2. History
• Acoustics, the signs of sound,
start as far as Pythagoras in the
thick century BC Road on the
mathematical properties of
stringed instruments.
• Echolocation in maths was
discovered by Italian priest and
physiologist Lazzaro
Spallanzani in 1794 when he
demonstrated that maths
hunted and navigated by
inaudible sound not vision
3. History
• The piezoelectric
effect, described
by Paul Jacques
Curie and Pierre
Curie in 1880.
• Nobel prize in
physics in 1903.
(for research in
nuclear physics)
5. WHAT IS SOUND?
Sound is a mechanical disturbance
from a state of equilibrium that
propagates through an elastic
material medium.
Cannot travel through vacuum.
Sound is the result of mechanical
energy traveling through matter
a wave producing alternating
compression and rarefaction.
Audible range: 20 Hz-20KHz
6.
7. What is ultrasound?
• Mechanical longitudinal wave with
frequency exceeding upper limit of
human hearing which is 20KHz.
• Diagnostic ultrasound- 1MHz to 20MHz
• Longitudinal waves- motion of particles in
the medium is parallel to direction of
wave propagation.
• Molecules of medium move back and
forth, producing bands of compression
and rarefaction.
• Ultrasound imaging and Doppler
ultrasound are based on thee scattering
of sound energy by interfaces of materials
with different properties through
interactions governed by acoustic physics.
8.
9. • The distance between
corresponding highest
points on the time pressure
curve is defined as the
wavelength.
• The time(T) to complete a
single cycle is called the
period.
• The number of complete
cycles in a unit of time is the
frequency(f) of the sound.
10. Velocity of sound
• Dependent on physical makeup of
material.
• Velocity is inversely proportional to
compressibility.
• Velocity is inversely proportional to
density.
• In body it is almost constant i.e.
1540m/s.
• The wave equation:
propagation velocity= frequency x
wavelength
Propagation velocity determined by
1.Density of medium and
2.Stiffness or elasticity of medium.
11. • In ultrasound frequency range, velocity of
sound is relatively constant in various soft
tissue when frequency is increased,
wavelength must decrease.
12. Amplitude or intensity of sound
• It is determined by the length of oscillation of
the particles. The greater the amplitude of
oscillation, the more intense is the sound.
• Ultrasound intensities are expressed in
watts/cm^2.
13. MECHANICS OF ULTRASOUND
• TRANSDUCER
It is a device that can convert one form of energy into another.
USG Transducer converts electric signal to ultrasonic energy and
vice versa
The most important component is the piezoelectric crystal
Piezoelectricity or piezoelectric phenomenon-
Certain materials when deformed or compressed,
generate a voltage, and conversely when subjected to a
voltage, becomes stressed or deformed’
(described by- Pierre and Jacques curies in 1880)
14. TRANSDUCER
• Front and back faces coated with thin
conducting film (gold/silver electrodes
for current to crystal – to cause strain)
• Strain: deformity of crystal caused when
voltage is applied
• Outside electrode- grounded
• Inside electrode- abuts against thick
backing block and absorbs sound waves
transmitted back in to transducer.
• housing- strong plastic.
• Acoustic insulator made of-rubber or
cork; prevents sound passing into
housing
15. CHARACTERISTICS OF PIZOELECTRIC
CRYSTALS/MATERIALS
• Innumerable dipoles
• Application of an electric field causes them to
realign, thus changing the crystal dimensions
• Compression of the crystal by the returning echo
induces voltage in between the electrodes
• Pressure electricity
• No current flows through crystal
• Plating electrodes- capacitors; voltage between
them produces electric field, causes crystals to
change shape
-voltage applied in burst/pulse-crystal vibrates
like cymbaal and generated sound wave.
- Backing block- dampens vibration.
16. Piezoelectric materials
• Naturally occurring materials
-Quartz
• Man made crystals(ferroelectrics)
-Barium titanate
- Lead zirconate titanate-PZT
• Vibration occurs in two mode
-Thickness mode- medical crystals
-Radial mode
17. CURIE TEMPERATURE
• Ceramic first heated at high temperature in the presence of a
strong electric field
• This brings the dipoles to the desired alignment.
