Coherent Image Formation, Absorption and Phase-Contrast Tomography

Coherent image formation

In radiography with paritally coherent X rays as they are available at the ESRF, both phase and absorption effects play an important role in the formation of a transmission image.    

Figure 1: Near Field, Fresnel Region, Fraunhofer Region

Four regions of the imaging distance z can be distinguished:

• Contact:
  The intensity distribution is a pure absorption image.
• Near field:
  Contrast is given by sharp changes in the refractive index, i. e. at interfaces. 
• Fresnel region:
  The image loses more and more resemblance with the object. 
• Fraunhofer region:
  The image intensity is the Fourier transorm of the object transmission function.

Absorption tomography

Figure 2: Conventional Absorption Tomography

In absorption-contrast tomography, the tomographic reconstruction yields the averaged attenuation coefficient for every voxel in the sample.

Phase-contrast tomography in outline mode

Figure 3: Phase-Contrast Tomography

Near-field coherent imaging gives phase contrast in regions of a highly-localized change in the refractive index of the sample, such as its borders and interfaces
between the sample matrix and inclusions. The tomogram is then an outlined image of those domains.

Outline phase-contrast tomography is especially useful for the investigation of weakly absorbing samples, samples with domains of similar absorption, but strongly differing refractive indices,  samples that must not absorb a high radiation dose.