X-ray grating interferometry is an extremely useful tool for investigating the structures of unknown biological samples. The interferometers work by firing x-rays through a phase grating that introduces an interference fringe at specific distances downstream. When an object -– such as a biomedical or material science sample –- is placed in the beam’s path, it modifies the observed interference pattern via absorption, refraction, and small-angle scattering. Once these signals picked up by the detector, technicians can algorithmically determine the sample’s properties.
In order to achieve a high sensitivity the period of the generated fringe needs to be in the micrometer range. Such fringes are challenging to be recorded directly over a large field of view.
To work around this, one widely-used method known as Talbot-Lau interferometry employs an absorption grating G2 placed right before the detector, and senses the distortions by a procedure known as phase stepping. Here, the absorption grating is scanned step by step for one or more periods of the interference fringe, each time recording an image which results in an intensity curve at each pixel. This way the interference fringe can be sensed indirectly and absorption, differential phase and small-angle scattering signals are obtained for each pixel.
This technique carries the advantage of decoupling the interferometer’s sensitivity from the detector’s resolution but ultimately causes a lower system dose efficiency due to photon absorption by G2.
Additionally, a slurry of manufacturing costs and mechanical complexities regarding G2, such as the area and aspect ratio of the gratings, further complicate its implementation.
To remedy this, researchers at the Institute for Biomedical Engineering in Zurich and the Swiss Light Source (SLS) have developed an interferometer which eschews the G2 grating, instead opting to directly exploit the fringe interference via higher resolution.
“We can perform differential phase contrast imaging with high sensitivity without the need for a G2 grating or a detector with small pixel size in order to resolve the fringe,” said Matias Kagias, a PhD student in the laboratory of Prof. Marco Stampanoni. Kagias and his colleagues present their work this week in Applied Physics Letters, from AIP Publishing.
The researchers’ experimental setup consisted of an x-ray source, a single phase grating, and a GOTTHARD microstrip detector developed by the SLS detector group –- a significantly simplified version of the traditional Talbot-Lau interferometer. The GOTTHARD detector uses a direct conversion sensor, in which x-ray photons are absorbed, the charge generated from one absorption event is collected by more than one channel for small channel sizes -– charge sharing.
“The key point to resolving the fringe is to acquire single photon events and then interpolate their positions using the charge sharing effect, which is usually considered as a negative effect in photon counting detectors,” Kagias said. By interpolating the position of many photons, a high resolution image can then be acquired.
When the researchers implemented the appropriate algorithm to analyze this recorded fringe, they found that fringes of a few micrometers could be acquired successfully while still retrieving the differential phase signal.
According to Kagias, this ultimately increases the interferometer’s flux efficiency by a factor of 2 compared to a standard Talbot-Lau interferometer, which may lead to faster acquisition times and potentially a dose reduction.
Future work for Kagias and his colleagues involves moving to large area pixel detectors, and improving the resolution and sensitivity of their setup.
The article, "Single Shot X-Ray Phase Contrast Imaging Using a Direct Conversion Microstrip Detector with Single Photon Sensitivity” is authored by M. Kagias, S. Cartier, Z. Wang, A. Bergamaschi, R. Dinapoli, A. Mozzanica, B. Schmitt, and M. Stampanoni. It appeared in the journal Applied Physics Letters on June 7, 2016 (DOI: 10.1063/1.4948584) and can be accessed at: http://scitation.aip.org/content/aip/journal/apl/108/23/10.1063/1.4948584.
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Applied Physics Letters features concise, rapid reports on significant new findings in applied physics. The journal covers new experimental and theoretical research on applications of physics phenomena related to all branches of science, engineering, and modern technology. See http://apl.aip.org.
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