Computed tomography

CT image versus a conventional radiograph

Computed tomography (CT) is an advanced radiological imaging technique that provides superior spatial information as compared to standard x-ray techniques.

In conventional x-ray radiography, information is provided in only two dimensions, (x and y), Information relating to the third or depth (z) direction (corresponding to the direction of propagation of the incident x-rays) is lost as the three-dimensional object is collapsed into a two-dimensional image projected onto film or a detector. In CT imaging this third dimension is preserved.

This is achieved by obtaining a large number of observations at different viewing angles, which allows a cross sectional image to be produced by tomographic reconstruction.

Schematic illustrations of back projection

Tomographic reconstruction may be thought of as an advanced form of triangulation.

Features buried within a sample detected by conventional projection radiography can be localized in two dimensions but not in three: the position along the path travelled by the x-ray beam from the source to the detector cannot be determined. However, if we make two measurements at different viewing angles, we can use the process of triangulation to begin to estimate the position of the object within the sample.

Figure a: If we acquire and image though a section of a sample containing two discrete objects, we obtain a profile of the attenuation of the x-ray beam by the objects.

Figure b: If we project the data back along a line corresponding to the direction in which the data was acquired, we obtain a shadow image representing attenuation at this viewing angle. We can then repeat this process at 90 degrees to the first measurement and calculate a second back projection.

Figure c: Combining these two back projections allows us to localize the objects in our sample.

Positional accuracy is even further enhanced by making additional radiographs from more viewing angles and triangulating viewing angles.

Example: CT images of an extracted human tooth
Densely mineralized materials such as teeth and bones are excellent candidates for study by CT imaging due to their strong attenuation of x-rays.

Shown here:

Figure a. Transaxial section from a CT scan of an extracted human tooth. The heavily mineralized outer layer of enamel is clearly distinguishable from the inner dentin layer. Microscopic cracks in the dentin are also clearly visible.

Figure b. Reconstruction of the transaxial slices to produce a 3-D representation of the tooth.

Figure c. 3-D reconstruction with mineralized material set to be opaque to allow visualization of the root canal structures, shown in pink.

Scanning parameters: resolution: =35 um, frame averaging=3, rotation per scan 0.7o, scanned over 360o

Example: Micro CT scan of a rat mandible

In this example, a rat mandible was scanned to assess bone mineral density. Two subscans were acquired and fused to generate the image. Total imaging time was 30 minutes.

Shown here:

Figure a. 3-D reconstruction

Figure b. Sagittal cross section

Figure c. Transaxial cross section

Scanning parameters: resolution: =18 um, frame averaging=3, rotation per scan 0.7o, scanned over 180o

Example: CT image of a mouse knee

Shown here is a three-dimensional reconstruction of a mouse knee from an in vivo scan of mouse. The image was generated by adjusting the attenuation threshold to display only mineralized tissue.

Scanning parameters: resolution= 9 um, filter = 0.5mm Al rotation 0.8o, scanned over 180o. Data was reconstructed using a ring artefact reduction correction=factor of 7, and a beam hardening correction of 30%

Example: Mouse embryo

This scan shows a three-dimensional image of a mouse embryo. Due to the limited mineralization of bone in the embryo, the sample was fixed in osmium tetroxide prior to scanning. It also reflects the same mouse with image clipped to show internal structures.