{ ECHO BASICS PHYSICS AND INSTRUMENTATION - DR. NAIR ANISHKUMAR P.K.V.
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Transcript of { ECHO BASICS PHYSICS AND INSTRUMENTATION - DR. NAIR ANISHKUMAR P.K.V.
{
ECHO BASICS
PHYSICS AND INSTRUMENTATION
- DR. NAIR ANISHKUMAR P.K.V
Mechanical vibration transmitted through an
elastic medium.
Spectrum of sound
Sound
• Ultrasound can be directed as a beam and
focused
• As ultra sound passes through a medium it
obeys laws of reflection and refraction
• Targets of relatively small size reflect
ultrasound thus can be detected and
characterised.
Advantages for Diagnostic utility
• Ultrasound is poorly transmitted through
a gaseous medium
• Attenuation occurs rapidly, Especially at
higher frequency.
Disadvantages
Particles of the medium vibrate parallel to the
line of propagation producing longitudinal waves.
Areas of compression alternates with areas of
rarefaction.
Amount of reflection , refraction and attenuation
depends on acoustic properties of medium
Denser medium reflect higher percentage of
sound energy
Mechanics :
Mechanics :
The loss of ultrasound as it propagates through a
medium is referred to as attenuation
It is the rate at which the intensity of the ultrasound
beam diminishes as it penetrates the tissue.
Attenuation has three components: absorption,
scattering, and reflection
INTERACTION BETWEEN ULTRASOUND AND TISSUE
Always increases with depth
It is affected by the frequency of the transmitted beam
and the type of tissue through which the ultrasound
passes
The higher the frequency is, the more rapidly it will
attenuate
Attenuation increases with increase in density of
medium.
Attenuation
Expressed as the half-power distance, which is a �measure of the distance that ultrasound travels before
its amplitude is attenuated to one half its original
value.
As a rule of thumb, the attenuation of ultrasound in
tissue is between 0.5 and 1.0 dB/cm/MHz.
Attenuation
The velocity and direction of the ultrasound beam as
it passes through a medium are a function of the
acoustic impedance of that medium
Acoustic impedance (Z, measured in rayls) is the
product of velocity (in meters per second) and
physical density (in kilograms per cubic meter).
Acoustic impedance
Acoustic impedance
The phenomena of reflection and refraction obey the
laws of optics and depend on the angle of incidence
between the transmitted beam and the acoustic
interface as well as the acoustic mismatch, i.e., the
magnitude of the difference in acoustic impedance’
Use of a acoustic coupling gel during transthoracic
imaging
Acoustic impedance Importance :
The interaction between an ultrasound beam and a
reflector depends on the relative size of the targets
and the wavelength of the beam
As the size of the target decreases, the wavelength of
the ultrasound must decrease proportionately to
produce a reflection and permit the object to be
recorded.
Specular echoes and scattered echoes
Specular echoes are produced by reflectors that are
large relative to ultrasound wavelength
The spatial orientation and the shape of the reflector
determine the angles of specular echoes.
Examples of specular reflectors include endocardial
and epicardial surfaces, valves, and pericardium
Specular echoes
Specular echoes
Targets that are small relative to the wavelength of
the transmitted ultrasound produce scattering
Such objects are referred to as Rayleigh scatterers.
The resultant echoes are diffracted or bent and
scattered in all directions.
Scattered echoes
Scattered echoes contribute to the visualization
of surfaces that are parallel to the ultrasonic
beam and also provide the substrate for
visualizing the texture of grey-scale images
The term speckle is used to describe the tissue-
ultrasound interactions that result from a large
number of small reflectors within a resolution
cell.
Scattered echoes
Without the ability to record scattered echoes, the
left ventricular wall, for example, would appear as
two bright linear structures, the endocardial and
the epicardial surfaces, with nothing in between .
High-frequency ultrasound though has good
resolution , is reflected by many small interfaces
within tissue, resulting in scattering, much of the
ultrasonic energy becomes attenuated and less
energy is available to penetrate deeper into the
body..
Importance :
THE TRANSDUCER
Piezoelectricity
A period of quiescence during which the transducer
listens for some of the transmitted ultrasound energy to �be reflected back is known as dead time.�
The amount of acoustic energy that returns to the
transducer is a measure of the strength and depth of
the reflector.
