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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5786_Библиотеки_им_академика_М_И_Перельмана.pdf
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World War I and World War II, the development of sonar (Sound Navigation and
Ranging System) and radar (Radio Detection and Ranging) took place. The latter
technique used electromagnetic waves rather than ultrasound.
The next important step was the use of ultrasound to detect flaws in metal
using high-frequency ultrasound. The metal flaw detectors became increasingly
important as World War II was approaching, but were reported after the war.
2,4
After World War II, Howry and Bliss, in Denver, started to experiment with
sonar equipment and amplifiers from the navy.7 They developed a pulse-echo
technique in 1948–49, and later produced cross-sectional images of a human
partly submerged in water. At the same time, Wild in Minneapolis developed a
breast scanner and actually made a diagnosis of breast lesions with his device.12
The Swedish physician Inge Edler and physicist Helmut Hertz, at the University
of Lund, borrowed a metal flaw detector from Kockum's Shipyard in Malmö,
Sweden. In 1953, they managed to trace the movements of the human cardiac valves by means of the sound waves emitted and received by their modified instrument.
technology.
5
This was the start of a new era in cardiology relying on sound
6
The next breakthrough was by the Scottish physician Ian Donald, in Glasgow,
who conducted the basic research for the development of a machine for clinical
use employing ultrasound to make two-dimensional images of human tissue.
Donald had served in the Air Force during World War II and his past experience
influenced his prototype machine, which consisted of two metal flaw detectors.
His Lancet paper of 1958, ‘Investigation of abdominal masses by pulsed ultrasound’, is considered to be one of the most important for the development of
clinical ultrasound.
3
Since the late 1950s, the development of ultrasound in medicine in general
and in the field of obstetrics and gynaecology in particular has continued in an
exponential way. Breakthrough advances have been repeatedly made in spite of
claims that the development of ultrasound in medicine has reached its physical
limits.
Physics and instrumentation
SOUND, WAVES AND PROPAGATION
Sound is a mechanical vibration in a medium. The medium may be, for example,
air, water or human soft tissue. The sound wave propagates through the medium
as a longitudinal compression wave. When we think of waves we may picture
a stone being thrown into a quiet lake and observe the concentric rings that propagate from the centre, or we may think of the waves in the ocean as seen from the
shore or from a boat. These waves are transversal waves. Sound waves, however,
are longitudinal waves and the medium that they travel through is subject to cyclic
variations in pressure as the medium is being compressed or rarefied (Fig. 1.1).
Make a small experiment by putting your index finger on the top of your larynx,
then make the sound of a z-z-z. With your finger you will feel the vibrations caused
by your vocal cords that are your own sound system, that cause the z-z-z to be
heard in the room. You have now produced longitudinal sound waves that travel
3
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Compression
λ
Pressure
Decompression
Distance
Distance
Moves with wave velocity, c
Ultrasound in obstetrics and gynaecology
Fig. 1.1 (Upper panel) A schematic illustration of a sound wave as it travels in a medium
causing periodic compressions and rarefaction of the medium. (Lower panel) The dislocation
of the particles.
through the room and cause compression and rarefaction of the air in their path.
When the sound waves hit the eardrums of someone in the room, the process is
reversed and causes the eardrums to vibrate and the person will hear your z-z-z.
The sound wave is a longitudinal wave caused by compression and rarefaction of
a physical medium in the direction of the movement of the wave.
This sound wave may further be described by intensity and frequency.
If you have a piano, you can carry out a small experiment in your living room
by hitting A above middle C. You will hear a chamber tone with a frequency of
440 Hz. If you move up one octave on your piano and hit A, you will hear it at a
frequency of 880 Hz. If you move up one more octave to the next A, you will hear
an A note with the frequency of 1760 Hz.
The frequency tells us about the degree of highness or lowness of a tone. The frequency is the number of vibrations per second that produce the sound.
Hit the A on your piano very lightly and you will barely hear the chamber tone
of 440 Hz; hit the key with force and you will hear the same chamber tone with
the frequency of 440 Hz, but much louder. This tells us that the same tone may
differ in intensity or loudness.
The intensity tells us something about the loudness or strength of the sound signal.
