Basic Physics of Ultrasound

lectured by

Dr.khitam Y. Elwasife

Ultrasound as all sound waves are caused by vibrations and therefore cause no

ionisation and are safe to use on pregnant women. Ultrasound is also able to

distinguish between muscle and blood and therefore show blood movement

WHAT IS ULTRASOUND?

• Ultrasound or ultrasonography is a medical imaging technique that uses high frequency sound waves and their echoes.

• Known as a ‘pulse echo technique’

• The technique is similar to the echolocation used by bats, whales and dolphins.

• Ultrasound cannot penetrate air or bone

• Ultrasound is sound with frequencies higher than about 20 kHz

• Propagation of ultrasound waves are defined by the theory of acoustics

• Ultrasound moves in a wavelike by expansion and compression of the medium through

which it travels

• Ultrasound waves travel at different speeds depending on material

• Ultrasound waves can be absorbed, refracted, focused, reflected, and scattered.

•

• in physics

• Characterized by sound waves of high frequency Higher than the range of human hearing

• Sound waves are measured in Hertz (Hz ),Diagnostic U/S = 1-20 MHz, Nondiagnostic medical

applications <1MHz

• Sound waves are produced by a transducer

Sound waves consist of mechanical vibrations containing condensations

(compressions) & rarefactions (decompressions)that are transmitted

through a medium.

Sound is mechanical. Sound is not electromagnetic.

Matter must be present for sound to travel

– Ultrasound as all sound waves are caused by vibrations and therefore cause no

ionisation and are safe to use on pregnant women. Ultrasound is also able to

distinguish between muscle and blood and therefore show blood movement.

When an ultrasound wave meets a boundary between two different materials some

of it is refracted and some is reflected. The reflected wave is detected by the

ultrasound scanner and forms the image.

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

• Sound requires a medium through which to travel

• 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

Ultrasound Physics

Ultrasound Test

Ultrasound is a test that uses reflected sound waves to produce an image of organs

and other structures in the body. It does not use X-rays or other types of possibly

harmful radiation.

For ultrasound testing, gel or oil is applied to the skin to help transmit the sound

waves. A small, handheld instrument called a transducer is passed back and forth

over the area of the body that is being examined.

The transducer sends out high-pitched sound waves (above the range of human

hearing) that are reflected back to the transducer. A computer analyzes the

reflected sound waves and converts them into a picture that is displayed on a TV

screen. The picture produced by ultrasound is called a sonogram, echogram, or

ultrasound scan. Pictures or videos of the ultrasound images may be made for a

permanent record.

Ultrasound is most useful for looking at organs and structures that are either

uniform and solid (such as the liver) or fluid-filled (such as the gallbladder).

Mineralized structures (such as bones) or air-filled organs (such as the lungs) do

not show up well on a sonogram

• Process Overview

– Transducer (electrical signal acoustic signal) generates pulses of ultrasound

and sends them into patient

– Organ boundaries and complex tissues produces echoes (reflection or

scattering) which are detected by the transducer

– Echoes displayed on a grayscale anatomical image(in heart)

• Each point in the image corresponds to an anatomical location of an echo

• generating structure

• Brightness corresponds to echo strength

Obstetrics and Gynecology

The development and monitoring of a developing foetus

Uses of Ultrasound

Cardiology

Seeing the inside of the heart to identify abnormal structures or functions and measuring blood flow through the heart and major blood vessels

Urology •measuring blood flow through the kidney •seeing kidney stones - detecting prostate cancer early

Acoustic 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 waves back at the probe

• Impedance mismatch = ΔZ

• 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

• 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

• Minimize refraction by scanning perpendicular to the interface that is causing the refraction

• Reflection: The production of echoes at reflecting interfaces between

tissues of differing physical properties.

• Specular - large smooth surfaces

• Diffuse – small interfaces or nooks and crannies

• Specular Reflection: Large smooth interfaces (e.g. diaphragm, bladder wall)

reflect sound like a mirror- Only the echoes returning to the machine are

displayed

• Specular reflectors will return echoes to the machine only if the sound beam is

perpendicular to the interface

Diffuse Reflector • Most echoes that are imaged arise from small interfaces within solid organs

• These interfaces may be smaller than the wavelength of the sound

• The echoes produced scatter in all directions

• These echoes form the characteristic pattern of solid organs and other tissues

Specular Diffuse

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.

