Especially structures of soft tissues are characterized by the similar value of absorption index, therefore the imaging methods based on X-ray intensity attenuation measurement often fail. These structures, however, usually differ in refractive index what can be successfully exploited for imaging. A big advantage of phase enhanced imaging is its non invasiveness as a sample itself should be transparent for used radiation i.e. minimal dose is spent in a sample. That is very desirable especially when imaging sensitive tissues as in mammography.

Part of termite head: image with edges enhanced by phase effects taken with Medipix2 at a distance of 600 mm revealing fine internal structure of antennae (tungsten X-ray tube, 40 kV).


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 type.

 
Images of mouse kidney taken with microfocus X-ray source and Medipix2 pixel detector.

Left side image was taken in contact geometry. Just absorption image without phase contrast enhancement.
The edges are enhanced in the magnified image. Image taken at distance of 60 cm. Source: Tungsten X-ray tube at 40kV.

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.

Termite knee:
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).

Neutronogram of edges 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).

Taken by Medipix2 device with 6LiF converter layer deposited on the detector surface. Neutron source was  ICON facility in PSI Villigen.