A new era of bioimaging with x-rays

Visible and infrared (IR) wavelengths have so much utility and provide so much value—why complicate things with other wavelengths? Of course, we all know that other wavelengths provide access to distinct energy regimes in atoms and molecules, giving unique information about biomolecular processes, cells, and entire organisms, but there can be a tendency to underestimate the capabilities of the x-ray spectrum. The 10 million diagnostic x-ray images taken every day may tend to contradict that statement, but although x-rays can produce unparalleled images of an object's optical (x-ray) density, there's only so much you can do with simple intensity contrast images. Right?

Well, today, x-rays are doing much more. And if lightsources.org has anything to say about it, you'll be hearing a lot about that.

The Lightsources collaboration is a cooperative effort of 19 different synchrotrons—large machines that accelerate electrons to very high speeds. As the electrons are accelerated they emit "synchrotron radiation": x-ray beams. A synchrotron light source will have several beamlines—x-ray extraction points where a particular magnet configuration creates beams of a given energy, collimation, and flux density. Those beams are then further conditioned with x-ray mirrors, slits, monochromators, and other optics. Although some light sources (as they are rather ambiguously referred to) downconvert beamline radiation to UV or IR wavelengths, most beamlines produce x-rays with high flux, low divergence, small beam size, monochromaticity, and temporal control that cannot be matched by any other source.

Those unmatched capabilities enable unmatched imaging performance, much of it turned on subjects of biological interest.

For example, the synchrotron radiation for medical physics (SYRMEP) beamline at the ELETTRA Light Source in Trieste, Italy, recently produced high-resolution microCT scans of xylem conduits in drought-stressed laurel trees. Xylem conduits carry water and nutrients from roots to leaves. They depend upon hydraulic pressure, and that in turn depends upon a continuous liquid flow from root to leaf. In drought, air embolisms can be introduced, which may irreversibly disrupt liquid flow. There's a hydraulic measurement method of estimating air embolisms, but until this study there was no way of validating that method.

The ELETTRA researchers imaged tree shoots using monochromatic 19 keV x-rays, rotating the sample in 0.33° steps over 180°. This particular phase-contrast imaging method relies on interference between scattered and transmitted beams through the sample itself—which provides excellent contrast in soft tissue samples because of the beam's high spatial coherence.

Other researchers use synchrotron beams to perform such biological tasks as imaging bone composition, where monochromaticity and small beam size are essential, and identifying viral protein structure, where high flux density is key.

A visit to the Lightsources website will introduce you to many more examples, and provide insight into the evolving capabilities of x-ray measurements.