Doppler Modalities

Question 1

what are the three standard Doppler Modalities

Answer 1

CW
PW
Colour

Question 2

what does spectral refer to

Answer 2

fact that the Doppler frequency shifts are separated and displayed as a “spectrum.”

Question 3

Doppler angle

Answer 3

The angular effect on the Doppler shift can be determined from the unit circle and the cosine function.

Question 4

reflection from red blood cells

Answer 4

Rayleigh scattering

Question 5

what is there a tradeoff between

Answer 5

There is a tradeoff between increased reflectivity and increased absorption with a shorter wavelength from higher frequency operation. Notice that as the depth increases beyond 3 cm, the transmit frequency really should be set as low as possible for optimal sensitivity because of the dominance of absorption.

Question 6

what is red blood cell aggregation called

Answer 6

rouleau effect

Question 7

what is the rouleau effect

Answer 7

With low shear flow states, blood cells form a rouleau formation (like a roll of coins), increasing the reflectivity. This increase in reflectivity often results in the ability to visualize the blood, referred to as spontaneous contrast. In this example, the rouleau formation is formed in the common femoral vein. (As also visualized in video clip presented in the next slide.)

Question 8

what is the doppler insonifcation angle meaured between

Answer 8

the direction of the flow and the line of observation (Doppler steered line). The following slides illustrate how the Doppler angle is specified for varying cases.

Question 9

doppler insonifcaton angle of 0

Answer 9

The flow is directly toward the transducer, so the angle between the flow direction and the steered line is 0 degrees. The resulting Doppler shift is positive.

Question 10

Doppler insonfication angle 180

Answer 10

The flow is directly away from the transducer, so the angle between the flow direction and the steered line is 180 degrees. The resulting Doppler shift is negative.

Question 11

with a doppler insonifcation angle of 90

Answer 11

will have no frequency shift

Question 12

what do parameters of the transmitted signal chRCERTISTIC depend on

Answer 12

which type of Doppler is being performed
 how the user has set the transmit power control 
 the sample (gate) volume in PW
 the gate depth in PW, 
 the steered line, etc.

Question 13

why is doppler a spectrum

Answer 13

Unlike the train model, with blood flow, there are many simultaneous frequency shifts (a spectrum) corresponding to the various blood velocities and angles to flow.

Question 14

amplification

Answer 14

When the reflected echoes are received, the amplitude of all the echoes is very low
The system automatically amplifies the signals before we are able to manipulate them (Pre-processing)

Question 15

Mixers: Doppler shift detection

Answer 15

The returning signal in Doppler is a combination of the transmitted frequency and the spectrum of frequency shifts as depicted in the last slide.
The Doppler system uses a process called “mixing to baseband” to remove the higher frequency transmitted signal component, so as to detect the lower frequency (baseband) Doppler shifted signals.

Question 16

Demodulation

Answer 16

The signals reflected from stationary structures are still at the same frequency as the transmit frequency – therefore they are subtracted from the combined signal leaving only shifted frequencies

Question 17

wall filters

Answer 17

high pass filters that pass signals with frequency shifts higher than the wall filter corner frequency and diminish signals with frequency shifts below the wall filter corner frequency

Question 18

Clutter signals

Answer 18

Specular reflectors cause extremely high amplitude signals relative to reflection from blood cells (Rayleigh scattering).
These signals may be as much as 160 dB stronger than the desired blood signals.

Question 19

after wall filter

Answer 19

Signal dynmic range is gretaly reduced

Question 20

different studies

Answer 20

require different wall filters

Question 21

venous stduies

Answer 21

lower wall filters because venous flow is typically at lower speeds

Question 22

arterial wall filters

Answer 22

higherwall filters

Question 23

typical wall fiolters settings

Answer 23

< 25 – 50 Hz Venous
50 – 100 Hz Arterial
200 – 600 Hz Adult Echo
600 – 800 Hz Pediatric Echo

Question 24

correct wall filter frequnecy to use

Answer 24

depends on the operating frequency of the transducer as well as the velocity of the desired signal and the undesired clutter.

Question 25

wall filter saturation

Answer 25

When the clutter signals are not adequately reduced by the wall filters, the circuit saturates. Saturation appears on the spectrum as a bright white signal relatively symmetric about the baseline. In the following spectrum, the circuit saturates as the specular reflection from the valve is evident for each cardiac cycle.

Question 26

variable gain

Answer 26

Once demodulation occurs more amplification can occur
This is called Doppler Gain
The Doppler gain is logarithmic
Operator control to amplify the spectral Doppler tracing

Question 27

Fast fournier technqiue

Answer 27

The full range of frequency shifts is divided into individual frequency bins

Question 28

what does each bin correspond to (FFT

Answer 28

Each bin corresponds to a narrow range of frequency shifts which by the Doppler equation can be converted into a calculated velocity

Question 29

last bin

Answer 29

corresponds to the maximum decectable velcoity

Question 30

first bin

Answer 30

The first bins signals are zero because of the wall filter, if the wall filter were increased more bins would have zero frequency shifts

Question 31

Grayscale compression and rejection

Answer 31

The Doppler signals span a greater dynamic range than is visible by the human eye (how many shades of gray can we detect?)
We can manually change the dynamic range on the machine while the Doppler is on or post-processing after image acquisition

Question 32

axes of spectral doppler presentation

Answer 32

three

Question 33

Frequnecy and amplitude are

Answer 33

unrelated!!!

Question 34

Frequnecy shift

Answer 34

The frequency shift is determined by all of the factors in the Doppler equation. Higher velocities produce higher frequency shifts.

Question 35

Amplitude

Answer 35

The amplitude is determined by the parameters affecting the power, the type and amount of reflection and the attenuation.

Question 36

Continuous wave sample volume

Answer 36

CW operation detects flow that occurs anywhere within the intersection of the transmitting and receiving beams of the transducer
The sample volume is also the overlapping region of the transmitting and receiving beams
Because of the large sample volume, the spectral tracing/Doppler can give complex presentations if multiple flows are found between the beams transmitting and receiving – no range ambiguity