Basic Principles of Ultrasound & Basic term
Produced by International Clinical Application Team
Defines Ultrasound (US) as :
sound with frequency greater than 20,000 cycles per second or 20kHz.
US can direct as a beam.
It obeys the laws of reflection and refraction.
It is reflected by objects of small size.
It propagates poorly through a gaseous medium.
The amount of US reflected depends on the acoustic mismatch.
Reflection and Propagation :
• Effect of propagation through gaseous zones
- poor propagation, inadequate imaging.
• Effect of propagation through dense zones
- nearly all of the US is reflected. Structures below dense zones are poorly imaged
Examples of dense materials - bone, calcium, metal.
• Cycle - the combination of one rarefaction and one compression equals one cycle.
the distance between the onset of peak compression or cycle to the next.
• Velocity - the velocity is the speed at which sound waves travel through a
particular medium. Velocity is equal to the frequency x wavelength.
The velocity of US through human soft tissue is 1540 meters per second.
• Frequency - the number of cycles per unit of time. Frequency and wavelength
are inversely related. The higher the frequency the smaller the wavelength.
• Acoustic Impedance - simply put, acoustic impedance is dependent on the
density of the material in which sound is propagated through. The greater
the impedance the more dense the material.
The portion of a sound that is returned from the boundary of a medium. (echo)
The change of sound direction on passing from one medium to another.
• Acoustic Mismatch
The boundary between two different media where reflection and refraction occurs
The decrease in amplitude and intensity as a sound wave travels through a medium
<Types of Echoes>
• Specular : echoes originating from relatively large, regularly shaped objects with
smooth surfaces. These echoes are relatively intense and angle dependent.
(i.e. IVS, valves)
• Scattered : echoes originating from relatively small, weakly reflective, irregularly
shaped objects are less angle dependant and less intense.(i.e.. blood cells)
Frequencies for adult imaging - 2.0mHz to 3.0mHz.
Frequencies for pediatric imaging - 5.0mHz to 7.5mHz and 10mHz.
Effect of higher frequencies on penetration - the higher the frequency the less
penetration, the lower the frequency the greater the penetration.
Acoustic Shadowing - the loss of information below an object because the
greater portion of the sound energy was reflected back by the object. This
occurs in objects like prosthetic valves.
Enhancement - the increase in amplitude from objects that lie behind a weakly
attenuating structure. Enhancement may occur in structures below a cyst.
Reverberation - produced from the multiple reflections from an object as the
sound energy bounces back and forth between the object and the transducers
face or dense structure.
• Lateral resolution
the ability to resolve objects side by side. Lateral resolution is proportionally affected by
the frequency, the higher the frequency the greater the lateral resolution. Higher
frequency transducers are used in fetal and pediatric echocardiography because the
lateral resolution displays the smaller structures in those patients and there is less need
for depth penetration. Lower frequencies are used for adults where structures are larger
and the need for greater depth penetration is important.
• Axial Resolution
Axial resolution is the ability to resolve objects that lie one above the other. Axial
resolution is inversely proportional to the frequency of the transducer depending on the
size of the patient. The higher the frequency the lower the axial resolution is in large
patients. This state results from the rapid absorption of the ultrasound energy with
lower penetration. Lower frequencies are utilized to increase depth of penetration.
• Depth of Penetration
Higher frequencies are attenuated by tissue more than lower frequencies. This means
that the higher the frequency the greater the resolution but the lower the depth of
penetration. User lower frequencies for adults and higher frequencies for children. The
advent of harmonic imaging allows the use of a lower frequency pulse to be picked up
and sampled at a higher frequency (the second harmonic) where the low frequency
allows greater penetration and high frequency provides better resolution.
Axial Decrease Increase
Lateral Decrease Increase
Penetration Increase Decrease
<Basic Components of the Imaging System>
the probe housing the elements, backing material, electrodes, matching layer
and protective face that both sends and receives the sound waves.
• Transmitter - the component that creates the impulses sent to the transducer
to generate sound energy. Also called the pulser.
• Receiver - the component that receives the current generated in the transducer
from the returning sound energy.
• Amplifier - the component that amplifies the returning signals and prepares
them to be displayed on the CRT.
• Depth : Adjusts the depth from which the returning ultrasound signals will be
displayed. Most ultrasound systems have a maximum depth of 24cm.
