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TRANSCRIPT
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