2D X-ray Dark-field Radiography

In order to examine larger objects on the microstructure level and detect details below the actual resolution limit of the X-ray system, dark-field imaging has been developed over the past years. With the use of optical gratings, variations of few micrometers within a material can be recognized by the scattering of X-rays.

As is the case with digital radiography, only a 2D image of the object is produced in dark-field radiography. The advantage is that the appearance of small fissures or fine porosities and other microstructural changes can quickly and reliably be detected. Inline systems for quality control based on this new technology are conceivable.

3D X-ray Tensor Tomography

In anology to dark-field radiography, the scattering of extremely small structures is measured by using optical gratings. In tensor tomography, the object is additionally rotated around all three spatial axes during the scanning process. This allows to obtain 3D results of the object.

By special processing and reconstruction of the acquired data, the median orientation of the object's microstructure can be displayed with this method. At the moment this kind of examination is limited to objects of a size of few centimeters due to the limited surface of the optical gratings.

X-ray Phase Contrast

For materials of low atomic numbers, which can be found in biological samples or plastic products, for example, conventional CT imaging does not produce the required contrast between different structures that might be needed for further data analysis. In these cases, the wave-optical properties of X-rays can be used to significantly increase image quality by the application of phase-sensitive methods.

In the case of submicro CT scans, the appropriate choice of parameters and special algorithms allow for the extraction of high-contrast phase-contrast images without further equipment or tools. If the refraction of X-rays needs to be detected with micro CT systems, optical gratings, as used in dark-field radiography and tensor tomography, are required.

Multi-energy X-ray Techniques

Multi-energy procedures can be employed to better characterise and determine the materials within an object. As the name suggests, the method involves the examination of an object with different X-ray energies. Since the interaction of X-rays varies with both the energy and the atomic number of the respective material, additional information can be gained.

On a practical level, objects in the hand luggage at the airport, for example, can be identified more reliably by dual-energy radiography. In CT imaging, the use of two X-ray spectra and suitable calibration measurements even allow the determination of density and atomic number of individual materials.

X-ray Contrast Media (Staining)

A further possibility to enhance contrast, particularly in biological tissue samples, is the application of suitable X-ray contrast agents, which have a high absorption capacity and a certain accumulation behaviour. In a special procedure called staining, the sample is prepared with the contrast medium prior to the measurement.

That way certain kinds of tissues such as tumors can be made visible and even individual cells can be displayed. This will facilitate new diagnostic techniques such as 3D histology for pathological diagnoses. In industry, the water transport in geological samples can for example be better observed by using X-ray contrast agents.