1. The ultrasound machine transmits high-frequency (1 to 12 megahertz) sound pulses into the body using a probe.

2. The sound waves travel into the body and hit a boundary between tissues (e.g. between fluid and soft tissue, soft tissue and bone).

3. Some of the sound waves reflect back to the probe, while some travel on further until they reach another boundary and then reflect back to the probe .

4. The reflected waves are detected by the probe and relayed to the machine.

In ultrasound, the following events happen:

Transducer ( probe)

– Piezoelectric crystal

• Emit sound after electric charge applied

• Sound reflected from patient

• Returning echo is converted to electric signal grayscale image on monitor

• Echo may be reflected, transmitted or refracted

• Transmit 1% and receive 99% of the time

5- The machine calculates the distance from the probe to the tissue or

organ (boundaries) using the speed of sound in tissue (1540 m/s)

and the time of the each echo's return (usually on the order of

millionths of a second).

6.The machine displays the distances and intensities of the echoes on the screen, forming a two dimensional image. All the energy comes from the transducer

-All we “see” are reflections and scatter

The Piezoelectric Effect

Piezoelectricity : is the electric charge that accumulates in certain solid

materials (such as crystals, and biological matter such as bone, DNA and

various proteins in response to applied mechanical stress. The word

piezoelectricity means electricity resulting from pressure.

piezoelectric means pressure electricity

The piezoelectric effect is understood as the linear electromechanical interaction

between the mechanical and the electrical state in crystalline materials with

no inversion symmetry. The piezoelectric effect is a reversible process in that

materials exhibiting the direct piezoelectric effect (the internal generation of electrical

charge resulting from an applied mechanical force) also exhibit the reverse

piezoelectric effect (the internal generation of a mechanical strain resulting from an

applied electrical field

The conversion of sound to electrical energy is called the piezoelectric effect

Discovered by Pierre and Jacques

Curie in 1880.

Ultrasound waves are produced using the piezoelectric effect.

When a potential difference is applied across certain crystals (piezoelectric) the

crystals themselves deform and contract a little. If the p.d. applied is alternating then

the crystal vibrates at the same frequency and sends out ultrasonic waves. For

ultrasound - lead zirconate titanate (PZT) crystals are used. This process also works

in reverse. The piezoelectric crystal acts a receiver of ultrasound by converting

sound waves to alternating voltages and as a transmitter by converting alternating

voltages to sound waves

A piezoelectric disk

generates a voltage

when deformed

Producing an image • Probe emits a sound wave pulse-measures the time from emission to

return of the echo

• Wave travels by displacing matter, expanding and compressing

adjacent tissues

• It generates an ultrasonic wave that is propagated, impeded, reflected,

refracted, or attenuated by the tissues it encounters

Important concepts in production of an U/S 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 density or stiffness

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

c=fλ

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

• Propagation velocity

Increased by increasing stiffnes

Reduced by increasing density

• Bone: 4,080 m/sec- Air: 330

m/sec--Soft Tissue Average:

1,540 m/sec

• Attenuation of ultrasound : • The loss of ultrasound energy as it travels through a medium (such as tissue) is

called attenuation. The loss of ultrasound energy is expressed as change in

ultrasound intensity.

• The units of ultrasound intensity are watts per centimeter squared.

• Decibels are the units for describing the difference between ultrasound

intensities.

• Decibels are used because they are small numbers (called logarithms) that can

describe large changes in intensity.

• For example, when the intensity of sound becomes one thousand times softer, the

attenuation is minus 30 decibels.

• When the intensity of sound decreases to one half of the original value, the

attenuation is minus 3 decibels.

•

• The distance that ultrasound travels in order for the intensity to

decrease to half the original value is called: half intensity depth. In

decibels, half intensity depth is the distance ultrasound travels to

achieve a three decibel loss.

• The rate of attenuation of ultrasound in soft tissue is one half the

frequency per centimeter. The rate of attenuation is called

the attenuation coefficient. For example, the attenuation coefficient of

a 12 MHz transducer in soft tissue is 6 dB per centimeter.

• The half intensity depth using this particular 12 MHz transducer is the

distance the ultrasound travels to achieve a three decibel loss. Since we

know that there is a 6 decibel loss after the ultrasound travels a

centimeter. The half intensity depth is 0.5 centimeter.

