DR.MERVAT MOSTAFA

ULTRASOUND

SOUND AND ULTRASOUND IN MEDICINE(PHR 177)COURSE

Prof. Dr. Moustafa. M. MohamedVice Dean

Faculty of Allied Medical SciencePharos University

Alexandria

Dr. Mervat MostafaDepartment of Medical Biophysics

Pharos University

Sound

• Sound is energy traveling though matter as a wave.

• Sound is a mechanical, longitudinal wave that travels in a straight line.

• The wave travels by compressing and rarefacting matter.

• Sound requires a medium through which to travel.

• Depending on the matter- the wave will travel at different velocities or directions.

a- Sound is produced by vibrations

• Sound source vibrates.

• Surrounding air vibrates.

• Vibrations travel in air.

• Ear drum vibrates and sound heard.

b- Sound can travel through solids, liquids and gases

• You can hear sound when swimming underwater.

• Floors, ceilings and brick walls also transmit sound.

c- Sound cannot travel through a vacuum

Sound waves are longitudinal waves

Speed of sound

• It increases with the temperature.• It varies with the medium:

Sound and light waves compared

• Difference between Sound and Light waves.

Sound and light waves compared

Sound waves Light waves

Travelling speed in Air 330 m/s 3*10^8

Wave Composition Longitudinal Transverse

Transmitting Medium All SubstancesEmpty Space And All

Substances Except Opaque Materials

Relation of Transmitting Medium

Velocity to Velocity

The Denser The Medium, The

Greater The Speed

The Denser The Medium, The Slower

The Speed

Sensations Produced Hearing Seeing

Audible sound and ultrasound

• a Audible sound

• Human beings can hear sound of frequency from about 20 Hz to 20 kHz.

• It is called audio frequency range.

• Many animals can hear sound waves within a wider range of frequencies.

Audible sound and ultrasound

B- Ultrasound• Audio freq. range: 20 Hz - 20 kHz

• fsound > 20 kHz (ultrasonic waves)

• Dolphins and bats can emit ultrasonic waves.

Ultrasound

• This is sound with a higher frequency than we can hear, i.e. above 20000 Hz.

• Uses include: industrial cleaning, breaking down kidney stones, industrial quality control, scanning of unborn babies and SONAR.

Uses of Ultrasound

• Ultrasound has many uses, especially in medicine where:

• it is used to scan the foetus.

• Measuring blood flow.

• Viewing organs and other tissues for abnormalities or information.

• Mapping and injecting drugs into the brain.

Basic Ultrasound Physics

oscillations/sec = frequency - expressed in Hertz (Hz)

Amplitude= The degree of variance from the norm

What is Ultrasound?

•Ultrasound is a mechanical, longitudinal wave with a frequency exceeding the upper limit of human hearing, which is 20,000 Hz or 20 kHz.

• Medical Ultrasound 2MHz to 16MHz.

High Frequency• High frequency (5-10 MHz) greater resolution less penetration• Shallow structures vascular, abscess

Low Frequency

• Low frequency (2-3.5 MHz)

greater penetration

less resolution

• Deep structures

Aorta, renal

ULTRULTRASOUND – How is itproduced?

• Produced by passing an electrical current through a piezoelectrical crystal (probe)

• U/S probes emit and receive the energy as waves to form pictures.

The Machine

• The basic components of the Ultrasound machine are:• A computer/CPU unit• Transducer Controls• Transducer

Transducer

• The Transducer is the main probe which sends and receives the sound waves.

• Can come In many different shapes and sizes, single-element, multiple-element, surface or insertion

Ultrasound Process

• The ultrasound can be external or internal depending on what is being examined• If external, then the skin is prepared with a mineral-oil based jelly to maximize contact of transducer to skin and allow better conduction for the waves• The Transducer or patient can be moved to obtain more images and angles of the subject

Ultrasound Production

• Transducer contains piezoelectric elements/crystals which produce the ultrasound pulses.

• These elements convert electrical energy into a mechanical ultrasound wave

The Returning Echo

• Reflected echoes return to the scanhead where the piezoelectric elements convert the ultrasound wave back into an electrical signal

• The electrical signal is then processed by the ultrasound system

Piezoelectric Crystals

• The thickness of the crystal determines the frequency of the scanhead

Frequency vs. Resolution

• The frequency also affects the QUALITY of the ultrasound image

–The HIGHER the frequency, the BETTER the resolution

–The LOWER the frequency, the LESS the resolution

• A 12 MHz transducer has very good resolution, but cannot penetrate very deep into the body.

• A 3 MHz transducer can penetrate deep into the body, but the resolution is not as good as the 12 MHz.

Interactions of Ultrasound withTissue

• Reflection

• Refraction

• Transmission

• Attenuation

Reflection

– The production of echoes at reflecting interfaces between tissues of differing physical properties.

– The ultrasound reflects off tissue and returns to the transducer, the amount of reflection depends on differences in acoustic impedance

– The ultrasound image is formed from reflected echoes

Refraction

• A change in direction of the sound wave as it passes from one tissue to a tissue of higher or lower sound velocity

• U/S scanners assume that an echo returns along a straight path

• Distorts depth reading by the probe

Transmission

– Some of the ultrasound waves continue deeper into the body

– These waves will reflect from deeper tissue structures

Attenuation

• The intensity of sound waves diminish as they travel through a medium

• In ideal systems sound pressure (amplitude) is only reduced by the spreading of waves

• In real systems some waves are scattered and others are absorbed, or reflected

• This decrease in intensity (loss of amplitude) is called attenuation.

Attenuation

Defined - the deeper the wave travels in the body, the weaker it becomes -3 processes: reflection, absorption, refraction

– Air (lung)> bone > muscle > soft tissue >blood > water

Producing an image

• Important concepts in production of an ultrasound image:• Propagation velocity.• Acoustic impedance.• Reflection• Refraction• Attenuation

Propagation Velocity

• Sound is energy transmitted through a medium-

• Each medium has a constant velocity of sound (c)

• Tissue’s resistance to compression

• Product of frequency (f) and wavelength (λ)

c = fλ

• Frequency and Wavelength therefore are directly proportional- if the frequency increases the wavelength must decrease.

Impedance

• Acoustic impedance (z) of a material is the product of its density and propagation velocity

Z= pc

• Differences in acoustic impedance create reflective interfaces that echo the u/s wavesback at the probe

• Impedance mismatch = ΔZ

Acoustic Impedance

• Homogeneous mediums reflect no sound

• acoustic interfaces create visual boundaries between different tissues.

• Bone/tissue or air/tissue interfaces with large Δz values reflect almost all the sound

• Muscle/fat interfaces with smaller Δz values reflect only part of the energy

Benefits and Risks of Ultrasound

• Benefits (Non-invasive, No use of Radiation, Widely Available and cheaper than CT, Real-time, Visualize structure, movement, and live function)

• Risks (Heat, Cavitations, Not many)

• development of heat - tissues or water absorb the ultrasound energy which increases their temperature locally

• formation of bubbles (cavitations) – when dissolved gases come out of solution due to local heat caused by ultrasound

ASSIGNMENTS

• up to this lecture you are assigned to assignments number 13 to 15