•Sound waves consist of mechanical vibrations
containing condensations (compressions) &
rarefactions (decompressions)that are transmitted
through a medium.
•Sound is mechanical.
•Sound is not electromagnetic.
•Matter must be present for sound to travel
WHAT IS ULTRASOUND?
• Ultrasound or ultrasonography is a medical
imaging technique that uses high frequency
sound waves and their echoes.
• Known as a ‘pulse echo technique’
• The technique is similar to the echolocation used
by bats, whales and dolphins, as well as SONAR
used by submarines etc.
Bats: Navigating with ultrasound
• Bats make high-pitched chirps which are too high for humans
to hear. This is called ultrasound
• Like normal sound, ultrasound echoes off objects
• The bat hears the echoes and works out what caused them
• We can also use ultrasound to look inside the body…
• Dolphins also navigate with ultrasound
• Submarines use a similar method called sonar
Bats: Navigating with ultrasound
• If a bat hears an echo 0.01 second after it makes a chirp, how
far away is the object?
• Clue 1: the speed of sound in air is 330 ms-1
• Clue 2: The speed of sound equals the distance travelled
divided by the time taken
• Answer: distance = speed x time
• Put the numbers in:
distance = 330 x 0.01 = 3.3 m
• But this is the distance from the bat to the object and back
again, so the distance to the object is 1.65 m.
1. The ultrasound machine transmits high-frequency (1
to 12 megahertz) sound pulses into the body using a
2. The sound waves travel into the body and hit a
boundary between tissues (e.g. between fluid and
soft tissue, soft tissue and bone).
3. Some of the sound waves reflect back to the probe,
while some travel on further until they reach another
boundary and then reflect back to the probe .
4. The reflected waves are detected by the probe and
relayed to the machine.
In ultrasound, the following events
5. The machine calculates the distance from the probe
to the tissue or organ (boundaries) using the speed
of sound in tissue (1540 m/s) and the time of the
each echo's return (usually on the order of
millionths of a second).
6. The machine displays the distances and intensities
of the echoes on the screen, forming a two
DIAGNOSTIC AND THERAPEUTIC USE
• Diagnostic ultrasound is applied for obtaining
images of almost the entire range of internal
organs in the abdomen.
• Include kidney, liver, spleen, pancreas,
bladder, major blood vessels and the foetus
during pregnancy.it is based on echo aspect
and doppler shift aspect.
• Therapeutic ultrasound are based on the
thermal effects and cavitation effects
developed during the irradiation of ultrasound
on the body.
• Ultrasonography – ultrasonic energy is used to
detect the state of internal body organs.
PHYSICS OF ULTRASONIC WAVES
• Like other forms of sonic energy,it exists as a
sequence of alternate compressions and
rarefractions of a suitable medium.
• It is propogated through the medium at some
• Also depends on frequency of the sonic
energy and the density of the medium
through which it travels.
• Characteristic or specific acoustic impedance
of a medium is defined as the product of
density of the medium with the velocity of the
sound in the same medium.
It determines the degree of reflection and refraction
at the interface between two media.
• The percent of incident wave energy which is
reflected is given by
Z1 = acoustic impedance of medium 1
Z2 = acoustic impedance of medium 2
provided the ultrasonic beam strikes the
interface at a right angle.
• Greater the difference in acoustic impedance,
greater the amount of reflected energy.
• For eg., the acoustic impedance of air and
tissue are 42.8g/cm2 and 1.6x105g/cm2
• This difference is so large that most of the
ultrasonic energy tends to be reflected at the
Wavelength & frequency
• Ultrasonics follow the wavelength &
frequency relationship as
V = propagation velocity of sound
n = frequency
ʎ = wavelength
Propagation of velocity and absorption
• It is determined by the density of the medium it is
travelling through and the stiffness of the medium.
• Depth of penetration = velocity of sound in the
medium x time / 2
• The reduction of amplitude of ultrasonic beam while
passing through a medium can be due to its
absorption by the medium and its deviation from the
parallel beam by reflection, refraction, scattering and
• Ultrasonic waves are projected in a medium as a
• The beam enters the medium and two regions (near
& far field) are defined.
• In near field,ultrasound radiated from different parts
of the element travels as spherical waves that
interfere constructively and destructively.
• In far field, the ultrasound diverges and appears to
be coming from a point source located at the centre
of the transducer.
• Resolution of an ultrasound system is the system’s ability to
distinguish between closely related structures.
• Axial resolution – is the minimal axial distance, parallel to the
• It is determined by the wavelength of the transmitted pulse.
• Smaller the wavelenth, higher the frequency and better the
• Lateral resolution – is the lateral distance, perpendicular to
the beam axis
• It is determined by the shape/divergence of the ultrasound
beam, produced by the probe.
GENERATION & DETECTION OF
• The physical mechanism normally use to generate & detect
ultrasonic waves is the piezo- electric effect
• Which have the property to develop electric potentials on
definite crystal surfaces when subjected to mechanical strain
and vice versa.
• The effect is demonstrated by crystals of materials like
quartz,tourmaline and rochelle salt.
• Quartz is replaced by barium nitrate & lead zirconate titanate.
• They can be moulded to any shape to obtain better focusing
action for producing high intensity ultrasonic waves.
• Depend upon its applications, materials with high Q factor are
suitable as transmitters whereas with low Q & high sensitivity
are preferred as receivers.
• PZT are much better than quartz crystals upto 15
MHz,because of its high electro- mechanical conversion
efficiency and low intrinsic losses.
• At frequencies higher than this,quartz is normally used
because of its better mechanical properties.
• PVDF is another ferro- electric polymer used effectively in
high frequency transducers
• There are three parameters that are important in optimizing
transducers for various types of applications.
• These are frequency,active element diameter & focusing
• Ultrasound in the medical field used in both diagnostic and therapeutic
• The therapeutic equipment is designed to agitate the tissue to the level
where thermal heating occurs in the tissue and found to be quite sucessful
in the treatment of muscular ailments.
• In this, the system operate at several W/cm2
• In diagnostic equipment considerably lower ultrasonic power levels are
• It is used either as continuous waves or in pulsed wave mode.
• Applications making use of continuous waves depend on doppler’s effect.
Eg.,used as foetal heart detecter and blood flow measuring instruments.
• The majority of modern diagnostic ultrasound is based on pulse-
• It is used for the detection & location of defects or abnormalities in the
structures at various depths of the body.
BASIC MODES OF TRANSMISSION
• Pulsed ultrasound
• Ultrasound is transmitted in short bursts at arepitition rate
ranging from 1 – 12 kHz.
• Returning echoes are displayed as a function of time which is
proportional to the distance from the source to the interface.
• Burst duration is generally about 1µsec.
• It is used in most imaging applications.
• Continuous doppler
• Continuous ultrasonic signal is transmitted while returning
echoes are picked up by a separate receiving transducer
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