Acoustic Impedance

Acoustic impedance (Z) of a material is given by:

impedance (Rayl) = speed of sound (m/s) • density of material (kg/m3)

in material

• the acoustic impedance unit is called the Rayl (kg/m2/s)

• acoustic impedance can be considered to be a measure of a material’s ability to

transmit acoustic energy (air and lung media have low values, and bone and metal

have high values)

• acoustic impedance is determined by the density and stiffness of a medium

• since the speed of sound is independent of frequency in the diagnostic ultrasound

range, acoustic impedance is also independent of frequency

• acoustic impedance determines the amount of energy reflected at an interface

• since the speed of sound in tissue is relatively constant in the diagnostic ultrasound

range, then the acoustic impedance of most tissues is also a constant, they typically

have values around 1.6 x 106 kg/m2/s (Rayls)

impedance density

impedance speed of sound

Echo Echo is something you experience all the time. If you shout into a well, the echo comes back a moment later. The echo occurs because some of the sound waves in your shout reflect off a surface (either the water at the bottom of the well or the wall on the far side) and travel back to your ears. A similar principle applies in cardiac ultrasound.

Echocardiography (echo or echocardiogram) is a type of ultrasound test that uses high-pitched sound waves to produce an image of the heart. The sound waves are sent through a device called a transducer and are reflected off the various structures of the heart. These echoes are converted into pictures of the heart that can be seen on a video monitor. There is no special preparation for the test.

Generation Of An Ultrasound Image

Ultrasound gel is applied to the transducer to allow transmission of the sound waves from the transducer to the skin

The transducer transforms the echo (mechanical energy) into an electrical signal which is processed and displayed as an image on the screen.

There are 5 basic components of an ultrasound scanner that

are required for generation, display and storage of an ultrasound image.

1. Pulse generator - applies high amplitude voltage to energize the crystals

2. Transducer - converts electrical energy to mechanical

(ultrasound) energy and vice versa

3. Receiver - detects and amplifies weak signals

4. Display - displays ultrasound signals in a variety of modes

5. Memory - stores video display

Machines

The transducer probe is the main part of the ultrasound machine. The transducer probe transmits and receives the ultrasound. The curved faceplate

shapes the ultrasound waves into a narrow beam.

Transducer probes come in many shapes and sizes. The shape of the probe determines its field of view, and the frequency of emitted sound waves

(controlled by the tuning device) determines how deep the sound waves penetrate and the resolution of the image. The ultrasound is pulsed. There must be a pause

to allow the reflected wave to be detected.

Ultrasound Transducers

Ultrasound Pulse Production and Reception

A transducer is a device that can convert one form of energy into another. Ultrasound

transducers are used to convert an electrical signal into ultrasonic energy that can be

transmitted into tissue, and to convert ultrasonic energy reflected back from the tissue

into an electrical signal.

The general composition of an ultrasound transducer is shown below:

• the most important component is a thin

piezoelectric (crystal) element located near the

face of the transducer

• the front and back face of the element is coated

with a thin conducting film to ensure good

contact with the two electrodes

• the outside electrode is grounded to protect the

patient from electrical shock

• an insulated cover is used to make the device

watertight

• an acoustic insulator made of rubber is

used to prevent the passing of sound into the

housing (i.e.: reduces transducer vibrations)

• the inside electrode is against a thick backing block that absorbs sound waves

transmitted back into the transducer

Piezoelectric Crystal

Certain material (or crystals) are such that the application of an electrical field causes

a change in their physical dimensions. The reverse effect, where an external pressure

causes a change in the crystal’s physical dimensions and thus induces a voltage

between electrodes, is called the piezoelectric effect. Piezoelectric means pressure

electricity.

• some naturally occurring materials posses piezoelectric properties (eg: quartz) but

most crystals used in diagnostic ultrasound are man-made ceramics like lead

zirconate titanate (PZT)

• the advantage is using ceramics is that they can be formed into different shapes

• piezoelectric crystals can be designed to vibrate in either the thickness or radial mode,

but in medical imaging it is the thickness mode that is used

•transducer crystals do not conduct electricity

Matching Layer A matching layer of material is placed on the front surface of the transducer to improve

the efficiency of energy transmission into the patient. The material used has an

impedance in between that of the transducer and tissue; and it has a thickness one forth

the wavelength of sound in the transducer crystal material (quarter wave matching).

