Applied ultrasound

Regarding the physics of ultrasound, which one of the following statements is

CORRECT?

a) Application of a direct current causes piezoelectric materials to vibrate

b) The speed of sound conduction through the human body is 940 m/s

c) Differences in acoustic impedence of different tissues causing refraction of the

incident beam is the basis of ultrasound imaging in the body

d) For most applications in anaesthesia, e.g. vessel cannulation and regional

anaesthesia, A-mode display format is employed

e) Anisotropy is an example of an ultrasound artefact where the echoic amplitude of a

structure varies with the angle of insonation

Answer: e

Explanation

The basis of ultrasound imaging in the body is the reflection of sound pulses beyond

the range of our hearing off the interface between tissues of different acoustic impedances.

Return is best when the angle of incident sound (or insonation) is at 90° to the

interface to be imaged. Refraction (the alteration of path direction on transition from

one medium to another) only occurs when the angle of incidence is different to 90° and

there is a significant change in the speed of sound between two media. The speed of

sound through body tissues is fairly consistent at 1540 m/s (940nm is the wavelength

of the infrared LED in a pulse oximeter). Most ultrasound scanners used by anaesthetists

use pulsed-wave emission and display in 2-D mode. A-, B- and M-mode display

modalities give a highly detailed cross-section in one plane of the structure and, in

M-mode, display its change over time. The application of an alternating current to a

piezoelectric crystal causes it to vibrate (DC causes it to deform only). Pulsed-wave

emission means that the probe will alternate between ‘transmitting’ and ‘listening’

tasks rapidly but will not do both at once. The frequency of the sound emitted in each

pulse will be in the order of MHz (depending on the desired penetration), the pulse

duration is typically 1 to 3 microseconds, and the pulse repetition frequency is how

many pulses it will transmit per second while listening in between. This is between 1

and 10 kHz, which makes the gap for listening between pulses between 0.1 and 1.0 ms.

This is mentioned to highlight that the probe spends much more time listening than

transmitting. The sciatic nerve is highly anisotropic and will be bright at a 90° angle of

insonation but virtually disappear at 80° or 100°.