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| The change of frequency is __________ to the velocity. |
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| Flow toward the beam, the frequency shifts __________ and flow away from the beam shifts __________. |
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| Do you get a stronger frequency shift whe the flow is toward or away from the beam? |
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| Neither. Shift is shift. Both are proportional to velocity. |
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| If the velocity of blood flow increases, the frequency shift __________. |
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| If the velocity of blood flow decreases, the frequency shift __________. |
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| Faster flow makes __________ shift. |
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| If the operating frequency of the transducer increases, the frequency shift __________. |
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| If the operating frequency of the transducer decreases, the frequency shift __________. |
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| Higher operating frequency makes __________ shift. |
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| If the angle of the Doppler beam relative to the direction of flow increases, the frequency shift __________. |
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| If the angle of the Doppler beam relative to the direction of flow decreases, the frequency shift __________. |
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| Higher angle makes __________ shift. |
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| Velocity through a mitral valve is 1.0 m/sec. Using a 2.5 MHz transducer, what frequency shift will be created? |
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3247 Hz
(2*2,500,000*1)/1540 = 3246.75 |
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| What is the cosine for a 0 degree angle? |
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| What is the cosine for a 45 degree angle? |
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| What is the cosine for a 60 degree angle? |
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| What is the cosine for a 90 degree angle? |
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| What is the worst Doppler angle possible and why? |
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| 90 degrees because the cosine is 0, and 0 times anything is 0; no frequency shift, no waveform. |
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| What is the Doppler angle that give the maximum frequency shift possible and why? |
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| 0 degrees because the cosine is 1.00. |
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| What is the speed of sound in tissue? |
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| Velocity in CCA is 100cm/sec, and operating frequency is 5 MHz. The of incidence is 60 degrees. What is the frequency shift? |
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Definition
3247 Hz
(2*5000000*0.5)/1540 |
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| What is the Doppler equation for frequency shift? |
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| What is the Doppler equation for solving estimated velocity? |
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| speed of sound through a medium |
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| If frequency shift increases, velocity __________. |
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| If frequency shift decreases, velocity __________. |
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| If operating frequency increases, velocity _________. |
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| If operating frequency decreases, velocity _________. |
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| If angle decreases, velocity _________. |
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| If angle increases, velocity _________. |
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| What is the estimated velocity if you use a 5 MHz transducer at a 60 degree angleto get a 4 KHz peak frequency shift from an ICA? |
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123 cm/sec
(4000*1540)/(2*5000000*0.5)=1.232 m/sec
1.232 m/sec * 100 = 123.2 cm/sec rounded to 123 cm/sec |
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| What are the elements of Doppler processing? |
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Definition
1. Audio output
2. Zero-crossing detector
3. Spectral analysis (FFT)
4. Color flow
5. Power Doppler |
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| What is a zero-crossing detector? |
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| An application that detects the frequency crossing of the zero baseline in order to give and average of shifts. AKA analog Doppler |
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| What is spectral analysis? |
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Analyzes (breaks down) the complex signal into component frequencies.
