Echo and Doppler Physics
Dr George Abraham, Royal Free Hospital.
• Vibrations that transfer energy from place to
place without the transfer of matter.
• Sound waves travel through a medium and set
up patterns of disturbance – vibrations.
• Light is a transverse wave and sound is a
• Amplitude is maximum disturbance from the resting state
• Frequency is the number of waves produced by the source every second
• Wavelength of a wave is the distance
between a point on one wave and the
same point on the next wave.
What’s the frequency?
• Measured in Hz – 1
• Normal range of human
• kHz = 10,000; MHz =
• Sound waves travel fastest in
solids, then liquids, then gases.
• The speed of sound in the
heart is 1560 m/s
Velocity of a wave
• 𝑉 = 𝑓 λ
• Velocity = frequency X wavelength
• Frequency = 1/time period
• Higher frequencies will thus give a shorter
• They can reflect off a smooth surface in a
• Rough surfaces scatter waves in all directions.
• Diffraction happens when waves pass through
• Refraction happens when waves change speed
entering a different density medium.
What can waves do?
US and Reflection
• Frequencies above 20kHz are Ultrasound.
• US Machines use frequencies 1.5-7 MHz.
• In ultrasound, a detector placed near the source of the
US can measure the time taken for the sound to
bounce back and hence the distance from the source.
• Pulse Repetition Period or PRP is the time between
the onset of one pulse till the onset of the next pulse.
This parameter includes the time the pulse is “on” and
the listening time when the ultrasound machine is
• Pulse Repetition Frequency is the number of
pulses/second and relates to frame rate.
Refraction happens when waves pass from one
medium to another at an angle and change speed.
• The resolution of a recording is the ability to
distinguish 2 objects close together.
• Higher frequencies have shorter wavelengths
hence higher resolution.
• As the ultrasound waves pass through a medium
they gradually lose energy as heat.
• In the body, the depth of penetration is around
• At 1 MHz, the penetration is 30 cm
• At 5 Mz, the penetration is 6 cm
• Tissue density X Propagation velocity
• This in addition to the angle of incidence,
determines how much of the US is reflected:
the strength of the signal.
High frequency beam with a short wavelength and
high pulse repetition frequency.
Depends on how narrow the
beam is: the ‘focus’
• PZT crystals are materials that vibrate and then transduce
an incoming signal to electricity.
• Backing materials reduce ringing after the current is gone
• Matching layers reduce impedance
• A single crystal
receiver modes with
• Thus gives excellent
and can be used to
• As sound waves move from a source towards
an observer the waves shorten in wavelength
and increase in frequency.
• As sound waves move from a source away
from an observer the waves lengthen in
wavelength and decrease in frequency.
• The change between the frequency
transmitted and received is the doppler shift
• The amount of doppler shift depends on the
velocity and this can be calculated
Doppler spectral analysis
• The frequency range of doppler shift is
• -10 to + 10 kHz hence within the audible range
• The spectral display is a speed/time graph with
red blood cell velocities detected along the
beam at any given time point are depicted
ranging from zero to the peak velocity
• The brightness of the signal denotes the
amplitude of the signal received corresponding
to that velocity
• In CW Doppler the echo machine uses
continuous transmission and receiving of US
signals by 2 dedicated crystals
• The CW Doppler obtains signals along the
entire cursor line
• It cannot assess flow at any one specific point
• It can measure high velocities without aliasing
The phenomenon of aliasing means PW doppler displays will not
represent the highest velocities accurately.
The sampling frequency of the PW probe has to be more than twice the
frequency of the wave.
Important Points for Revision
• Relationship of velocity to frequency and wavelength
• Know the speed of sound in tissues
• Understand principles of image formation by
measurement of reflected US waves from the
• Understand how moving sources give a doppler shift
• Know the limitations of PW doppler and CW doppler
• Know the simplified Bernoulli equation
- David Houghton
Frequency is number of oscillations per second and is measured in Hertz.
Wavelength is the distance between two successive waves. This is measured in metres.
Velocity is the average speed of movement of the wave across a medium. This is 1540 m/s in the heart.
The wavelength is the propagation velocity divided by the frequency.
Concept of compression waves
There are areas in a medium where the particles are closer together which is compression. There are other areas where the particles are further apart. In all of these areas there will be variation in the wave conducted across the medium e.g velocity will be greater or less.
Wavelength is the distance between two identical points on the waves and is measured in metres
The Ultrasound waves used for echocardiography are between 1.5-7 MHz. The Velocity of these waves in 330 m/s in air and 1570 m/s in blood.
Doppler shifts are an increase in the frequency of sound coming from an object moving toward you. This is evidenced in our daily lives e.g if a train is coming toward you the sound it generates increases in frequency. In the same way if blood is coming toward you its frequency increases. If blood is moving away from you its frequency decreases. This is used as the principle for colour doppler in which the areas in which the frequency is changed are colour labelled. This is usually blue for blood moving away from you and red for blood moving toward you. If this is displayed as a graph then velocity toward you is displayed above and velocity toward is displayed below the baseline.
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