DOPPLER ECHOCARDIOGRAPHY-1
BASIC PHYSICS,PULSE WAVE AND
CONTINUOS WAVE DOPPLER
DR PRADEEP SREEKUMAR
• Sound is a mechanical vibration transmitted through
an elastic medium
• Ultrasound-portion of sound spectrum having
frequency greater than 20,000 cycles /sec
• Use of ultrasound to study the structure and function
of heart and great vessels-echocardiography
• Advantages of ultrasound
Can be directed as a beam and focussed
Obeys laws of reflection and refraction
Produce longitudnal waves
Generation Of An Ultrasound Image
Machines
There are 5 basic components of an ultrasound scanner that are required for
generation, display and storage of an ultrasound image.
1.
Pulse generator - applies high amplitude voltage to energize the crystals
2.
Transducer - converts electrical energy to mechanical (ultrasound) energy
and vice versa
3.
Receiver - detects and amplifies weak signals
4.
Display - displays ultrasound signals in a variety of modes
5.
Memory - stores video display
• Depicted as sine wave-peaks and troughs
• One cylce=one compression + one rarefaction
• Distance between 2 similar points represent
wavelength
• 0.15 to 1.5 mm in soft tissue
• Frequency- number of wavelengths per unit time
• V=f X λ(v=velocity,f =frequency, λ is wavelength)
• Velocity of sound=1540 m/sec in soft tissue
• Wavelength=1.54/f
• Amplitude
Measure of strength of the sound wave
Indicated by height of sine wave above and below
baseline
• Higher the frequency greater the resolution
• Higher frequency,lesser the penetration
• Loss of ultrasound as it propogates through a
medium is called attenuation
PRICIPLES OF PEIZO ELECTRIC CRYSTALS
The charges in a piezoelectric crystal are exactly
balanced, even if they're not symmetrically arranged.
The effects of the charges exactly cancel out, leaving
no net charge on the crystal faces
the electric dipole moments—vector lines separating
opposite charges—exactly cancel one another out.
If you squeeze the crystal , you force the charges out
of balance.
• Now the effects of the charges (their dipole
moments) no longer cancel one another out and net
positive and negative charges appear on opposite
crystal faces.
• By squeezing the crystal, voltage is produced across
its opposite faces- piezoelectricity
• The piezoelectric effect was discovered in 1880 by two French
physicists, brothers Pierre and Paul-Jacques Curie, in crystals of
quartz, tourmaline, and Rochelle salt (potassium sodium tartrate).
They took the name from the Greek work piezein, which means "to
press."
• The phenomenon of generation of a voltage
under mechanical stress is referred to as the
direct piezoelectric effect
• mechanical strain produced in the crystal
under electric stress is called the converse
piezoelectric effect.
• Ferro electrics,barium tianate,lead zirconate titanate
are used as peizo electric crystals.
• Dampening material-shortens the ringing response
Also absorbs backward and laterally transmitted
acoustic energy
• Frequency emitted by transducer is directly
proportional to propagation speed within crystal and
inversely related to thickness
• Important feature of ultrasound is ability to direct or
focus the beam as it leaves the transducer
• Proximal cylindrical and distally divergent
• Proximal zone –Fresnel zone
• Divergent field is called Fraunhofer zone
• Imaging is optimal in near field
• Decreasing wavelength or increasing transducer size
increase near field
Haemo”dynamics”
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Blood flow is a complex phenomenon
Not a uniform liquid
Flow pulsatile
Vessel walls are elastic
Properties of Blood
• Density-mass of blood per unit volume
• Measure of resistance to accelaration
• Greater the density,greater the resistance to
flow
• Viscosity:resistance to flow offered by fluid in
motion
• 0.035 poise at 37 degree.
