THE PHYSICS OF ULTRASOUND - PART 1 - ANAESTHESIA TUTORIAL OF THE WEEK 199 - 4TH OCTOBER 2010

Sep 22, 2026

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Anaesthesia Tutorial of the Week issue 199 presents a structured physics overview of ultrasound for clinical trainees. It reviews core sound-wave properties—wavelength, amplitude, period, frequency, and velocity—linking them to human hearing limits, the megahertz range used clinically, and the relationships among frequency, period, and wavelength. The tutorial explains acoustic behavior through the wave equation and tissue speed differences, then describes how probes generate ultrasound via transducer arrays and the piezo-electric effect.

LanguageEnglish

CategoryExam

FormatPDF

Pages8

Reading Time20 min

AccessFree to download

QUESTIONS

INTRODUCTION

BASIC PRINCIPLES OF SOUND WAVES

The Wave Equation

GENERATING ULTRASOUND AND IMAGE PRODUCTION

Structure of the probe

The piezo-electric effect

THE PHYSICS OF ULTRASOUND-PART 1 ANAESTHESIA TUTORIAL OF THE WEEK 199 4TH OCTOBER 2010 Dr M MacGregor1 , Dr L Kelliher1 , Dr J Kirk-Bayley2 1: Specialist Registrar, 2: Consultant, Royal Surrey County Hospital, Guildford, Surrey. [Correspondence to](Correspondence to mark.macgregor@nhs.net)[ ](Correspondence to mark.macgregor@nhs.net)[mark.macgregor@nhs.net](Correspondence to mark.macgregor@nhs.net) QUESTIONS Before continuing, try to answer the following questions. The answers can be found at the end of the article, together with an explanation. 1. Regarding the properties of sound waves, which of the following statements is correct? a. „Wavelength‟ refers to the time taken for one complete wave cycle to occur. b. The speed of a sound wave is unrelated to the medium through which it travels. c. Frequency is inversely related to wavelength. d. Frequency is directly related to the period e. The SI unit of frequency is the candela. 2. When considering clinical ultrasound, which of the following statements are correct? a. Clinical ultrasound uses sound waves in the frequency range 1 – 20 MHz. b. The average velocity of clinical ultrasound through the soft tissues ofthe body is 1540 m/s c. B-mode is the most commonly used image modality. d. Piezo-electrical materials are unaffected by pressure changes. e. The Doppler effect refers to the change in frequency for an observer moving relative to the source of the sound wave. 3. The difference in acoustic impedance between the air/soft tissue interface is? a. Large. b. Small. c. Smaller than the muscle/liver interface. d. Smaller than the blood/water interface e. Not detectable. INTRODUCTION Imaging with ultrasound is finding ever increasing applications in modern medicine. The use of oesophageal doppler to monitor cardiac output and guide fluid therapy is now widespread and the use of ultrasound devices in ICU extends to assessment of cardiac function via trans-thoracic/ transoesophageal ECHO. In anaesthesia the utilization of ultrasound is well established for obtaining vascular access and performing a myriad of peripheral nerve blocks.Ultrasound-guided techniques are helping to increase success rates and reduce complications. Understanding the physical principles of ultrasound forms part of the core knowledge in training in anaesthesia. BASIC PRINCIPLES OF SOUND WAVES Sound is mechanical energy that is transmitted through a medium by vibration of molecules. Sound waves are termed „longitudinal‟ waves, which refers to the fact that the molecules oscillate in a line which is in the same direction as the propagated wave. Sound waves are characterized by the following parameters: Wavelength (λ)–This is the distance over which a wave‟s shape repeats, determined by measuring the distance between two consecutive corresponding points of the same phase. It is expressed in units of distance (m, cm, mm, etc.) Amplitude (A)–This is a measure of the maximum height of the wave and refers to the particle displacement. Period (τ)– This is the time taken for one complete wave cycle to occur. Frequency (f)– This is the number of complete cycles per second. The SI unit of frequency is the hertz (Hz) . One hertz equals one cycle per second. Humans hear sound within a range of 20 Hz to 20 kHz. Ultrasound has a frequency above 20 kHz, i.e. above the threshold of human hearing. Clinical ultrasound is in the range 1 – 20 MHz (1 MHz = 1000000 Hz) . Frequency is inversely related to the period. f 􀀠 1T Velocity (c)–This is the speed at which sound waves pass through a medium. The velocity of a sound wave is determined by the physical properties of the medium it is passing through and may be calculated using the equation: 􀁅 c 􀀠 ~~ ~~ 􀁕 Where c is the speed,β is the stiffness (Bulk Modulus) and ρis the density. Audible sound waves are transmitted through air at a speed of 330 m/s. Clinical ultrasound is transmitted through the soft t

FAQs

What are the key parameters that characterize sound waves in this tutorial?

The tutorial covers wavelength, amplitude, period, and frequency, and explains how each relates to measurable physical behavior of the wave.

How does ultrasound frequency relate to period and wavelength?

Frequency is inversely related to the period, and with constant soft-tissue velocity (assumed 1540 m/s) higher frequency corresponds to shorter wavelength.

How is ultrasound generated and received by the probe?

The probe uses an array of transducers that emit pulsed ultrasound bursts and then switch to detect returning echoes; piezo-electric materials convert electrical charge into sound and pressure back into electrical signals.

Sep 22, 2026

Uploaded by Callum