The electron beam of the SEM is used to create a cone of X-rays by focusing the beam onto a metal target (usually platinum).
The sample, which is off-set to one side of the beam, is placed within the X-ray cone.
The resulting X-ray absorption image is projected onto a charge-coupled device (CCD) behind the specimen. This CCD output is essentially a 2D X-ray image where differences in grey-scales reflect changes in density of the specimen:
lighter areas indicate that the X-rays are less absorbed and therefore consist of a material of low-density
darker areas indicate that the X-rays are more absorbed and therefore consist of a material of high-density
A full 3D CT dataset is created by collecting 2D images over 360 degrees at 1º increments of rotation.
These data are then reconstructed to produce ‘virtual slices’ through the specimen and a virtual 3D volume can be visualised.