(Unlike analog Dopper, this allow all the frequency shifts to be displayed rater than just and average.) |
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| What are the elements of FFT? |
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Definition
X=time
Y=frequency shit/estimated velocity
Z=echo strength at a given frequency shift at a given moment indicated by pixel brightness |
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| What is aliasing with Doppler? |
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Definition
| When the display shows cut-off waveforms with the peaks "wrapped around on the other side of display. |
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| What causes aliasing with Doppler? |
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| When the frequency shift exceeds the Nyquist limit. |
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| What is the basic concept of understanding the Nyquist limit? |
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| To get an accurate reproduction of frequency shift in real time, we need to sample twice the highest frequency of the original signal. |
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| What does PRF stand for and what does it mean? |
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Pulse Repetition Frequency
(pulses of ultrasound per second) |
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| PRF = 2 times __________? |
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| Nyquist limit = 1/2 __________? |
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| If you have Doppler running with a 12 KHz PRF, what is the upper limit of frequency shift that can be processed with aliasing? |
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6 KHz
12,000/2 = 6000 Hz
Anything above 6KHz will alias |
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| What influences the likelihood of aliasing? |
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| Anything that changes PRF or frequency shift. |
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| Name some ways you can reduce the likelihood of aliasing and why? |
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Definition
1. Raise the angle of incidence. (decrease cosine, decrease frequency shift)
2. Lower the operating frequency. (decrease frequency shift)
3. Find an approach that lessens depth. (pulse returns quicker)
4. Change to CW. (no aliasing with CW) |
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| Echo techs must switch to CW Doppler at velocities above ~2m/sec, while vascular techs can use PW to assess over 600cm/sec. How can this be? |
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| Echo depth 12-18 cm. Vascular depth 3-8 cm depth. PRF will be lower for echo due to much more depth. Why? Travel time. |
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Definition
| Each sample is compared to the previous one to arrive at a rough estimate of mean velocity, not peak. Used in color flow processing. |
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| What is the usual color flow assignment for echo? |
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Definition
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| What is the usual color flow assignment for vascular? |
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| Make arteries red, veins blue (usually, not always possible) |
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pulses per second
(usually expressed in KHz) |
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pulse repetition period
(beginning of one pulse to the beginning of the next pulse) |
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pulse duration
(the length of time of the pulse itself from beginning to end) |
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| If frequency is 10 Hz, and you create 2 pulses per second with 2 cycles per pulse, what is the PD, PRF, and PRP? |
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Definition
PD= P*(#cycles per pulse)
P= 1/10= 0.1 seconds
PD= 0.1 *2 = 0.2 seconds/pulse
PRF= 2 Hz (2 pulses per second)
PRP= 1/PRF
PRP= 1/2 = 0.5 seconds |
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| What is the typical PD for diagnostic ultrasound transducers? |
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| If the operating frequency is 2 MHz and each pulse has 200 cycles, what is the PD? |
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Definition
PD=P*(# cycles/pulse)
P=1/F = 1/2,000,000 = 0.0000005 seconds
PD=0.0000005*200= 0.0001 seconds = 100 μsec/pulse |
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Definition
| Duty Factor: Proportion of time the transducer is actually vibrating to produce ultrasound (percent of time the transducer actually works) |
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| If the frequency is 20 Hz, there are 2 pulses per second, and there are 2 cycles per pulse, what is the DF? |
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Definition
DF = PD/PRF or PD*PRF
P= 1/F = 1/20= 0.05 sec/cycle
PRF= 2 Hz (cycles/pulse)
PD= 0.05 sec/cycle * 2 (cycles/pulse)= 0.1 sec/pulse
DF= PD/PRP
PRP= 1/PRF = 1/2 = 0.5 sec
DF= 0.1sec/0.5sec = 0.2 * 100= 20% |
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| If DF=0.25% and F=3.5 MHz, How many cycles per second are being produced? |
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Definition
8750 cycles/sec
0.25%=0.0025
3500000*0.0025=8750 |
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Spatial pulse length
(distance of one pulse) |
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cycles per pulse*λ
(remember λ=c/f) |
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| What is wavelength if frequency is 5 MHz? |
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Definition
308 μm/cycle
λ = 1540/5000000 = 0.000308 =308 |
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| High frequency has a __________ wavelength. |
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Definition
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| Low frequency has a __________ wavelength. |
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Definition
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| Medium with faster propagation speed has a __________ wavelength. |
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Definition
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| Medium with slower propagation speed has a __________ wavelength. |
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| If f=2MHz and there are 25 cycles per pulse, what is SPL? |
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Definition
0.02 m/pulse SPL
λ=1540/2000000=0.00077 m/cycle
SPL= 0.00077*25=0.01925--->0.02 |
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| Wavelength is to SPL as... |
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