basics of ultrasound physics, components transducers or probe and its types, gel, ultrasound beam and image acquisition, a mode, b mode, m mode

1. ULTRASOUND
PHYSICS
DR.ANUSHUYA
JUNIOR RESIDENT
STAR

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3. PROPERTIES OF
ULTRASOUND
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4. ULTRASOUND
• Ultrasound is a mechanical, longitudinal wave
with a frequency exceeding the upper limit of
human hearing.
A Difference between x ray and ultrasound wave ?
• Ultrasound Cannot travel throughVacuum.
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5. WAVE MOTION
◍ Longitudinal wave
◍ Bands of compression and rarefaction.
◍ Motion of particles in the wave is parallel to
the direction of wave propagation.
◍ Each repetition of this movement – a cycle.
◍ Wavelength –distance bw two bands of
compression or rarefaction.
◍ Frequency – Number of cycles per second.
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6. WAVE MOTION
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7. ULTRASOUND FREQUENCY
Ultrasound by definition has a frequency of
greater than 20,000 cycles per second.
 Audible sound ?
 20 – 20,000 cycles per sec
 Man’s voice ?
 100 cycles per sec
 Diagnostic Imaging ?
 1,000,000 to 20,000,000 cycles per sec
 Hertz? Megahertz ?
 1 Hz - One cycle/s / 1MHz - a million cycles/s7

8. ULTRASOUND VELOCITY
◍ Independent of frequency
◍ Dependent on characteristics of trans.
Medium
 Compressibility
 Density
o Inversely proportional to each other/
velocity.
o All liquids transmit sound within a narrow
range of velocities.
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10. ULTRASOUND INTENSITY
watts/ cm 2
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11. ULTRASOUND
RELATIVE INTENSITY
◍ Comparison of relative power of two beams
expressed logarithmically using a bas e of 10.
◍ I0 – Reference intensity
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13. INTERACTIONS
OF ULTRASOUND
REFLECTION
ABSORPTION
REFRACTION
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ATTENUATION
SCATTERING

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15. ACOUSTIC IMPEDANCE
◍ The product of the tissue’s density and the sound
velocity within the tissue.
◍ Velocities:
 Soft tissues = 1400-1600 m/sec
 Bone = 4080
 Air = 330
◍ Amplitude of returning echo is proportional to the difference
in acoustic impedance between the two tissues
◍ Thus, when an ultrasound beam encounters two regions
of very different acoustic impedances, the beam is reflected
or absorbed
– Cannot penetrate
– Example: soft tissue – boneinterface15

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25. “
COMPONENTS OF USG
AND
TRANSDUCERS
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COMPONENTS

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TRANSDUCERS

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TRANSDUCERS

29. BACKING MATERIAL
AIR BACKED
CRYSTALS
◍ Reverberation
extends pulse
duration
◍ Conttinuous
wave doppler
and pulse
doppler
PHYSICAL
DAMPING
◍ Tungsten powder
mixed with epoxy
resin
◍ Rubber to increase
absorption
◍ Sloped surface
ELECTRONIC
DAMPING
◍ Resistor on
either side
◍ Dynamic
damping with a
voltage pulse of
transducer action
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30. PIEZO-ELECTRIC CRYSTAL
◍ PE EFFECT : Application of electric field –
change in the dimension.
◍ Pierre and Jacques curie in 1880.
◍ Natural – Quartz.
◍ Ferroelectrics – Ceramic materials with
innumerable dipoles that can be made into
diiferent shapes and made to vibrate in
either thickness or radial mode.
◍ Barium titanate and lead zirconate titanate
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31. CURIE TEMPERATURE
◍ Ceramic crystals are heated to a high
temperature in a strong electric field.
◍ At a high temperature the dipoles are free to
move and the electric field brings them into the
desired geometric alignment to produce PE
effect.
◍ Then the crystals are gradually cooled while
subjected to high constant voltage.
◍ The curie temperature is the temperature at
which this polarization is lost.
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Never be
autoclaved !!!!

