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Imaging technology

The intricacies of insect structure

München, 08/14/2015

Munich physicists have built a compact X-ray source driven by laser light. When combined with phase-contrast X-ray tomography, the system permits detailed three-dimensional imaging of tissue structures within organisms.

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A new X-ray imaging system devised by physicists from LMU, the Max Planck Institute of Quantum Optics (MPQ) and the Technische Universität München (TUM) is capable of revealing even the finest hairs on the wings of an insect. The experiment, carried out by research teams led by Professors Stefan Karsch (LMU, MPQ) and Franz Pfeiffer (TUM), is a pioneering achievement. For the first time, scientists have coupled their state-of-the-art technique for generating X-rays using extremely short pulses of laser light with phase-contrast X-ray tomography to visualize biological tissues. The result is an unprecedentedly detailed three-dimensional view of the external and internal cuticular structures of the insect.

The X-rays were generated by accelerating electrons to nearly the speed of light over a distance of approximately 1 cm, using ultrashort laser pulses, each lasting around 25 femtoseconds (1 fs is one millionth of a billionth of a second). The laser pulses have a power of approximately 80 terawatts (80 x 1012 W). By way of comparison, a typical nuclear power plant generates 1500 megawatts (1.5 x 109 W) of power. Up until now, such radiation could only be produced in extremely costly ring accelerators measuring several kilometers in diameter. In contrast, the laser-driven system that served as the basis for soft-tissue imaging in the new study can easily be accommodated in a university laboratory.

Surfing electrons

The train of high-energy laser pulses first ionizes a sample of hydrogen gas, creating a plasma consisting of positively charged atomic cores and their electrons. As the pulses plow through the plasma, they produce a wake of oscillating electrons, like the wash of turbulent “whitewater” generated by a racing speedboat. This electron wave creates a trailing, wave-shaped, electric field structure on which the electrons surf, and are thus very rapidly accelerated. The particles then begin to vibrate, emitting X-rays as they do so. Each light pulse generates an X-ray pulse. Moreover, these X-rays have very special properties: They have a wavelength of approximately 0.1 nanometers, a duration of only about 5 fs, and are spatially coherent, i.e. they appear to come from a point source. This last feature is particularly significant. For, unlike conventional radiography, which is based on the absorption of X-radiation, the phase-contrast imaging method developed by Franz Pfeiffer makes use of X-ray refraction to accurately image the shapes of objects, including soft tissues. Spatial coherence of the imaging pulses is essential for the success of this technique.

The new laser-based imaging system enables the researchers to visualize structures with dimensions down to about one hundredth the diameter of a human hair. Another advantage is the system’s ability to create three-dimensional images of objects using tomography, i.e. by combining individual views taken from different angles. After each X-ray pulse, the specimen is rotated slightly, and about 1500 such images were taken of the specimen, which were then assembled to form a 3D data set.

Due to the ultrashort nature of the X-ray pulses, this technique could be used in future to freeze ultrafast processes on the femtosecond time scale e.g. in molecules - as if they had been illuminated with a femtosecond flashbulb.

The technology is of particular interest for medical applications, as it is able to distinguish differences in tissue density. Cancer tissue, for example, is less dense than healthy tissue. The method therefore opens up the prospect of detecting tumors in an early stage of growth, when they are less than 1 mm in diameter, before they can spread through the body and exert their lethal effect. For this purpose, however, researchers must shorten the wavelength of the X-rays produced even further in order to penetrate thicker tissue layers. (MPQ/LMU)
(Nature Communications 2015)