8. Properties and biomedical applications of ultrasound.

Ultrasound

defined as sound with frequencies above 20 kHz
US image is an echo image of sound reflections from inside the body, assembled by a powerful computer

US diagnostics

A non-invasive imaging technique based on the use of low-intensity US
It is mainly used to image the surface of soft tissues, muscles or bones
Methods based on the Doppler principle can also be used to determine blood flow velocity

US therapy

Utilizes the thermal effect of high-intensity US to destroy abnormal tissue (e.g. tumors) or the mechanical destructive effect to crush kidney stones

Propagation in a vacuum

no sound propagation

gas and liquid propagation

only longitudinal sound waves can occur

soft tissue sound propagation

longitudinal waves

solids and at the surface of liquids propagation

transverse waves also can be formed

Interaction of medium and sound: in a medium

Absorption, dispersion

Interaction of medium and sound: border of medium

reflection, refraction, breakdown

Generation of ultrasound

inverse piezoelectric effect
electrostriction
magnetostriction

Diagnostic sonography

Generation of ultrasound
Interaction of US with human tissues
Detection of the echos
Data processing → image formation

Generation of ultrasound (step1)

piezoelectric transducers – inverse piezoelectric effect pulse vs. continuous techniques

Interaction of US with human tissues (step2)

reflection, scattering → signal (echo) absorption, reflection, scattering, refraction → attenuation of US refraction → artifact production

Detection of the echo

piezoelectric transducers – piezoelectric effect intensity, arrival time and frequency of the echos

Data processing → image formation

amplification displaying the signal/image formation spatial/temporal resolution of the image

direct piezoelectric effect

conversion of mechanical stress into voltage (charge separation!)
alternating mechanical stress (dimensional change) alternating electrical signal US detection

inverse piezoelectric effect

conversion of electrical stress to mechanical stress

application of alternating stress periodic dimensional change/vibration US generation

Pulse-echo methods

1. One-dimensional A-scans (amplitude), distance measurement
2. One-dimensional B-scan (brightness) transfer to more complex images
3. TM-mode (M-mode)
4. Two-dimensional B-scan, US tomography

Doppler effect

when the wave source and the observer move relative to each other, the observer detects a frequency different from the original frequency.

Doppler effect-based measurements

vasoconstriction, shunt and tissue perfusion

spatial distribution of blood vessels, blood supply to organs, developmental anomalies and major flow disorders

examination of low or high resistance areas

Disadvantages of US

obstructed by intestinal gases and lung tissue imaging

examination-dependent, difficult to reproduce

inaccurate measurements

Therapeutic applications of US

High intensity focused US therapy (HIFU)
Extracorporeal Shock Wave Rock Crushing (ESWL)