A sound wave travelling in a medium produces compression and rarefaction of
the medium as shown in Figure 1.1. The velocity of propagation of the sound wave
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is dependent on the medium and is 330 m/s in air, 1480 m/s in water, 1589 m/s in
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λ =
v
f
muscle and 3500 m/s in bone. The hardness or stiffness of the medium is the main
factor determining the propagation velocity of sound.
Ultrasound machines are now standardized and calibrated to use 1540 m/s as
the speed of sound in human tissue. Based on the propagation of the sound wave
in a particular medium (v) with a particular frequency (f), we arrive at the first
important equation for the wavelength λ:
(1)
A chamber tone (440 Hz) has a wavelength of 0.75 m, propagating in air at the
velocity of 330 m/s. It is obvious from equation 1 that the wavelength will vary with
the frequency and velocity of sound in the tissue. The higher the frequency, the
shorter the wavelength; the higher the velocity of sound, the longer the wavelength.
Because the speed of sound in human tissue has been standardized at 1540 m/s in
the equation, the wavelength will vary with the frequency (Table 1.1).
The higher the frequency of ultrasound in human tissue, the shorter the wavelength.
An ultrasound wave with a frequency of 5 MHz (M is the Greek abbreviation
for mega which means big, but used in acoustics it means million) has a wavelength of 0.31 mm.
It is important to understand what really happens when a sound wave moves
through the medium. A scene we all are familiar with will demonstrate the principle (Fig. 1.2).
When a sound wave propagates through a medium, the wave moves while the
medium remains in place. Thus, when ultrasound propagates through human tissue, it
is the wave that moves, not the tissue.
Let's go back to the sound waves. Low-frequency sound (a human voice, music)
will spread all over a room. You can easily hear the voice of a person talking with
his back turned to you. Very high-frequency sound behaves like light – it moves
like a beam along a straight line.
High-frequency ultrasound propagates through tissue in a relatively narrow beam
and may be focused by acoustic lenses.
In order to make a simple ultrasound machine, we need to be able to produce
high-frequency sound. In the 1880s the Curie brothers discovered the piezoelectric effect which implies that a crystal, for example a quartz crystal, will produce
an electrical current if subject to mechanical pressure. Conversely, an electrical current that is applied to a quartz crystal will cause the crystal to change its shape. The
change in shape will have an impact on the surrounding medium. If alternating
Physics and instrumentation
Table 1.1 Various ultrasound frequencies and the corresponding wavelength
Frequency (MHz) Wavelength (mm)
3.5 0.44
5 0.31
8 0.19
10 0.15
5
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Wave motion
Fig. 1.2 The person on the shore throws a stone into the water. The stone creates waves in
Ultrasound in obstetrics and gynaecology
the form of concentric rings that approach the cork. Instead of being ‘pushed away’ the cork
moves up and down as the wave passes by.
current is applied to the crystal, the crystal will repeatedly change its shape and the
movements of the crystal will produce a wave transmitted through the medium.
By using a piezoelectric material (quartz crystal) it is possible to produce highfrequency sound waves that emerge from the crystal into human tissue. The same
crystal can be made to pick up the echoes emerging from the depth of the tissue. Such
echoes will have an impact on the crystal that produces an electric pulse that we may
detect and process further.
If you have been at an outdoor rock concert in front of a full-blast subwoofer,
you will have experienced the impact that sound can have on your body, in particular on your air-filled chest cavity. Imagine the sound level scaled down to an
impact you cannot feel and then a very sensitive instrument introduced to detect
the sound waves; then you have a demonstration of the basic principle of receiving low-impact echoes. Making images with sound is about sending and receiving
sound waves in the form of a pulse (Fig. 1.3).
We now have enough knowledge to make a one-dimensional ultrasound image
of the fetal skull the way it was done in the late 1950s and early 1960s. It was
called A-mode (A stands for amplitude) (Fig. 1.4).
In the early days of the clinical use of ultrasound, A-mode technology made it
possible to measure the fetal biparietal diameter and the conjugata vera, to locate
the placenta, including placenta praevia, and to diagnose polyhydramnion, detect
the fetal heart activity, diagnose a molar pregnancy and a variety of other diagnoses. The interpretation of such images was difficult and required extensive training and imagination of the examiner. Still, sophisticated diagnoses were made by
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dedicated pioneers.
