Clinical Doppler Ultrasound

359 Pages·2006·85.85 MB·English

by  Paul L. Allan

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Clinical Doppler
Ultrasound
SECOND EDITION
Paul Allan
BSc, DMRD, FRCR, FRCPE
Consultant Radiologist and Clinical Director of Radiology
Department of Medical Radiology
Royal Infirmary of Edinburgh
Edinburgh, UK
Paul A. Dubbins
BSc, MBBS, FRCR
Consultant Radiologist
Department of Radiology
Derriford Hospital
Plymouth, UK
W. Norman McDicken
PhD, FIPEM
Professor of Medical Physics and Medical Engineering
Medical Physics
The University of Edinburgh
Edinburgh, UK
Myron A. Pozniak,
MD
Professor of Medical Imaging
Department of Radiology
University of Wisconsin
Madison, WI, USA
iii
An imprint of Elsevier Limited
© 2006,Elsevier Limited.All rights reserved.
First published 2006
First edition 2000
The right of Paul Allan,Paul A Dubbins,W Norman McDicken and Myron Pozniak to be identified
as authors of this work has been asserted by them in accordance with the Copyright,Designs and
Patents Act 1988.
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ISBN-13:978-0-443-10116-8
ISBN-10:0-443-10116-7
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Notice
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iv
List of contributors
Paul Allan BSc,DMRD,FRCR,FRCPE Fred T.Lee,Jr,MD
Consultant Radiologist and Clinical Director of Robert A. Turrell Professor of Medical Imaging
Radiology Department of Radiology
Department of Medical Radiology University of Wisconsin
Royal Infirmary of Edinburgh Madison, WI
Edinburgh, UK USA
Jonathan D.Berry BSc,MBBS,FRCR W.Norman McDicken PhD,FIPEM
Specialist Registrar Professorof Medical Physics and Medical Engineering
Department of Radiology Medical Physics
King’s College Hospital The University of Edinburgh
Denmark Hill Edinburgh, UK
London, UK
Imogen Montague MB ChB,MRCOG
Paul A.Dubbins BSc,MBBS,FRCR Consultant Obstetrician and Gynaecologist
Consultant Radiologist Department of Obstetrics and Gynaecology
Department of Radiology Derriford Hospital
Derriford Hospital Plymouth, UK
Plymouth, UK
Myron A.Pozniak,MD
Karen Gallagher AVS,BN Professor of Medical Imaging
Senior Clinical Vascular Scientist Department of Radiology
Vascular Laboratory University of Wisconsin
Royal Infirmary of Edinburgh Madison, WI
Edinburgh, UK USA
Peter R.Hoskins BA,MSc,PhD,FIPEM Paul S.Sidhu BSc,MBBS,MRCP,FRCP,DTM&H
Consultant Medical Physicist Consultant Radiologist and Senior Lecturer
Medical Physics Department of Radiology
The University of Edinburgh King’s College Hospital
Edinburgh, UK Denmark Hill
London, UK
viii
Preface
The first edition of Clinical Doppler Ultrasound brought up to date and one or two small errors
was designed as a practical introduction to the from the first edition have been corrected.
principles and practice of Doppler ultrasound Although other vascular imaging techniques,
in the clinical setting.Since then there have been such as computed tomography angiography
a number of advances which have required a and magnetic resonance angiography have seen
revision of the original text.These include signif- dramatic evolution during the last few years,
icant technical advances in ultrasound equip- the authors remain convinced of the primary
ment; the evolution of echo-enhancing agents role of Doppler ultrasound in the investigation
and their wider availability;and new applications of vascular disorders and this role has developed
of Doppler ultrasound have been developed. and expanded with time,rather than contracted.
