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Imaging

The non-destructive nature of X-ray imaging has permitted a wide range of applications in testing and diagnosis for science, medicine and industry. Additionally, differing X-ray wavelengths may be utilized to selectively increase visibility of features of i nterest.

Advances in the X-ray imaging area were limited in the first fifty years by focusing and detection technologies. The first commercial image intensifier was introduced in the 1950s for medical imaging. The introduction of computer control in the 1960s opened up new fields. For example, in 1971 the first Computed Tomography unit was used by Drs Hounsfield and Ambrose for successful diagnosis of a tumor. The years following saw significant improvements in beam emission, detector systems, and the required computing power and algorithms.

High resolution X-ray imaging

The relatively long wavelength of visible light forms a natural limit to the resolution achievable by conventional light microscopy. Special techniques (e.g. confocal scanning and near-field scanning microscopies) can yield a modest improvement by a factor of about two, but beyond this a fundamentally different approach is needed. Following the discovery of X-rays a hundred years ago attempts were made to utilize X-rays for micro-imaging and microscopy. In principle, the very much smaller wavelength of X-rays could yield resolutions smaller by orders of magnitude than those achievable by optical microscopy. In addition the much greater penetrating power of X-rays could give microscopic information on objects opaque to visible light, as it was doing already for macroscopic imaging. Nevertheless the practical development of X-ray microscopes was slow, due mainly to the absence of optical elements, such as lenses and mirrors, operating in the X-ray region. Even today such elements are difficult to make and use, and of limited availability. Accordingly, lens-less microscopy techniques were investigated for X-ray applications.

Lens-less Techniques

The XuM is essentially a development of one such technique—point projection X-ray microscopy. The latter has a long history (Nixon, 1957; Cosslett and Nixon, 1960; Horn and Waltinger, 1978). The earliest attempts were based on a pinhole placed in front of a standard X-ray source. Later (1933 on) microfocus tubes were developed and applied to X-ray microradiography and microscopy. Significant advances were made by Cosslett and co-workers in the 1950s with the use of magnetic electron lenses , allowing submicron focusing of the electron beam. Next came the realization that a scanning electron microscope (SEM) could be utilized for the purpose of producing a fine X-ray source (Horn and Waltinger, 1973) suitable for X-ray microscopy. Several groups are currently working in this area (Yada and Takahashi, 1992; Cazaux et al., 1993; Yoshimura et al., 2000). Most recently synchrotron sources are being used for this as well as other types of X-ray microscopy.

The projection X-ray microscope

The operating principles of the projection X-ray microscope can be succinctly summarized as follows (Horn and Waltinger, 1978):

‘The object is fixed close to a point source of X-rays and its image is projected on to a photographic film. The magnification factor follows from the geometry of central projection. The resolving power depends on the diameter of the source, and the exposure time depends upon its intensity.’ The physical principles are also simple—imaging (i.e. projection) is described by ray optics, with contrast produced by absorption in the specimen.

This physical picture (for both macro- and microscopic X-ray imaging) has been the standard one in use since the discovery of X-rays by Röntgen over 100 years ago.

This is an extract for the XuM Principles and Appllications manual, please email us if you would like a copy of the full document.

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