Factors determining flow
• Flow rate is determined by
– Pressure gradient
– Resistance
• Viscosity of blood
• Radius of lumen
• Length of vessel
Types of flow
Laminar flow
Shape of parabola
Concentric layers,each parallel to vessel wall
Velocity of each layer differs
Maximal velocity is at centre of vessel
Decreasing profile towards peripheries
Turbulent flow
• Obstruction produce increased velocities, flow vortices
• Whirlpools shed off in different directions producing
variable velocities- chaos
• Predicted by Reynolds number
• Reynolds number depends on
Re=( ρ x c x D)/v
ρ-Density of blood
D-Vessel diameter
c-Velocity of flow
V-viscosity
The Reynolds number is dimensionless
If Re is less than 1200 the flow will be -laminar
1200-2000 flow is described as -transitional
Greater than 2000 -turbulent
Doppler Principle
First described by Johann Christian Doppler, an
Austrian mathematician and scientist who lived in the
first half of the19th century.
Doppler’s initial descriptions referred to changes in the
wavelength of light as applied to astronomical events.
In 1842, he presented a paper entitled "On the Coloured Light of Double Stars and
Some Other Heavenly Bodies" where he postulated that certain properties of light
emitted from stars depend upon the relative motion of the observer and the wave
source.
• Doppler effect describes the frequency shift of the
signal in relation to the relative motion of a source
and an observer.
• The wave generated by a source that moves away
from an observer/receiver appears to him to be of
lower frequency than the wave generated by a
stationary source, or generated by a source moving
toward the observer.
• The frequency of the signal detected by the receiver
moving toward the still source is higher, compared to
the frequency detected by the still receiver, or a
receiver moving away from the source.
• Applied in echocardiography to determine flow
direction,flow velocities,flow characteristics
• Stationary rbc-zero doppler shift(received
frequency= transmitted frequency)
• Positive doppler shift-RBCs moving towards
transducer ,received frequency >transmitted
frequency
• Negative doppler shift:RBC’s moving away from
transducer- transmitted frequency more than
receiving frequency
• Doppler shift represents difference between
received and transmitted frequencies ,which
occur due to motion of RBC’s relative to the
ultrasound beam
• Fd = (2f V cos Ø)/C
Why the factor 2?
• Double doppler shift
1st shift-transducer stationary source,RBC the
moving receiver
2nd shift is when,RBCs are moving source and
transducer is the stationary receiver.
Doppler equation- rearranged
Factors affecting doppler equation
• Estimation of blood flow velocity is dependent on
incident angle between ultrasound beam and blood
flow
• When RBCs parallel-maximum velocity
• When RBCs perpendicular-no doppler shift
• When angle between ultrasound beam and blood
flow is less than or equal 20 degree,cosine close to 1
and percent error is less than or equal to 7%
The Effect of Angle
Angle
0
10
20
30
60
90
Cosine
1
0.98
0.94
0.87
0.5
0
Percentage error
0
2
7
13
50
100
Angle correction
• It is possible to correct for angle
• Not recommended as in most cases its possible to
align ultrasound beam parallel by utilising multiple
views, serial assessment difficult unless same angle
correction used
• It is assumed that angle between ultrasound beam
and direction of blood flow is parallel
• By adjusting according to the direction of assumed
flow, it changes the angle calculations in the Doppler
equation resulting in different estimates of flow
velocity.
• The use of this control does not actually change the
direction of the Doppler beam and its use does not
alter the quality of either the audio output or the
spectral recording
Effect of frequency
• Lower the frequency,higher the velocity detected
• A 2 MHz transducer detects higher velocity
compared to a 5 MHz transducer
SPECTRAL DOPPLER DISPLAY
Flow velocity
• Displayed on y axis
• Velocity of RBCs within sampled volume is calculated
• Absence of velocity-zero baseline
Spectral velocity recordings
Direction of flow
• Flow direction also displayed on Y axis
• Positive doppler shift-flow towards transducer
Traditionally displayed above baseline
• Negative doppler shift-flow away from transducer
Displayed below zero baseline
Intensity or amplitude
• Blood cells do not move at equal velocities
• Produce different frequency shifts
• Amplitude or intensity of doppler signal reflects
number of blood cells moving within a range of
velocities at a particular point of time
• Bright signal-strong doppler shift frequency at a
particular point of time .
