Characteristics of Sound
• Requires medium for its transmission.
• Compression and rarefaction.
• Longitudinal waves
• Length of wave
• Wave once generated continue in original
direction until it is either reflected, refracted or
Velocity of Sound
• Independent of frequency
How velocity is related to:
• Converts an electrical signals into ultrasonic
energy that can be transmitted to tissues & vice
• Constituents :-
1. Piezoelectrical crystals
2. Two electrodes
3. Backing block
4. Acoustic insulator(rubber )
5. Plastic housing
• Two basic modes of transducers
1. Continuous mode – doppler studies
2. Pulse mode
Transducer Q factor
purity of sound & length of sound
•High Q transducer
•Low Q transducer
Advantage of High frequency over low frequency
Depth resolution is superior & near zone is long
Tissue absorption increases with increased
• Reflected wave sound produces image
• It depends on
1. Acoustic impedance
2 Angle of incidence
• Impedance is product of tissue density with
velocity of sound in the material.
Angle of incidence
Higher the amount of angle less is the amount
of sound reflected
Specular interface Examples
diaphragm,walls of the vessels
boundaries of many organs
• Bending of waves as they pass from one medium
to another is called refraction
• Interfaces that are either smaller than the
wavelength or not smooth are nonspecular
• It means conversion of ultrasonic to thermal
• It depend on
1. Frequency of sound
2. Viscosity of medium
3. Relaxation time
• A Mode– displayed as spikes projecting from baseline. Spike
height is proportional to echo intensity. It is static one-
• TM Mode– echoes produced by moving structures as dots.
• B Mode– produces a picture of a slice of tissue. Echoes
displayed as dots.
• Gray scale Imaging– It’s a B mode type displaying large
amplitude of echoes arising from tissues as varying shades of
• Real time imaging–
produces multiple images in a very short period
• Two types
1. Mechanical– Oscillating & Rotating wheel
2. Electronic array– Linear array & phased or
• Doppler ultrasound is a technique for making
non-invasive velocity measurements of blood
• Christian Doppler was the first to describe the
frequency shift that occurs when sound or
light is emitted from a moving source.
• Ultrasound is transmitted into a vessel and
the sound that is reflected from the blood is
detected. Because the blood is moving, the
sound undergoes a frequency (Doppler) shift.
Uses of doppler
• Determination of anatomy.
• Determination of small vessel flow distribution.
• Detection of vascular dilatation, obstruction.
• Evaluation of intravascular disease and
• Detection of extra vascular flow.
1) Continuous wave doppler
• Without B MODE imaging
2) Pulsed wave doppler
• Pulsed Doppler ultrasound is a technique for measuring
the velocity of blood in a small sample volume
• Used in general & obstetric ultrasound & also used to
provide data for doppler sonograms & colour flow images.
Disadvantage – “Aliasing”
3) Color Doppler
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