Ch 2 Doppler Flashcards
What is the doppler effect?
Change in frequency due to motion of a sound source, receiver or reflector
List factors that the doppler effect depends on?
-Relative motion of reflector/source
-Motion direction
-Velocity of moving objects
-Angle b/w direction of wave propagation + motion direction of reflector
-Transmitting frequency
-Propagation speed of sound in a medium (assumes 1540 m/s in soft tissue)
Flow moving to the right is what kind of doppler shift?
Positive shift, results in a positive number
(motion of reflector is traveling towards probe)
Flow moving to the left is what kind of doppler shift?
Negative shift, results in a negative number
(motion of reflector is traveling away from probe)
What is a doppler shift?
Change in frequency of returning echoes with respect to the emitted frequency
What is doppler shift proportional to?
-Blood flow speed
-Operating frequency
List what the variables mean in the doppler shift equation: DS = Fr-Ft
Frequency change (DS) = Received frequency (Fr) - Transmitted frequency (Ft)
(ex 8MHz - 10MHz = -2MHz, a negative shift)
At which angle will we receive the most accurate doppler shift?
Either 0 or 180 degrees to a vessel
(avoid 90 degrees)
What is the only thing the doppler equation tell us?
Only indicates if relative motion is moving directly towards or away from the probe
List what each variable means in the complete doppler equation: F (DS) = 2 Fo V (cos) / c
F: doppler shifted frequency
Fo: operating frequency
V: velocity of moving reflectors (RBCs)
C: speed of sound in soft tissue (1540 m/s)
2: round trip travel time
Cos: doppler angle of incidence or angle correct
What angle is doppler dependent on + what is this angle called?
-Angle b/w direction of blood flow + direction of sound propagation
-Known as the insonification/insonation angle
What is the insonation angle?
Angle b/w direction of blood flow + direction of sound propagation
If doppler angle increases, the doppler shift frequency will increase or decrease?
Decrease (they are inverse of each other)
The doppler shift depends on what?
The cosine of the doppler angle
(see chart in slides - as the doppler angle increases, the cosine decreases and gets less accurate)
Flow speed measurements incorporate/require what?
Proper doppler angle correction
Cosine 1.0 is accurate or not?
100% accurate
If we are not getting any color flow in a vessel, what should we do with our probe?
Heel/toe it to try and create an angle under 60 degrees
Are cosine + doppler inverse of each other?
Yes
What doppler angle should we ALWAYS stay under?
Under 60 degrees (50% cosine) b/c it is more accurate
What should we do if we cant get a doppler angle of 60 degrees?
Go lower, never higher b/c way less accurate
A doppler angle of 30 degrees has a cosine of 0.87, what is this telling us?
That only 87% of the true shift is detected
A doppler angle of 90 degrees has a cosine of 0, what is this telling us?
No motion towards or away from probe, therefore no doppler effect!!
(Never have an angle over 60 degrees)
Perpendicular angles (90 degrees) create no doppler shift, how does this affect our CD display?
Poor/no color shown on screen b/c it is referencing nothing
(best to have parallel angle of 0 or 180 degrees)
What can we do as sonographers if we can’t align our doppler beam to be parallel to a vessel?
-Use angle correct on machine
-Line up the doppler arrow so it is parallel with vessel flow
