Pulse Wave Doppler Flashcards
What is the Doppler effect?
the change in frequency or wavelength of echo signals that occurs when there is a relative motion between the sound source (probe) and reflecting surfaces (rbcs)
What does Doppler effect determine?
the velocity of the moving reflectors and the direction of flow
the Doppler effect/shift is the difference between what?
the received frequency and the transmitted frequency
what is the audible range
20Hz - 20,000Hz
Doppler shift formula
∆F(Doppler shift) = 2Fov*cosθ / c
What units are used for:
- frequency shifts
- velocities
- kilohertz kHz
- meters/sec
What units are used for:
- frequency shifts
- velocities
- kilohertz kHz
- meters/sec
what is the cosine when the angle is
- 0
- 60
- 90
1
.5
0
As the angle between the transducer and flow decreases - what happens to the Doppler shift?
it increases as we get closer to 0 angle / cosine of 1
what does it mean by a wide bandwidth
a large range of frequencies
relationship between the flow velocity and Doppler shift
faster velocity = higher Doppler shift
CW Doppler
2 crystals - the beam is directional with a region of beam overlap
what determines the CW frequency?
the applied voltage
main disadvantage to CW Doppler - and why
range ambiguity - inability to tell where the Doppler signal is coming from
because of the long pulse length - no backing material in CW Doppler, resulting in poor axial resolution
advantages of CW Doppler
measures high velocities, no aliasing, small probe sizes, use of high frequencies
main advantage of PW Doppler
range resolution - allows tech to place and know where the Doppler information is derived from
definition of Nyquist limit
the level at which aliasing will begin to occur
1/2Doppler PRF
how does aliasing occur?
if the velocity of the reflector, RBCs, are moving too fast for the sampling rate (PRF) - aliasing will occur
when aliasing does occur… what can be done to correct it?
- adjust the spectral baseline and or scale
- find different window with shallower depth
- lower the frequency
disadvantage of PW Doppler
can’t accurately measure high velocities due to aliasing
on spectral analysis - what does the vertical axis represent? the horizontal?
- vertical: frequency shift or velocity
- horizontal: time
flow direction is determined within where?
the demodulator of the Doppler receiver
what causes spectral broadening?
turbulent flow
tortuous vessels
sample gate too close to vessel wall
just distal to stenosis
waveform magnitude or waveform brightness
known as the amplitude of the signal
shown by how “bright” the spectral disaplay is
what does Fast Fourier Transformer do? how?
it performs signal processing or spectral analysis
-does so by using a mathematical technique to make a conversion from the Doppler shift information into the visual spectral analysis
what does peak flow speeds and spectral broadening mean?
a vertical thickening of the spectral trace
pulsatility index formula
PI = max velocity - min velocity / mean velocity
Resistivity index formula
RI = max velocity - min velocity / max velocity
systolic/Diastolic ratio formula
ratio = peak systolic velocity / end diastolic velocity
what is a packet?
the color information is obtained by positioning many pulsed Doppler lines and gates over the area of interest
packets are composed of how many pulses per line of color?
3 and 30 - the more pulses, the slower the frame rate
