Technical Principles of Diagnostic Ultrasound

All ultrasound techniques rely on the processing of reflected sound waves. A wave motion results from the periodic, medium-dependent propagation of the vibration of particlesaround their resting positions. Sound waves are pressure waves that spread by alternately compressing and decompressing the medium they are traveling in. The speed of sound wave propagation is a function of the compressibility and density of the medium. Various modes of sound propagation are distinguished, again depending on the medium. Transverse waves occur when the particles vibrate around a resting position perpendicular to the direction of propagation, longitudinal waves occur when the vibration is parallel to the direction. In solid media, sound propagates in the form ofboth transverse and longitudinal waves. In fluids and gases, only longitudinal waves occur because no shearing forces are present in these media. As the human body mostly consists of water, the effect of transverse waves is negligible. Particlesexcited in the ultrasound range vibrate around their resting positions at a rate of 20,000 to 1 billion times per second. The frequencies used in vascular ultrasonography range from 2–10MHz (frequency f: 1 hertz = 1/s).The velocity of sound waves, C, is the product of wavelength and frequency f:C = • f Wavelength is the shortest distance between two vibratingparticles in identical states of motion (Table 1.1). The average speed of sound in human tissue is about 1,540 m/s. The wavelengths occurring in diagnostic ultrasound are determined by the frequency emitted by the transducer (carrier frequency)and range from0.7mmat a frequency of 2MHz to 0.15mmat 10 MHz.Parameters defining a sound wavePeriod: Duration of a complete vibrationWavelength: Spatial extension of a periodFrequency: Number of periods per secondAmplitude: Measure of sound energy