• Crystal is then gradually cooled while subjected to the same
high voltage.
• Dipoles then become fixed as the room temperature is
reached and crystal possesses piezoelectric properties
• Curie temp is the temp at which this polarization is lost
• Transducers should never be autoclaved.
19. THE BACKING BLOCK
• Made up of a combination of tungsten and
rubber powder in epoxy resin.
• The proportion of the ingredients depends on
the transducer’s frequency.
• An ideal Backing material should accept all the
sound waves reaching it and not reflect it back
into the crystal.
25. TRANSDUCER PROPERTTIES
• Resonant frequency
• Transducer is designed to be maximally sensitive to a
certain natural frequency called is resonant frequency.
• Determined by the thickness of the piezoelectric crystal.
• Thinner the crystal, higher is the frequency.
• The crystal is designed so that its thickness=1/2 wavelength
• in medical ultrasound the transducer is driven at its
resonant frequency.
• Outside the resonant frequency the intensity of sound
decreases
26. TRANSDUCER Q FACTOR
• Refers to two characteristics of PZ crystals, and
Length of time the sound persists.
• High Q factor- pure sound with narrow range of
frequency.
• Low Q factor- spectrum of sounds, wide range of
frequencies.
• The interval between initiation of the wave and
complete cessation of vibration is called the RING
DOWN TIME.
27. High Q: useful for Doppler USG transducers
because it furnishes narrow range of sound
frequencies
Low Q: useful for organ imaging because it can
furnish short ultrasound pulses and will
respond to a broad range of returning
frequencies.
The Q factor can be controlled by altering the
characteristic of the damping block.
28. • Damping block consist of powered rubber and
tungsten blended with an epoxy resin.
• Ratio of tungsten to resin is chosen to satisfy
the impedance requirements
• Rubber is added to increase the attenuation of
sound in the backing block.
29. Formation of a wavefront
• Wavesmove away from
Point of origin as
concentric circles
• The distance at which
the wavefront is
formed depends on
wavelength.
• Shorter the wavelength
closer on the front
forms to the transducer
Imposition of waves to form wavefront
30. Characteristics of Ultrasound Been
• Intensity of Ultrasound varies longitudinally
along length of the beam.
• Parallel component (near/ fre-nel zone)
• Far zone
• X: transition point
33. REFLECTION
• Both ultrasound and light obey the law of reflection, the angle of incidence and the
angle of reflection are equal
• The factor that determines the percent of the incident beam undergoing reflection is a
property, peculiar to various tissues, called acoustic impedance
• Acoustic impedance z = p u rayls, where
p is density
u is velocity of sound in cm/sec.
The velocity of sound in all soft tissue is virtually same 1540 m/sec.
As sound waves pass from one tissue to another, the amount of reflection is determined by
the difference in the impedances of the two tissues.
Soft tissue – Air interface 99.9%of waves reflected.
Acoustic gel is used to avoid transducer air impedance.
34. TRANSDUCER JELLY/COUPLING AGENT
Jelly acts as a special aqueous conductive medium for
the sound waves
Prevents the formation of bubbles between the
transducer and the patient’s skin
Acts as a lubricant
It is non allergic, odorless, non staining, harmless with
neutral pH and easily removable with tissue or towel.
Content: water, carbomer, EDTA, propylene glycol,
glycerin and trolamine, colorant.
35. Types of reflectors
Specular reflectors
If the interface is large and relatively smooth it
reflects sound much as a mirror reflects light.
ex- 1.diaphragm
2. urine filled bladder wall
3.endometrial stripe
36. Diffuse reflectors
• The echoes from diffuse reflectors scattered in
all directions.
• The acoustic interfaces involve structures with
individual dimensions much smaller than the
wavelength of sound.
