Chapter 4: Doppler Imaging Concepts

Question 1

What is the formula for Ohm’s law?

Answer 1

Flow (I) = Voltage (V) / Resistance (R)
or
Blood flow (Q) = Pressure change (P) / Resistance (R)

Question 2

What are the following doppler artifacts and how are they corrected?

Question 3

Clutter

Answer 3

A heartbeat with a bunch of crap underneath. Acoustic noise eliminated with high-pass wall filters.
Increase the wall filter.

Question 4

Aliasing

Answer 4

When the top of the heartbeat wave is on the bottom of the screen. (wraparound) Caused by insufficient sampling of flow.
Increase PRF, decrease frequency shift, or use CW

Question 5

Mirror image

Answer 5

Reproduction of spectral or color information opposite a strong reflector.
Decrease color/spectral gain or decrease angle to <90 degrees

Question 6

Color bleeding

Answer 6

Overgained color or spectral waveform causing signal to be larger than it should be.
Decrease gain or transmit power

Question 7

High PRF/Depth ambiguity

Answer 7

HPRF setting in PW Doppler means the PW is emulating a CW probe and is unsure where echoes came from.
Change depth or Doppler angle

Question 8

Flash artifact

Answer 8

Excessive color signal in surrounding tissue caused by movement during power Doppler.
Eliminate the source of motion if possible (breath hold)

Question 9

What is the point at which Reynolds number predicts turbulence?

Answer 9

2000

Question 10

Oscillator

Answer 10

The component of a continuous-wave Doppler device that produces the voltage that drives the transducer.

Question 11

fast Fourier transform

Answer 11

A mathematic process used for analyzing and processing the Doppler signal to produce the spectral waveform

Question 12

Phase quadrature

Answer 12

The component of the Doppler device that determines positive opposed to negative frequency shifts and, therefor, direction of blood flow.

Question 13

What is the Resistive Index and how is it calculated?

Answer 13

Doppler measurement used to quantitate the resistiveness of the distal bed.
(Peak sys velocity - End dias velocity) / PSV

Question 14

What is the pulsatility index and how is it calculated?

Answer 14

Doppler measurement used to determine how pulsatile a vessel is over time.
(Peak sys velocity - End dias velocity) / Mean

mean = average of the velocities between peak sys and end dias

Question 15

Power doppler

Answer 15

aka Amplitude doppler
Doppler mode in which the signal is determined by the amplitude (strength) of the shift, not the frequency. Amplitude is directly proportional to the number of RBC. No aliasing. Can use 90 degree angle of incidence.

Question 16

Continuous wave doppler

Answer 16

Doppler device that uses CW US transmission. Needs 2 elements.

Question 17

Color doppler

Answer 17

Doppler shift information presented as a color superimposed over the grayscale image.
Can measure mean velocity, but not peak or trough.
Uses autocorrelation for signal processing

Question 18

Pulsed wave doppler

Answer 18

Doppler technique that uses pulses of sound to obtain Doppler signals from a user-specified depth.

Question 19

Frequency shift

Answer 19

The difference between the transmitted and received frequencies

Question 20

What factors would increase frequency shift?

Decrease?

Answer 20

F = 2fv cos0 / c
above the line: frequency of transducer, velocity of blood, and cosign of the doppler angle
below the line: propagation speed through soft tissue
*note: as doppler angle increases, cosign of angle decreases

Question 21

Nyquist limit

Answer 21

The highest frequency, after which you would get aliasing.
Equal to 1/2 the PRF.
If PRF is 2500 Hz, then a doppler shift >1250 Hz will alias.

Question 22

Ensemble length/Packet size

Answer 22

The number of pulses per scan line in color doppler. As low as 3. Typically 10-20
Higher length = more pulses per scan line, higher sensitivity to slow flow, more accurate mean velocity, slower frame rate, worse temporal resolution

Question 23

What signal processing technique is used for color Doppler?

For spectral doppler?

Answer 23

Autocorrelation

Fast Fourier transform