In this second edition we have remained true It is important that those performing Doppler
to our initial aim to provide practical information ultrasound examinations have a clear under-
and advice on the practice and role of Doppler standing of the underlying principles of this
ultrasound. The contributors to this book all powerful tool and we have tried to encourage
have significant clinical experience in diagnostic such understanding with this book.
ultrasound and all the chapters have been re-
written in the light of more recent knowledge Paul L.Allan
and developments; a new chapter on echo- Paul A.Dubbins
enhancing agents has been included.The images W.Norman McDicken
have been updated and expanded,the references Myron Pozniak 2006
vii
1
Physics: principles, practice
and artefacts
W. Norman McDicken and Peter R. Hoskins
A number of techniques have been developed a moving structure within the body,it experiences
which exploit the shift in frequency of ultra- a Doppler shift in its frequency and returns to
sound when it is reflected from moving blood. the receiving (detecting) crystal.Reflected ultra-
This frequency shift is known as the ‘Doppler sound is also detected from static surfaces within
effect’.1Five types of diagnostic Doppler instru- the body but it has not suffered a Doppler shift
ment are usually distinguished: in frequency. After the reflected ultrasound is
1. Continuous wave (CW) Doppler received, the Doppler instrument separates the
2. Pulsed wave (PW) Doppler signals from static and moving structures by
3. Duplex Doppler exploiting their different frequency.
4. Colour Doppler imaging (CDI;colour velocity Motion of the reflector towards the transducer
imaging) produces an increase in the reflected ultrasonic
5. Power Doppler imaging frequency, whereas motion away gives a reduc-
tion. The system electronics note whether the
The characteristics of an ultrasound beam, detected ultrasound has a higher or lower
the propagation of ultrasound in tissue and the frequency than that transmitted, and hence
design of transducers as found in B-mode imaging extracts information on the direction of motion
are all relevant for Doppler techniques.2–6 relative to the transducer.
When the line of movement of the reflector is
at an angle θ to the transducer beam, then the
THE DOPPLER EFFECT AND ITS
Doppler shift,f ,is given by:
APPLICATION D
f = f – f = f .2.u.cosq
For all waves such as sound or light the Doppler D t r t
c
effect is a change in the observed frequency of
the wave because of motion of the source or where f is the transmitted frequency, f is the
t r
observer. This is due either to the source received frequency,c is the speed of ultrasound
stretching or compressing the wave or the and ucosq (i.e. u × cosine q) is merely the
observer meeting the wave more quickly or component of the velocity of the reflecting agent
slowly as a result of their motion. In basic along the ultrasonic beam direction (Appendix
medical usage of the Doppler effect,the source A1).For a typical case of blood flow in a super-
and observer (receiver) are a transmitting and a ficial vessel:
receiving crystal usually positioned next to each Transmitted frequency,f = 5 MHz = 5 ×106Hz
t
other in a hand-held transducer (Fig. 1.1a). A Velocity of sound in soft tissue,c= 1540 m s-1
continuous cyclic electrical signal is applied to Velocity of blood movement,u= 30 cm s-1
the transmitting crystal and therefore a corre- Angle between ultrasonic beam and direction
sponding CW ultrasound beam is generated. of flow,q= 45°
When the ultrasound is scattered or reflected at The Doppler shift is therefore: 1
1
s f = (5 ×106×2 ×30 ×cos45)/154 000 the system electronics can extract a Doppler shift
t D
c
a = 1372 Hz signal from the samples.The Doppler equation
f
e again applies to this Doppler shift and can be
t
r
a The shift in frequency is small and within the used to calculate the speed of the reflector.7

d
n audible range. In an ultrasonic Doppler instru- A bonus of PW Doppler is that since pulsed
a
ment,the electronics are designed to extract the ultrasound is employed,the range of the moving
e
c difference in frequency,f = f – f (the Doppler target may be measured from the echo-return
ti D t r
c shift frequency).The instrument can therefore time,as well as its speed from the Doppler shift.
a
pr feed a signal of frequency f to some output The range can be measured from one echo
D
s, device such as a loudspeaker, or frequency signal; however, the calculation of the Doppler
e
pl analyser as discussed later. shift and hence speed typically requires 50–100
i
c So far we have considered an ultrasound echoes. As for the CW case, a group of blood
n
ri beam being reflected from a structure moving cells moving with different velocities produce a
p
: at a fixed speed and hence generating a Doppler range of Doppler shift frequency components in
s
c shift of one particular frequency. In practice, the output signal.
i
s
y there are many reflecting blood cells and their It was noted above that the frequency of
h
P speeds are different. The ultrasound signals reflected ultrasound is shifted upward or down-
returned to the detector from the different cells ward depending on whether the motion of the
therefore have suffered different Doppler shifts reflector is toward or away from the transducer.
