A specially-structured device called a Fresnel zone plate (FZP) efficiently quickly produces X-ray images in color. This technique is applied in non-destructive industrial testing and analysis of materials. Apart from that, it is also used in nuclear medicine and radiology.
X-rays are commonly used to determine the chemical composition of materials, thanks to the characteristic “fingerprint” that different fluorescence substances are emitted when exposed to X-ray light. This imaging technique currently requires focusing on the X-rays and scanning the whole sample. However, focusing an X-ray beam down to small areas is challenging, especially with typical laboratory X-ray sources, making images time-consuming and expensive to produce.
Single exposure and, thus, eliminating the need for focusing and scanning
This new method, developed by Jakob Soltau and colleagues at the Institute for X-ray Physics at the University of Göttingen, Germany, allows an image to generate from a large sample area with just a single exposure while eliminating the need for focusing and scanning. The method includes using an X-ray color camera and a gold-plated FZP placed between the object being imaged and the detector. FZPs are opaque in structure and have transparent zones, often used to focus X-rays. However, in this experiment, the researchers were interested in the shadow FZP casts on the detector when the sample is illuminated.
After measuring the intensity pattern that reaches the detector after passing through the FZP, the researchers found information on atom distribution that fluoresce at two different wavelengths. They then decoded this distribution using a computer algorithm.
Soltau says, “We know the set of algorithms that can be used favorably for this very well from phase-retrieval in coherent X-ray imaging. We apply this to X-ray fluorescence imaging using the X-ray color camera in our experiment to distinguish between the different energies of the detected X-ray photons.”
The researchers say that just one image acquisition is enough to determine the chemical composition of a sample. While the acquisition time is currently on the order of several hours, they hope to reduce this in the future.
Potential in imaging biological tissues
The team says the new technique has many potential applications. These include nuclear medicine and radiology; materials analysis; non-destructive industrial testing; determining the compositions of chemicals in paintings and cultural artifacts to verify their authenticity; analysis of soil samples or plants; and testing the quality and purity of semiconductor components and computer chips. This technique could be used to image incoherent radiation sources such as inelastic X-ray (Compton) and neutron scattering or gamma radiation, which would be helpful in nuclear medicine applications.
“As a research group, we are very interested in the three-dimensional imaging of biological tissues,” Soltau tells Physics World. “Combining tomographic imaging, for example, with a detector recording the transmitted X-ray beam to obtain a map of the electron density (a technique known as phase contrast propagation imaging) with our novel full-field fluorescence imaging approach would allow us to image structures and (local) chemical compositions of the sample in one scan.”
In the first demonstration of this new technique, which is detailed in Optica, the Göttingen team achieved a spatial resolution of about 35 microns and a field of view of around 1 mm2. While the number of resolution elements imaged in parallel remains relatively low, with smaller zone widths or increasing the sample area being illuminated towards a more extensive field view, this could be improved using the FZP. Another challenge will reduce the acquisition time without increasing unwanted background noise from elastically-scattered radiation.
The researchers would now like to try their technique with synchrotron radiation, which is much more intense than the X-ray light available in most laboratories. A further advantage is that synchrotron radiation consists of high-energy beams of charged particles generated using electric and magnetic fields. This gives it a narrow bandwidth that should allow higher spatial resolution and shorter acquisition times. The team has booked time on DESY’s PETRA III synchrotron beamline in June for this purpose.
