Osnove ultrazvuka - fizika i artefakti

Abstract

Ultrasound (US) represents mechanical, longitudinal, acoustic waves with frequencies above 20 000 Hz. Ultrasound waves of frequency from 2 to 20 MHz are used in diagnostics. US waves have the following characteristics: propagation velocity, frequency, wavelength and amplitude. By passing through human bodies, in interaction with different tissues, US loses energy-attenuation. This is due to: absorption, reflection, refraction, dispersion and diffraction of ultrasonic waves. Reflection is responsible for the occurrence of an ultrasound image. In the ultrasonic transducer there are piezoelectric crystals that have the ability to convert electrical impulses into mechanical (ultrasonic) waves and vice versa. By connecting an US with a circuit, piezoelectric crystals convert electrical impulses into US waves. In the interaction with matter, some of them are reflected/rejected. These echoes are again detected by piezoelectric crystals in US transducer and converted into electrical impulses which are displayed on the screen in the form of points. The US transducer is both a generator and a US wave receiver. On the basis of reflected echoes in the form of points, a picture on the screen is formed. The intensity of reflected echoes represents echogenicity, i.e. the ability of a structure to create US waves. Thus, we distinguish anechoic, hypoechoic, hyperechoic and isoechoic structure. Based on the frequency, resolution and penetration of ultrasonic waves, the probe is divided into: convex, linear and sector. The basic rule is that the higher the frequency of the US transducer the better the image resolution, but the smaller the penetration of the US wave. Forms that do not represent anatomical structures appear as a result of US examinations but are the result of specific inspection techniques and are called artefacts. These include: acoustic enhancement, acoustic shadow, reverberation and the phenomenon of 'mirrors'.