The physics of

ultrasound

Dr Graeme Taylor

Guy’s & St Thomas’ NHS Trust

Physics & Instrumentation

Modern ultrasound

equipment is

continually evolving

This talk will cover

the basics

What will be covered?

• The basic properties of ultrasound

• Generating and detecting it

• The common imaging modalities• A , B & M mode

• Spectral Doppler

• Colour flow & Power Doppler

• Safety issues

What is ultrasound?

• Just a normal ‘sound’ wave but at a higher

frequency than we can hear (>1 MHz)

• It is a pressure wave which travels through

a medium

• In medical imaging, longtitudinal waves are

commonly used

What are its major properties?

• It travels through a medium with a

characteristic wave velocity

• It reflects off interfaces between media of

different acoustic impedance

• It refracts (bends) when passing obliquely

between media of different sound velocity

• It scatters in media containing small objects

• It is absorbed by most media

Sound velocities

• Air – about 300 m/s

• Water – about 1450 m/s

• Soft tissue – about 1560 m/s

• Fat, muscle, blood all have

slightly different velocities

• Bone – about 3000 m/s

It takes about 6 μsec for sound to travel

through 1 cm of soft tissue

Sound reflection

• When sound travels across a junction

between two media, some bounces

back and some travels onward

• This depends on the product of the

density ρ and sound velocity c of each

medium ( its impedance = ρc)

• As the impedances differ more, so more

is reflected and less transmitted.

No sound travels across air-tissue boundary

Sound refraction

• Unlike X rays – sound does not always

travel in a straight line – though imagers

assume that is what’s happening

• When passing at an oblique angle

between media with different velocity, the

sound can change direction (like light)

• This can distort the ultrasound image

Important when using it to guide interventions!

Scattering

• Scatter (sound bouncing off in all

directions) occurs when a medium

contains lots of small (<1 wavelength)

items of a different impedance.

• This is what gives tissues their

‘greyness’ on an ultrasound image

(Wavelength at 1 MHz = 1.5mm)

Absorption

• This is where energy is lost to the tissues

• It means deeper structures return weaker

echoes

• Soft tissue absorption is approximately

proportional to frequency

• Bone has high absorption, nearly

proportional to the square of frequency

To image deep structures – use lower frequency

Absorption – the consequences

• All imagers need to compensate for

absorption with depth

• Imaging close to the surface can use

higher frequencies (up to 10 MHz)

• Large folk need to be imaged at lower

frequency

• Imaging through bone (skull) is only

practicable at 2 MHz or less

Resolution – Depth compromise

Imagers – some available modes

• B Scan

• Colour flow mapping

• Spectral Doppler

Imagers – not forgetting M mode

• B Scan

• M mode

Used to visualise the movement of moving reflecting objects

• The fundamental principle is that of

Directional pulse-echo location

• The distance to a sound reflector is

proportional to the delay in the echo

Depth α delay time = distance / sound velocity

The basis of imaging - echo ranging

Instrumentation – A Scan

To make a successful A (amplitude) Scan tracing one needs:

– a ‘short’ ultrasound pulse

– a narrow ultrasound beam

A scanEcho

Amplitude Time

Instrumentation – B Scan

To make a successful B (brightness) Scan

image one also needs:

a way to steer/move beam to cover a two

dimensional area

All scanners assume some fixed sound velocity – is it

correct for your patient

What frequency to use?

Shorter sound pulses have better resolution

but higher frequency content

So the shortest pulse is limited by the

absorption of sound in tissue

To obtain echoes from deep structures one

needs to use low frequency pulses

1 - 4 cm deep use 7 -12 MHz

10 - 20 cm deep use 2 - 5 MHz

(Guide – depth resolution ~ 2 wavelengths)

How big a transducer ?

The beam width is defined by the shape of

the active transducer

A small transducer has a narrow beam

close up, but widens out & weakens

quickly

A trade off between resolution and penetration

Instrumentation – Swept gain

‘Swept gain’ or ‘depth gain compensation’

must be used to compensate for quieter

distant echoes

Echo amplitude

Depth

This is inbuilt in all scanners

but assumes some ‘average’

attenuation which may be wrong

Gain increased

with time after

transmission

Practical Transducers

The all electronic transducer is the basis of

the modern ultrasound B scanner

No moving parts

Array of small elements

Wide frequency range

Electronically controlled

Instrumentation - transducers

The all electronic transducer is a large

array of small elements

Each is individually

wired to the scanner

Transducers

Each element is a simple piezo-electric

transducer

electrical → sound → electrical

They are not individually ‘directional’

They must work in ‘unison’

Instrumentation - transducers

The transducer is made directional by a

‘beam former’ – an electronic process which

synchronously controls the action of many of

the independent elements simultaneously

To make a simple forward looking

directional beam - just use plenty

of elements together

Instrumentation – linear arrayTo model a rectilinear scanner –

just repeat pulse-echo cycle with

different sets of active elements Linear

array

Instrumentation – phased array

To model a sector transducer, one only

needs to add short time delays between

firing elements and also on receiving ehoes.