• E.g.- solid organ- liver, kidney, spleen
37. REFRACTION
This occurs when an ultrasound beam
passes, at an angle other than 90
degrees, from one tissue into
another with change in velocity.
It increases with the increasing angle
of incidence.
It passes deeper into the body where
it gives rise to artifacts.
If angle of incidence is less than 3
degrees, very little refraction seen.
38. ABSORPTION
• Due to friction among molecules in their back- forth
movement, reduction in intensity of the ultrasound
beam occurs as it traverse matter.
• Friction results in degradation of part of molecules
kinetic energy to heat.
• The greater the frequency, the greater the
attenuation coefficient. This means high frequency
beam shows less penetration than a low frequency
beam.
• Attenuation in soft tissue is 1 dB/cm/ MHz
41. ULTRASONIC DISPLAY
• Ultrasonic image is the electronic
representation of data generated from the
returning echoes displayed on a TV monitor
A mode >> TM mode>> B mode >>
gray scale>> real time
42. A(amplitude) mode
• Echoes project spikes vs.
time
• Amplitude of spike- echo
intensity
• Separation between the
peak – depth
• Used in – ophthalmology
echocardiography
43. M- mode and TM mode
• Here spikes– dots
• Detects motion of structures
• M- mode does not have time
factor and is not meaningful
• TM- mode has time
• More useful in echo
cardiography.
44. B-mode
• It produces picture of slice of tissue
• Contact screening
simple sector scan
compound contact scan
Image is assembled by a computer which gets
information from the transducer by an arm with 3
joints.
Disadvantage- no shades of gray, only light and dark
areas seen
45. GRAY SCALE IMAGING
• To display variation of amplitude of echoes
arising from tissues as varying shades of gray
• Transducer>> echoes>>stored in scan
converter>>produces digital signal>> visible
image on TV monitor.
46. REAL TIME ULTRASOUND
• System have frame rates fast enough to allow
movements to be followed.
• Real time transducer can produce multiple frames in
a very short time.
• At least 16 frames/sec
• Types 1. mechanical scanners
2. electronic array scanners
linear array
phased array
48. TIME GAIN COMPENSATION
Reduced echo pulse reaching the Deep
A way to overcome ultrasound
attenuations time gain
compensation(TGC) in which signal
gain is increased as time passes from
one emitted wave pulse. This
correction makes equally echogenic
tissues look the same even if they are
located in different depths.
Slope of TGC adjusts degree of
amplification.
49. • Three components govern amplitude
without discrimination of depth
• 1.INTENSITY
• Determines P.D across transducer.
produces energetic beam
stronger echoes at all levels
2.COARSE GAIN
All echoes are enhanced nearly twice,
proportionately
regulated height of echoes from all
depths.
3.REJECT CONTROL
Rejects useless weaker siggnals
Improves clarity
50. • DELAY
Regulates the depth at which TGC
begins to augment the weaker signals
NEAR GAIN CONTROL
Diminishes near echoes, dampens them
FAR GAIN CONTROL
Increases all distant echoes
ENHANCEMENT CONTROLS
Enhances specific part of TGC.
51. USG ARTEFACTS
• In radiological imaging, the term arti fact is used to
describe any part of an image that doesn't
accurately represent the anatomic structures
present within the subject being evaluated.
• It may cause structures to appear in an image that
are not preset anatomically or a structure that is
present anatomically may be missing from image.
• US artifacts may also show structures as present
but incorrect in location, size or brightness
52. Ultrasound display equipment relies on physical assumptions.
To assign the location and intensity of each received echo.
1. Echoes detected, originated from within the main
ultrasound beam
2. An echo returns to the transducer after a single reflection.
3. the speed of sound in human tissue is constant
4. the sound beam and its echo travel in a straight path
5. The acoustic energy in an ultrasound field is uniformly
attenuated.
53. • In clinical sonography, these assumptions are often not
maintained, when this occurs, echoes may be displayed
erroneously and perceived as artifacts. Artifacts thus arise
secondary to these errors.
