Introduction

Physics ultimately rules the processes in living organisms and thus is in that sense fundamental for understanding medicine. To cite some examples involving different branches of physics:

• the dynamics of forces in joints, e.g. the dependence of stress in an articulation → mechanics

• the microstructure of bones and the role of compounds with high elasticity and high tensile strength → solid-state physics of articulations

• control of blood circulation and the variable viscosity of blood and blood plasma → hydrodynamics

• passive molecular transport through membranes via osmosis → thermodynamics

• signal conduction in nerve cells → electrodynamics

• image formation on the retina → optics and

• hearing → acoustics and mechanics.

Many physical properties of tissues, substances, cells, and molecules and of their mechanisms of operation are exploited for diagnostics and therapy (e.g. ultrasonic imaging, electro- or magneto-encephalography, high-frequency electromagnetic-radiation therapy). Rather than these manifold relationships between physics and the phenomena of medicine and biology, medical physics today covers that part of physics in which new phenomema are exploited and new techniques developed explicitly for use in diagnostics and treatment. The largest area has to do with diagnostic methods, the most prominent being transmission radiography with X-rays, which was introduced soon after the discovery of X-rays by Röntgen in 1895. Many sections of this chapter cover the principles and recent developments of diagnostic tools, weighing their virtues as well as possible disadvantages (e.g. adverse side effects). The various techniques also provide complementary information.