In x-ray imaging, x-rays are passed through the object
and a shadow image is recorded with the help of a digital x-ray
camera, the so-called x-ray detector.
Since the x-rays are influenced when traversing the object, inner structures can be made visible and a three-dimensional, extremely detailed computer model can be produced.
Depending on the x-ray method applied, possible defects can be discovered, materials can be distinguished and micro structure parameters can be determined. Due to the variety of possibilities of x-ray imaging, it is widely used across all branches.
Digital x-ray radiography (radiographic examination) is used to get a first impression of the inside of an object or to carry out a quick test of components. To that purpose, the object is irradiated by x-rays and a magnified two-dimensional shadow image is produced with the help of an x-ray detector. This relatively simple method makes it possible to detect different defects as air pockets, cracks or broken cables. Since three-dimensional object is reduced to a two-dimensional image by this method, inner structures may overlap and conceal each other. Also, this technique does practically not allow quantitative statements concerning the quality and condition of an object.
If one wants to obtain a three-dimensional image of the object, one has to irradiate it from different directions and record a great number of shadow images. With the data and the help of specific reconstruction algorithms one can produce a three-dimensional model on the computer. The term computer tomography is used, when the sample is measured and presented with a spatial resolution of about 5 µm up to some 100 µm. The resolution correlates with the size of the object to be examined. If the complete object is to be displayed, the following rule of thumb for the achievable resolution applies: width of object divided by 1000 or 2000, depending on the detector system used. Depending on the required resolution and the materials the object consists of, objects of a size up to 40 - 50 cm can be examined with this method.
With the help of submicro computed tomography structures ranging in size from below 1 µm to 5 µm, can be made visible which remain hidden in regular computed tomography. This is achieved either by strong geometric magnification of the object or by optical magnification within the x-ray detector. The high resolution that can be achieved by this method allows for example the investigation of individual fibers in fiber reinforced plastic (CFRP or CFK), for instance, or the detection of small micro-cracks and fine porosities in materials and components. Also, larger-sized objects of few centimeters can be examined, if only a certain region of interest needs to be looked at to answer certain questions.
In order to examine larger objects on the micro-structure level and to detect details below the resolution limit of the x-ray system, darkfield imaging has been developed in the past years. With the use of optical gratings, changes of few micrometers in the material quality can be recognized by the scattering of x-rays. As is the case with digital radiography, only a two-dimensional image of the object is produced in darkfield imaging, too. The advantage is, that the appearance of small fissures or fine porosities and other microstructural changes can quickly and reliably be detected. With this technology, inline systems for quality management are conceivable, too.
The extension of darkfield imaging to produce three-dimensional results is called tensor tomography. The scattering of extremely small structures is measured by using optical gratings. As opposed to (what happens) in radiographic darkfield imaging, rotation of the object around all three spatial axes allows x-rays to pass through it in all possible directions. By special processing and reconstruction of the acquired data, the entire object and the median orientation of its microstructures can be displayed with this method. At the moment this kind of examination is limited to objects of a size of few centimeters such as fiber reinforced components (CFK) due to the limited surface of the optical gratings.
Multi-energy procedures are also relatively new developments in x-ray imaging and are used to better characterise and determine materials an object is composed of. As the name suggests, this method involves the examination of an object with different x-ray energies or x-ray spectra, respectively. This method makes use of the fact that interaction of the x-rays vary with both the energy and the atomic number of the respective material varies. On a practical level, objects in the hand luggage at the airport, for example, can be identified more reliably. In computed tomography (µCT and sub-µCT), the use of two x-ray spectra and suitable calibration measurements even allow the determination of density and atomic number of individual materials.
For materials of low atomic numbers, which can be found in biological samples or plastic products, for example, conventional micro computed tomography does not produce the required contrast between different structures necessary for further data analysis. In these cases, the optical properties of x-rays can be used to significantly increase image quality by the application of phase-sensitive methods. In the case of high-resolution submicro computed tomography measurements, 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 computed tomography systems, an optical grating, as used in darkfield imaging and tensor tomography, is required.
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 attach themselves to the sample in a tissue-specific manner. In a preparatory procedure, also called staining, the sample is prepared in the appropriate way and a contrast agent is administered before the measurement. That way certain kinds of tissues such as tumors can be made visible and even individual cells can be displayed. In future this will facilitate new diagnostic techniques such as 3D histology for pathological diagnoses.