Ultrasound uses high-frequency sound waves to create images of the inside of the body. Sound waves above the range of human hearing, between 2-18 MHz, are used. The speed of sound varies in different tissues, and piezoelectric crystals in the transducer generate and detect sound waves to assess depth and direction of structures. Doppler ultrasound can detect the frequency shift of echoes from moving structures like blood cells to assess blood flow. Technological advances include 3D imaging, portable scanners, and digital storage and sharing of ultrasound images.

1. Ultrasound
Sarah Gillies
Sarah.gillies@ekht.nhs.uk

2. Sound: What is it?
- A longitudinal, ‘pressure’ wave
• Wavelength,  – length of one
complete cycle of a wave (m)
• Period, T – the time required to
complete a full cycle
• Frequency, f – the number of
cycles per second = 1/T
• Amplitude, A – maximum
pressure displacement
• Speed, c – distance travelled
by a given point on the wave in a
given interval of time (m/s)

3. Ultrasound
Ultrasound is acoustic (sound) energy in
the form of waves which have a frequency
above the human hearing range.
Human hearing frequency range:
The highest frequency that the human ear
can detect is approximately 20 thousand
cycles per second - 20,000 Hz.
Diagnostic ultrasound frequency range:
Typical diagnostic ultrasound scanners
operate in the frequency range of 2 to 18
megahertz (MHz)

4. Speed of Sound
Speed, c = distance = λ = f λ
• Wave speed, c, depends on the density and compressibility
of the medium:
• A denser medium → slower speed
Harder materials are more difficult to compress, this means the
material impedes the formation of compressions and rarefactions.
Vs
c f
 

5. Speed of sound in the body.
Medium Speed (m/s)
Air 330
Water 1480
Fat 1460
Liver 1570
Muscle 1600
Soft tissues (average) 1540
The speed of sound in a given medium remains fixed.
Therefore…

6. Frequency
High Frequency:
Low Frequency:
if  decreases …. f increases (and vice versa)
Double the frequency half the wavelength

7. Sound
 Sound needs a medium to travel through
 In diagnostic ultrasound, this medium is
provided by using……?
Gel !

8. Using sound-waves to create an
image.
 Transducer placed onto gel
applied to skin.
 Short bursts of ultrasound are
sent into the patient.
 As the pulses travel into the body they are
reflected and scattered, generating echoes.
 Some echoes travel back to the transducer where
they are detected-these echoes are used to form a
B-Mode image.

9. Echo Ranging
To display each echo in a position that
corresponds correctly to the target
detected, the ultrasound system needs to
know:
 The distance of the target from the transducer
 The direction of the target from the transducer
The range of the target from the transducer
is measured using the pulse echo
principle.

10. Pulse Echo Principle
 The same principle is used in echo sounding equipment in boats to
measure the depth of water:
 Transducer transmits pulse of ultrasound, it travels through water to
bottom of seabed, a reflection/echo is produced and is detected on its
return.

11. Pulse Echo Principle
t = 0 t = d/c t = 2d/c
To measure send & return time a clock is
started as the pulse is sent (t = 0)
If the speed of sound in water is c and the
depth d, then the pulse reaches the seabed
at time t = d/c
The returning echo also travels at speed c
and takes a further time d/c to reach the
transducer where it is detected.
Therefore the echo arrives back at a total of t
= 2d/c
d = ct/2
Therefore system calculates distance by
measuring arrival time t of an echo assuming
a fixed value for the speed of sound
Depth is calculated from
the time of transmission of
pulse to reception of echo,
taking into account the
speed of sound.

12. The B-Mode Image
Cyst in breast tissue
A sonographic study of
valves in a patient's heart

13. Creation of an image
 The B-mode image is formed from a a large number of B-mode scan
lines where each line is produced by a pulse-echo sequence.
 In a typical linear array transducer, the beam is stepped across the
transducer array producing an image line of echoes which are
displayed on-screen as bright spots.

14. How do we create the sound
waves for diagnostic ultrasound?
 Sound generally comes from a vibrating device.
 The source of this vibration for diagnostic ultrasound
comes in the form of a small wafer of piezoelectric
material which vibrates a millions of times per second.
 The piezoelectric material (approx
128 individual piezoelectric
elements in a transducer) is the
main component within the
ultrasound transducer.

15. Piezoelectrics
 Convert electrical
energy into mechanical
energy (sound) to
produce ultrasound.
 Mechanical energy into
electrical energy for
ultrasound detection.
It does this by
physical deformation
of the crystal/ceramic
structure.

16. Transducers
Linear array: linear probe
Linear array:
curved probe
Intra-cavity probe
Phased array

17. B-mode images
Gall
Stones
Twin Pregnancy
at 10 weeks
Breast cysts

18. The Doppler
Effect
The doppler effect is the change in
the observed frequency of the sound
wave (fr) compared to the emitted
frequency (ft) which occurs due to the
relative motion between the observer
and the source.

19. Doppler example
 The sound the driver hears will
remain the same.
 The observer located in front of the
car will hear a higher-pitched
noise. Why?
 Because the sound waves
compress as the vehicle
approaches the observer located
in front. This increases the
frequency of the wave, and the
pitch of the vroom rises.
 The observer located behind the
car will hear a lower-pitched noise
because the sound waves stretch
out as the car recedes. This
decreases the frequency of the
wave, and the pitch of the vroom
falls.

20. Using doppler in medical
ultrasound
 In a Doppler ultrasound
examination, sound waves of
a certain frequency are
transmitted into i.e. the
heart.
 The sound waves bounce off
blood cells moving through
the heart and blood
vessels.
 The movement of these
cells, either toward or away
from the transmitted waves,
result in a frequency shift
that can be measured.

21.  The diagram shows a Doppler
transducer placed on the skin
and aimed at an angle, θ,
towards a blood vessel, which
contains blood flowing with a
velocity of u m/s, at any
instant.
 The transducer emits
ultrasound waves of frequency,
fo, and echoes generated by
moving reflectors in the blood,
e.g. red blood cells, have a
frequency, fr.
 The difference between these
two frequencies, Δf, is related
to the velocity of the flowing
reflectors through the following
equation:

22. A higher-frequency Doppler
signal is obtained if the
beam is aligned more to the
direction of flow.
In the diagram, beam (A) is
more aligned than (B) and
produces higher-frequency
Doppler signals.
The beam/flow angle at (C) is
almost 90° and there is a very
poor Doppler signal.
The flow at (D) is away from
the beam and there is a
negative signal.
Effect of the Doppler
angle in the sonogram.

23. The Doppler Effect

24. Technological Advances
3D imaging
Portable scanners
PACS
Picture
Archiving &
Communications
System