• Darker regions-weak doppler shift
Timing
• Time is displayed along x axis
• Displayed along with ECG.
• Change in blood velocity,flow direction can be
accurately timed in relation to cardiac cycle.
Doppler Audio signals
• Doppler shift frequencies are in audible range
• Guide for localising blood flow and for proper
aligning ultrasound beam parallel to flow
• Laminar flow-smooth tone
• Turbulent flow-harsh sound.
Pulsed and Continuous Wave Doppler
Continuous Wave Doppler
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older and electronically more simple
continuous generation of ultrasound waves
continuous ultrasound reception
two crystal transducer
Blood flow along entire beam is observed
• ADVANTAGE
ability to measure high blood velocities
accurately
• DISADVANTAGE
1)lack of selectivity or depth discrimination
2)no provision for range gating to allow selective
placing of a given Doppler sample volume in
space
Pulsed Wave Doppler
• Ultrasound impulses are sent out in short bursts
or pulses
• transducer that alternates transmission and
reception of ultrasound
• ability to provide Doppler shift data selectively
from a small segment along the ultrasound
beam- sample volume can be selected.
• The transducer functions as receiver for a limited
time period
• Time corresponds to the interval required for sound
to return from specified area.
• Another burst of sound waves are not transmitted
until previous impulses are received.
• Pulse repetition frequency (PRF)–frequency at which
transducer transmits pulses.
• PRF determines sampling rate.
Inability to accurately measure high blood
flow velocities- aliasing
“Alias” means false
Aliasing
• The aliasing phenomenon occurs when
the velocity exceeds the rate at which
the pulsed wave system can record it
properly
Fig.1.24
Aliasing is represented on the spectral trace as a cut-off of a
given velocity with placement of the cut section in the opposite
channel or reverse flow direction
Nyquist Limit
• The Nyquist limit defines when aliasing will
occur using PW Doppler.
• The Nyquist limit specifies that measurements
of frequency shifts (and, thus, velocity) will be
appropriately displayed only if the pulse
repetition frequency (PRF) is at least twice the
maximum velocity (or Doppler shift frequency)
encountered in the sample volume.
The Nyquist Limit
The simplest sound wave is an oscillation between two
amplitudes.
A sampled waveform thus needs at least two sample
points per cycle.
Thus the wave's frequency must not be above half the
sampling frequency.
This limit is called the Nyquist limit of a given sampling
frequency
• Shannon's sampling theorem
(Claude E. Shannon, born 1916, American
mathematician)
Also known as the Nyquist criterion, a general
"rule" for measurement of frequencies, stating
that the measurement (sampling) frequency
must be at least twice the maximum
frequency to be measured.
Whenever Shannon's sampling theorem is not
fulfilled, aliasing occurs
• Nyquist limit specifies the maximum velocity
that can be recorded without aliasing.
Avoiding aliasing
Increase the Nyquist limit1)altering variables in Doppler equation
2)high PRF mode
3 )Change from PW to CW
• V =
C × PRF
4 f COS Ø
(PRF= Δf× 2=
2f V cos Ø × 2)
C
Max velocity can be increased by
1)Increasing PRF
2)Decreasing transmitted frequency
3)Increasing speed of sound in tissue
4)Decreasing cosØ
High PRF Doppler
• It is desirable to use as high a PRF as possible
for recording abnormally elevated velocity
jets.
• Maximum PRF is limited by the distance the
sample volume is placed into the heart.
Range ambiguity
• some of the range selectivity used in precisely
locating the sample volume is lost.
• pulsing sequence is carried on over and over,
some data is returned to the transducer
• data from all these volumes are added
together
Baseline shift ("zero shift" or "zero offset" )
Electronic cut and paste
Moves the aliased doppler signal upward or
downward(unwrapping)
Repositioning baseline effectively increases the
maximum velocity at the expense of other direction.
