Module 3 : Principles Of Color And Spectral Doppler Flashcards
Who first described the Doppler effect/ Doppler shift
- Christian johann Doppler
What was the Doppler guy studying when he discovered the Doppler principle
- he was studying the varying color of the stars to determine if they were moving toward or away from the earth
What is the Doppler effect
- change in frequency of sound, light, or other waves caused by the motion of the source or the observer
Ultrasound Doppler shift is caused by what
- change in frequency of sound caused by the motion of red blood cells
The Doppler shift is the difference between what two frequencies
- difference between the transmitted frequency and the received frequency
What is the transmitted frequency
- transducer frequency
What is the received frequency
- reflected or echo frequency
Frequency shift when the RBCs move toward the transducer
- the echo frequency will be larger/higher than the transducer frequency (antegrade flow)
Frequency when the RBCs are moving away from the transducer
- the echo frequency will be smaller/lower than the transducer frequency (retrograde flow)
What is beat frequency
- method used to measure the Doppler shift is based on wave interference
- the reflected wave will vary slightly from the original transmitted wave the algebraic sum give off beat frequency
In which part of the vessel are cells moving the fastest
- the cells at the center of the vessel move the fastest while those at the sides move the slowest
How is a spectral trace created with the beat frequency
- there are many different frequencies within the beat frequency due to the different velocities of RBCs that are plotted on a graph to form spectral trace
Complicated Doppler shift equation
^F = 2Fo x V x Cos0 / C
^ F
Doppler shift
Fo
Transducer operating frequency
V
- velocity if moving RBCs
Cos0
- angle of insonation
C
- speed of sound through soft tissue
What is the Nyquist limit
- upper limit to the Doppler shift that can be detected by pulsed instruments
- 1/2 PRF
What must be exceeded if aliasing occurs
- the nyquist limit hast to be exceeded
Why does aliasing occur
- PRF is too slow for the true Doppler shift and is sampling the flow to late so its only capturing parts of it
Aliasing on color Doppler
- color wraps around the color bar showing both colors with no separation
5 ways to compensate for aliasing
- move the baseline
- increase PRF (scale)
- increase Doppler angle (not good)
- lower the operating frequency
- change to CW
What is high PRF mode
- allows the machine to send out subsequent scan lines before the first is returned
High PRF mode can lead to what downfall
- range ambiguity
What is range ambiguity
- machine does not know what depth the echoes are being returned and could show misrepresented echoes on the display
3 ways Doppler info can be represented
- spectral
- color
- sound
Characteristics of spectral Doppler
- graphed information shown on display
- QUANTITATIVE (can measure speed) using FFT
What is fast Fourier transformation (FFT)
- means if taking complex beat frequency and dissecting it into its component waves
- each wave is the result of a reflection off a moving blood cell within the sample volume
Color Doppler characteristics
- visualize flow by sending out two set of scan lines (one for 2D one for color)
- Doppler info is OVERLAID the 2D
- obtained using autocorrelation
- qualitative
What velocity does color Doppler represent
- mean flow velocity
- different colors represent positive or negative Doppler shift and give the direction of flow
Characteristics of Doppler sound
- Doppler info can be heard
- helps us detect changes in flow
Characteristics of CW
- sounds waves emitted continuously from ONE crystal and is continuously received by another crystal
- zone of sensitivity
What is the zone of sensitivity
- in CW the velocities received come from a large overlap zone
- area where transmitted and received sound is overlapped
Advantages to CW
- no nyquist limit so can scan really high velocity with no aliasing
Disadvantages to CW
- no range resolution
- no 2D image
PW characteristics
- one crystal is used to fire and then listens for returning echoes
Advantage to PW
- we know how fast the sound is traveling so we can determine at what depth we would like to sample
PW sample volume increase in size causes what
- increases ring time
- increases spectral broadening
PW sample size decrease in size does what
- decreases ring time
- samples a more discrete area of flow and the spectral waveform displays less spectral broadening
Power Doppler characteristics
- overlay of color onto the 2D image
- power Doppler uses amplitude of returning Doppler signals to detect flow
- very sensitive to low flow states and independent of angle
- directional power Doppler has been introduced
