Module 7 : Doppler Principles

Question 1

what is a doppler shift

Answer 1

• a change in pitch results form a relative motion of the source of the sound or the reciever
• when a sound source is moving towards you the wave crests are closer together and pitch is higher or moving away from you the crests are further apart and pitch is lower

Question 2

what 3 things does doppler give us information on

Answer 2

• presence of flow
• direction of flow
• quality of flow

Question 3

what are three different types of doppler

Answer 3

• spectral trace
• color image
• audible sound

Question 4

how is the Doppler effect produced with blood

Answer 4

• sound reflects of moving RBCs which alter the frequency that is returned
• the amount of change is related to the direction and speed of the RBCs

Question 5

what does the intensity of the signal relate to

Answer 5

• proportional to the number of RBCs and thus indicates the quantity of blood in the sample

Question 6

what is the doppler shift equation

Answer 6

• doppler shift = received - transmitted

Question 7

how is the doppler shift determined

Answer 7

• uses the beat frequency

- reflected wave differs from transmitted frequency

Question 8

what is the more detailed doppler shift equation

Answer 8

^F = 2Fo x v x cos0 / c

Question 9

does doppler use a higher or lower frequency

Answer 9

• lower frequency

- RBCs very small so already reflect very little of the echoes

Question 10

what is the most accurate angle of insonation for doppler

Answer 10

• most accurate would be 0º

- we use 60º accurate and reproducible

Question 11

what is the nyquist limit

Answer 11

• reached when RBC velocity faster than machines ability to sample
• pulses only sent out as fast as they can be returned and are limited by the speed of sound in tissue
• causes aliasing

Question 12

what is the nyquist limit equation thing

Answer 12

1/2 PRF

Question 13

what is aliasing

Answer 13

• wrapping of the color or the spectral trace around the baseline

Question 14

what are the five ways we can correct for aliasing

Answer 14

• move the baseline
• increase the PRF
• increase doppler angle
• lower the operating frequeny
• change to continuous wave

Question 15

what two methods o correct for aliasing do we use in practice

Answer 15

• move baseline

- increase PRF

Question 16

where should the sample volume be placed in the vessel

Answer 16

• middle of vessel

- where the highest velocity is

Question 17

what are the 3 components of the sample volume

Answer 17

• beam width (3D)
• receiver gate length
• length of emitted pulse

Question 18

what decreases the gate length

Answer 18

• pulse length reduced

- improve spectral display

Question 19

do you want long or short pulse for doppler

Answer 19

• long pulses to interact with the RBCs longer

Question 20

instead of range gating what is used with continuous wave doppler

Answer 20

• no image so uses zone of sensitivity

Question 21

what is the zone of sensitivity

Answer 21

• intersection of transmitted and received beam

Question 22

what is the spectrum

Answer 22

• array of components of a wave arranged in order of increasing frequency over time

Question 23

what are the three components of the spectral trace

Answer 23

• time (x)
• frequency or velocity (y)
• power (z)

Question 24

what is the power (Z axis) of the spectral trace

Answer 24

• represented by brightness of the pixels

- proportionate to the number fo blood cells moving at one velocity at a given time

Question 25

what is the way in which a system creates a spectral trace

Answer 25

- fast Fourier transform

Question 26

what is the Y axis of the spectral trace

Answer 26

- represent either frequency of the doppler shift or velocity - velocity is preferred

Question 27

what is the flow direction of RBCs relative to

Answer 27

- the transducer

Question 28

what does antegrate flow mean

Answer 28

- towards the probe expressed as positive number

Question 29

what is retrograde flow

Answer 29

- flow away from the transducer expressed as a negative number

Question 30

what are the 3 different categories of pulsatility

Answer 30

- high - moderate - low

Question 31

what are the characteristics of high pulsatility or high resistance flow

Answer 31

- tall narrow sharp systolic peak | - reversed or absent diastolic flow

Question 32

where is high resistance flow seen in body

Answer 32

- arteries of the extremities

Question 33

what are the characteristics of low pulsatility or low resistance flow

Answer 33

- broad systolic peaks | - forward flow through diastole

Question 34

where do we see low resistance flow

Answer 34

- vessels that feed the vital organs

Question 35

characteristics of moderate pulsatile or moderate resistance flow

Answer 35

- tall narrow sharp systolic peak | - forward flow through diastole

Question 36

what is the pulsatility index equation

Answer 36

- PI = A (peak velocity) - B (lowest velocity) / mean

Question 37

what is the resistivity index equation

Answer 37

- RI = A - B / A

Question 38

what is the systolic diastolic ratio

Answer 38

S/D = A/B

Question 39

acceleration equation

Answer 39

acceleration = ^v / ^t

Question 40

what is spectral broadening

Answer 40

- vertical thickening of the spectral trace or envelope

Question 41

what is false spectral broadening

Answer 41

- artifact that occurs when doppler gain is set too high