PHYSICS AND INSTRUMENTATION
• Sound waves are mechanical vibrations that
induce alternate reductions and compressions on
their passage through any physical medium.
• Sound waves are described by the following
terms: • Frequency (f): number of cycles per
• Wavelength (λ): distance between cycles (mm)
• Amplitude: extension of cycles, „loudness“
• Propagation velocity (c): depending on the
medium in which the sound waves travels(m/sec)
• Ultrasound is defined as sound with frequencies above
the for humans audible range between 20 Hz and
• Diagnostic medical ultrasound uses frequencies
between 1.000.000 and 20.000.000 Hz = 1 to 20
• By selecting transducers with different frequencies and
by adjusting the frequency on the machine display one
can select the emitted wave length.
• According to the wave equation: c = λ •f, a change in
frequency results in a reciprocal change in wave length.
• As the propagation velocity in the heart is 1540 m⋅s-1,
the emitted wavelength can be calculated as λ = c / f or
λ (mm) = 1,54 / f (f in MHz).
• Importance of wavelength for the ultrasound
• Image resolution: maximal 1 -2 wavelengths
(approx. 1 mm). The shorter the wavelength, the
higher the image resolution will be.
• Depth of penetration: proportional to the
wavelength, inversely related to thefrequency:
short waves travel short distances, long waves
travel long distances.
• Knobology: Select transducer and adjust
frequency to match the required penetration
depth while allowing for optimal image
• Interaction of ultrasound waves with tissue
• Reflection: a part of the ultrasound wave is
thrown back towards the transducer by an
• Scattering: a part of the ultrasound wave is
diffused into all directions by an object.
• Refraction: the direction of the ultrasound
wave is deflected from the straight path by an
• Attenuation: the energy of the ultrasound
wave is absorbed by conversion into heat.
An ultrasound transducer consists of many small,
carefully arranged piezoelectric elements that
are interconnected electronically. The frequency
of the transducer is determined by the thickness
of these elements. Each element is coupled to
electrodes, which transmit current to the
crystals, and then record the voltage generated
by the returning signals. An important
component of transducer design is the
dampening (or backing) material, which
shortens the ringing response of the
piezoelectric material after the brief excitation
The principles of piezoelectricity
• A piezoelectric crystal will vibrate when an
electric current is applied, resulting in the
generation and transmission of ultrasound
• Conversely, when reflected energy encounters
a piezoelectric crystal, the crystal will change
shape in response to this interaction and
produce an electrical impulse.
Resolution is the ability to distinguish between two
objects in close proximity.
It has at least two components: spatial and temporal
Spatial resolution is defined as the smallest distance that
two targets must be separated by for the system to
distinguish between them.
It, too, has two components:
Axial resolution refers to the ability to differentiate two
structures lying along the axis of the ultrasound beam
(i.e., one behind the other), and lateral resolution
refers to the ability to distinguish two reflectors that lie
side by side relative to the beam .
• A third component of resolution is called
contrast resolution. Contrast resolution refers
to the ability to distinguish and to display
different shades of gray within the image. This
is important both for the accurate
identification of borders and for the ability to
display texture or detail within the tissues.
• Temporal resolution, or frame rate, refers to the ability
of the system to accurately track moving targets over
time. It is dependent on the amount of time required
to complete a scan, which in turn is related to the
speed of ultrasound and the depth of the image as well
as the number of lines of information within the image.
Generally, the greater the number of frames per unit of
time, the smoother and more aesthetically pleasing the
realtime image. Factors that reduce frame rate, such as
increasing depth of field, will diminish temporal
resolution. This is particularly important for structures
with relatively high velocity, such as valves. Temporal
resolution is the main reason that M-mode
echocardiography is still a useful clinical tool
Pre processing versus post processing
• Preprocessing controls adjust transmission
and acquisition and formatting of ultrasound
signals to convert into electrical signals.
• changes in these affect the information to the
scanner on the basis of which image is
• post processing settings affect the manner in
which preprocessed information is displayed
on the monitor
• Monitor Calibration
• As for ordinary PC monitors, echo monitors
can be calibrated only in the room in which
the scanning will be performed. A bright room
will have different calibration results than a
dark room. It is important because when it’s
not calibrated bright enough you may lose
low-intensity signals and if it’s too bright
signals from strong reflectors may be over-
represented. With correct calibration you can
use all the brightness range the monitor can
• GAIN amplitude of electrical signals
• Generated by returning usg signals.
