Basics of Tomography

What is Tomography ?

In conventional radiography, X-rays pass through the investigated object, and the transmitted intensity is recorded as a two-dimensional image. The information contained in this radiograph is a projection of the absorption density in the sample onto the plane perpendicular to the X-ray beam direction.

With coherent illumination, the image also contains infomration on the refractive index of the sample, apparent in what is called phase contrast.

Figure: Principle of computized tomography (schematic)

If now the sample is imaged several times in different orientations, three-dimensional (volume) information on the sample structure can be obtained using computer algorithms.

This is called a tomographic reconstruction or tomography. It enables us to look at slices of the investigated object without physically cutting it.

High Resolution Micro-Tomography

In hard X-ray microimaging, the spatial resolution is limited by the detector system. More precisely, in a conventional detection system consisting of a scintillator screen used to convert X rays into visible light, a microscope optics to magnify the visible-light image, and a CCD camera to record the magnified image, the resolution is normally limited by scattering in the scintillator.

In tomographic experiments, the wobble of the rotation axis is another factor that may significantly spoil resolution.

Current detection systems achieve a resolution of around one micrometer. </P