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Fig. 1.3 (Bottom) An electric current is applied to the transducer and a pulse is sent out. (Top)
A pulse is received and generates an electric current that can be displayed by the instrument.
The stronger the returned pulse (echo), the higher the amplitude of the electric current.
The natural step forward was to make two-dimensional images. The strength of
the echoes was then displayed as a white dot instead of as an amplitude; the higher
the intensity of the returned echo, the larger the dot. This was called B-mode
(B stands for brightness). In a one-dimensional system, these signals were impossible to interpret but moving the transducer in a plane across the area to be examined (scanning) during sending and receiving made it possible to display all the
echoes emerging from structures in that plane. Together, these were converted
into a relatively easy-to-read two-dimensional image (Fig. 1.5). This manual
scanning made it much easier to produce and interpret two-dimensional images
produced with ultrasound. The image quality was further improved by the development of the analogue scan converter, so that grey scaling could be applied as
well as scaling of the image and calliper movements on the screen.
The next technical step was to produce real-time two-dimensional images.
This was achieved mechanically in the 1960s by Krause and Soldner in Erlangen,
Germany.10 A more sophisticated way was to align a set of crystals to make a
linear transducer, described by Nicolaas Bom in Rotterdam, in 1971.1 The principle was further developed by Martin Wilcox who produced a clinically most
successful real-time scanner in 1972 (Fig. 1.6).
The principle of displaying the returned signals appropriately is simple: the speed
of sound is known and the time from when a pulse is emitted until it comes back can be
calculated. It is obvious that each submitted pulse will hit many structures in the path
of the beam, thus many echoes will be returned separated by a short time interval.
Electronic real-time scanning implies that the transducer sends a pulse, and
then it switches to the listening mode. A linear transducer may typically have 196
or more crystals aligned in a row. Typically crystals number 1–50 are fired, and
then number 2–52, etc. The examiner is presented with an image frame rate of
approximately 30 per second, which for the human eye will make the on-screen
image appear flicker free with movements in real time.
Physics and instrumentation
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Ultrasound in obstetrics and gynaecology
Fig. 1.4 A-mode. A single ultrasound beam is sent through the fetal skull and, in sequence
reflected from the parietal bone closest to the transducer, the falx cerebri, the skull bone
distal to the transducer and, finally, the posterior uterine wall. Depending on the strength
of the returned pulses (echoes), the quartz crystal will generate a high- or
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low-amplitude current.
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Fig. 1.5 Twin pregnancy. B-mode image obtained in 1964 by Diasonograph, Nuclear
Enterprises Ltd, Edinburgh, UK. The image is made by ‘compound scanning’, i.e. by rocking
the probe back and forth during the process of moving the scanning arm slowly across the
pregnant abdomen. Reproduced by permission from Bertil Sundén.
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Finally, we need to understand the physical principle of M-mode (M stands
for motion). M-mode is used to trace the movement of a structure. For example,
tracing the movement of a heart valve or the movement of the atrial wall and
the ventricular wall of the fetal heart simultaneously on the very same image
makes it possible to discriminate a dissociation of the rhythm, i.e. supraventricular tachycardia, various kinds of AV block, etc. During an M-mode recording,
we register the movements of the echoes along one single line in our image (the
y-axis) while time runs along the x-axis. The principle is easy to understand if
we imagine that we put a long paper strip on our desk, hold a pen against the
paper and move the pen up and down while pulling the paper strip in a direction
perpendicular to the movement of the pen. In our example, the pen represents
the moving echoes and the up-and-down movement of the pen will result in a
curved line on the paper reflecting the movements of the pen. An M-mode scan
is shown in Figure 1.7.
Physics and instrumentation
ONE TRANSDUCER FOR EACH PURPOSE
A variety of sizes and shapes of transducers have been produced for the various
applications of ultrasound in medical diagnosis. Transducers have various sizes of
‘footprints’, i.e. the part of the transducer that touches the skin or other tissue.
Transducers with a small footprint are necessary in, for example, cardiology, for
sending a beam between the ribs to reach the heart as a target organ. To reach the
heart and thoracic aorta, even an oesophageal transducer may be used; in urology
and proctology, the prostate or lower part of the intestines is reached by inserting
a transducer into the rectum. The gastroenterologist may examine the liver from
the surface of the abdomen or insert a slim transducer through the gastric scope to
reach the surrounding organs including the ductus pancreaticus and the pancreas.