The time required for the ultrasound pulse to make the
round-trip from transducer to target and back again
allows calculation of the distance between the
transducer and reflector
Piezoelectricity
Piezoelectric ceramics : ferroelectrics, barium
titanate, and lead zirconate titanate
Piezoelectric elements 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.
Piezoelectricity
The dampening material shortens the ringing
response of the piezoelectric substance after the
brief excitation pulse.
An excessive ringing response (or ringdown) �lengthens the ultrasonic pulse and decreases range
resolution.
Thus, the dampening material both shortens the
ringdown and provides absorption of backward and
laterally transmitted acoustic energy
Backing material
At the surface of the transducer, matching layers
are applied to provide acoustic impedance
matching between the piezoelectric elements and
the body.
This increases the efficiency of transmitted
energy by minimizing the reflection of the
ultrasonic wave as it exits the transducer surface.
Matching layers
An ultrasound beam as it leaves the transducer is
parallel and cylindrically shaped beam. Eventually,
however, the beam diverges and becomes cone
shaped .
The proximal or cylindrical portion of the beam is
referred to as the near field or Fresnel zone.
When it begins to diverge, it is called the far field or
Fraunhofer zone.
Wave motion
Imaging is optimal within the near field
The length of the near field (l) is described by the
formula:
where r is the radius of the transducer and λ is the
wavelength of the emitted ultrasound.
Near field
From the above formula optimal ultrasound imaging :
large-diameter & high-frequency transducer maximize
the length of the near field.
Near field
Factors preventing this approach from being
practical.
1) The transducer size is predominantly limited by
the size of the intercostal spaces.
2) Although higher frequency does lengthen the near
field, it also results in greater attenuation and lower
penetration of the ultrasound energy
Near field
the ultrasound beam is both focused and steered
electronically
it is primarily achieved through the use of
phased-array transducers, which consist of a
series of small piezoelectric elements
interconnected electronically
MANIPULATING THE ULTRASOUND BEAM
By adjusting the timing of excitation, the beam
can be steered
Dynamic transmit focusing
Near field Focusing
An undesirable effect of focusing is its effect on
beam divergence in the far field. Because focusing
results in a beam with a smaller radius, the angle
of divergence in the far field is increased.
Divergence also contributes to the formation of
important imaging artefacts such as side lobes
Resolution is the ability to distinguish
between two objects in close proximity.
two components:
spatial
temporal.
Resolution
It is defined as the smallest distance that two targets
can be separated for the system to distinguish
between them.
Two components:
Axial resolution
lateral resolution
Spatial resolution
Ability to differentiate two structures lying along the
axis of the ultrasound beam
The primary determinants are the frequency of the
transmitted wave and its effect on pulse length.
Axial resolution
the ability to distinguish two reflectors that lie
side by side relative to the beam
affected by the width or thickness of the
interrogating beam, at a given depth
lateral resolution diminishes as beam width
(and depth) increases.
Lateral resolution
The distribution of intensity across the beam profile
will also affect lateral resolution
both strong and weak reflectors can be resolved within
the central portion of the beam, where intensity is
greatest.
Lateral resolution
Gain is the amplitude, or the degree of
amplification, of the received signal.
Gain
Contrast resolution refers to the ability to distinguish
and to display different shades of grey
For accurate identification of borders display texture or detail
within the tissues.
Useful to differentiate tissue signals from background noise.
Dependent on target size.
A higher degree of contrast is needed to detect small
structures
Contrast resolution
Ability of the system to accurately track moving
targets over time.
It is dependent on speed of ultrasound and the depth
of the image as well as the number of lines of
information within the image.
Greater the number of frames per unit of time, the
smoother and more aesthetically pleasing the real-
time image.
Temporal resolution
CREATING THE IMAGE
The pulse, which is a collection of cycles traveling
together, is emitted at fixed intervals
TRANSMITTING ULTRASOUND ENERGY
How one can use ultrasound to obtain an image of an object.
Modes :
SIGNAL PROCESSING
Dynamic range is the extent of useful ultrasonic
signals that can be processed to reduce the range of
the voltage signals to a more manageable number
It is defined as the ratio of the largest to smallest
signals measured at the point of input to the display
It is expressed in decibels
Concept of dynamic range
The range of voltages generated during data
acquisition, by post-processing, is transformed to 30
shades of grey which the human eye is able to
distinguish
Grey scale :
The new frequencies generated due to nonlinear
interactions with the tissue ,which are integer
multiples of the original frequency, are referred to
as harmonics.