32. SOUND WAVE PRODUCTION
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◍ An electric dipole is a distorted molecule
that appears to have a positive charge in
one end and a negative charge on the other.
◍ Electric field will cause the dipoles to realign
causing a change in dimension to a few
microns.
◍ Voltage applied in sudden burst or pulses
generates sound waves.

33. “PRESSURE ELECTRICITY”
◍ Reflected sound waves from the body carry
energy and they transfer the energy to the
transducer.
◍ Causing compression of the crystal element.
◍ Compression forces the tiny dipoles to
change their orientation which induces a
voltage between the electrodes.
◍ The voltage is amplified and serves as the
ultrasonic signal.
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34. RESONANT FREQUENCY
◍ The thickness of piezoelectric crystal determines its natural
frequency, called its resonant frequency.
◍ The crystal is designed so that its thickness is equal to
exactly half the wavelength of the ultrasound to be produced
by the transducers.
◍ Thickness = wavelength/2
◍ Thick crystal will produce ?
◍ Low frequency ultrasound.
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35. Calculate
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36. TRANSDUCER
Q FACTOR
◍ Two characteristics :
 purity of sound &
 length of time that the sound persists.
◍ High Q transducer - nearly pure sound made up of
narrow range of frequencies / longer time.
◍ Low Q transducer - whole spectrum of sound
covering wider range of frequencies /shorter time.
◍ The interval between initiation of the wave and
complete cessation of vibration is called the “ ring
down time “.
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39. TRANSDUCER
Q FACTOR
◍ High Q : useful for doppler USG transducers
because it furnishes continuous 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.
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40. SPATIAL PULSE LENGTH
◍ The length of the sonic pulse.
◍ Number of waves multiplied by their
wavelength.
◍ The sonic pulse from an unsupported high
Q crystal is long because it persists for a
longer time.
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41. QUARTER WAVE
MATCHING
◍ The thickness of the matching layer must be
equal to one-fourth the wavelength of
ultrasound.
◍ The impedance of the matching layer must
be about the mean of the impedances on
either side (transducer and tissue)
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42. TRANSDUCER JELLY/COUPLING AGENT
◍ Air and other gases impede sound waves
◍ At tissue-air interface, more than 99.9% of
the beam is reflected so none is available
for further imaging
◍ 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
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43. PROPERTIES
◍ Non allergenic
◍ Odourless
◍ Non staining
◍ Harmless
◍ Neutral ph
◍ Easily removable with tissue or towel
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44. USG GEL
◍ Water
◍ Carbomer : synthetic high molecular weight polymer of
acrylic acid cross linked with allyl sucrose and containing
50-68% of carboxylic acid groups. Neutralized with alkali
hydroxide to make it water soluble.
◍ EDTA
◍ Propylene glycol : organic oil compound that doesnot
irritate the skin and helps retain moisture
◍ Glycerine and trolamine : neutral colorless gel that
absorbs moisture from air
◍ Colorant : occasionally used, usually blue color44

46. ULTRASOUND
BEAM
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47. WAVEFRONT
◍ Piezoelectric crystals behave as a series of
vibrating points.
◍ Each vibrating point produces multiple
concentric rings or waves that eventually
form a continuous front.
◍ The distance at which the waves become
synchronous depends on their wavelengths.
◍ The shorter the wavelength, the closer the
front forms to the surface of the transducer.
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48. WAVEFRONT
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49. BEAM ZONES
◍ The intensity of ultrasound varies
longitudinally along the length of the beam.
◍ The beam travels as a parallel bundle for a
certain distance, beyond which it diverges.
◍ FRESNEL or NEAR ZONE –parallel portion.
◍ FRAUNHOFER or FAR ZONE – diverging
portion.
◍ Fresnel zone is longest with large
transducer and high frequency sound.
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51. IMAGE
ACQUISTION
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56. THANK YOU
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