Chapter 1: Diagnostic Medical Imaging: X-ray projection technique, image subtraction method, direct digital x-ray imaging, computed tomography (CT).

In 1896 W.C. Röntgen announced the discovery of the potential of an at that time yet unknown type of radiation to penetrate materials and to cause opacification of photographic plates (after development). It turned out that various biological tissues attenuated this radiation differently such that the generation of projections of the human body onto a photographic plate was possible (Fig. 1.1). As the physical nature of the radiation was unknown, it was said to consist of “x-rays” – a designation which is still in use today. A further milestone was the introduction of computed x-ray tomography by G.N. Hounsfield in 1973 which for the first time allowed to produce cross sections of the human body for diagnostic purposes in vivo and noninvasively apart from the application of a radiation dose (Fig. 1.2). The x-rays used for diagnostic purposes in medicine are electromagnetic waves with a photon energy between 30–200 KeV typically. “X-ray” was until a few years ago the most often applied medical imaging technology. Since then, methods on the basis of ultrasound (in contrast to x-ray non-ionizing) have become more often used. In addition, MRI (Magnetic Resonance Imaging) is of rapidly increasing significance in medical imaging, in particular as with the combination MRI/MRS (MR Imaging and Spectroscopy) also functional examinations can be made. The combination of x-ray CT (Computed Tomography) with PET (Positron Emission Tomography), in turn, has in recent years fostered significant and novel clinical applications for functional investigations. The radiation dose which is delivered to the patient is always of concern; it should be as small as possible. An enormous progress has been achieved through the years such that today a “simple” x-ray image can be produced with the aid of a radiation dose which can be considered harmless (Fig. 1.3).