7. Pulse-echo principle
D
2t
t
transducer
target
Delay time, T = 2t
Delay time, T = 2t
D=(v)(t)
D=(v)(t)
D = vT/2
D = vT/2
8.
9.
10. Ultrasound Transducers
Can be used both to transmit & receive ultrasound
Coaxial cable
Transducer housing
Acoustic absorber
Backing block
Electrodes
Piezoelectric crystal
Matching layer
12. Acoustic pulse production
A medical transducer produces a “characteristic”
frequency.
For each electrical impulse, a pulse “train” that
consists of N sinusiodal cycles is produced.
The “Q” of a transducer is a measure of the
number of cycles in a pulse train.
14. Ultrasound definition
Infrasound < 15 Hz
15 < Sound < 20 kHz
Ultrasound> 20kHz
2 MHz < Medical ultrasound<20 MHz
Internal local use about 50 MHz
15. Velocity of Sound
Velocity of sound is an important parameter
Two material qualities decide the velocity
– bulk modulus, B and density, ρ
Bulk modulus (compressibility) is defined as
– ratio of increase in pressure to a change in volume
– units are N/m2
» Air, B = 1.5×105 N m-2, ρ = 1.27 kg m-3
v = 345 m s-1 ( at room temperature & pressure)
» Water, B = 2.05×109 N m-2, ρ = 1×103 kg m-3
v = 1432 m s-1 ( at room temperature & pressure)
16. Ultrasound propagation properties
Velocity of sound in “soft tissue” is
nearly constant = 1500 m/sec.
Velocity of sound in bone and air
differ greatly from soft tissue.
Velocity = Frequency x Wavelength
“Ultra”sound implies f > 1 MHz
Wavelength = Velocity/Frequency
Wavelength < 1.5 mm
17. Speed of sound in different materials
dry Perspex
air gelatine (10%)
tooth brass steel
natural rubber
bone glass
lung gall stone
0 1000 2000 3000 4000 5000 6000
speed of sound (ms-1)
skin
muscle
brain
saline
water blood eye lens tendon
fat
18. Sound Intensity & Attenuation
Intensity of a wave:
– Energy per unit time per unit area
» Units: Wm-2; Symbol: I
Sound is scattered & absorbed by matter
– Reduction in intensity called attenuation
– change in intensity ∝ distance × intensity
≈ µ = attenuation constant, dependent on material
∆I = −µI∆x
19. Attenuation of Sound
− µx
Io
Integrating gives:
Io is the original intensity I = I oe
gµ
Intensity
a sin
re
D ec
D istance
20. Attenuation Coefficient
Attenuation of sound is usually expressed as decibel (dB)
Change in decibels (dB) is defined as: 10 log10 ⎛ I ⎞
⎜ ⎟
⎝ Io ⎠
I = e − µx
Io
log(I/Io) = -µx * log(e)
10* log(I/Io) = -µx * 10 * log(e) = -µx *4.343
Attenuation coeff. in dB/m (α) = 4.343 µ (m-1)
21. Attenuation against Frequency
1000
Attenuation Coefficient (dBm-1)
ng
air
100 lu
skin
tis
en
tes n
bi
le
sp
10 l o
og
m
ae
r
H
te
wa
1.
0
0.
1 1.0 10 100 1000
Frequency (MHz)
22. Safety Issues
High intensity ultrasound causes heating
Could damage body tissues
– Diagnostic ultrasound always used at low
intensities
100
Intensity (W/cm2)
10
“Potentially harmful zone”
1 “Safe zone”
0.1
Diagnostic Ultrasound levels
0.01 Exposure time (seconds)
1 10 100 1,000 10,000
Time of exposure (s)
24. Scattering of Ultrasound
Attenuation made up from:
absorption (heating)
scattering
depends on relative size of particle (a) wavelength (λ)
Scale of Frequency Scattering Examples
Interaction Dependence Strength
a >> λ f 0=1 (no Diaphragm, large
geometrical dependence) Strong vessels, soft
region tissue/bone, cysts
a~λ Predominates for
Stochastic variable Moderate most structures
region
a << λ f4 Weak Blood
25. Reflection
Z1 = ρ1v1 Z2 = ρ2v2
1
T =1-R
R
Z = acoustic impedance
Z=ρv
2
R = [(Z1-Z2)/(Z1+Z2)]
26. Acoustic Impedances
Material Impedance, Z
(kg m-2 s-1)
Air 0.0004 × 106
Blood 1.61× 106
Brain 1.58× 106
Fat 1.38× 106
Human soft tissue 1.63× 106
Kidney 1.62× 106
Liver 1.65× 106
Muscle 1.70× 106
Skull Bone 7.80× 106
Water 1.48× 106
29. Ultrasound reflection properties
Acoustic energy is reflected at interfaces between
tissues with differing acoustic impedances (Z).