Creating a sound wave from an electrical pulse

When a positive voltage (A) is applied across the surface of the crystal, it creates an

electric field across the crystal surface which cause the molecules (dipoles) in the crystal

to realign and thus changing the shape (width) of the crystal.

When the voltage polarity is changed from positive to negative, there is a point in time

when the electric field across the crystal is zero (at voltage equal to zero) and the crystal

relaxes (B). When the voltage polarity is reversed (i.e.: negative) the crystal realigns

once again and changes its width once again (C).

A B C

Positive

Negative

Voltage Pulse

Time

When an electric current is applied to these crystals, they change shape rapidly.

The rapid shape changes, or vibrations, of the crystals produce sound waves that

travel outward. Conversely, when sound or pressure waves hit the crystals, they

emit electrical currents. Therefore, the same crystals can be used to send and

receive sound waves. The probe also has a sound absorbing substance to

eliminate back reflections from the probe itself, and an acoustic lens to help

focus the emitted sound waves

CPU (Central Processing Unit )

is the brain of the ultrasound machine. The CPU is basically a computer that

contains the microprocessor memory, amplifiers and power supplies for the

microprocessor and transducer probe. The CPU sends electrical currents to the

transducer probe to emit sound waves, and also receives the electrical pulses from

the probes that were created from the returning echoes. The CPU does all of the

calculations involved in processing the data. Once the raw data are processed, the

CPU forms the image on the monitor. The CPU can also store the processed data

and/or image on disk.

The net effect the alternating voltage pulse has on the crystal is to make it oscillate back

and forth about its width. This change in shape of the crystal increases and decreases

the pressure in front of the transducer, thus producing ultrasound waves.

Ultrasound wave direction

Ultrasound wave direction

Compression region created when crystal

surface is expanding (more pressure on surface)

Rarefaction region created when crystal

surface is contracting (less pressure on surface)

wavefront diagram

The transducer pulse controls allow the operator, called the ultrasonographer, to

set and change the frequency and duration of the ultrasound pulses, as well as

the scan mode of the machine. The commands from the operator are translated

into changing electric currents that are applied to the piezoelectric crystals in

the transducer probe.­

Creating an electrical signal from a sound wave

When the compression region (A) of the ultrasound wave is incident on the front surface

of the crystal, it induces a high pressure region on the surface which in turn compresses

the crystal. This cause the molecules in the crystal to re-align and induce an electric field

across the crystal which generates an electrical voltage signal that is proportional to the

intensity of the compression region.

A B

When the rarefaction region (B) of the ultrasound wave is incident on the front

surface of the crystal, it induces a low pressure region on the surface which in turn

relaxes the crystal.

Compression region compresses crystal

surface (more pressure on surface)

Rarefaction region relaxes crystal surface

(less pressure on surface)

wavefront diagram

The net effect the ultrasonic wave has on the crystal is to make it oscillate back

and forth about its width. This change in shape of the crystal induces a voltage signal

that also varies in time and in amplitude.

NOTE

A transducer can function both as a transmitter and a receiver of ultrasound energy, but

it can not transmit and receive at the same time.

Transmitter Mode Receiver Mode

Transducer Characteristics

Transducer Thickness

A transducer can be made to emit sound of any frequency by driving it (in continuous

mode) with an alternating voltage of that frequency. However, a transducer vibrates

most violently and produces the largest output (pressure amplitude) of sound when

= 2 • t

where the is wavelength of sound and t is the thickness of the piezoelectric crystal.

The frequency of the emitted sound waves is then given by

frequency = v = v

2 • t

where v is the speed of sound in the piezoelectric crystal.

operating frequency crystal thickness

Why should the transducer thickness be equal to 1/2 of the desired wavelength?

When the piezoelectric element is driven by a alternating voltage the crystal

vibrates (i.e.: the width of the crystal moves back and forth). The front face of the

crystal emits sound both in the forward and backward directions as does the back

surface.

Front surface Back surface

Thickness (t)

A B C D

• wave front (A) will get absorbed by the transducer’s backing material

• wave front (D) will enter into the patient

• the wave front (C) is reflected at the back face of the disk, and by the time it joins

wave front (D), it has traveled an extra distance 2t. If this distance equals a

wavelength the wave fronts (D) and (C) reinforce for they are in phase, and

constructive interference or resonance occurs.