"baseline shift"
Pulse Wave VS Continuous Wave Doppler
Depth resolution
Sample volume
High velocity detection
Aliasing
Sensitivity
Control of sample volume placement
CW
no
large
yes
no
more
poor
PW
yes
small
no
yes
less
good
Continuity equation
• The continuity equation states that the
amount of blood flow through one cardiac
chamber (or valve orifice) is the same as the
blood flow through the other chambers and
orifices
• It is based on the principle of conservation of
mass.
• “Whatever mass flows in must flow out.”
The Continuity Equation
VELOCITY TIME INTEGRAL
• Calculation of volumetric flow is complex as
flow velocity is not constant
• Blood flow is pulsatile
• Hence integrated velocity over time is taken
• VTI is equal to area under curve cm/sec X sec.
• Measure of distance that blood moves with
each heart beat.
• VTI is also referred to as “stroke distance”
• Volumetric flow or stroke volume can be thus
calculated as
SV=CSA X VTI
Continuity equation can be rewritten asCSA1 X VTI 1=CSA2 X VTI 2
CSA2 =(CSA1 x VTI 1) /VTI 2
• VTI is obtained by tracing the leading edge of
velocity spectrum.
Clinical Applications
• Calculation of valve areas
• Calculation of regurgitant volumes and fractions
• Calculation of regurgitant orifice areas
• Calculation of intra cardiac shunt ratios
LIMITATIONS OF CONTINUITY
PRINCIPLE
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Erroneous determination of CSA
Measurement of diameter during wrong phase of
cardiac cycle.
Inconsistent annulus measurements
Erroneous determination of VTI
Incorrect placement of pulse wave sample
volume
Significant angle between doppler beam and
blood flow
Incorrect filter/gain settings
• Bernoulli's principle is named after Swiss
mathematecian Daniel Bernoulli who published his
principle in his book Hydrodynamica in 1738
Bernoulli equation
• Bernoulli described the conversion of energy in a fluid from
one form to another, as occurs when fluid flow in a tube
that suddenly either increases or decreases its diameter.
• Bernoulli's law states that total energy at all points along a
tube is the same—conservation of energy.
• Energy (the ability to do work) is composed of pressure
energy (pressure x the volume of fluid),kinetic energy (fluid
in motion has kinetic energy ,which is proportional to the
mass and the velocity squared; mass is density x volume),
and gravitational energy (the product of density, volume,
height above a surface, and the gravitational constant).
Limitations of Bernoulli Equation
• Significant flow acceleration-prosthetic valves
• Significant viscous forces-muscular VSD,tunnel
subaortic stenosis
• Increased proximal velocity-AS+AR,AS + HOCM
• Altered blood viscosity-polycythemia
Discrepancies between Catheter
derived and Doppler derived
Pressure Gradients
• Pressure gradients derived at cardiac
cathetrisation and measured by Doppler may
differ.
• Catheter derived gradient is peak to peak
gradient between LV and Aorta
• Non simultaneous measurement
• Doppler gives peak instantaneous gradient
and is greater than peak to peak gradient.
PRESSURE RECOVERY PHENOMENON
• Complex hemodymanic concept- pressure of fluid
decreases as velocity increases.
• Once flow passes through a narrowing pressure
drops and increases towards original value
• Rate and magnitude of pressure recovery is variable
• In prosthetic valves,3 effective orifices-2 large
orifices by sides and a central small orifice.
• Maximum velocity and lowest pressure is at
narrowest orifice.
• Immediately distal to orifice, pressure increases
(recovers)and velocity decreases.
• Doppler gradients are measured at narrowest orifice,
while catheter gradients are recorded downstream
to prosthetic valve where the pressure has already
recovered.
• Hence, Doppler derived pressure gradients are more
compared to catheter derived pressure gradients.
Determination of Pressure Gradients
Pressure gradients are useful for assessment of
severity of valvular stenosis and estimation of
intracardiac pressures .
