Principles of Doppler Echo

Definition of Doppler Echocardiography

method for detecting the direction and velocity of blood flow

Uses of Doppler echo

to evaluate normal vs abnormal flow states and acquire quantitative data

Laminar blood flow

normal, parabolic blood flow with max velocity <1.5 m/s

Turbulent blood flow

abnormal, disordered direction of flow with many RBC velocities

Frequency

the number of waves that pass a given point in one second (Hertz or unit of cycle/second)

↑ high Frequency =

↓ short wavelength

↓ low Frequency =

↑ long wavelength

What is the sound speed in tissues?

1540 m/s

How does conventional 2D echo work?

The machine calculates distance by timing how long the echo takes to return back from the tissue. This repeats thousands of times per second to create an image

The best images in 2-D are obtained when the target is _____ to the sound waves

perpendicular

How does doppler echo work?

depends on the measurement of relative change in the returned/reflected ultrasound frequency when compared to transmitted frequency

characteristics of disturbed blood flow that doppler echo measures

direction, velocity, turbulence, distinguishes normal and abnormal flow patterns and quantitates them

Red blood cell moves TOWARD the transducer

↑ higher frequency + positive doppler shift

Red blood cell moves AWAY from the transducer

↓ lower frequency - negative doppler shift

The Doppler equation is used to calculate the velocity of blood flow based on

• transmitted frequency

• Angle of transducer

• Velocity of sound in blood (1540m/s)

High pitched sounds

large doppler shifts - presence of high velocities

Low pitched sounds

lesser doppler shifts

Laminar flow audio

smooth, pleasant

Turbulent flow audio

high pitched, whistling, harsh/raspy

A spectral Doppler display

is a graphical representation of blood flow velocity over time, generated by an ultrasound machine

Flow velocity toward the transducer is

positive or upward

Flow away from transducer

negative or downward

Laminar flow (plug flow) doppler profile

narrow doppler spectrum / Clear frequency window

turbulent blood flow doppler profile

Spectral broadening - thicker frequency window that can fill the window depending on grade of narrowing or stenosis

True velocity of blood flow is measured when the angle of Doppler beam direction is

Parallel to the flow

Plug flow

flattened flow during systole - velocity is constant across entire CSA

Continuous Wave Doppler

Continuous generation of ultrasound coupled with continuous ultrasound reception

Advantage of CW

measures high velocities accurately

Disadvantage of CW

lack of selectivity or depth discrimination

Pulsed Wave Doppler

alternates transmission and reception of ultrasound

Advantages of PW

provides doppler shift data selectively from a small segment along the ultrasound beam

Sample volume

small, operator defined area within tissue where blood flow velocity is measured

Range gating

transmits short pulses then listens for returning echos only from a specific depth/range

PW mapping

visual map of blood flow

Disadvantage of PW

inability to accurately measure high blood flow velocities over 1.5-2.0 m/s

aliasing

occurs due to sampling rate being too low with respect to Nyquist Rate

Nyquist limit =

PRF/2

Nyquist limit defines…

the maximum velocity that can be measured without aliasing.

PRF

pulse repetition frequency - the number of pulses the ultrasound machine transmits per second

Max PRF is limited by -

The depth of the sample volume from the transducer.

^{The deeper the sample volume, the longer the ultrasound pulse takes to travel and return, which forces the system to use a lower PRF}

The closer the sample volume is located to the transducer-

the higher the max PRF can be used

The Nyquist limit is controlled by 2 factors: 

Depth into the tissue and transducer frequency