PW AND CW DOPPLER- Doppler Physics

what are the 3 advantages of continuous wave Doppler?

1. accuracy in measuring high velocities (able to measure high velocities accurately. It is capable of detecting virtually unlimited velocit ies whereas pulsed-wave Doppler suffers from aliasing. 2. increased sensitivity (ability to detect smaller Doppler shifts) 3. no aliasing (fundamental advantage of continuous-wave relative to pulsed wave Doppler)

increased likelihood of aliasing: Doppler shift

1. higher velocity of blood 2. insonification angle close to 0 degrees or 180 degrees 3. higher transmit frequency

what are the four ways to eliminate TRUE aliasing?

1. use continuous wave Doppler 2. select a lower frequency transducer 3. select a new view that has a shallower sample gate depth 4. increase the scale

For pulse wave Doppler, the sampling rate (frequency at which sampling occurs) is the _______.

PRF

for pulse wave Doppler, the Nyquist limit is the _________ divided by _________

PRF, 2

in continuous-wave doppler, one transducer element is _________________ while the other ________________

continuously transmitting a signal, continuously receiving echoes

since the Doppler shift is proportional to the operating frequency, a lower operating frequency results in a lower Doppler shift, __________ the likelihood of aliasing

decreasing

increased likelihood of aliasing: PRF

deeper gate depth

aliasing _______ occur with continuous-wave transducers

does not

increasing the scale only _____________________ is the current scale is underdriven

eliminates aliasing

what is the definition of aliasing?

errors in measuring high velocities high velocities will appear negative

aliasing appears when the measured Doppler shift ___________ the Nyquist Limit

exceeds

what is the sampling rate?

frequency at which sampling occurs

if the Doppler scale (PRF) is lowered , the spectrum will appear was a _________ percentage and hence more appropriately scaled. the result is more accurate image with the ability to more accurately measure the velocity

greater

according to the Doppler equation, higher velocity flow, higher operating frequencies, lower propagation velocities, and insonification angles closer to 0° or 180° all result in ____ Doppler frequency shifts

higher

based on the Nyquist Limit equation, ________ Doppler frequency shifts are more likely to cause aliasing

higher

eliminating display aliasing:

it can be eliminated by adjusting the baseline

what is the nyquist limit (nyquist frequency)?

it represents the maximum Doppler shift frequency that can be correctly measured without resuming in aliasing in color or pulsed wave doppler

what is the disadvantage of pulsed wave doppler?

its inability to accurately depict high velocities this is known as aliasing

since the Doppler shift is angle dependent, when the Doppler angle is other than 0° or 180°, the calculated velocity is ____ than the actual velocity. By the user indicating the Doppler angle, the system can calculate the cosine of the angle and correct the calculated velocity.

less

since the transmit is continuous (no pulses are emitted and there is no "___________"), there is no image created

listening time

__________ PRFs result in a lower Nyquist limit, which _________ the likelihood of aliasing

lower, increases

CW Doppler does not permit us to determine where the wave is reflected. Echoes arise from the entire length of overlap between the transmit and receive beams, what does this mean?

range ambiguity the term "range" means depth

what is the advantage of pulsed wave doppler?

range resolution/specificity doppler signals are received primarily from the doppler gate location, as set by the sonographer

in pulsed wave doppler, echoes are being generated from only the area of interrogation, the sample volume, or the _________

sample gate

decreasing the sample gate depth increases the PRF. Since PRF is proportional to the Nyquist Limit, an increased PRF results in an increased Nyquist Limit, decreasing the likelihood of aliasing

selecting a new view that has a shallower sample gate depth

pulsed-wave Doppler is performed by transmitting pulses from either a _______ crystal or a __________. each pulse is followed by a dead time, known as the ___________

single, group of crystals, listening time

what if the maximum PRF is much higher than the Doppler frequency shift detected?

the ability to accurately measure the velocity of the spectrum is clearly compromised by the scale being too high even a slight difference in cursor placement means a significant difference in a measured velocity

continuous-wave Doppler utilizes continuous transmission and reception of ultrasound waves. This is accomplished by ___________

two dedicated transducer elements

the difference between display aliasing and true aliasing is that display aliasing can actually be ________by _______ the scales or shifting the Doppler baseline for the specific spectrum

unwrapped, increasing

sonographic appearance of aliasing:

when true aliasing occurs, the spectrum appears to wrap from either the top of the spectrum to the bottom or vice versa. Flow toward the transducer will extend up to the top of the spectrum and then continue at the bottom of the spectrum. With true aliasing, it is not possible to increase the Doppler scales anymore so as to unwrap the true peak velocity.

when does aliasing occur?

whenever high velocities appear negative

a display alias implies that the signal is __________ in the spectrum

wrapped