# Doppler Flashcards

What is a doppler shift?

A change in sound frequency caused by motion between the transmitted and received frequency

Difference between the transmitted and received frequency

Doppler shift = received - transmitted

What is another word to describe doppler shift?

Doppler frequency

What does a positive doppler shift indicate?

The source is moving to the transducer

The reflected frequency is higher than transmitted frequency

What does a negative doppler shift indicate?

The source is moving away from the transducer

The reflected frequency is lower than transmitted frequency

What are typical values for Doppler shift?

20-20,000 Hz

Audible

What is demodulation?

It extracts the lower frequency doppler shift from the higher frequency transducer

What is the doppler equation?

Doppler shift = (2 x velocity of blood x transducer freq. x cos0)/prop. speed

How is doppler shift related to velocity?

Doppler shift directly related to velocity

Faster the velocity, the greater the doppler shift

How is doppler shift related to frequency?

Doppler shift directly related to transmitted sound frequency

Transducer frequency doubled, doppler shift doubled

What happens if doppler angle is not parallel (0 or 180) to the direction of flow?

The doppler shift is not 100%

Velocity is determined by the cosine of the beam angle compared to motion

What is the equation for measured velocity?

Measured velocity = true velocity x cos0

What is the cosine of 0?

1

What percent of doppler shift is measured at 0 degrees?

100%

What is the cosine of 60?

0.5

What is the percentage of doppler shift at 60 degrees?

50%

What is the cosine of 90?

0

What is the percentage of doppler shift at 90 degrees?

0%

What type of signal processing is used for bidirectional doppler?

Phase quadrate or quadrate detection

Flow to transducer is above line, flow away is below line

What does speed measure?

Only magnitude

What does velocity measure?

Magnitude and direction

How many crystals does CW use?

Two

One always transmitting

One always receiving

What are the advantages of CW doppler?

Accurately measure high velocities w/ no aliasing

Higher sensitivity for detecting smaller shifts

What is the disadvantage of CW doppler?

Cannot determine the exact location or depth of the moving blood cells (range ambiguity)

How many crystals does PW doppler use?

Only one crystal must be used

Alternates between sending and receiving sound pulses

What are the advantages of PW doppler?

Select the exact location where velocities are measured

Adjustable receive gate with echoes from one area

What is the disadvantage of PW doppler?

Aliasing occurs and we cannot accurately measure high velocities

What term is used to describe systems that perform imaging and PW doppler at the same time?

Duplex imaging

When does aliasing occur?

When high velocities of PW doppler are incorrectly displayed as being negative

If PRF is too low relative to doppler shift or when doppler shift reaches nyquist limit

What is the Nyquist limit?

The doppler frequency at which aliasing occurs Nyquist limit (kHz) = PRF/2

What are the two main ways to avoid aliasing?

Reduce doppler shift

Raise Nyquist limit