Ultrasonic Ultrasonic
Doppler ModesDoppler Modes
Piero TortoliPiero Tortoli
Information Engineering DepartmentInformation Engineering Department
Università degli Studi di Firenze
Christian CachardChristian Cachard
CREATISCREATIS
Université Claude Bernard Lyon 1
OutlineOutline
• Doppler effect
• CW/PW Doppler systems building-blocks
• Pulsed Wave (PW) mode:
PRF, sample volume, spectral broadening, mean frequency estimation…
Advanced Doppler systems and methods:
�Single-gate (TCD, Duplex)
�Multi-gate
�Flow-imaging
�Power, Harmonic & Tissue Doppler imaging
• Doppler artefacts (aliasing, blooming…)
Doppler effectDoppler effect
Change in the observed frequency of a wave, due to motion
Fixed Tx and Rx
Rx approaching
Tx
Rx receding from
Tx
fr = ft fr > ft fr < ft
A moving reflector / scatterer returns echoes with:
• higher frequency if it is approaching the source/receiver
• lower frequency if it is moving away from the source/receiver
If the transmitter and the receiver are still but a reflector or scatterer is moving, the Doppler effect
takes place in the same way as in A) or B)
Doppler effectDoppler effect
Doppler effectDoppler effect
Doppler effectDoppler effect
f0 (or ft ) transmitted frequency
ϑ angle between directions of sound propagation and of target path
c sound wave velocity
f0 = 5 MHz
c= 1500 m/s
ϑ= 60°
v = 30 cm/s
fd ≅ 1 kHz
vc
ffd ×= ϑcos2
0
Doppler frequency: difference between Tx and Rx frequencies:
0d Rf f f= −
Doppler equationDoppler equation
The Doppler frequency shift is proportional to:• the target velocity, v• the transmitted frequency, f0• cos ϑ (angle between directions of sound propagation and of target path)
� it decreases as ϑ� 90°� in particular is 0, when ϑ = 90° (probe
perpandicular to the vessel axis)
vc
ffd ×= ϑcos2
0
Doppler equationDoppler equation
The frequency ratio is equal to the velocity ratio
vc
ffd ×= ϑcos2
0
Simple case: ϑ = 60°; cos ϑ = 1/2
0
df v
f c=
Vessel Speed range (cm/s) Doppler frequency range (Hz)
Carotid artery 100 - 150 2000 - 3000
Ascending aorta 20 - 290 400 - 5800
Descending aorta 25 - 250 500 -5000
Abdominal aorta 50 - 60 1000 - 1200
Femoral artery 100 - 120 2000 – 2400
Inferior cava vein 15 - 40 300 - 800
Arterioles 0.5 - 1 10 - 20
Capillaries 0.02 - 0.17 0,4 - 3,4
Blood velocitiesBlood velocities
02 cosd
ff v
cϑ= × f0 = 3 MHz
ϑ = 60°; cos ϑ = 1/2
Doppler frequency detection:Doppler frequency detection:
Audio outputAudio output
All Doppler frequencies fall in the audio range
�The sound produced by loudspeakers
provides immediate (but qualitative and
operator-dependent) information on blood
movement
Jugular vein Common carotid artery
Spectral analysis of the Doppler signal allows distinct velocity contributions to be discriminated
In Doppler spectrograms, subsequent spectra are grey-scale coded and displayed in adjacent vertical lines
Fre
quen
cy (
kH
z)
Time (s)
Doppler frequency detection: Doppler frequency detection:
Spectral AnalysisSpectral Analysis
Doppler instrumentationDoppler instrumentation
� 2 - 6 MHz
Abdominal ultrasound, obstetrical and gynaecological
exam, echocardiography, transcranial Doppler;
� 7.5 - 14 MHz
Small parts, vascular Doppler;
� 10 - 20 MHz
Ophthalmology, special vascular exam;
� 20 - 50 MHz
Intra-Vascular UltraSound (IVUS), ultrasound
biomicroscopy (ophthalmology, dermatology);
Ultrasound Doppler equipmentUltrasound Doppler equipmentHandheld systems
(fetal monitoring,
PAOD…)
Portable systems (TCD, bedside echo-
cardiography…)
Advanced systems (assessment of stenosis, hemodynamics, heart valve function, TDI…)
Integrated Ultrasound equipment
Doppler systems building blocks: Doppler systems building blocks:
Continuous Continuous WaveWave (CW) systems(CW) systems
Transmitter
Flow
Distincttransmitting and receiving transducers
US energy is
continuously transmitted into the body
Receiver
Processing
Audiooutput
Display
1515
CW Doppler (Continuous Wave Doppler)
( )
++=+= t
DtDtPtdtpt
rs ωωω
0cos
00cos
0)()(
p(t): signal from non moving tissue,
transmitted frequency
d(t): signal from moving tissue,
transmitted frequency + Doppler frequency
f0= 3 MHZ
f0++fD = 3,001 MHZ
CW systems: benefits and drawbacksCW systems: benefits and drawbacks
TXRX
• Large investigated volume
� easy transducer positioning
Flow
� no possibility of selecting the region for investigation
� no discrimination between
different flow contributions
� No aliasing (no ambiguity)
� strong “clutter”
Echoes backscattered from the region where TX and RX
beams overlap, are
integrated in the receiver
Used only in cardiologyUsed only in cardiology
2nd Flow
1717
• The velocity of wall vessel is in the range 5 à 10 mm/s
• The Doppler frequency induced by the wall motion is in the range 10 to 30 Hz
• High pass filter: wall filter
Wall motionWall motion
1818
Frequency spectrum of the Doppler signal
Doppler systems building blocks: Doppler systems building blocks:
Pulsed wave (PW) systemsPulsed wave (PW) systems
Transmitter
Flow
·A single transduceracts as transmitter and receiver
Bursts of US energy are transmitted into the body at rate PRF
GateReceiver
Processing
Display
Audiooutput
• Selection of the R.O.I. �
� possible discrimination
between different flow
contributions
� possible difficulties in
transducer positioning
� Risk of aliasingFlow
Only the echoes
backscattered from the selected sample volume are
gated in the receiver
PW systems: benefits and drawbacksPW systems: benefits and drawbacks
Used in most advanced instrumentsUsed in most advanced instruments
Doppler equationDoppler equation
vc
ffd ×= ϑcos2
0
A moving scatterer returns echoes with:
• fr > f0 ; fd = (fr – fo) > 0 if it is approaching the source/receiver • fr < f0 ; fd = (fr – fo) > 0 if it is moving away from the source/receiver
Doppler receiver architectureDoppler receiver architecture
× LPF HPF
LPF HPF
90°
L.O.
AFromtransducer
Q
I
Complexdemodulator Sample
& Hold
Band-pass filters
Gate
Low-noiseamplifier
•The In-Phase & Quadrature channels are needed to distinguish positive from negative Doppler shifts
•In recent systems, the echo-signal is sampled at rf (digital processing)
×
The wall filter suppresses
the signal from tissue
PW receiver: Gate (Sample and Hold)PW receiver: Gate (Sample and Hold)
Range gate
TX burst
Received echoes
1st wall 2nd wall
vesselUS transducerskin
•Bursts are transmitted at PRF rate: for each TX burst, one
sample of the Doppler signal is obtained (time sampling).
• The electronic gate selects the information backscattered
only from the region of interest (sample volume).
PW systems: Sample Volume (SV) PW systems: Sample Volume (SV)
SV depth
SV length
Transducer
Blood/tissue volume contributing to the Doppler signal
• The depth and the length can be set by the operator
• The width depends on the transducer features/settings
� Better resolution
� Worst S/NSmall SV
SV width
Pulse Repetition Frequency Pulse Repetition Frequency
(PRF)(PRF)
The rate (1/PRI) at which the bursts are transmitted is called PRF
• The delay of each echo is proportional to the depth of
the target• The PRF should be low enough that a new burst is not
transmitted before the last echo from the max depth (Dmax) has not come back (PRFmax = c / (2xDmax)
For each Pulse Repetition
Interval (PRI), one burst of few cycles at frequency fo is transmitted.
