Phase-contrast Imaging

Phase-contrast imaging is one of the highest resolution imaging technique which is widely used in many different areas. This imaging technique exploits the fact that different material has a different refractive index. The light (EM wave) passing through a different material under observation, will be bent according to the respective refractive index of the material. This introduces phase variation which is detected to infer the required physical parameters of the material.

Phase-contrast imaging has got a wide application in biological, medical and geological science. Let us glance through two important applications of it, namely in X-Ray Imaging and Transmission Electron Microscopy (TEM).

X-Ray Imaging

X-rays imaging is performed by measuring differences in the attenuation of X-rays by the tissues. The material properties are inferred from the differences in the number of photons detected at the end of defined paths in the sample. Naturally, then this form of X-ray imaging concentrates almost entirely on the particle side of X-ray behaviour. This can be considered as a photo-electric effect. The main drawback of photo-electric effect based X-ray imaging is that the image contrast depends on the atomic number and density of the tissue. For instance, let us consider a bone and soft tissue. Here the conventional X-ray provides good contrast as there is a considerable difference in the atomic number and density of bone and soft tissue. But what happens if we consider thin tissues with a very similar atomic number and only small density variation? Obviously the resulting image- contrast will be poor.

X-ray phase-contrast image of spider [1]

Phase-contrast imaging tries to address this issue. It makes use of the fact that X-rays also exhibit wave-like behaviour in addition to the particle nature.  When the X-ray waves interact with a material, the interactions induce a phase shift in the wave. We have sophisticated technology and well-developed detectors available today to detect these phase shifts. The phase-contrast based X-ray imaging shows much better performance in terms of image contrast compared to the conventional X-ray imaging.  

Transmission electron microscopy (TEM)

Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through an ultra-thin specimen, interacting with the specimen as it passes through it. These interactions are captured in the form of an image onto an imaging device.

In addition to the contrast in the transmitted beam due to the electronic interaction, we can introduce diffraction contrast by applying the principle of phase-contrast imaging. This ability arises from the fact that the atoms in a material diffract electrons as the electrons pass through them, causing diffraction contrast.

  A TEM image of the polio virus [2]

References


[1]  https://en.wikipedia.org/wiki/Phase-contrast_imaging

[2]  https://en.wikipedia.org/wiki/Transmission_electron_microscopy

[3]  http://users.ox.ac.uk/~atdgroup/technicalnotes/Phase%20Constrast%20X-ray%20imaging.pdf