• if wave fronts (D) and (C) are not in phase, then there will be some destructive

interference

• same reasoning applies to wave front (B)

Patient Backing

Block

Constructive Interference

(waves A & B add to form a

new wave of amplitude A + B)

Destructive Interference

(waves A & B add to form a new

wave of amplitude A + B = 0)

If wave B is wave front (C) and

wave F is wave front (D) then we

see that when transducer

thickness is one half the

wavelength, both wave fronts are

in phase and constructive

interference (ie: their individual

amplitudes add) occurs.

Ultrasound Transducer

Transducer

• A transducer only generates a useful ultrasound beam at one given frequency

• This frequency corresponds to a wavelength in the transducer equal to twice the

thickness of the piezoelectric disk – This is due to a process known as Resonance!

• Choice of frequency is important – remember that attenuation increases with

increasing frequency

• Image resolution increases with frequency

• Therefore, there is a trade-off between scan depth and resolution for any particular

application

Ultrasound Transducer(Beam Shape – Diffraction

•In the near field region the beam energy is largely confined to the dimensions of the transducer

• Need to select a long near field length to achieve good resolution over the depth you wish to

scan too. Near field length increases with increasing transducer radius, a, and decreasing

wavelength, . Short wavelength means high frequency – not very penetrating

• Large transducer radius – Wide beam (poor lateral resolution)

NEAR FIELD FAR FIELD

NFL

a Near Field Length, NFL = a2 /

a = radius of transducer

= Wavelength

Beam Focusing. An improvement to the overall beam width can be obtained by

focusing . Here the source is designed so that the waves converge towards a point in

the beam, the focus, where the beam achieves its minimum width

the beam diverges again but more rapidly that for an unfocused beam with the same

aperture and frequency

Ultrasound Transducer

Beam Focusing

W a

F

Beam width at focus, W = F / a

At focal point:

• Maximum ultrasound intensity

• Maximum resolution

Beam Focusing

For a single element source, focusing can

be achieved in one of two ways:

1) A curved source

A curved source is manufactured with

a radius of curvature of F and hence

produces curved wave fronts which

converge at a focus F cm from the

source F

Source Focus

Ultrasound Transducer

Hull and East Yorkshire Hospitals

NHS Trust

Beam Focusing

For a single element source, focusing can be achieved in one of two ways:

2) An acoustic lens

An acoustic lens is attached to the face of a flat source and produces curved

wave fronts by refraction at its outer surface (like an optical lens). A convex lens

is made from a material with the lower speed of sound than tissue.

Source Focus

Lens

Hull and East Yorkshire Hospitals

Multiple Zone Focussing

• Fire transducer several times with different

focus to compile better image

•more focus points decreases frame rate

Transducer Pulse Controls

They offer the potential for a doctor who is a specialist technical device or by

entering a value and time-frequency sound pulses issued by the probe, which must

be defined in advance by Member to be filmed. As well as the control of this unit

scanning mechanism used by the device to show the picture

Disk Storage: The processed data and/or images can be stored. Storage can

include hard disks, compact disks (CDs), digital video disks (DVDs), or a

network drive. Most of the time, ultrasound machines store data with the patient's

medical records.

Printers

Most ultrasound machines have thermal printers connected to them. Ultrasound

images are in motion, but a still can be captured at any point in time to send the image

to the printer.

How an ultrasound is done?

In an ultrasound scan, a real-time scanner forms a continuous range

of images of the subject on a screen. A transducer is used for

releasing these waves. The recurring beams of the ultrasound scan

the subject and go back to the transducer. The data obtained from

the different reflections recomposes in the form of a picture on

display screen.

Ultrasound imaging is a complex medical procedure that requires

prior training due to the possible health risks. The high frequency

waves generated during the process are potentially damaging to

body tissue and nerves if exposure is too lengthy.

Is Ultrasound Safe?

Yes. Although occupational exposure to ultrasound in excess of 120 dB (loudness) may lead

to hearing loss, and exposure in excess of 155 dB may produce heating effects that are

harmful to the human body, and it has been calculated that exposures above 180 dB may lead

to death, the loudness of medical ultra sound waves is much quieter albeit a higher pitch

(above 20,000 hertz).

ultrasound poses no known risks to the patient. ultrasonic energy has two potential

physiological effects: it enhances inflammatory response; and it can heat soft tissue.

