Doppler

Question 1

What is the main limitation of continuous wave doppler?

Answer 1

It’s inability to accurately define the sample volume. (Depth )

Question 2

Describe continuous wave doppler

Answer 2

2 seperate transducer are used, one to transmit and the other to receive echoes. Ultrasound is continuously transmitted and received, the region where the beams intercept is called the sample volume. The CW sample volume is large, often extending without limit.

Question 3

Explain pulsed doppler mode

Answer 3

In PW doppler, the machine transmits pulses rather than continuous ultrasound. This enables it to acquire doppler information in a small well defined volume of blood. Echoes from all other depths are ignored

Question 4

Why is the transmit pulse duration longer for pulsed doppler.

Answer 4

It increased the transmitted power to compensate for the weak scattering of blood.

Question 5

What does the colour in a doppler image represent?

Answer 5

The doppler shift NOT the flow velocity.

Question 6

Compared to pulse doppler, colour doppler has …

Answer 6

• less effective wall filtering
• less accurate doppler determination
• poorer ability to detect weak signals from small or deep lying vessels.
• poorer ability to detect slow moving blood.

Question 7

The _______ effect is the change in
______ due to the relative motion between the source and observer.

Answer 7

doppler, frequency.

Question 8

If a source emits a sound of frequency “f”, an observer moving away from the source will experience:

Answer 8

Sound of a lower frequency than f

Question 9

The detected Doppler _______
is the difference between the
_________ frequency and the received frequency.

Answer 9

Shift, transmitted.

Question 10

The Doppler shift is:

Answer 10

• proportional to the moving target velocity
• Proportional to the cosine angle between the beam and the target

Question 11

According to the doppler equation, the doppler shift is zero when….

Answer 11

1. the target velocity is zero
2. the angle between the beam and the direction of motion is 90 degrees.

Question 12

The highest doppler frequency shift that can be measured is equal to

Answer 12

1/2 PRF

Question 13

The _____ limit is the ______ doppler frequency shift which can be detected

Answer 13

Nyquist, maximum

Question 14

The estimated doppler frequency is dependant on the:

Answer 14

cosine of the angle.

Question 15

What signal types does the received ultrasound signal consists of?

Answer 15

• echoes from stationary tissue
• echoes from moving tissue
• echoes from stationary blood
• echoes from moving blood

Question 16

Colour doppler displays all of the following information, except:
a) echo location
b) doppler shift
c) direction of shift
d) flow velocity

Answer 16

d) flow velocity

Question 17

In colour doppler imaging, the assignment of red and blue colour to positive or negative channel is:

Answer 17

completely arbitrary

Question 18

What are the general considerations for setting the colour scale?

Answer 18

Should be set low enough to ensure good sensitivity, should be set high enough to avoid aliasing.

Question 19

Where should the angle correction be set during spectral doppler sampling?

Answer 19

The angle correction should be set parallel to the flow

Question 20

What general recommendations would you make about the baseline during a spectral doppler examination>

Answer 20

It should be adjusted so that all the spectral doppler window is in view.