Phase-contrast imaging

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.

PBI has the simplest measurement geometry, similar to conventional attenuation-contrast imaging and requires no optical elements. The only difference is that free-space distances are kept between the object, sample and detector. This leads to that the phase can generate intensity variations, visible as edge enhancements. The image represents a mixing of the absorption-contrast signal and the phase-contrast signal. Implementing PBI in a compact laboratory-scale setup puts high requirements on the spatial coherence of the source, detector resolution and long distance between the sample and the detector. Both the MetalJet and the NanoTube sources meet the requirements for doing laboratory-based high-resolution PBI. Furthermore, the high brightness helps to keep the exposure time down, while still enabling high resolution.

To obtain an image that represents the object thickness, the phase-contrast image must be treated with a process called phase retrieval. There are many methods for phase retrieval, most of them based on convolutions. If using a single image, assumptions on the object must be made, such as assuming a single material object. With multiple images acquired at different propagation distances, this assumption can be circumvented. On the other hand, multiple images introduce a higher complexity in the experiments.

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!

SBI exploits the near-field speckle pattern generated by illuminating a random wave front modulator, also called diffuser, with spatially coherent X-rays. The multi-contrast images can be obtained by retrieving local image distortions of the acquired patterns due to the presence of the sample. The local distortions carry the information for absorption, phase shift and scattering of the sample. The resolution of SBI can be improved by doing so called speckle scanning, which consists of a 1D or 2D raster scan, but consequently needs longer acquisition time.

In similarity to GBI, a MetalJet source contributes to SBI by providing high spatial coherence to resolve the µm-sized speckles and high brightness to reduce exposure time.