Just like visible light, X-rays also experience refraction. The refraction is caused by what is called phase shift through the material the radiation passes. Even though this effect is much weaker for X-rays than for visible light, it can be used to create contrast for imaging.

Phase-contrast imaging requires special techniques to be detected but can be very beneficial. For materials that have low absorption, such as soft biological matter, polymers and many other organic compounds, the phase can give more than 1000 times stronger contrast than absorption. Without going into the detailed physics behind phase contrast, we can still understand the methods that are used to exploit it.

To make phase-contrast imaging, the phase shift induced by the object must be measured. The unfortunate catch is that X-ray detectors don’t measure phase, but only intensity. The phase shift is instead converted to intensity variations, that can be detected. There are several methods to do this, which are all good in different ways.

Compared to absorption-contrast imaging, the requirements on the equipment are in general higher for phase-contrast imaging. To be able to obtain contrast, an X-ray source with high transverse coherence is needed. This means that the source either has to have a small emission spot or has to be moved far away from the object. Both options are traditionally related to very low flux, which leads to very long exposure time. To keep the exposure time short, an X-ray source with a small emission spot that can still maintain a high flux is very beneficial. Another commonly used metric is the source brightness. The source brightness is a measure that includes both coherence and flux and is important to have high in many cases!

Phase-contrast imaging was originally developed for biomedical applications, where it is very beneficial for imaging of soft tissues. Recently, it has seen a growing interest also in materials science, engineering and industrial non-destructive testing.

Simple example of how the phase shift through an object perturbs the wavefront. The wavefront cannot be directly measured but is here converted to intensity variations at a distance. The contrast arises from both absorption and phase. The edge enhancements introduced by the phase shift is apparent along all contours. 

Grating-Based Phase-Contrast Imaging (GBI)

The core of GBI is the Talbot self-imaging effect: when spatially coherent X-rays pass through a grating (G1) with periodic line pattern, the line pattern will repeat itself at certain distances. By inserting a second grating (G2) that matches the self-image from G1, small differences introduced by a sample present in the beam can be detected. The second grating is shifted in several steps between where it is in phase and out of phase with the self-image from G1. This allows three properties of the object to be measured separately; X-ray attenuation, phase shift and dark field originating from ultra-small angle scattering. Separate attenuation and phase signals allow better discrimination of materials and the dark-field signal visualizes the presence of small structures below the imaging resolution.

By retrieving the changes in the self-image measured by stepping the G2 grating, the three image modalities can be measured simultaneously. GBI can often be implemented in a laboratory setup, but often requires a grating directly in front of the source, referred to as G0. The reason for this is that most sources don’t have the spatial coherence to enable image contrast in GBI, but each slit in the G0 grating works as a spatially coherent source but mutually incoherent to the other slits. Naturally, the G0 grating causes a loss in flux of more than 50%. A MetalJet X-ray source reaches the sufficient spatial coherence to enable GBI without the G0 grating, thus avoiding the loss of flux.

The most common way to acquire images in GBI is the phase stepping procedure that requires several images every projection. This has the downside that it associates GBI tomography to very long exposure times. In this context, it is of deepest concern to keep the flux high!

Application examples:

X-ray GBI microscope with MetalJet source

The joint research by scientists from KTH and ETH/PSI proved the advantage of using a MetalJet source in GBI, which significantly improved flux and visibility compared to a conventional microfocus tube. Moreover, a preliminary exploration of its biomedical application has been shown by tomography on a rat.

A comparison between phase-contrast (a) and attenuation-contrast (b) slices from GBI tomography based on a MetalJet source. The sample was a rat brain scanned in a liquid paraffin bath.

• Exposure time = 1 min/ stepping image
• Voxel size = 20 µm
• Tube voltage = 50 kV, tube power= 40 W
• G1 is designed for π-phase shift at 25 keV, period= 4.12 µm
• CCD camera with Gadox scintillator

Reproduced from T. Thüring, et al., “X-ray grating interferometry with a liquid-metal-jet source“, Appl. Phys. Lett. (2013) with the permission of AIP Publishing.

Brochure

Publications