• Increase in gain increases the amount of
• 2D GAIN
• Increasing the 2D GAIN, which will increase
the amplitude of the returning ultrasound
signal,compensates for signal attenuation. The
cost of increasing the 2D gain is a decrease in
spatial resolution secondary to increased
• Adjust to the minimum required to obtain an
adequate image. There should be a complete
range from low (dark gray) to high (white)
Too low gain settings
• Only bright signals like pericardium are visible.
• Very low amplitude signals like SEC are
• Grey scale bar graph on right side of monitor
image guides to adjust low amplitude dark
gray to high aplitudes white signals.
• Clinical pearl :eliminate the effects of bright
ambient room lights
• TIME GAIN CONTROL (TGC)
• Also called depth compensation, these levers controls
gain for individual sectors of the display in a vertical
• TGC compensates for changes associated with variation
of US penetration at increasing depth; thus ensuring
that all signals will be of similar intensity regardless of
• The TGC allows amplification of the weaker signals
returning from the far field more than the signals
returning from shallower depths.
• Usually set so that controls for near field are lower
and far field higher - a “” shape. Some ultrasound
units automatically compensate for the attenuation
of the far field and therefore require a lower setting
in both the far field and near field—set these units in
• LATERAL GAIN COMPENSATION (LGC)
• The LGC control, which is not found on all
ultrasound machines, involves the horizontal
• LGC allows for selectively modulating the gain
at the lateral aspect of the image where there
may be more attenuation of signal due to
increasing scatter of the ultrasound waves
• This control adjusts the amount of acoustic
power transmitted by the US transducer. Since
acoustic power equals acoustic energy/time it
becomes evident that US can produce heat.
Adjust the power control to highest power
level within thermal limits (mechanical index
of approximately 0.3).
• DEPTH controls the depth of the image
displayed in one-centimeter gradation.
• The greater the depth,the less the resolution.
At a higher depth, the transducer needs to
cover a longer distance, therefore the frame
rate and the resolution are both lower.
• Set the depth at the minimum required to
visualize all structures of interest.
• DYNAMIC RANGE (DR) COMPRESSION
• The DR is the range of useful US signals expressed as
the ratio between largest and smallest signals.
• Usually about 100db of ultrasound information is
available, but the monitor can only display a much
smaller range, on the order of ~ 30 db.
• Therefore, in order to display the range of ultrasound
signals detected by the transducer the dynamic range
control allows for compression of the wide spectrum of
amplitudes. These compressed signals are then
displayed on the monitor as varying shades of gray.
• Awide system DR is necessary to display very weak
signals such as the endocardium and very strong
signals such as calcified valves.
• Logarithmic DR compression is a method to record all
ultrasound signals, i.e. a method to increase the DR of
the US system.
• Increasing the DR yields a higher number of
gray scale levels (increased spatial resolution
by increased contrast levels) and increased
image detail and smoother images.
Decreasing the dynamic range increases the
contrast of the image, with more black and
white areas than shades of gray. Set so that
blood filled cavities appear dark.
Decreased dynamic range Increased dynamic range
• The transmission FOCUS optimizes the ultrasound
intensity in near and far field resulting in
• Enables to focus the USG beam at aselected
distance from the transducer by altering the
sequences of electrical impulses sent to
transducer elements typified by phased array
• Goal as thinner beam improves lateral resolution
so beam should be narrowest at SOI
• CLINICAL PEARL
• Basically tells which depth we like to have the
best image resolution When evaluating for
PFO focus should be placed at this level of
atrial septum as structures distal to it appear
fuzzy or abnormally thick
• Denotes transmitted usg frequency which can be
adjusted according to proximity of structures to the
transducer esp in TEE “Res” translates to the highest
frequency band available on the transducer. These
settings are used for superficial imaging. “Gen”
represents mid-range frequencies that are often the
default setting. “Pen” represents the transducer’s
lowest range of frequencies and is for deeper tissue or
• For shallow structures use higher frequencies and for
deeper structures use lower frequencies.
• In an obese patient, the ultrasound waves have farther
to travel and are attenuated along the way. The lower
end of the frequency range should be used in obese
patients, which can compromise detail but allows for
When sound waves of a given frequency pass through tissue, harmonics are
produced at multiples of the initial frequency.
The tissue harmonics setting interprets one of these harmonics, filtering
out reverberation echoes allowing for a cleaner image with better contrast
and less artifact.
Certain applications, such as M mode LV measurements in these instances
tissue harmonics should be turned off.
Harmonics imaging allows the ultrasound to identify body tissue and reduce
artifact in the image. It does this by sending and receiving signals at two
different ultrasound frequencies. For example, with harmonics on, a probe
would emit a frequency of 2MHz, but it would only “listen” for a 4MHz
frequency. This improves image quality because body tissue reflects sound
at twice the frequency that was initially sent, which results in a cleaner
image that better displays body tissue without extra artifacts
• Post processing tool works in conjunction with
preprocessing dynamic range settings to alter
the range of displayed grey scale.