and add together to give a complex signal con- A numerical example illustrates this point and
taining a range of frequencies.The Doppler shift emphasises the small changes in frequency that
frequencies are extracted from the detected the instrument must distinguish.When 2 MHz
complex signal and can be fed to a loudspeaker ultrasound is reflected from an object travelling
where they can be interpreted by listening.High- at 30 cm s-1toward the transducer,it returns to
frequency (high-pitch) components in the audible the receiver with a frequency of 2.00078 MHz,
sound are related to high speeds, whereas low- a shift of +0.00078 MHz.If the object moves at
frequency components correspond to low speeds. 30 cm s-1 away from the transducer, the ultra-
Strong signals, that is of loud audible volume, sound returns with a frequency of 1.99922 MHz,
correspond to strong echoes that have received a a shift of -0.00078 MHz.Virtually all Doppler
Doppler shift.Strong signals could be due to the instruments which measure velocity preserve this
detection of many blood cells, say in a large direction information.
vessel,or to echoes from tissue.Later it is noted
that an output display called a spectral display
CONTINUOUS AND PULSED
or spectrogram is often used to portray the
WAVE DOPPLER INSTRUMENTS
frequency content of Doppler signals.
In the PW Doppler technique, the electrical Doppler blood flow instruments are required to
excitation signal is applied to the crystal as pulses, be extremely sensitive and to be capable of
each containing say 10 cycles,at regular intervals detecting weak signals from moving blood in the
and therefore a corresponding train of pulses of presence of much stronger signals from static
ultrasound are transmitted.For example,10-cycle or moving tissues; the latter give rise to low-
pulses can be transmitted, separated by non- frequency Doppler shift ‘clutter’ signals. The
transmission intervals of duration 20 times that magnitude of the scattered signal from blood is
of each pulse.Regularly spaced echoes are then typically 40 dB below that received from soft
received back from a reflector and they can be tissues,i.e.the blood echo amplitude is typically
regarded as samples of the signal which would one-hundredth of the soft tissue echo ampli-
be received if a continuous wave had been trans- tude.The dB unit is a measure of the size of a
2 mitted as discussed above.If the reflector is moving signal relative to another signal;the second signal
1
Physics: principles, practice and artefacts
is often a reference signal or perhaps the input repetition frequency (PRF) of 10 000 s-1, i.e. C
o
signal to an amplifier to which the output is 10 kHz.The highest velocity that the instrument n
t
compared. Blood flow signals may be detected can measure is directly proportional to its PRF in
u
even though the vessel is not clearly depicted,for (see aliasing artefact),therefore the PRF is made o
u
instance in the fetal brain,or the renal artery of as high as possible while still avoiding overlap s

a
the neonate. between successive echo trains.Echo signals,i.e. n
d
The transducer of a basic CW Doppler unit trains of echoes, are produced as a transmitted p
u
has two independent piezoelectric crystals.Since pulse passes through reflecting interfaces and ls
e
the transmitting crystal is continually driven to regions of scattering targets.After amplification, d

generate a continuous wave of ultrasound, a successive echo signals from a specific depth are w
a
second crystal is used to detect the reflected selected by electronic gating and the Doppler v
e
ultrasound.When a CW Doppler mode is imple- shift frequency is extracted as described above. D
o
mented as part of an ultrasound system which Pulsed Doppler devices can be used on their p
p
uses array transducers,separate groups of array own by altering slowly the beam direction or the le
crystal elements are used for transmission and gated range depth while listening to the output, r
in
reception. On extraction of the Doppler shift for example in transcranial blood flow studies. s
t
r
frequency a filter,the ‘wall-thump’filter,is often Identification of vessels is made easier by com- u
m
used to remove large, low-frequency compo- bining the PW Doppler mode with a real-time e
n
nents from the signal,such as those from slowly B-scan mode to form a duplex system;however, t
s
moving vessel walls.Typically in a Doppler unit this obviously adds to the cost and complexity.
