Principles of ultrasound

The term ultrasound refers to sound with a frequency above that which can be detected by the human ear. The audible frequency range lies between 20Hz and 20kHz (one hertz equals one cycle per second, one kilohertz equals one thousand cycles per second), whereas the frequencies of sound waves used for diagnostic applications in medicine are of the order of one thousand times higher than this, with a range between 1 and 10MHz (megahertz = one million hertz). Ultrasound imaging relies on the so-called pulse echo principle, which involves emitting a short burst of ultrasound and then listening for the returning ‘‘echo’’ after the sound has been reflected off appropriate surfaces. This is exactly the mechanism which has been employed by bats for millions of years to navigate their way around dark caves and to catch flying insects. Human interest in navigation using sound waves was significantly enhanced (if not initially inspired) by the sinking of the Titanic, which occurred when the ship collided with an iceberg in April 1912. Within a few years, ships were widely equipped with SONAR (Sound Navigation And Ranging) devices, which emit sound waves beneath the surface of the sea, and detect echoes from large objects within a radius of several miles. The technology advanced considerably during both world wars as it was utilized to detect submarines and mines. Ultrasound imaging of patients began to evolve in the late 1940s, and over the following decade simple (A-mode) systems were developed that could detect midline shift in head injury and the presence of foreign bodies in the orbit. These devices emitted ultrasound pulses and detected echoes along a single line through the tissue. To generate twodimensional images, however, echoes must be acquired over multiple lines. Various methods of scanning the beam were explored, and Holmes [1] describes how some early imaging systems required subjects to be immersed in a water bath for the duration of the scan. Reflection of sound from a moving object gives rise to a change in the observed frequency. This phenomenon, known as the Doppler effect, was first described by Austrian physicist Christian Doppler in 1842. By measuring the change in frequency one can estimate the velocity of the moving object. That this is possible with some accuracy is well known to those who have been caught driving above the legal speed limit by police officers armed with a speed gun. Doppler measurement of blood flow has now become a standard feature of ultrasound systems, which commonly use color to display the velocity and direction of blood superimposed on conventional anatomical ultrasound images.