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 =