Acoustic impedance = product of velocity of
sound (v) and physical density (ρ).
The unit of acoustic impedance is the “Rayl.”
Strength of acoustic reflection increases as
difference in Z increases.
For soft-tissue/air, soft-tissue/bone and bone/air
interfaces, almost total reflection occurs.
30. Transmission
velocity = v decreased velocity
Frequency is unchanged during propagation.
Therefore, wavelength must change as velocity of medium changes.
32. Refraction
reflected
refracted
incident
Angle of incidence = angle of reflection.
Refracted wave changes direction.
33. Geometrical region (a>>λ)
Sound reflected & refracted like light
laws of reflection
& refraction hold
θi θ
θi = θ r
r
sound velocity = v1
sound velocity = v2 sin θi v 1
=
θt sin θr v 2
35. Doppler Ultrasound
Waves reflected off moving surfaces have changed
frequency
– fractional change ∝ velocity
» vsurface= velocity of surface
» v = velocity of sound
» fs = frequency of source
» ∆f = change in frequency
Measuring frequency of returned signal gives
velocity
36. Doppler effect
Moving source of sound changes perceived
wavelength (frequency).
Shift in frequency is termed “Doppler shift.”
Change in frequency = 2f(S/v)cosθ.
– f = frequency
– S = source velocity
– v = velocity of sound
– θ = angle between “view” direction and
direction of motion.
37. Doppler Ultrasound
Used to monitor heartbeats, blood flow, etc.
Can produce images showing motion
– i.e. Imaging beating heart
38.
39.
40.
41.
42. Pulse-echo principle
A short pulse is send out, and the time for the
return pulses is measured
– called A-scan
transmitter/ Original pulse
Echoes
receiver
Amplitude
A B C
A
B
Time ( depth )
C
43. Depth (axial) resolution
2d
transducer
tw
d
To resolve distance, d,
To resolve distance, d,
vtw<2d
vtw<2d
44. Frequency and Resolution (axial resolution)
This is for linear array transducers with parallel beams
MHz Axial resolution Lateral resolution Wave length (mm)
3.0 1.1 mm 2.8 mm 0.5
4.0 0.8 mm 1.5 mm 0.375
5.0 0.6 mm 1.2 mm 0.3
7.5 0.4 mm 1.0 mm 0.2
10.0 0.3 mm 1.0 mm 0.15
For harmonic imaging the input frequency doubles the output frequency
(it works just for low frequencies)
45. Axial resolution
“Axial” resolution is defined as the ability to
distinguish between two objects along the axis of
the sound beam.
For a given frequency, axial resolution improves
as Q decreases.
For a given Q, axial resolution improves with
increasing transducer frequency.
49. Time-gain compensation
transducer
target
Attenuation of soundwave (dB)
Attenuation of soundwave (dB)
is approximatley proportional to
is approximatley proportional to
distance (delay time).
distance (delay time).
54. Multi-element Transducers
Ultrasound focused
– time of arrival of pulse at each transducer gives
direction. Called a B-scan
Electrical pulse
variable D D D D D D D D D
delays 1 2 3 4 5 6 7 8 9
transducer
array
Focused Wavefront
55.
56.
57. Two Dimensional Imaging
Using multi-element array, 2-D image can be
constructed - called B mode imaging
X
B mode
imaging system
X Y
Transducer
array
Y Computer display
65. Ultrasound and contrast
Contrast agent
A material which, when introduced into blood or tissue, causes one
or more its acoustic properties to change significantly. The most
common of these properties is backscatter coefficient. Intravascular
contrast agents usually comprise microbubbles which increase the
blood echo level and can hence enhance the detectability of blood
flow. Microbubble contrast agents emits harmonics and can be
disrupted by ultrasound, both of which phenomena form the basis of
nonlinear imaging.
82. Doppler Ultrasound
Waves reflected off moving surfaces have changed
frequency
– fractional change ∝ velocity
» vsurface= velocity of surface
» v = velocity of sound
» fs = frequency of source
» ∆f = change in frequency
Measuring frequency of returned signal gives
velocity
83. Doppler effect
Moving source of sound changes perceived
wavelength (frequency).
Shift in frequency is termed “Doppler shift.”
Change in frequency = 2f(S/v)cosθ.
– f = frequency
– S = source velocity
– v = velocity of sound
− θ = angle between “view” direction and
direction of motion.