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Ultrasound in obstetrics and gynaecology
10
Fig. 1.6 (Upper panel) The ADR linear scanner (in Europe manufactured under the name
of ADR-Kranzbühler, image by courtesy of the company, 1980). (Lower panel) The basic
principle of scanning in real time. The crystals fire the sound beams, which travel into the
human tissue, hit structures and are reflected. The reflected echoes are picked up and
displayed accordingly on a screen.
In vascular surgery, imaging through a catheter has been developed for target organs
such as the neck vessels and coronary arteries. In the field of obstetrics and gynaecology, curvilinear transducers are extensively used for transabdominal examination
(Fig. 1.8). The shape of the transducer fits well to the pregnant and non-pregnant
abdomen, the footprint is small, while the view deep in the tissue is wide due to
the sector-shaped image. The use of a convex transducer also reduces the effect
of reverberations and wave front aberrations (see later). Transducers designed for
transvaginal scanning make the early pregnancy and the non-pregnant uterus accessible at a close range; thus, they are widely used in gynaecology and obstetrics.
Transducer technology has become complex. The essential unit, the sound-emitting crystal, was made of natural materials such as quartz. Nowadays most of the
crystals are made of artificial ceramics mixed with plastic materials with various
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A-mode M-mode
Movement with time
Movement with time
Time
Grey-scale amplitude
along beam at fixed time
Depth
Fig. 1.7 The principle of M-mode. The basic principle is described in the text.
Physics and instrumentation
Fig. 1.8 Sector, curvilinear, linear and transvaginal transducers.
forms of damping material to produce a clean pulse and a pulse of short duration.
The electrical excitement is made through thin silver electrodes connected to the
ceramic material. The basic principle for producing a pulse wave and receiving an
echo, which generates a current, remains the same, as illustrated in Figure 1.8.
THE ULTRASOUND BEAM
NEAR FIELD AND FAR FIELD
Ideally, an ultrasound beam would emerge from a crystal, be narrow and circular and shoot into the tissue along a straight line. Then it would return along the
same line from structures it may hit, to the very same crystal, which would be
excited by the echoes and produce an electrical current. In real life, the beam
is not ‘narrow and circular’ but advanced engineering has, over time, worked to
11
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d =
r
2
λ
BW =
×Frλ
2
Near field Far field
Transducer
d
2r
modify the beam towards the ideal form. Put simply, the beam has a near field
and a far field. In the near field we may influence the shape of the beam by focusing. In the far field we cannot do that. When we make our images we are operating in the near field. So from an imaging point of view, we would like the beam
to have a long near field (Fig. 1.9).
The depth (d) at which the transition of the beam from the near field to
the far field takes place is given by equation 2. r is the diameter of the circular
transducer:
(2)
This equation tells us that the near field is relatively long if the diameter of the
circular transducer is large and/or the wavelength (λ) is short, i.e. the frequency is
high. It follows that the near field is relatively short if the transducer has a small
diameter and/or the wavelength is long, i.e. the frequency is low.
Ultrasound in obstetrics and gynaecology
FOCUSING
This brings us to the next important feature, which is the focusing of the beam.
The required effect of focusing the beam is to reduce the width of the beam.
Focusing may be achieved by employing lenses in various forms.
(3)
Figure 1.10 shows the trade-off of having a narrow beam width as an effect of
focusing: an increased divergence of the beam distal to the focal distance.
Considering equations 1–3, we may conclude that a focused transducer with a
large diameter (aperture) and a high frequency (short wavelength) will provide
a narrow beam in our region of interest (at the focal distance). So why do we not
settle for transducers with a large aperture and a high frequency?
The quick answer is that a large aperture may not be acceptable for a particular
application and high-frequency ultrasound is absorbed to a greater extent than
low-frequency ultrasound. The range of a relatively low-frequency transducer is
longer than for a relatively high-frequency transducer.
Fig. 1.9 Schematic illustration of an ultrasound beam emerging from a transducer with
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a circular surface with a diameter 2r. The beam has a near field reaching into the depth of
d and a far field.