast in the resulting 2D images, which are detailed shadow pictures called radiographs.
A detector, such as a flat-panel or CCD, captures these radiographs by recording the intensity of the X-rays passing through the sample. Hundreds or thousands of these projection images are taken from numerous angles as the object completes a rotation of at least 180 degrees. The process requires a microfocus X-ray tube to produce a fine beam, a precision rotation stage, and a sensitive detector.
Generating 3D Microscopic Images
After collecting the 2D projection images, the data undergoes tomographic reconstruction. Specialized algorithms, such as filtered back-projection, analyze the radiographs to calculate the X-ray attenuation value for every point within the sample’s volume. This process translates the 2D images into a complete three-dimensional dataset.
The output of this reconstruction is a 3D map composed of volumetric pixels, or “voxels.” Each voxel contains a grayscale value representing the material density or attenuation at that specific coordinate inside the object. This creates a high-resolution digital model of the specimen’s internal and external structure.
This 3D dataset can be viewed as a series of cross-sectional slices, allowing for a virtual examination of the object’s interior in any direction. The entire volume can also be rendered as a complete 3D model that can be digitally rotated, magnified, and analyzed on a computer.
Diverse Applications of XMT
X-ray microtomography’s ability to visualize internal structures non-destructively has led to its adoption in numerous fields.
- In life sciences, it is used to examine bone microstructure for osteoporosis research and visualize the anatomy of small animals.
- Materials science uses it to detect internal flaws like voids or cracks in composites, foams, and ceramics.
- Geoscientists apply it to study pore networks in rock samples for oil and gas exploration.
- Paleontologists and archaeologists can examine the internal structures of delicate fossils and artifacts without causing damage.
- Industrial quality control relies on it to inspect complex parts like electronic components for hidden defects.
Sample Considerations and Preparation
While a wide range of materials can be analyzed, solid objects are the most common. The main constraint is the sample’s size, which must fit inside the scanner and be small enough for X-rays to penetrate, often in the millimeter to centimeter range. Powders and liquids can also be imaged if held within a stable, X-ray transparent container.
Clear images require sufficient density contrast between materials in the sample. When natural contrast is low, such as in biological soft tissues, contrast-enhancing staining agents can be used. These agents contain heavy elements that strongly absorb X-rays, making specific structures visible.
The sample must remain perfectly still during the scan, which can last from minutes to many hours, to ensure sharp images. This is achieved by mounting the specimen on a pin or securing it inside a small tube. Although the technique is non-destructive for most materials, the radiation dose can be a concern for sensitive biological samples or materials prone to radiation damage.