• 1.Inherent to the ultrasound beam charactristics
• 2.the presence of multiple echo paths
• 3.velocity errors and
• 4.attenuation errors.
54. • Artefacts associated with ultrasound beam
characteristics
• Artefacts associated with multiple echoes
• Artefacts associated with velocity errors
• Artefacts associated with attenuation errors
55. Focal zone- a region
within the transmitted
sound beam in which
the beam narrows to its
min size. Lat resolution
is best within focal zone
of beam
• Us image processing
assumes- echoes
detected originated from
within the Mai.
Ultrasound beam.
• However beam exits the
transducer as a complex
3D bow tie shape with
additional off axis low
energy beams, referred to
as side lobes and grating
lobes
56. Beam width artefact
• Physics mechanism- poor lateral resolution
• Alteration of artefact - can be reduced by
adjusting the focal zone to the depth level of
interest and by placing the transducer at the
centre of the object being studied.
• Clinical relevance- lateral blurring of point
targets, aberrant echoes from adj highly echo
genie objects.
57.
58. Side lobe artefact
• Physics mechanism- echoes transmitted outside
primary beam are reflected back to transducer
• Alteration of artefact- can be reduced by Decreasing
gain, using multiple windows, advanced transducer
design, using multiple windows
• Clinical relevance- mimics debris in anechoic
structures side lobe artefacts are echogenic, linear or
curvilinear artefacts. Strong reflectors include bowel
gas adjacent to the gall bladder or urinary bladder.
59.
60. Artefacts associated with multiple
echoes.
• Reverberation artefact
• Comet tail artefact
• Ring down artefact
• Mirror image artefact
61. Reverberation artefact
• Physics- reflections between highly reflective
interfaces in parallel
• Alteration of artefact- can be reduced by using
THI , decreasing gain, using multiple windows.
• Clinical relevance- usually unwanted, mimics
debris within cystic structures, may be useful,
as in abnormal air
62.
63. Comet tail artefact
• Physics- reverberations from closely spaced highly
reflected interfaces
• Alteration- may be more visible using THI and less by
using SCI( spatial compound imaging)
• Spatial compound imaging is a method that obtains
sonography information from several different angles
of intonation and combines them to produce a single
image.
• Clinical relevance- adenomyomatosis, colloid nodules,
calcification, nephrocalcinosis.
64.
65. Ring down artefact
• Physics- resonant vibrations within air bubbles
• Alteration may be reduced by using SCI
• Clinical relevance- abnormal air( abscess,
pneumoperitonium, emphysematous
infections , pneumobilia, normal air( bowel
loops, appendix)
68. Mirror image artefacts
• Physics- reflections off a strong specular
reflector produce a mirror image of an object
• Alteration- reduced by decreasing gain,
changing AOI , using multiple imaging windows
• Clinical relevance- mimics disease,
pseudothickened bowel wall and consolidated
lung
69.
70. Speed displacement artefacts
• Physics- constant speed of sound is used to
calculate depth in soft tissues, but actually
there is some variability among diff tissues.
• Alteration- modern us units allow manual
speed of sound correction
• Clinical relevance- inaccurate locations and
measurements of structures in axial
dimension, loss of lateral resolution.
71.
72. Refraction artefact
• Physics- refraction at oblique interfaces causes
altered location of objects, duplication of
underlying structures, and or shadowing of
edges.
• Alteration- reduced by using different imaging
window-
• Clinical relevance- false position of lesion during
biopsy, can mimic duplication of structure.
73.
74. Posterior acoustic enhancement
• Physics- increased intensity of echoes distal to
low attenuating structure
• Alteration- may be increased by using THI
• Clinical relevance- differentiation of cystic
from solid structures
75.
76. Acoustic shadowing
• Phy- reduction in echo strength distal to a
highly attenuating or a reflective object
• Alteration- increases with increased frequency
and THI decreases with inappropriate FZ
placement, excessive beam width, and SCI
• Clinical relevance- Detection of stones,
calcification and air
