Introduction

Ultrasound imaging or ultrasonography has been used in clinical practice for more than half a century. It is noninvasive, relatively inexpensive, portable, and has an excellent temporal resolution. Imaging by means of acoustic waves is not restricted to medical imaging. It is used in several other applications such as in the field of nondestructive testing of materials to check for microscopic cracks in, for example, airplane wings or bridges, in sound navigation ranging (SONAR) to locate fish, in the study of the seabed or to detect submarines, and in seismology to locate gas fields.

The basic principle of ultrasound imaging is simple. A propagating wave partially reflects at the interface between different tissues. If these reflections are measured as a function of time, information is obtained on the position of the tissue if the velocity of the wave in the medium is known. However, besides reflection, other phenomena such as diffraction, refraction, attenuation, dispersion, and scattering appear when ultrasound propagates through matter. All these effects are discussed below.

Ultrasound imaging is used not only to visualize morphology or anatomy but also to visualize function by means of blood and myocardial velocities. The principle of velocity imaging was originally based on the Doppler effect and is therefore often referred to as Doppler imaging. A well-known example of the Doppler effect is the sudden pitch change of a whistling train when passing a static observer. Based on the observed pitch change, the velocity of the train can be calculated.

Historically, the first practical realization of ultrasound imaging was born during World War I in the quest for detecting submarines. Relatively soon these attempts were followed by echographic techniques adapted to industrial applications for nondestructive testing of metals. Essential to these developments were the publication of The Theory of Sound by Lord Rayleigh in 1877 and the discovery of the piezoelectric effect by Pierre Curie in 1880, which enabled easy generation and detection of ultrasonic waves. The first use of ultrasound as a diagnostic tool dates back to 1942 when two Austrian brothers used transmission of ultrasound through the brain to locate tumors. In 1949, the first pulse-echo system was described, and during the 1950s 2D gray scale images were produced.