1st Exam Review Flashcards
If you have a pulse rate frequency of 15,000 Hz what is the upper limit of Delta F you may measure without aliasing?
Delta F = 7500 Hz
What are the axis of FFT display?
x = Time
y=Delta F
Z=Echo strength, Demonstrates quantity of echoes returning indicated by pixel brightness
What is zero crossing detection?
Analog Doppler averages shifts by counting crossings of zero producing a single line tracing. Example: parks Doppler.
What are the cosine values for incidence angles: 0° 45° 60° 90°
What does the Theta symbol represent?
0° = 1.0
45° =0.707
60° =0.500
90°=0.000
Theta represents the value assigned to an angle of incidence. This factors in the angle of two correct the Delta F report. Less accurate at angles above 60°
How does the angle of Doppler Beam effect the report of frequency shift?
A higher angle detects a smaller shift. 90° detects no shift. 0° detects the highest most accurate shift.
Who discovered the frequency shift due to motion?
Christian Andres Doppler 1803-1853
What is the complete Doppler equation?
Delta F = 2FoV (cos)/C
What is the velocity estimate Doppler equation?
Ve = Delta F C/ 2Fo(cos)
Why do you not get aliasing with continuous wave Doppler?
Continuous wave Doppler has no PRF
It’s always sending and always receiving, therefore there is no aliasing with CW
What is the difference between pulse duration and duty factor?
Pulse duration is the transmit time within the PRP. Duty Factor is the ratio of transmit time to Pulse Repetition Period time. They essentially reference the same thing in different ways.
Duty factor = PD/ PRP x 100
Reminder: Pulse duration is transmit time only. Pulse repetition period Includes the pulse duration and the listening time.
Define PRP
PRP is the pulse repetition period.
The length of time from the beginning of a pulse to the beginning of the next pulse.
What can you do to reduce the likelihood of aliasing?
Raise the angle of incidence, it lowers the Delta F which may work with the Nyquist limit.
Lower the operating frequency, It lowers the Delta F
Decreased the scale, lowers the PRF, reduces the Nyquist limit.
What is the Nyquist limit at any given time?
Nyquist limit = one half the PRF
Or the PRF must be at least two times the Nyquist limit
The Nyquist limit is the result of the PRF
What causes aliasing?
Measuring velocities that exceed the Nyquist limit for the operating frequency being used.
How do you avoid aliasing?
Use a pulse rate frequency at least two times the peak the velocity being sampled.
What is spectral analysis?
Fast Fourier Transform.
Analyzes complex signal into component frequencies.
Displayed as spectral waveforms.
What are three diagnostic advantages of Doppler ultrasound?
Estimate arterial stenosis.
Estimate cardiac Valve pressure gradient. 4(V2)
Evaluating flow character qualitatively.
What influences frequency shift? (3 things)
Higher velocity of blood makes a bigger shift
Higher Operating frequency creates a bigger shift
Angle of the Doppler Beam
What should you remember about SPL and wavelength?
Wavelength is to SPL as period is to pulse duration.
The distance of one cycle And the distance of one pulse.
What is bandwidth in ultrasound?
The bandwidth represents the range of frequencies in the signal created by the transducer, the number of frequencies produced.
What is quality factor?
Quality factor is an index of how clean and efficient the signal is. The ratio of center frequency to bandwidth.
What is the equation for Quality factor?
Quality factor = operating frequency/ bandwidth
What is the simple Doppler equation?
Delta F = 2FoV/C
Define PRF
PRF = pulse repetition frequency, or the number of pulses per second. Usually reported in kilohertz
What is duty factor?
To proportion of time to transducer is actually vibrating to produce ultrasound relative the pulse repetition period.
What is the relationship of PRF and PRP?
PRF and PRP are inverse.
PRF=1/PRP
PRP=1/PRF
Similar to relationship between period and frequency.
What are the duty factory equations?
Duty factor = PD/PRP
Since PRP =1/PRF, Duty factor also = PDx PRF
What is the magic relationship for wavelength?
Wavelength = C/ f
C =Wavelength x f
F =C/Wavelength
What is spatial pulse length? SPL
The distance of one pulse
SPL =Cycles per pulse x Wavelength
Remember Wavelength is equal to Propagation speed/Frequency.
What is pulse duration?
The link of time the pulse itself from beginning to end of the group of cycles.
PD =Number of cycles in the pulse x Time of one cycle (period).
What influences the likelihood of aliasing?
Anything that changes PRF or frequency shift.
At which velocity must echo techs switch to CW?
Due to depth/ PRF, echo must use CW Doppler at 2m/second to avoid aliasing.
What advantage does PW have over CW Doppler?
PW uses range gating whereas CW average is everything a long it’s beam path.
How does the Nyquist limit manifest in color Doppler?
Aliasing in color: The color reaches the top of the scale and wraps around to the brightest, bottom, of the opposite side of the scale.
In Color Doppler what does color indicate?
Color maps direction you towards or away from the transducer as set by the operator.
In Color Doppler what does brightness indicate?
Brightness of color indicates shift/ velocity
Brighter=Higher
Darker= Lower
What are the seven parameters of ultrasound?
Frequency period Wavelength Propagation speed Amplitude Power Intensity
Describe the quality factor pyramid.
Q = F/BW
What is the equation for period?
Period = 1/F
What is the equation for frequency?
Frequency = 1/P
What is the equation for wavelength?
Wavelength = C/f
What is the propagation speed of sound in soft tissue?
1540 m/sec
What kind of Doppler analysis is used with color flow?
Autocorrelation.