This effectively ‘bends

the beam’

Repeating the pulse-echo sequence with

differing delays gives a sector scan

sector

Focal zones

• It is possible to narrow the beam width at a

certain depth for better resolution

• This is done by adding symmetrical time

delays to inner elements in the group

selected when transmitting and receiving

• Greater delay difference = closer focus

Multiple focal zones

• Can optimise beam width of pulse for a given

depth range (the focal zone)

• To improve resolution at various depths,

make separate images, each with different

focal zone placement, then cut-&-paste

This reduces the frame rate

Real-time compounding

• If one can CHANGE DIRECTION of the

transducers’ line-of-sight, one can interrogate

tissue from different directions & combine to

give better image

Removes ‘speckle’

improves echoes

from non-aligned

specular reflectors

Image showing advantage of compounding

Conventional

image of breast

(Speckle &

shadow)

With

compounding

‘SonoCT’

transducer bandwidth

A transducer’s ‘bandwidth’ refers

to the range of frequencies

which it can transmit & receive

Older transducers typically had

limited bandwidths

This limited their use to a fixed

depth range and resolution

F

wide bandwidth

‘Wideband’ transducers have been

developed, along with scanners

which control the range of

frequencies transmitted and

received at any one time

Vary receive bandwidth with depth of echo

One transducer - various centre frequencies

Harmonic imaging - Send at low frequency,

receive at 1st harmonic

Modern image possibilities

Spectral Doppler

•Basic B Scan techniques do not work well with blood, it appears dark and it travels too fast

•Movement sensitive techniques are needed

•Spectral Doppler

•Colour Flow Mapping

•Power Doppler

Christian Andreas Doppler

Spectral Doppler

• Red cells scatter back a small proportion

of incident ultrasound

• MOVING cells will reflect

ultrasound with a DOPPLER

SHIFT in FREQUENCY

• This small shift is proportional

to the red cell velocity

θ

Spectral Doppler

If the transmit frequency is in the low

MegaHertz range, then the Doppler-shift

frequency for blood flow is in the audio

range

Basic B scan equipment can be adapted,

and it is helped if the transducer has a

wide bandwidth and the beam can be

steered to get a good ‘Doppler angle’

Usually ‘Pulsed Doppler’ is provided

Duplex - Spectral Doppler

Spectral Doppler techniques use similar pulses, beam shaping & steering methods to Bscan

The basic difference is that the pulse-echo process is repeated over and over, but for just one line-of-site.

Duplex Spectral Doppler

These two techniques are combined with a method for displaying the sound spectrum - a sonogram

Note – it is common to show the vertical axis in cm/sec – though it really is only known in kHz !

Duplex - Colour flow

The data is

colour coded

according to

frequency

(velocity) and

superimposed

on the B scan

image

Be aware of direction of flow detection and aliasing

Duplex - Power Doppler

Velocity and direction can sometimes confuse, and

one can choose ‘Power Doppler’ mode

This displays the

areas where flow is

detected but with the

brightness proportional

to the POWER of the

movement detected.

Good for slow flow and when vessels hard to see on B scan

Some safety issues

•Thermal & mechanical effects

‘Significant deleterious bio-effects on either

patients or operators of diagnostic ultrasound

procedures have not been reported in

literature’

• ALARP (as low as reasonably practicable)

• MI – mechanical index (likelihood of cavitation)

• TI – thermal index (1.0 = 1ºC temperature rise)

Some safety

issues

Use BMUS

‘GUIDELINES FOR THE SAFE USE OF

DIAGNOSTIC ULTRASOUND EQUIPMENT’

Eg: avoid

- an embryo less than eight weeks after conception;

- the head, brain or spine of any foetus or neonate;

- an eye (in a subject of any age).

Imaging summary

B SCAN

pulse echo, short pulses,

displays anatomy

M MODE

displays mechanical motion

SPECTRAL DOPPLER

displays flow signal at one

point

COLOUR FLOW

displays flow over image

area