• Reducing compression gives largest dynamic
range of grey shades like “ matt finish”
• Increasing compression eliminates the display
of shades of grey at each spectrum resulting in
softer smoother image like “glossy finish”
Ultrasound Gray Maps
• Adjusting gray maps on your image has a
similar effect on an ultrasound image as
changing the dynamic range., but they are
different. While Dynamic Range adjusts the
overall number of shades of gray, a gray map
determines how dark or light you prefer to
show each level of white/gray/black based
upon the strength of the ultrasound signal
• This control enables elimination of greater
number of low intensity signals termed as
acoustic noise coming from refraction signals
from within the body and electric noise from
equipment itself or ventilators.
• Clinical pearl:increase reject control to
eliminate random echoes from low intensity
areas but care should be taken not to miss
pathology like thrombus in formation
• Adjusts the updating and averaging of consecutive
frames on the screen to reduce noise and speckling.
Increased persistence will smooth out the images
but sacrifices crispness of moving structures.
Decreased persistence will give a grainier image.
Increased Persistence Decreased Persistence
Higher levels of persistence are more desirable for slow moving structures and lower
levels for rapidly moving structures
• It controls the angle of the sector displayed on the monitor.
• Wide angles >90⁰ are used to survey broad array of cardiac
structures in a single frame
• But wider the sector size lower the frame rate and thus
reduced temporal resoltion
• For fast moving structures sector width should be kept
narrow so as to allow higher frame rates for better
resolution. Line Density
• Line density adjusts the number of scan lines in your
ultrasound image. A higher level provides better resolution
in the image (more scan lines), but reduces the frame rate.
• ROI is the area that defines where color will be
• Depth and width of ROI is inversely
proportional to frame rate and therefore
temporal resolution of color Doppler image.
• Similar to 2D gain as an amplifying tool but
probably more imp. Clinically because a low set
gain may miss flows of small leaks paravalvular or
across small pfo.high gain settings lead to
increased size of color signals such as of
regurgitant jets leading to overestimation.
• Clinical pearl adjust color gain until specles of
color lie outside ROI and then reduce one otr two
settings until the speckles go away also increase
gain if heart rate is increased >110bpm
• Displays the range of color velocities.
• It should be adjusted acc to expected
velocities at ROI if ROI is pulmonary veins then
low velocity blood flow is expected so
decrease color scale
• For PISA measurements baseline of color scale
nyquist limit is adjusted to get hemisphere
• This setting is given so as to distinguish
between laminar blood flow (which appears
as standard shades of blue or red depending
on its direction of flow in relation to
transducer)from turbulent flow jets by giving
it mosaic colors in shades of yellow and green.
• The Edge control varies the degree of
sharpness of borders and edge enhancements
within an image.
• Lower edge levels produce smoother
delineation with less noise. Higher edge
levels produce sharper images
• The zoom function allows magnifying region
of interest on the screen. It is important to
distinguish between ordinary zoom when the
image appears larger, but the resolution of the
magnified area does not change, and high
resolution zoom when the resolution in the
region of interest is increased. This function is
important when performing precise measures
such as left ventricular outflow tract or PISA
• Calipers are available for taking
measurements in each of the major operating
• This control enables tracing of cardiac
structures (2D) and velocity envelopes
Speckle Reduction Imaging
• Speckle Reduction Imaging (SRI,XRES,) uses
an algorithm to identify strong and weak
ultrasound signals. By evaluating the image on
a pixel-by-pixel basis, it attempts to identify
tissue and eliminate “speckle.” Weak signals
that seem to be astray are eliminated, while
strong signals are enhanced/brightented. It
provides a smoother, cleaner image
• Compound Images combines three or more
images from different steering angles into a
single image. Traditionally, transducers send
ultrasound signals in a single “line of sight.”
This means it sends a sound signal
perpendicular to the probe head, then listens
for the echo. With compound imaging, the
ultrasound sends signals at multiple angles,
allowing it to “see” tissue from multiple angles
and eliminate artifact
• Depth 15-17
• Frame rate near 100 /sec
• Sector width narrow
• Compression low
• Optimal 2d and color gain (50-60)
• Zoom for smaller measurements
• Decrease 2d gain for zoomed images
• Increase sweep speed for time measurements
• Use sweeps and decrease near field TGC to eliminate
• Use focus for smaller structures evaluation
• Power settings adjustment for contrast studies.