The deeper the depth the slower the frame rate of the image. A shallow depth
results in a higher frame rate.
• Output gain (power) : Adjusts the amount of power used to send ultrasound into
the body. The higher the output gain the stronger the returning signal and the
better The signal to noise ratio.
• Receiver gain : Adjusts the amount of power used to amplify the returning
ultrasound signal. Higher receiver gains result in higher levels of signal and noise
in the image.
• TGC : Depth gain compensation adjusts the amount of power used to amplify the
returning ultrasound signals at a specific depth. Higher TGC levels result in higher
levels of signal and noise at a given depth. The TGC sliders are used to ensure a
uniform display of the gray scale intensity throughout the display. Strong signals
can be decreased and weak signals can be increased at specific depths
• Focal zone : Modern phased array transducers can be dynamically focused.
As the focal zone control is adjusted up and down a marker can be seen moving
up and down a marker can be seen moving up and down pointing at the
current zone of focus.
• Transmit frequency : Most of today's cardiac ultrasound systems have phased
array transducers which use wide band width technology. These wide band width
Transducers can be set to selectively receive anywhere from 2 to 4 frequencies
within a given range.
• Reject :
Filters out low-level signals so that only the stronger returning signals are
displayed. Reject can be used to decrease the amount of noise in the image if the
noise has a significantly lower intensity than the signals returning from
• EE (Edge Enhancement) :
The boundaries between strong and week returning signals are enhanced
by increasing the strong signal and decreasing the weak signal.
• DR (Dynamic range) :
Determines the number of gray shades used to map the gray scale image on
the display. Higher compression results in more shades of gray (a softer looking
image). Lower compression results in a more bi-stable (black and white)
image with fewer shades of gray.
• Post processing :
Sometimes referred to as gamma curves. Post processing
of the gray levels allows the user to determine the gray scale intensity that will
be assigned to the different returning signals.
• Persistence (Frame Average) :
Normally a frame is built by memory and then immediately sent to the display.
Real image data can be enhanced and noise can be averaged out by performing
• Sweep speed :
The display rate for the M-mode or Doppler data. Common display rates are
12.5, 25, 50 and 100 mm/sec
• Sample size :
The length of the pulsed Doppler sample gate can be adjusted from 1 to 10 mm
on most ultrasound systems. Increasing the sample gate, increases the amount
of time that the system samples (listens for) returning signals.
• Sample position :
The pulsed and continuous wave Doppler cursor position can be adjusted from side
to side across the field of view. The pulsed Doppler sample volume can be adjusted
up and down within the field of view but is limited to the range of the selected PRF.
• PRF :
The pulse repetition frequency is the number of times that a pulse (pulsed wave
Doppler) is repeatedly sent during one second. The PRF determines the scale of the
pulsed Doppler display and is labeled ‘scale’ on most ultrasound systems.
• Filter :
The low velocity component of the pulsed or continuous wave Doppler signal can
be rejected by adjusting the Doppler filters.
• Baseline shift :
Flow towards the transducer is displayed above the spectral Doppler baseline.
Flow away from the transducer is displayed below the spectral Doppler baseline.
The position of the baseline can be adjusted up and down within the spectral Doppler
Pulsed wave Doppler (PW) :
Pulsed wave Doppler uses a single transducer element which sends out short bursts of
ultrasound. Range resolution (the ability to localize and analyze blood flow at specific
depths) can be achieved by sampling the returning bursts of ultrasound at various time
intervals. The major limitation of standard PW Doppler
Is its inability to measure high velocities.
Advantage Is range specific thus allowing for the measurement of various flow
characteristics in small selected regions of interest.
Disadvantage Spectral wrap around (aliasing) at relatively low velocities
Continuous wave Doppler (CW) :
Continuous wave Doppler uses two side by side transducers elements. One element
continuously sends (transmits) while the other element continuously receives. This
method will allow maximum velocities to be measured but has no range resolution CW
Doppler samples all blood flow in its path. A spectrum of Doppler frequencies are
observed. Only the highest velocity in the Doppler path can be clearly measured. It is
possible however to distinguish between flow patterns which have different time and
Advantage Can display high velocities without aliasing.
Disadvantage Displays all velocities in its path. The non-imaging probe requires a
High degree of skill to operate.