Ultrasound energy produces a mechanical pressure wave through soft tissue. This pressure

wave may cause microscopic bubbles in living tissues and distortion of the cell membrane,

influencing ion fluxes and intracellular activity.

When ultrasound enters the body, it causes molecular friction and heats the tissues slightly.

This effect is typically very minor as normal tissue perfusion dissipates most of the heat, but

with high intensity, it can also cause small pockets of gas in body fluids or tissues to expand

and contract/collapse in a phenomenon called cavitation.

Ultrasound produces heating, pressure changes and mechanical disturbances in tissue.

Diagnostic levels of ultrasound can produce temperature rises that are hazardous to sensitive

organs

• Acoustic impedance (AI) is dependent on the density of the material in which

sound is propagated

- the greater the impedance the denser the material.

• Reflections comes from the interface of different AI’s

• greater of the AI = more signal reflected

• works both ways (send and receive directions)

Medium 1

Medium 2

Medium 3

Tra

ns

du

ce

r

Interactions of Ultrasound with

Tissue

• Strong Reflections = White dot Diaphragm, tendons, bone

Reflected Echo’s

Weaker Reflections = Grey dots Most solid organs, thick fluid

• No Reflections = Black dots Fluid within a cyst, urine, blood

• What determines how far ultrasound waves can travel?

• The FREQUENCY of the transducer

– The HIGHER the frequency, the LESS it can penetrate

– The LOWER the frequency, the DEEPER it can penetrate

– Attenuation is directly related to frequency

Basic Principles of Image Formation

Hull and East Yorkshire Hospitals

NHS Trust

Pulse-Echo in Tissue

• Ultrasound pulse is launched into the first tissue

• At tissue interface a portion of ultrasound signal is transmitted into the

second tissue and a portion is reflected within the first tissue (termed an echo)

• Echo signal is detected by the transducer

Transducer

Can transmit

and receive US

Tissue 1 Tissue 2 Tissue 3

Basic Principles of Image Formation

B-Mode Image

• A B-mode image is a cross-sectional image representing tissues and organ boundaries

within the body

• Constructed from echoes which are generated by reflection of US waves at tissue

boundaries, and scattering from small irregularities within tissues

• Each echo is displayed at a point in the image which corresponds to the relative

position of its origin within the body

• The brightness of the image at each point is related to the strength (amplitude) of the

echo

• B-mode = Brightness mode

Hull and East Yorkshire Hospitals

NHS Trust

Doppler effect: change in wavelength with speed

• Ultrasound, like normal sound, is a wave.

• If a source of sound moves towards the listener, the waves begin to catch up

with each other. The wavelength gets shorter and so the frequency gets higher

– the sound has a higher pitch.

• We use this principle to work out how fast blood cells move. Ultrasound

reflects off the blood cells and causes a Doppler shift

• The ultrasound probe emits an

ultrasound wave

• A stationary blood cell reflects the

incoming wave with the same

wavelength: there is no Doppler shift

• The ultrasound probe emits an

ultrasound wave

• A blood cell moving away from the

probe reflects the incoming wave

with a longer wavelength

• In reality, there is actually two

Doppler shifts. The first one occurs

between the probe and the moving

blood cell (not shown here) and

the second one occurs as the red

blood cell reflects the ultrasound.

• Now, the blood cell moves

towards the probe. It reflects

the incoming wave with a

shorter wavelength

Doppler imaging: combine imaging

and Doppler

Use BOTH normal ultrasound imaging

and Doppler imaging Used to image

blood flow

•The ultrasound probe

emits an ultrasound wave

•A stationary blood cell

reflects the incoming wave

with the same wavelength:

there is no Doppler shift

•The ultrasound probe

emits an ultrasound wave

•A blood cell moving away

from the probe reflects the

incoming wave with a

longer wavelength

•In reality, there is actually

two Doppler shifts. The

first one occurs between

the probe and the moving

blood cell (not shown

here) and the second one

occurs as the red blood

cell reflects the

ultrasound.

•Now, the blood cell moves

towards the probe. It

reflects the incoming wave

with a shorter wavelength