Introduction

Medical imaging plays a central part in the diagnostic examination of patients, as well as in invasive radiological procedures, which are increasingly dependent on accurate diagnostic information. Such information can be gained by applying transmitted, emitted or reflected electromagnetic radiation or mechanical vibration (ultrasound). The following physical phenomena are the basis of modern imaging:
- X-rays are absorbed in tissue (X-ray examination),
- radio frequency radiation is yielded by the excitation of odd atomic nuclei in a magnetic field (magnetic imaging based on nuclear magnetic  resonance, NMR),
- radioactive isotopes concentrated in certain tissues emit gamma radiation (nuclear medicine imaging or isotope imaging),
- high frequency beams of compression and rarefaction are reflected back towards a transmitter sensor (ultrasound examination),
- infrared radiation is emitted spontaneously by tissues (infrared imaging, thermography).

All of these methods except ultrasound are based on electromagnetic (em-) radiation in different energy domains. Ultrasound imaging is based on the detection of vibration, which is generated in a piezoelectric crystal. Isotope, ultrasound, and magnetic examinations were developed into useful imaging methods in the seventies and eighties, whereas X-radiation was discovered one hundred years ago in 1895.

Those types of radiation (X, gamma, beta and alpha radiation) which can impart more energy than light (2 - 4 eV), have the capacity to ionize atoms and dissociate molecules and therefore cause biological damage. The use of non-ionizing methods like ultrasound and magnetic resonance should generally be preferred, because of their inherent safety without taking account of their other advantages.

X-ray imaging methods can be grouped in the following way:
- radiography with a screen and film or a cassette (about one hundred years old), and today with a digital cassette
- fluoroscopy or transillumination (from the beginning of the century) with an image intensifier and a television camera tube which began in the
sixties, and
- computerized axial imaging methods (which began in the seventies).

Imaging methods can also be grouped according to whether a volume of tissue or a thin slice of tissue is imaged. In a conventional X-ray examination a 3-dimensional object is projected into a 2-dimensional image. Shadows of different organs are summated on film. The entire tissue volume is exposed, which means that much scattered radiation is generated which degrades image quality, particularly by reducing contrast.

In axial imaging (e.g. computed tomography, CT) radiation is directed only into a thin disc or slice of tissues. Conventional tomography is an older method, whereby shadows of objects outside the focal plane are blurred by synchronous movement of the X-ray tube and screen-film combination.

Computed Tomography (C T) was invented in 1973. A computer was used for the first time to measure, analyze and display an axial image in digital units. The greatest advantage of this method is good contrast resolution. Axial imaging is also used in magnetic resonance imaging and in nuclear medicine to a lesser extent. Ultrasound imaging is also restricted to one plane, but an image is not formed with reconstruction techniques like in the other axial modalities. Different imaging methods based on different physical principles therefore project different views of the anatomy and physiology of organs.

Digital computers play a pivotal role in all modem imaging methods. They will assume even more importance in the nineties both in digital bone and thorax imaging as well as in converting all image information (measurement, display, archiving, transmission and communication) into digital form.

The interactions of different energy forms with matter are scrutinized in this chapter, as well as the physical principles underlying imaging methods. The construction, function, and use of X-ray generating devices, the biological consequences of radiation, and radiation protection, and factors affecting image quality are also discussed.

Aaro Kiuru