OTHER DESIGNATIONS: Radiography, phase contrast imaging, scanning micro/nanoprobe, full-field microscopy, diffraction enhanced imaging (DEI), x-ray tomography, topography.

PURPOSE: Hard x-ray imaging nondestructively visualizes samples, frequently the internal or hidden components of the samples. It is applicable to nearly all fields of science from the life sciences to engineering to archaeology. A few uses are:

• Human and animal physiology (amplitude- or phase-contrast imaging and DEI)
• Mapping of magnetic domains in two dimensions (micro/nanoprobe)
• Mapping of composite materials in three dimensions (tomography)
• Properties of individual grains in a polycrystalline material (micro/nanoprobe)
• Mapping of the distribution of elements in cells (micro/nanoprobe)
• Strains in near-perfect crystals (diffraction imaging topography)
• Time-resolved imaging of sprays (radiography)

HOW THE TECHNIQUE WORKS: There are two basic experimental methods:

1. Imaging. A full-field image can be recorded, not unlike the images taken by your doctor or dentist.
2. Scanning. A very small illuminated spot is created on the sample using focusing devices. The image is then built up by “raster scanning” the sample through the illuminated spot.

In either case, there has to be a physical cause of the contrast. This can be due to changes in absorption, elemental composition, or refractive index of the sample.

UNIQUENESS: What are the relative merits of x-ray microscopes and electron microscopes? Electron microscopes will always have better spatial resolution, but they will be limited in the range of samples that can be studied. The uniqueness of x-ray imaging resides in its deeper penetration, enabling the study, for example, of buried interfaces and wet biological samples.

EXAMPLES:

Growth Modes of Oxide Films on Textured Metal Substrates
Diffraction-Enhanced Imaging Improves Cancer Detection
X-Ray Imaging of Shock Waves Generated by High-Pressure Fuel Sprays

Orientation maps from a deposited oxide film and a textured nickel substrate obtained from x-ray Laue microdiffraction.

Interactions between small, tightly packed single crystals at their boundaries ultimately determine the structural and electronic behavior of most polycrystalline materials. X-ray microbeam studies of individual grains can now characterize the local three-dimensional structure, orientation, and strain with submicrometer resolution, as demonstrated in epitaxially grown cerium oxide (CeO₂) films on textured nickel (Ni) substrates. Understanding the microstructure of such heteroepitaxial materials is crucial to the development of superconductor technology based on deposited films. Analysis of diffraction patterns revealed two distinct growth regimes, and large-area orientation mapping showed that crystallographic tilting associated with the complex interfaces gave rise to enhanced texture. The tilt mechanisms should apply to many oxide/metal systems, including thermal barrier coatings, solar cells, and interfaces in electronic devices, suggesting that the benefits of intentionally enhanced texture are achievable in many coated materials.

J.D. Budai, W. Yang, N. Tamura, J.-S. Chung, J.Z. Tischler, B.C. Larson, G.E. Ice, C. Park, and D.P. Norton, “X-ray microdiffraction study of growth modes and crystallographic tilts in oxide films on metal substrates,” Nat. Mater. 2, 487 (2003).

Digital radiograph (top) and diffraction-enhanced image (bottom) of breast specimen.

In conventional mammograms, differences in tissue densities and composition show up as contrasting areas due to x-ray absorption, allowing doctors to see tumors or changes in tissue. However, differences between healthy and cancerous tissues are very small, and scattering of x-rays can lead to blurring and even lower contrast. Researchers have developed a new mammography technique called diffraction-enhanced imaging (DEI) that uses ultrabrilliant x-rays and provides a dramatic contrast between normal tissues and tumors. The DEI method uses a single-energy beam of x-rays instead of the broad-energy beam used in conventional imaging. The key to the new imaging method is an analyzer crystal placed between the tissue under study and the x-ray detector. The analyzer can differentiate between x-rays that are traveling much less than one ten-thousandth of a degree apart. This method of line-scan imaging reduces scatter and helps visualize low-contrast areas that otherwise would be lost. This technology offers great hope for early detection of breast cancer and therefore allows for higher survival percentages. DEI could be used in experimental clinical trials within five years and possibly in routine mammography in ten years. In addition to mammography, potential applications of DEI include other low-contrast tissues and organs such as kidneys, and the nondestructive testing of materials.

E.D. Pisano, R.E. Johnston, D. Chapman, J. Geradts, M.V. Iacocca, C.A. Livasy, D.B. Washburn, D.E. Sayers, Z. Zhong, M.Z. Kiss, and W.C. Thomlinson, “Human breast cancer specimens: Diffraction-enhanced imaging with histologic correlation—Improved conspicuity of lesion detail compared with digital radiography,” Radiology 214, 895 (2000).

Time-resolved radiographic images of fuel sprays and attendant shock waves.

High-pressure, high-speed fuel sprays are a critical technology for many applications, including fuel injection systems, where the structure and dynamics of the fuel sprays are the key to increasing fuel efficiency and reducing pollutants. But because liquid sprays are difficult to image, particularly in the region close to the nozzle, high-pressure fuel sprays have never been considered as supersonic under typical fuel injection conditions. Synchrotron x-ray radiography and a fast x-ray detector were used to record the time evolution of transient fuel sprays from a high-pressure injector, capturing the propagation of spray-induced shock waves in a gaseous medium and revealing the complex nature of the spray hydrodynamics. The x-ray radiographs also allow quantitative analysis of the shock waves that would be nearly impossible with optical imaging. Under injection conditions similar to those found in operating engines, the fuel jets can exceed supersonic speeds and result in gaseous shock waves. This work sets the stage for study of the entire range of fluid dynamics inside and close to high-pressure liquid sprays. The methods used here may also be applied to the characterization of highly transient phenomena in optically dense materials.

A.G. MacPhee, M.W. Tate, C.F. Powell, Y. Yue, M.J. Renzi, A. Ercan, S. Narayanan, E. Fontes, J. Walther, J. Schaller, S.M. Gruner, and J. Wang, “X-ray imaging of shock waves generated by high-pressure fuel sprays,”Science 295, 1261 (2002).