Pressure gradients commonly determined by
Doppler include
1)Maximum instantaneous pressure gradient
2)Mean pressure gradient
• Maximum Instantaneous pressure gradient =4v2
(v=peak velocity)
• Mean pressure gradient cannot be obtained from
the mean (or average) velocity, but must be
calculated by making multiple instantaneous gradient
(and hence pressure calculations) measurements,
and then averaging those instantaneous pressures.
Estimation of RVSP by TR
• TR doppler signal represents the pressure difference
between RV and RA during Systole
• RVSP-RAP=4(V TR 2 )
• In absence of RVOT obstruction
RVSP=Pulmonary artery systolic pressure.
RVSP= BP SYSTLOLIC -4(V vsd )2
PASP=BP SYSTOLIC -4(V PDA )2
Mean Pulmonary Artery Pressure=4 (V PR-PEAK)2
PAEDP=4 (V PR-ED)2 + RVEDP
LAP= BP SYSTOLIC -4 (V MR)2
LVEDP =BP DIASTOLIC - 4 (V AR-ED)2
Pressure half-time and
deceleration time
The pressure half-time (PHT) is defined as the time (in
milliseconds) required for the peak initial pressure to
drop by one half .
Doppler signal measures velocity
The modified Bernoulli equation is applied to convert
velocity to pressure.
PHT is the time (in milliseconds) for the velocity to drop
to 0.707 of the maximum velocity .
V2 = (0.707) x V1
 The DT is the time required for the
velocity,beginning with the peak value, when
extrapolated, to cross the zero baseline
 The PHT is clinically used most frequently in
evaluating mitral stenosis and aortic
regurgitation
MVA = 220/PHT
PHT = 0.29 xDT
MVA
=220/0.29 x DT
=759/DT
Limitations of PHT in calculation of
MVA
• Non linear early diastolic slope
• Post balloon mitral valvuloplasty
• Significant Aortic regurgitation
• Cardiac rhythm disturbances
Non linear early diastolic slope
• Non linear or curvilinear decay –lead to
erroneous calculation of PHT.
• Part of slope considered most representative
should be chosen.
Post balloon mitral valvuloplasty
• Accuracy of calculated MVA by PHT declines
immediate post BMV.
• PHT is directly related to chamber compliance
and peak transmitral gradient
• Following BMV,abrupt changes in left atrial
pressure and compliance occur altering the
relationship between PHT and Mitral valve
area.
• This effect on PHT lasts 24 to 48 hours.
Significant Aortic regurgitation
• Severe AR shortens PHT.
• This is due to markedly elevated LVEDP which
reduces diastolic pressure gradient between
LA and LV.
• Mitral valve area is thus overestimated.
Cardiac rhythm disturbances
• Tachycardia-deceleration slope is
shortens,PHT shortens and thus valve area is
overestimated.
Proximal Isovelocity Surface Area
(PISA)
• Proximal Isovelocity Surface Area (PISA) method is
based on the continuity equation.
• When a flow passes through a narrow orifice, as it
approaches the narrowest region, there is a flow
convergence and flow acceleration.
• PISA is the surface area of the hemisphere at the
aliasing region of the flow convergence.
• PISA increases with lower aliasing velocity.
• Radius is measured from the orifice to point of
colour change.
• If the flow convergence is not a true hemisphere, the
angle subtended by the flow convergence at the
orifice has to be measured and divided by 180 to get
a correction .
• Good correlation between angiographic estimates of
regurgitant flow and PISA based estimates have been
reported.
Valve area calculation
A0=(2 Л r2x VN)/V0
A0=area of narrowed orifice
V0=velocity at narrowed orifice
R=radius of shell
VN=aliased velocity identified as Niquist limit
Angle correction
PISA principle is for flow approaching narrow
planar surface
In Mitral Stenosis,mitral leaflets may be funnel
shaped
To account for altered shape,angle corection
factor ά /180 is applied.
