The phase of spatially coherent X-ray
waves passing through an object is shifted according to effective
refractive index. Variations in the effective refractive index cause
variations in the phase of transmitted waves. This phase shift deforms
the wave-front of the transmitted radiation in a way that the rays are
deflected from their original propagation direction and a loss of
intensity is detected in a forward direction. This process causes edge
enhancement in the obtained image.
The technique can be used with
monochromatic and even polychromatic X-ray source. The spatial
coherence can be assured by small size of transmission point in a
source e.g. utilizing X-ray tube of microfocus or nanofocus
For a correct picture interpretation it is necessary to distinguish an absorption picture from a phase shift (refractive) picture. In measurements both pictures are mutually superimposed. This can cause serious problems in tomographic reconstruction, for instance. Since pixel Medipix detectors are energy sensitive and since both effects of absorption and refraction depend on energy of radiation in a different way, it is possible to distinguish between these two effects contributions by recording few pictures at different values of energetic discrimination in a detector.
A - picture taken by CCD camera at contact geometry (absorption),
B - picture taken by Medipix2 revealing fine structure (phase enhanced)
The effect of phase enhanced imaging can be used also in radiography with slow neutrons. With respect to wave nature of neutron radiation it is possible to define refractive index here as well - with all implications (see following picture).
of two aluminum blocks: 4 mm thick (left) and 10 mm (right). The signal
caused by phase contrast enhancement (B) is significantly larger than
the absorption signal (A).