Question

Explain the term acoustic impedance and state the tissue properties which determine its value.

Answer 1

Accoustic Impedence:

Accoustic impedence is a measures of opposition that a system presents to the accoustic flow resulting from an accoutic pressure given to the system, i.e. it is defined as the ratio of acoustic pressure p to acoustic volume flow U. So we can write Z=p/U. High Z means that a small acoustic flow generates (or requires) a large pressure; small Z means the reverse. In an instrument, the magnitude of Z varies strongly (by a factor of 1000 or so) when you scan through the frequency.

There is a close analogy of Accoustic impedence with Electrical impedence, which measures the opposition that a system presents to the electrical flow resulting from an electrical voltage applied to the system.The SI unit of accoustic impedence is pascal second per cubic metre (pa.s/m3) or the ryle per square meter.

Properties of Tissue:

Accoustic impedence is also a physical measure or property of tissue. For tissue it describes how much resistance an ultrasound beam encounters as it passes through a tissue.

Acoustic impedance depends on (1) the density of the tissue (d), and (2) the speed of sound waves inside the tissue(c).

and the relation is Z= d*c. i.e. if the density of tissue increases the accoustic impedence increases similarly for sound wave inside tissue if the speed increases the accoustic impedence increases.