MVA= ( ά /180 ) x (2 Л r2x VN)/VMS
Limitations of PISA method
Errors in measurement of radius
Valve area is proportional to square of radius-even
small errors are magnified.
Errors in measurement of angle
Measurement is done offline using a protractor
Angle measured in one dimension may not be true
representation of valve leaflet geometry.
THANK YOU
MCQs
Maximum detectable velocity for a 5 MHz
probe ,(assuming Doppler shift frequency as 5
KHz ,speed of sound in tissue as 1500m/sec
and doppler beam being parallel to blood
flow) ,would be
a)0.62 m/sec
b)0.75 m/sec
c)0.92 m/sec
d)1.0 m/sec
Percentage error when ultrasound beam
makes an angle of 20 degrees to blood column
a)3
b)7
c)15
d)30
When frequency of ultrasound probe is
reduced,
a)Resolution increases
b)Maximum detectable velocity increases
c) Attenuation increases
d)All of the above
Following statements are true about pulse
wave doppler compared to continuous wave
doppler except
a)Better depth resolution
b)Larger sample volume
c)More problem of aliasing
d) Less sensitivity
Resistance to flow through a tube is directly
proportional to
a)Fourth power of radius
b)Second power of radius
c)Length of tube
d)None of the above
Turbulent flow typically occurs when reynolds
number just exceeds
a)800
b)1000
c)2000
d)5000
Pulse wave is preferred to continuous wave
doppler to assess flow in all except
a)Pulmonary vein
b)Superior venacava
c)Pulmonary valve
d)Aortic valve
Doppler shift is directly proportional to all except
a)Speed of sound in tissue
b)Velocity of blood flow
c)Frequency of transducer
d)Cosine of angle between interrogating beam and
blood flow
True about Mitral valve area assessment by
PHT in severe AR is
a)PHT increases
b)Valve area is underestimated
c)MS severity is underestimated
d)Increases diastolic gradient between LA and
LV.
Mean pulmonary artery pressure of a patient
with TR jet velocity of 3 m/sec, Peak PR
velocity of 2m/sec is (in mm Hg)
a)36
b)16
c) 9
d)20
KEY
Maximum detectable velocity for a 5 MHz
probe ,assuming Doppler shift frequency as 5
KHz ,speed of sound in tissue as 1500m/sec
and doppler beam being parallel to blood
flow, would be
a)0.62 m/sec
b)0.75 m/sec
c)0.92 m/sec
d)1.0 m/sec
Percentage error when ultrasound beam
makes an angle of 20 degrees to blood column
a)3
b)7
c)15
d)30
When frequency of ultrasound probe is
reduced,
a)Resolution increases
b)Maximum detectable velocity increases
c) Attenuation increases
d)All of the above
Following statements are true about pulse
wave doppler compared to continuous wave
doppler except
a)Better depth resolution
b)Larger sample volume
c)More problem of aliasing
d) Less sensitivity
Resistance to flow through a tube is directly
proportional to
a)Fourth power of radius
b)Second power of radius
c)Length of tube
d)None of the above
Turbulent flow typically occurs when reynolds
number just exceeds
a)800
b)1000
c)2000
d)5000
Pulse wave is preferred to continuous wave
doppler to assess flow in all except
a)Pulmonary vein
b)Superior venacava
c)Pulmonary valve
d)Aortic valve
Doppler shift is directly proportional to all except
a)Speed of sound in tissue
b)Velocity of blood flow
c)Frequency of transducer
d)Cosine of angle between interrogating beam and
blood flow
True about Mitral valve area assessment by
PHT in severe AR is
a)PHT increases
b)Valve area is underestimated
c)MS severity is underestimated
d)Increases diastolic gradient between LA and
LV.
Mean pulmonary artery pressure of a patient
with TR jet velocity of 3 m/sec, Peak PR
velocity of 2m/sec is (in mm Hg)
a)36
b)16
c) 9
d)20