operating at 5 MHz, Doppler shift frequencies Since the ultrasound is pulsed and the excita-
below 100 Hz are removed by filtering.Basic CW tion time is short,a stand-alone PW unit uses a
Doppler instruments are small and inexpensive; single crystal transducer for transmission and
CW Doppler mode facilities are incorporated reception (Fig. 1.1b). On setting the electronic
into some array systems to allow them to detect gate to select a signal from a specific range,reflec-
high velocities (see section on aliasing artefact). tors within a volume,known as the sample volume,
The transmitted ultrasound field and the zone contribute to the signal.The shape and size of
of maximum receiving sensitivity overlap for a the sample volume are determined by a number
particular range in front of the transducer (Fig. of factors:the transmitted pulse length,the beam
1.1a). Any moving structure within this region width, the gated range length, and the charac-
of overlap will contribute a component frequency teristics of the electronics and transducer.The
to the total Doppler signal. The shape of the sample volume is often described as a tear drop
region of overlap (the beam shape) can be in shape (Fig.1.1b).Sample volume lengths are
considered as having a crude focus which depends usually altered by changing the gated range length.
on the field and zone shapes and on their angle In a blood flow unit for superficial vessels, the
of orientation to each other.In practice,the beam sample volume length may be as short as 1 mm,
shapes are rarely well known for CW Doppler whereas in a transcranial device it can be 1 or 2 cm;
transducers. A 5 MHz blood flow instrument however,the precise lengths are rarely known.
might be focused at a distance of 2 or 3 cm from The ultrasonic output intensity of pulsed
the transducer and a 10 MHz device at a distance Doppler instruments varies considerably from
of 0.5–1 cm.CW Doppler instruments normally unit to unit.The intensity (I -see safety section,
spta
have ultrasonic output intensities (I ) of less pp. 23–24) may typically be a few hundred
spta
than 10 mW cm-2 although they may be signifi- mW cm-2but can be as high as 1000 mW cm-2,
cantly higher when used in conjunction with particularly when they are required to penetrate
duplex systems to measure high velocities. bone,as in transcranial Doppler.At present the
A PW Doppler instrument, operating with most common use of stand-alone PW units is in
5 MHz ultrasonic pulses, may have a pulse transcranial examinations of cerebral vessels. 3
1
s Fig.1.1 Sample volumes in
t
c Doppler techniques. (a) For dual
a
f crystal continuous wave Doppler
e Extended sample volume
rt unit. (b) For pulsed wave Doppler
a
unit. (c) Neighbouring sample
d
n volumes along a beam for imaging
a
e (a) Doppler units.
c
i
t
c
a Tear-drop sample volume
r
p
,
s
e
l
p
i (b)
c
n
ri Consecutive sample volumes
p
:
s
c
i
s
y
h
P (c)
Technical factors in the use of CW and signal from vessel walls and any other
PW Doppler moving tissue.
1. Doppler beams are subject to the same 7. The final result in many cases should be a
physical processes in tissue as B-mode distinct display, called a ‘spectrogram’ or
beams,i.e.attenuation,refraction,speed of ‘sonogram’ (see section on the spectrum
sound variation,defocusing,etc. analyser),with a clearly defined maximum-
2. Since the stand-alone CW and PW units are velocity trace.
used blind,the beam direction and also the 8. Since the beam–vessel angle is unlikely to be
sample volume in the PW case must be known,the sonogram cannot be calibratedin
systematically moved through the region of velocity and the vertical axis remains as
interest to maximise both the volume and Doppler shift frequency.
pitch of the audible Doppler signal. 9. Care should be taken to ensure good acoustic
3. PW Doppler is subject to the aliasing arte- coupling between the transducer and the
fact in the measurement of high velocities, patient.Since there is no associated image it
CW Doppler is not. is not always apparent that a weak signal
4. The sensitivity (gain,transmit power) of the may be due to a lack of coupling agent.
Doppler unit should not be so high that 10. If possible,information should be obtained
noise detracts from the signal quality. on the shape of the sample volume for both
5. The instrument should be assessed on CW and PW beams.The sample volume size
normal vessels where the blood flow pattern can then be related to the size of the vessel
is known and the expected Doppler signal under study.With CW Doppler there is very
well understood. little depth discrimination.With PW Doppler
6. The wall-thump filter should just be high the sample volume depth and size are set by
enough to remove the strong low-frequency the user.
4