The returning signal contains both fundamental
and harmonic frequencies. By suppressing or
eliminating the fundamental component, an image
is created primarily from the harmonic energy
Tissue harmonic imaging
After destructive interference the remaining
harmonic energy can then be selectively
amplified, producing a relatively pure harmonic
frequency spectrum.
The strong fundamental signals produce intense
harmonics and weak fundamental signals
produce almost no harmonic energy thus
reducing artefacts.
The net result is that harmonic imaging reduces
near field clutter ,the signal-to-noise ratio is
improved and image quality is enhanced.
Tissue harmonic imaging
Side lobes occur because a portion of the energy
concentrate off to the side of the central beam and
propagate radially, a phenomenon known as edge
effect
A side lobe may form where the propagation
distance of waves generated from opposite sides of
a crystal differs by exactly one wavelength.
Side lobes are three-dimensional artefacts, and their
intensity diminishes with increasing angle.
ARTIFACTS : Side lobes
The artefact created by side lobes occurs because
all returning signals are interpreted as if they
originated from the main beam.
A prerequisite for a dominant side lobe artefact is
that the source of the artefact must be a fairly
strong reflecting target like The atrioventricular
groove and the fibrous skeleton of the heart
ARTIFACTS : Side lobes
Result from the beam reflecting from the
transducer or from other strong echo-producing
structures within the heart or chest .
Typically, a reverberation artefact that originates
from a fixed reflector will not move with the
motion of the heart.
It appears as one or more echo targets directly
behind the reflector, often at distances that
represent multiples of the true distance
ARTIFACTS : reverberations
Shadowing occurs beyond a region of unusually high
attenuation, such as a strong reflector.
It results in the absence of echoes directly behind
the target .
Eg: prosthetic valves & heavily calcified Native
structures.
ARTIFACTS : shadowing
ARTIFACTS : shadowing
Ring down artefact arises from high-amplitude �oscillations of the piezoelectric elements.
This only involves the near field
Eg : right ventricular free wall or left ventricular
apex
ARTIFACTS : near field clutter
ARTIFACTS : near field clutter
Doppler imaging is concerned with the direction,
velocity, and then pattern of blood flow through
the heart and great vessels.
The primary target is the red blood cells
It focuses on physiology and hemodynamics
The Doppler equations rely on a more parallel
alignment between the beam and the flow of
blood.
DOPPLER ECHOCARDIOGRAPHY
The increase or decrease in frequency due to
relative motion between the transducer and the
target is referred to as the Doppler shift.
It is the mathematical relationship between the
magnitude of the frequency shift and the velocity
of the target relative to the source
Doppler shift
the Doppler shift (∆f) depends on the transmitted
frequency (f₀ ) of the ultrasound, the speed of sound
(c ), the intercept angle between the interrogating
beam and the flow ( ө ), and, finally, the velocity of the
target (v ).
Doppler shift
Because the velocity of sound and the transmitted
frequency are known, the Doppler shift depends on
the velocity of blood and the angle of incidence, ( ө )
Doppler shift
Transducer or carrier frequency is the primary
determinant of the maximal blood flow velocity
that can be resolved
A lower frequency is advantageous because it
allows high flow velocity to be recorded.
Doppler shift
Five basic types
1 ) continuous wave Doppler
2 ) pulsed wave Doppler
3) color flow imaging
4 ) tissue Doppler
5 ) duplex scanning
Doppler Formats
It is similar to echocardiography. Short, intermittent
bursts of ultrasound are transmitted into the body
and listens at a fixed and very brief time interval �after transmission of the pulse.
This permits returning signals from one specific
distance from the transducer to be selectively
received and analysed, a process called range
resolution
Pulsed wave Doppler
The number of pulses transmitted from a Doppler
transducer each second is called the PRF.
To accurately represent a given frequency, it must
be sampled at least twice, that is
This formula establishes the limit (Nyquist limit)
below which the sampling rate is insufficient to
characterize the Doppler frequency.
Aliasing
This Imaging simultaneously transmits and receives
ultrasound signals continuously.
2 types
1) Transducer employs two distinct elements: one to
transmit and the other to receive
2) With phased-array technology, one crystal within
the array is dedicated to transmitting while another
is simultaneously receiving.
