Ch 19 Flashcards
Doppler Shift or Doppler frequency
The change in frequency
Used to measure velocity of blood in the circulation
a low frequency that rides on top of a much higher transducer frequency
range from 20 Hz to 20 kHz and are in the audible range
When does the frequency of sound change?
when the sound source and the receiver move closer together or father apart
When doesn’t the frequency change?
the distance between the sound source and receiver remain constant
Demodulation
the process of extracting the low Doppler frequency from the transducer’s carrier frequency
Creation of a frequency shift
transmitted sounds waves strike moving red blood cells
Positive Doppler Shift
Moves toward transducer
Doppler Shift is positive
Reflected frequency is higher than the transmitted frequency
Negative Doppler Shift
Blood cells move away from the transducer
Doppler Shift is negative
Reflected frequency is lower than the transmitted frequency
Speed vs Velocity
Doppler Frequency indicates which
Velocity not speed
Speed vs Velocity
Speed
magnitude
indicates the distance that a red blood cells moves in 1 sec
cm/s
Speed vs Velocity
Velocity
magnitude and direction
Doppler shift equation
2 x velocity of blood x transducer frequency x cos / propagation speed
Doppler Shift relations to velocity of blood
directly related
First shift during Doppler Shift exam
occurs when the sound waves from the transducer strikes moving blood cells
Second shift during Doppler Shift exam
after the sound waves strikes blood cells, reflections return to the transducer
results from transducer reception of sound waves from moving blood cells
Doppler Shift relation with frequency of the transmitted sound
directly related
Doppler Shift units
Hz
Measurement of the Doppler frequency depends on
the relationship between the direction of the blood flow and the direction in which the sound waves propagate
Doppler Shifts represents
100% of the true velocity when blood flow is parallel to sound beam
could be moving directly to or away from transducer
Doppler shifts when there is an angle
Doppler measures something less than the true velocity
Cosine
Percentage of the true velocity that is measured depends on the cosine of the angle between the sound beam and the direction of motion
Measured velocity =
true velocity x cosine
Doppler Shift relation with the cosine of the angle between the direction of flow and the direction of sound
directly related
0 and 180
flow is parallel to sound beam and measured velocity are equal
Cosine 0
flow toward transducer
Cosine 180
flow away from transducer
Perpendicular, 90 degrees
measured velocity is zero because cosine of 90 is zero
Doppler can’t be measured
Cosine 60 degree is
0.5, 1/2 the actual velocity
Flow toward the transducer is displayed _____ baseline
above
Flow toward the transducer is displayed _____ baseline
below
Phase quadrature, or quadrature detection
commonly used signal processing technique for bidirectional Doppler
Bidirectional Doppler distinguishes
which direction the flow is going
Continuous Wave Doppler
requires two crystals, one transmits and one receives reflections
Continuous Wave Doppler
Advantages
ability to accurately measure very high velocities
Continuous Wave Doppler
Disadvantages
exact location of the moving blood cells can’t be determined
Range Ambiguity
Signals arise from all blood cells in the region of overlap between the transmitted and receive beam
Continuous Wave Doppler
Disadvantages
Lack of TGC
located deeper will have a lower amplitude than reflections from blood cells that are at shallower depths
Doppler shifts produced by deeper blood cells may be incorrectly interpreted as having been created by fewer blood cells
Duplex Imaging
Simultaneous anatomic imaging and Doppler
Continuous Wave Transducer
A dictated continuous wave does not create anatomic images
Continuous Wave Transducer don’t use a backing layer which results in
undampened transmitted signal
narrow bandwidth
high quality factor
higher sensitivity
Matching layer is used in
both duplex and dedicated continuous wave transducer
Dedicated Continuous Wave Transducer
have increase sensitivity
detect low amplitude reflections and small Doppler shift
Pulsed Wave Doppler
Only one PZT is necessary
alternate between sending and receiving the sound pulses
Pulsed Wave Doppler Advantages:
Range Resolution
Able to select the exact location where velocities are measured
range specificity, or freedom form range ambiugity artifact
Pulsed Wave Doppler Disadvantages
inaccurate measurement of high velocity is signaled
Sample volume or gate
sonographer positions a small marker on image
then calculated the time of flight for a sound pulse traveling to and from the gate
transducer emits a sound pulse and then waits that time of flight before briefly listening for a reflection
Pulsed Wave Transducer
Contains backing layer
low quality factor
lower sensitivity
wide bandwidth pulses
Aliasing
most common error associated with Doppler ultrasound
false identity
Very high velocities in one direction are incorrectly displayed as going in the opposite direction
Two ways aliasing is created
Pulsed Doppler
Doppler sampling rate is too low in comparison to the measured blood velocity
Nyquist limit equation
PRF / 2
Nyquist frequency or Nyquist limit
highest Doppler frequency or velocity that can be measured without the appearance of aliasing
Aliasing occurs when
Doppler sampling rate is too low in comparison to the measured blood velocities
Penalty
issued when speed is over the limit
Two ways to avoid penalty
raise speed limit
reduce the speed
Aliasing occur when
the Doppler shift exceeds the Nyquist limit
Two ways to avoid aliasing
raised the Nyquist limit
reduce the Doppler shift
