Doppler Echocardiography

Joyce Meng M.D.

7/16/2008

Doppler vs. B-mode Echo- complementary roles

Primary target is the red blood cell

Examine the direction, velocity, and pattern of blood flow through the heart and the great vessels.

Primary target are the myocardium and the heart valves

Provides information about the shape and movement of cardiac structures.

Outline

Doppler EffectContinuous wave DopplerPulse wave DopplerColor DopplerTissue Doppler

Christian Doppler

Australian mathematician and physicist

Published his notable work on the Doppler effect at the age of 39

Was Gregory Mendel’s physics professor in the University of Vienna.

Doppler Effect

The pitch of sound was affected by motion toward or away from the listener

Sound moves toward the listener, frequency increases, pitch rises.

Sound moves away from the listener, frequency decreases, pitch falls.

Doppler effect applied to Echocardiography Transducer emits

ultrasound reflected from RBC.

If RBC (flow of blood) moves toward transducer, frequency of the reflected sound’s wavelength increases

If RBC (flow of blood) moves away from the transducer, frequency of the reflected sound’s wavelength decreases

Mathematical relationship

Fd: Doppler shift= F[r] (received frequency)- F[t] (transmitted frequency)

F0: Transmitted frequency of ultrasound V: velocity of blood. : intercept angle between the interrogation beam and the target Can solve for V=Fd(C)/2f0(cos

Why do we care about the velocity of blood flow?

Modified Bernoulli’s equation: P= 4v2

Gives us the ability to estimate pressure differences between two chambers (i.e, TR) Stenotic valves (i.e. AS)

Angle of the Doppler beamcos (0°)= 1cos (10°)= 0.98cos (20°)= 0.94cos (30°)= 0.87cos (60°)= 0.5cos (90°)= 0

Fd= 2f0(V)(cos )/C

Fd V(cos ) Misalignment of the

interrogation beam will lead to underestimation of the true velocity

Becomes significant when is >20°

Carrier frequency

V=Fd(C)/2f0(cos ) If Fd stays the same, the lower the f0 (carrier frequency),

the higher the velocity of the jet that can be resolved. Unlike B-mode imaging where higher frequency

transducer gives better resolution, here lower frequency transducers gives better resolution.

Spectral analysis

The difference in waveform between the transmitted and backscattered signal is compared.

A process called fast Fourier transform (FFT) displays this information into a “spectral analysis” (spectral display of entire range of velocities)

Time- x axis Velocity- y axis Toward the transducer is

positive, away from transducer negative.

Amplitude is displayed as “brightness” of the signal.

Continuous wave doppler

Two dedicated crystals- one for transmitting and one for listening

Receives a continuous signal along the entire length of the ultrasound beam

Disadvantage- don’t know where the signal comes from.

Advantage- can measure very high Doppler shift/velocities.

Most useful when trying to discern maximal velocity along a certain path (AS, TR…etc).

Clinical example- AS

The position of the doppler beam is 2-D guided.

In the GE system, it’s indicated by a single line

Profile is usually filled in- velocity along the path that is below the maximal velocity also represented.

Problematic cases

Don’t know where the maximal velocity comes from

Serial stenosis- LVOT obstruction or AS?

Problematic cases

AS or MR?

Pulse wave doppler

Short intermittent busts of ultrasound are transmitted. Only “listens” at a brief time interval Permits returning signal from one specific distance to be

selectively analyzed- “range resolution” Sample volume

Clinical Examples

position of doppler beam 2-D guided

In GE system, the sample volume is indicated by double lines

Spectral envelope not filled in

Common use- mitral inflow velocity and LVOT velocity

Aliasing

Sampling rate is inadequate to resolve the direction of flow

PRF (pulse repetition frequency)- number of pulse transmitted from the transducer/second

Nyquist limit= PRF/2 Cannot resolve higher frequency (velocity) sound waves

Aliasing

Tends to happen at higher velocity jets Doppler shift is has higher frequency- needs

higher PRF to resolve the direction of the wave.

Aliasing

Tends to happen in at greater depth

Sample volume at a shallow site- can interrogate more frequently (higher PRF)

Sample volume at deeper site- cannot interrogate as often (lower PRF)

High PRF imaging

Shallower sample volume associated with a higher PRF- less likely to have aliasing

Listening window will also sample returning signal from twice that depth Velocity from both sites will be recorded Disadvantage: ambiguity Advantage: Higher velocities can be analyzed without aliasing

Color Doppler

pulse wave Doppler with multiple sample volume along multiple raster lines

direction, velocity and variance determined for each sample volume

Color Doppler

Displayed as color information- Amplitude- intensity Direction- red vs blue (toward or away from

transducer) Velocity- brightness (bright blue higher velocity) Variance (turbulence)- coded green to give a

mosiac apperance. Overlays this information on 2D images Time consuming (temporal resolution is

especially poor with a large sector window) Different vendors have different algorithms for

generating color Doppler

Example of Color Doppler

Color Doppler jet with aliasing in the center due to high velocity

Color Doppler jet encoded with variance

Semiquantitative method

Important to remember that color codes velocity and not actual volume!

Angiography- contrast is actual regurgitation

Color doppler encodes “billard ball effect”- color may encode non-regurgitant blood that is “pushed around” by the regurgitant jet.

Semiquantitative method

Measures velocity, not regurgitant orifice area (ROA)

Velocity can be inversely proportional to ROA

Larger ROA may lead to lower velocity

Jet looks smaller than a those with smaller ROA.

Color gain

Same jet with different color gain appears different.

Color gain is turns up or down the amplitude of the color jet.

Color gain

To optimize color gain, turn it up until you see speckles in the tissues-

then turn it down slightly

Color scale/ Nyquist limit

By changing the color Nyquist limit, the jet appearance and size can appear different

Should set the Nyquist limit to the highest a given depth allows (generally >0.6 cm/s)

Color Doppler M-mode imaging

Pulse Doppler interrogation done along a single line

Doppler velocity shift recorded and color coded

Provides high temporal and spatial (but still not velocity) resolution to the assessment of flow

Color Doppler M-mode

Small amount of left to right flow during systole

Tissue Doppler Imaging

Routine Doppler targets blood flow High velocity Low signal amplitude

Tissue Doppler (assessing the movement of the myocardium) targets tissue Low velocity High signal amplitude

Different Filters

Example of pulse TDI

Velocity of tissue along a particular sample volume

Example of Color TDI

Velocity of tissue coded by color superimposed on 2-D image

Can derive information such as strain, strain rate, dyssynchrony…etc.