Module 7 : Doppler Flashcards
is the frequency lower in doppler or 2D why
- doppler
- account for attenuation for the attenuation from the RBC’s
how many pulses are sent out for spectral doppler
- 2
- one for 2D that is shorter
- one for spectral that is longer
+ more time to interact with the cardiac cycle
what is doppler shift detection the result of
- quadrate phase detector
what is a quadrate phase detector
- transmitted signal (reference) is duplicated and stored
- the reference signals are the same except they are 1/4 OUT OF PHASE WITH EACH OTHER
- returning signal is compared to the reference signal to determine the positive or negative shift
what two devices can the resultant doppler signal be sent to after quadrate phase detection
- loudspeaker
- spectral analyzer for image processing
what does the doppler shifted signal contain and is it simple or complex
- all of the shifts that occur in the sample gate
- complex
what does the spectrum analyzer do
- separates the individual doppler shifts from the complex beat frequency
what is the spectrum analyzer synonymous too
- prism separating white light into a rainbow
what is fast Fourier transform FFT
- the process through which doppler signals are separated into doppler shifted frequencies
what does FFT break the complex doppler shift signal into
- not its component/separate frequencies displaying both MAGNITUDE AND AMPLITUDE
how many spectra per second can FFT produce
100-1000
what axis are the operate doppler shifts plotted on
- the y axis
what is the magnitude of the doppler shift
- amount of positive or negative shift
what is a + 4 kHz doppler shifts magnitude equal too
- same as magnitude of -4kHz shift
what are the 3 factors affecting magnitude
- RBC velocity
- angle of insonation
- operating frequency
what is the amplitude(brightness) of the doppler shift
- represents the number of RBCs that are contained with the sample volume
what are the 4 factors affecting amplitude
- RBC density
- attenuation
- power
- gain
what does an increase in RBC density affect amplitude
- increase amplitude
what does an increase in attenuation affect amplitude
decrease amplitude
what does an increase in power and gain affect amplitude
- decrease in amplitude
what three things is the sample volume comprised of
- beam width
- gate length (range gated)
- pulse length
what is an important consideration to take into account for with PW sample volume in regards to the beam characteristics
- its a 3D sample volume that changes in size relative to depth
- when placed in the farfield it becomes larger
why does aliasing occur
- pulsed doppler is a sampling system and it is possible that the velocities of the RBCs will exceed the sampling limit of the machine
what limits the amount of sampling
- DEPTH
- limits the PRF
what is the limit exceeded when aliasing occurs and what value is it
- nyquist limit
- 1/2 PRF
what are the 5 ways to correct for aliasing
- change baseline (depending on direction of flow)
- increase PRF (scale) (aka velocity range)
- lower frequency (already lowest)
- increase doppler angle (inaccurate)
- switch to CW (only w/ a capable probe)
what 2 methods do we use clinically to correct for aliasing
- changing the baseline
- increase PRF
what happens when a machine operates in a high PRF mode
- allows machine to fire next pulse before all of the echoes from the first pulse have been returned
- raises the nyquist limit
what is one consequence of using high PRF mode
- range ambiguity
- if more than one vessel is on the screen we may pick up a trace from more than one vessel
what are fast sweep speeds used for
- intracycle measurements due to caliper placements and increased accuracy for systolic events
what are slow sweep speeds used for
- intercycle measurements
- rate and rhythm
what mechanisms control the CW probe
- has an oscillator that produces an continuous AC voltage for motion detection
is doppler only sensitive to RBC flow
- no
what is wall filter used for
- reject the low frequencies from wall motion of pulsatile vessels
what can be a consequence of too high of wall filter
- if too high it can reject real blood velocities in the lower frequency range
what is the main thing that color doppler provides us with
- qualitative blood flow analysis over a wide field of view
what 2 things does color doppler combine
- anatomical detail of 2D image and physiological hemodynamics with color overlay
what are 4 other names for color doppler
- color doppler imaging CDI
- color flow imaging CFI
- color flow mapping CFM
- color doppler CD
what are the 2 processes required to display color doppler
- standard 2D processing
- doppler processing
how many pulses per scan line are required for a color doppler image
- 4 pulses
- 1 for 2D
- 3 for color
- HUGE IMPACT ON FRAME RATE
how is color doppler processed
- autocorrelation
what is autocorrelation
- mathematical means of rapidly determining the sign, mean, power and variance of retuning signal
what is ensemble length
- aka packet size
- number of pulses used for each color scan
what does color data represent
- mean or average of the velocities present
what does a longer ensemble length mean for the frame rate
- slower frame rate
to calculate a mean velocity what is the minimum number of pulses that need to be sent out and what is the usual number sent out
- 3
- 10 or 20
what 4 things does autocorrelation yield
- sign (HUE)
- magnitude (SATURATION)
- power (LUMINANCE)
- VARIANCE
what is the hue
- the color you see and represents the sign or direction of flow
- tells us a bit about magnitude too
what is the saturation
- amount fo color in a mix with white and represents the magnitude of the doppler shift or how fast the blood is traveling
what does a more white saturation mean for flow velocity
- faster flow
what is luminance
- brightness of the hue and saturation presented
what does luminance represent
- power of flow and RBC density within the field of view
what will more RBCs travelling at the same velocity appear as
- brighter on display
what is variance
- displays the amount of velocities that are different than the mean
- how far away from the mean we are
how can a variance map help us more than mean velocity
- give greater detail
- can pinpoint an area of disease
what is color priority
- determines whether or not the color overlay will overwrite the low level echoes
what is given preceded in color priority is low
- non doppler shifted weak echoes
what is given precedence when color priority is set high
- color will overwrite the low 2D echoes
what is power doppler also known as
- energy mode
- color power angio CPA
what is power doppler
- color displayed represents the strength of the doppler shift as opposed to the velocity
what are 3 advantages to power doppler
- free of aliasing
- less dependant on angle
- sensitive to low flow states
what are 4 disadvantages to power doppler
- non direction
- flow speed
- character information (laminar flow or not)
- very susceptible to blooming artifacts