Continuous wave Doppler
A major advantage of continuous wave Doppler
imaging is that aliasing does not occur and very high
velocities can be accurately resolved.
A form of pulsed wave Doppler imaging that uses
multiple sample volumes to record the Doppler
shift
By overlaying this information on a two-
dimensional or M-mode template, the colour flow
image is created.
Based on the strength of the returning echo ,
flow velocity, direction, and a measure of
variance are then integrated and displayed as a
colour value
Colour Flow Imaging
The primary determinant of jet size is jet momentum,
which depends on both flow rate and velocity. Thus,
factors that affect velocity, including blood pressure, will
also affect jet size.
If colour Doppler imaging is performed when blood
pressure is either very high or very low, this clinical
information should be noted and taken into account
when the study is interpreted.
Technical Limitations of Color Doppler Imaging
The eccentric jets that become entrained along a
wall, making them appear smaller than they actually
are (Chamber constraint ).
For similar reasons, chamber size can also influence
the apparent area of a colour flow jet
Technical Limitations
By adjusting the colour scale, PRF is altered, and jet
size can change dramatically.
By lowering the scale (or Nyquist limit), the lower
velocity blood at the periphery of the jet becomes
encoded and displayed, making the jet appear larger.
Increasing the wall filter will reduce the jet size by
excluding velocities at the periphery.
Technical Limitations : Instrument settings
Power and instrument gain will also alter jet size.
Increasing these settings will increase jet area.
Transducer frequency has a complex effect on
colour jet area.
The jet size will tend to increase with high carrier
frequency because of the relationship between
velocity and the Doppler shift. On the other hand,
greater attenuation at higher frequency will make
jets appear smaller.
Technical Limitations : Instrument settings
Doppler imaging records velocity, not flow. It cannot
distinguish whether the moving left atrial blood
originated in the ventricle (the filled triangles) or
atrium (the filled circles), simply
that it has sufficient
velocity to be detected.
(billiard ball effect)
Related directly to the Doppler principle. For
example, aliasing occurs when pulsed wave
Doppler techniques are applied to flow velocities
that exceed the Nyquist limit
Mirror imaging / crosstalk :the appearance of a
symmetric spectral image on the opposite side of
the baseline from the true signal
Doppler Artifacts
Shadowing may mask colour flow information
beyond strong reflectors
Ghosting is a phenomenon in which brief swathes
of colour are painted over large regions of the
image
It is produced by the motion of strong reflectors
such as prosthetic valves..
Doppler Artifacts
Too much gain can create a mosaic distribution of
color signals throughout the image.
Too little gain eliminates all but the strongest
Doppler signals and may lead to significant
underestimation of jet area.
Doppler Artifacts
By adjusting gain and reject settings, the
Doppler technique can be used to record the
motion of the myocardium rather than the
blood within it
1) adjusting the machine to record a much lower
range of velocities
2) additional adjustments to avoid oversaturation
because the tissue is a much stronger reflector of the
Doppler signal compared with blood.
Tissue Doppler Imaging
One obvious limitation is that the incident angle between
the beam and the direction of target motion varies from
region to region.
This limits the ability of the technique to provide absolute
velocity information, although direction and relative
changes in tissue velocity are displayed.
The biologic effects of ultrasound energy are
related primarily to the production of heat
the amount of heat produced depends on the
intensity of the ultrasound, the time of exposure,
and the specific absorption characteristics of the
tissue.
BIOLOGIC EFFECTS OF ULTRASOUND
The perfusion of tissue have a dampening
effect on heat generation and physically allow
heat to be carried away from the point of
energy transfer.
Limited imaging time, occasional repositioning
of the probe, and constant monitoring of the
probe temperature help to ensure an
impeccable safety record
BIOLOGIC EFFECTS OF ULTRASOUND
Cavitation : Formation and behaviour of gas
bubbles produced when ultrasound penetrates
into tissue
Because of the relatively high viscosity of blood
and soft tissue, significant cavitation is unlikely.
BIOLOGIC EFFECTS OF ULTRASOUND
Few reports have suggested that some changes
might occur at the chromosomal level that would be
relevant to the developing foetus .
No evidence that any of physical phenomena
(oscillatory, sheer, radiation, pressure, and micro-
streaming ) has a significant biologic effect on
patients.
BIOLOGIC EFFECTS OF ULTRASOUND
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