1 / fo
Application of spectral analysis:Application of spectral analysis:
Stenosis detection in the carotid arteryStenosis detection in the carotid artery
a. Healthy
subject
b. Patient with proximal stenosis
(turbulence)
(Courtesy of Johan Thijssen)
The flow is interrogated by a range of angles around the
nominal Doppler angle, ϑ
beam axis
flow direction
ϑ +∅∅∅∅
∅∅∅∅
The corresponding Doppler spectrum
extends over a range around the nominal Doppler frequency
ϑ ----∅∅∅∅
Geometric spectral broadeningGeometric spectral broadening
Maximum and mean frequencyMaximum and mean frequency
Mean frequency curveMean frequency curve
Mean frequency estimation:Mean frequency estimation:
CrossCross--correlationcorrelationτ
SN(t) : Nth Doppler echo
SN+1(t): [N+1]th Doppler echo
τ can be estimated as the value maximizing:
dt ) (tS (t)S 1NN τ++∫
When such echoes are stored in a digital memory, τcorresponds to the shift needed to make them overlap
Quantification of Doppler Quantification of Doppler
waveformswaveforms
Pulsatility index (1-10):
PI = (A-B)/mean
= (peak systole-peak diastole)mean
Resistance index (0-1):
RI = (A-C)/A
= (peak systole-end diastole) peak systole
(Courtesy of Johan Thijssen)
Peripheral vascular Peripheral vascular ““damping factordamping factor””
Damping factor:
DF = PI2 / PI1
(Courtesy of Johan Thijssen)
EchoEcho--Doppler (Duplex) PW systemsDoppler (Duplex) PW systems
• A (M-mode) scan line
can be superimposed
to a B-mode image
• Over the scan line, a
specific sample volume
can be selected
• The Doppler signal
produced from the
gated sample volume
is analysed
PW-Mode
Ideal for stenosis assessment
EchoEcho--Doppler (Duplex) examplesDoppler (Duplex) examples
Healthy Common Carotid Artery Stenotic Internal Carotid Artery
Flow imaging modeFlow imaging mode
• Real-time 2D velocity maps are obtained
– By firing several pulses for each scan line
– By estimating the mean frequency (velocity) detected at each depth
– By color-coding consecutive pixels according to the detected mean frequencies
– By scanning a 2-D region
• Frame rate limitations
• Poor sensitivity
• Unsuitable for low velocities Multigate processing applied to multiple scan lines
Power Doppler modePower Doppler mode
• For each scan line, the power of Doppler echoes is detected and integrated (persistance)
☺High S/N
☺ Ideal for small vessels
� All movements (including “slow” blood and tissue movements) are detected
� Qualitative (velocity magnitude is ignored)
Same hardware as for
Doppler imaging: software differences
Un
idirectio
na
l P
DB
idirection
al P
D
Doppler modesDoppler modes
withwith
contrast agentscontrast agents
Echo enhancement Echo enhancement
Injection of US contrast agents (microbubbles in a shell) generates strong backscattering (echo enhancement)
Useful for small, deep, hardly accessible
vessels (eg:TCD analysis)
Harmonic Doppler modeHarmonic Doppler mode
• Non-linear behaviour of US contrast agents yields 2nd harmonic (2×ft ) components much stronger than those generated by tissue
Need for wideband transducers – Suitable for perfusion assessment
Harmonic Doppler modesHarmonic Doppler modes• Detection of 2nd harmonic echoes allows to reverse the roles of blood & tissue in US images• More sophisticated TX-RX strategies (eg: pulse inversion) allow further increments of blood/tissue ratio to be obtained
ConventionalUS imaging
HarmonicDoppler imaging
Pulse inversion
Doppler imaging
Tissue Doppler ImagingTissue Doppler Imaging
(TDI)(TDI)
Tissue Doppler imaging (TDI)Tissue Doppler imaging (TDI)
Signal
from
tissue Signal from blood
Conventional
wall filter
• The wall filter partially suppresses
the echo-signal from tissue
• In TDI, the blood signal is suppressed!
Frequency/velocity
Power spectral density
Signal
from
tissue Signal from blood
Conventional
wall filter
Power spectral density
Tissue Doppler images of left ventricle Tissue Doppler images of left ventricle
Diastole Systole
High signal, low velocity image
(Courtesy of Johan Thijssen)
Doppler angle ambiguitiesDoppler angle ambiguities
• Frequency/color changes due to a change in the angle of insonation
• If the angle is not known, the frequency cannot be
converted to velocity
The detected frequency depends on the Doppler angle
ϑcos2 0fc
vfd =
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