Day 8 - X-ray Imaging

Welcome to day 8 of the Diamond Industrial Liaison advent calendar. Behind today's door is a glimpse into X-ray imaging.

Rendered visualisation of 'Florian' the fly

Modern synchrotron radiation (SR) sources such as Diamond Light Source have dramatically fostered the use of SR-based X-ray imaging. Vital information such as density, chemical composition, chemical states, structure, and crystallographic perfection can mapped in two, or, increasingly, in three dimensions. The ongoing developments in this field have led to a dramatic increase in both the speed and resolution of X-ray imaging techniques pushing spatial resolution down towards the nanoscale.

X-ray imaging visualises samples, frequently the internal or hidden components of a sample and is applicable to nearly all fields of science from the life sciences to engineering to archaeology. It can probe the interior structure of materials, cells and molecules to address problems in areas such as cell biology, nanomagnetism, chemical identification and molecular identification, environmental science and soft matter.

Imaging is defined as "the ​process of ​producing an ​exact ​picture of something". In the case of synchrotrons, imaging isn't just about seeing what is on the outside of an object, but what is inside of it. Whether it's metals, rocks, biological samples or even delicate archaeological samples, X-ray imaging allow us to view what is contained within the sample, at a molecular level.

Hard X-ray imaging allows detailed information to be gathered from below the surface of a material through either full-field imaging, where the whole sample is illuminated, or through scanning, where the beam is focused to a small spot which is scanned across the sample. The high intensity and energy of synchrotron X-rays makes it possible to image a much larger range of materials and sample thicknesses than conventional X-ray sources, and the brilliance of the synchrotron source produces very high resolution images. The high intensity X-rays also permit very fast measurements for high speed imaging experiments. A technique called X-ray computed tomography can create three dimensional reconstructions of the internal sample volume. This makes it possible to view any cross-section of the virtual image at any angle.

At Diamond, the main microscopy and imaging techniques employed are detailed below:

• Absorption Contrast Imaging
• Phase Contrast Imaging
• X-ray Tomography
• X-ray Fluorescence (XRF)
• Cryo-electron Microscopy
• Coherent X-ray diffraction
• PhotoEmission Electron Microscopy (PEEM)
   

This is the most common imaging technique, and is the technique used in hospital X-ray imaging. An absorption contrast image is essentially a shadowgraph, the contrast being generated by the different attenuating power of materials in the sample. The small spot size and high intensity of synchrotron X-rays also make it possible to scan samples and provide a composite image in much finer detail than from conventional sources. Combining absorption contrast imaging with other X-rays techniques also allows detailed complementary information to be gathered, as in diffraction enhanced imaging, or to recreate three dimensional objects from two-dimensional scans as in tomography.

Absorption contrast imaging has a wide range of applications, in bio-medicine, materials science, engineering, environmental science and nanotechnology.

Phase contrast imaging takes advantage of the fact that different materials have different refractive indices. This produces a phase shift in the X-rays passing through the sample. By placing the imaging detector at a specific distance from the sample, interference between waves can be used to enhance contrast in the image.

Phase contrast imaging is really useful for the examination of crack growth and fatigue in materials under stress, which is of particular importance in the aerospace and engineering industries. It is particularly useful for enhancing the contrast of surfaces and interfaces in samples, which would not be visible using absorption contrast.

Price, S. et al. DOI: 10.1002/anie.201504227

X-ray Fluorescence (XRF) occurs when the inner shell electrons of atoms in the sample get excited by the X-rays and subsequently release X-ray photons when the system relaxes, that is when the electrons transition from the higher energy levels of the atom to the vacant inner shell. The beauty of this process is that each secondary X-ray photon (sometimes called characteristic radiation) emitted from the sample has a specific energy which is a fingerprint of the atom from which it has originated. By measuring the energy of the secondary photons it is possible to establish the elemental composition of the sample at the point where the X-ray beam hits the sample.

Typically a special type of detector called energy-dispersive detector is used to precisely measure the energy of each photon. The plot of the number of photon counts versus their energy, the X-ray spectrum, typicallly shows a number of peaks which are directly associated with specific elements, so by just glancing at the spectrum it is possible to quickly deduce which elements are present in the sample. 

X-ray fluorescence spectroscopy can be used to study the chemical composition of virtually anything and is becoming an advanced and essential analytical technique in life and environmental sciences, medical applications, archaeological and cultural heritage applications, forensic chemistry, industrial applications, and earth and planetary sciences. Some of the research taking place at synchrotrons like Diamond has helped scientists to study chemical processes, develop highly nutritious food, and investigate Alzheimer’s disease.

Electron microscopes work by generating a beam of electrons that electromagnetic lenses focus and this beam is then fired at a sample. The electrons then interact with nano-scale components, allowing us to investigate and visualise samples minute detail. 

Regular electron microscopy requires samples to be prepared in complex ways – techniques include coating samples in substances that protect them from radiation, sectioning them into tiny slices, or dehydrating them to prevent the interaction between electrons and water molecules. Cryo-electron microscopy, or cryo-EM, so called as the samples using this method under really cold conditions, at -200 °C using liquid nitrogen. This enables scientists to see biological elements as a whole and in an active state. Furthermore, rather than studying individual components of a sample and piecing a wider picture together, cryo-EM enables scientists to look at big, complex biological systems.

The technique allows scientists to study objects that – because of their size, complexity, or sheer awkwardness – would be virtually impossible to scrutinise with other techniques. In biology, applications of cryo-EM now span a wide spectrum, ranging from imaging intact tissue sections and plunge-frozen cells, to individual bacteria, viruses and protein molecules. It’s quick and flexible; and when used in tandem with other techniques, cryo-EM is a supremely powerful tool.

Coherent X-ray diffraction is an imaging technique which overcomes some of the limitations encountered with using lenses. Instead, a series of X-ray diffraction patterns are combined to mathematically reconstruct a three dimensional image of the structure being studied. With highly coherent synchrotron X-rays, this approach can provide spatial resolution on a nm scale. In the past this technique has been applied to model repeating crystal structures, but it is now being used to examine small, non-periodic samples.

The technique is in the very early stages of development and available only to experts through the peer review access route at this stage.

PhotoEmission Electron Microscopy (PEEM) is a non-destructive imaging technique that involves shining linearly or circularly polarised X-rays onto the surface of a sample to provide spectroscopic information on a nm scale. This information can be used to study nanostructures significant for sensors, catalysts, magnetic materials and nanoscale devices and phenomena such as nanomagnetism.

Would you like to know more about imaging and how you can apply it to your research? Do you perhaps have a structural problem that you are unable to solve in your lab or a material you wish to find out more about? Then please get in touch with the Industrial Liaison Team at Diamond.

The Industrial Liaison team at Diamond is a group of professional, experienced scientists with a diverse range of expertise, dedicated to helping scientists and researchers from industry access the facilities at Diamond. We’re all specialists in different techniques and have a diverse range of backgrounds so we’re able to provide a multi-disciplinary approach to solving your research problems. We offer services ranging from full service; a bespoke experimental design, data collection, data analysis and reporting service through to providing facilities for you to conduct your own experiments.

We’re always happy to discuss any enquiries or talk about ways in which access to Diamond’s facilities may be beneficial to your business so please do give us a call on 01235 778797 or send us an e-mail. You can keep in touch with the latest development by following us on Twitter @DiamondILO or LinkedIn