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Coherence comes into the fold

vv5079X-rays have been at the heart of imaging since their discovery at the end of the nineteenth century. Now, Pierre Thibault and colleagues at University College London, UK and the Paul Scherrer Institute in Switzerland, hope that a new twist on an old favorite will extend and give them dose-limited resolution and sensitivity through the development of X-ray ptychography.

X-ray ptychography is a scanning coherent diffractive imaging technique, the team explains. The technique first suggested in the 1970s involves illuminating the sample with a structured, often confined source and measuring the resulting diffraction pattern for different overlapping positions of the sample, the term is from the Greek "fold" and "writing". Ultimately, ptychography promises to solve the diffraction-pattern phase problem in X-ray studies.

Coherent diffractive imaging (CDI) techniques, of which ptychography is just one, are all underpinned by the lack of a lens to focus the image. Instead of focusing, a mathematical algorithm is used to reconstruct the image of the sample from the collected diffraction patterns. Such a lensless system thus bypasses many of the technical constraints of lenses, which for X-rays are often inefficient, may introduce aberrations, and strongly limit resolution. Lensless, however, means phase is lost, which is where the overlapping folds of ptychography are exploited.

"Ptychography may approach imaging speeds familiar from full-field methods while retaining its inherently quantitative nature and metrological versatility," the team explains Thibault et al. (2014). J. Synchrotron Rad. 21,1011-1018; doi:10.1107/S1600577514015343. "Beyond promises of high throughput, spectroscopic applications in three dimensions become feasible, as do measurements of sample dynamics through time-resolved imaging or careful characterization of decoherence effects." The team suggests that additional technological and analytical advances in bright X-ray sources are now needed to help this field mature and to allow it to enter the realm of high-throughput studies and even three-dimensional spectroscopy.

"Ptychography's active development and sustained rate of successes hints at its potential as an important player in contemporary questions on data acquisition strategies, information content and feature extractions," the team reports, hinting that so-called "big data" methods of the kind usually reserved for particle physics and high-speed tomography, will come to the fore.

"In a way there are many next steps," Thibault told the IUCr. "In the paper we mention 'quantitative improvements', namely improve speed and size of field of view, and 'qualitative improvements', or improving resolution and sensitivity. Beyond this, the goal would be moving to four dimensions, that is adding one extra axis, either spectrum (spectro-tomography) or time (tomographic movies).

The Thibault paper forms part of the special issue in the Journal of Synchrotron RadiationDiffraction-Limited Storage Rings and New Science Opportunities. Guest Editors: Mikael Eriksson and J. Friso van der Veen.