SPI review 5 Flashcards
flow towards the transducer
increased flow
flow away from trans
decreased flow
the doppler shift ranges from
20-20000Hz
the process by which the doppler freq (shift) is extracted from the trans freq
demodulation
tran freq high, doppler freq is low
doppler shift = rcvd freq - transmitted freq
FD = FR - FE
doppler equation
doppler shift=2xreflector speedxincident freqxcos(angle)/prop speed
doppler shift is directly related to the
red blood cell speed
freq of transducer
cosine of the angle between flow and the sound beam
doppler shift is inversly related to
the speed of the sound in the medium
what does the 2 in the doppler equation represent
double doppler shift, 1st occurs when the sound stikes the cell the 2nd the moving blood cell reflecting the wave back to the transducer
in order to accuratley determine velocity
the angle between the directions of flow and sound beam must be known
doppler measures velocity not
speed
velocity is
magnitude and direction
doppler freq depends on direction. The magnirude of shift depends upon the
cosine of the angle between the sound beam and the direction of motion
angle is
0
60
90
the cosines are
1
0.5
0
doppler shift is inversly related to
the speed of sound in a medium
non coherent processing is used with
unidirectioinal dopper
a basic doppler system (non or unidirectional dopper) that simply identifies the blood flow by the presence of
a freq shift, it does not distinguish blood flowing towards or away from trans
more sophisticated doppler systtem which distinguishes
flow towards and away from trans
bidirectional doppler distiguishes to and from trans by
the measurment of positive and negative doppler shifts
____ __________ is used in bidirectional doppler
phase quadrature
number of crystals in continuous wave trans
two
one is transmitting and one is receiving
advantage of continuous wave dopper
able to measure very high velocities accurately
disadvantage of continuous wave dopper
range ambiguity-echoes arise from entire length of overlap between the transmit and rcv beams
range means
depth
other advantages of continuous wave doppler
higher sensitivity and ease in detecting small doppler shifts
pulsed wave doppler number of crystals
one crystal that alternates between sending and rcving
advantages of pulsed wave doppler
range resolution aka
range specificity
freedom from range ambiguity
echoes arise only from the area o interrogation, the sample volume or gate. we adjust the receive gate
disadvantage of pulsed wave doppler
aliasing, errors in measuring high velocities
imaging and pulsed doppler can be performed with a single crystal trans. Simultanious imaging and doppler is know as
duplex ultrasound
high velocities appear negative. with pulsed doppler, high velocity measurements are inaccurate if the pulsed doppler sampling rate (PRF) is too low in comparison to measured blood velocity
aliasing
eliminating aliasing
improves the ability to measure the max velocity with doppler
the doppler freq at which aliasing occurs, = to 1/2 the PRF.
nyquist freq
nyquist limit (kHz) =
(kHz) = PRF/2
deeper samples are more
likely to have aliasing
w/ _____ freq transducers, aliasing is more likely to occur
higher
5 ways to eliminate aliasing
adjust the scale (increases the PRF and Nyquist limit)
get trans w/ lower freq
get a new window/view with shallower sample volume
switch to continuous wave doppler
adjust baseline(only appears gone)
smaller sample volumes (gates) create doppler spectra with ____ _____ ______
larger cleaner envelopes
larger sample volumes (gates) create doppler spectra with ____ _____
smaller envelopes (spectral broadening)
x axis on doppler spectrum
time
y asxis of doppler spectrum is
doppler shift or velocity
doppler shift and trans freq are _____ related
directly
with a 7MHz probe doppler shift is 3000Hz, what will the shift be with a 3.5MHz probe
1,500Hz
gray shades on a doppler spetrum are related to
amplitude of the reflected signal
number of red blood cells creating the reflection
color doppler is pulsed ultrasound technique and is subject to
range resolution or specificity
aliasing
color doppler is
semi quantitative
color doppler reports
average velocities aka mean velocities
color doppler gain setting has greatest effect on
jet size
doppler packets
multiple pulses are needed to determine RBC celocites by dopplers, single measurement is not enought
multiple doppler pulses are call
packet
aka
ensemble length
the packet must balance between accurate velocity measurements and
temporal resolutions, this is why color doppler measures mean velocity
disadvanage to doppler packets
more time needed
frame rate is reduced
temp res is decreased
advantages of doppler packets
greater accuracy
sensitivity to low flows
spectral doppler (pulsed and continuious) measures _______ -______ while color flow measures _____ ______
peak velocity
mean velocity
color power doppler aka
energy mode and color angio
advantages
increased sensitivity to low flows
not affected by doppler angles unless its 90*
no aliasing
color power doppler limitations
no velocity measurement
slower frame rate and temp res
susceptible to motion of trans, patient or soft tissues, called flash artifact
spectral analysis
performed to extract or edentify the individual freq making up the complex signal
spectral analysis for pulsed or CW doppler
fast fourier transform (FFT)
spectral analysis for color flow doppler
autocorrelation or correlation function
FFT and autocorrelations are
digital techniques performed by computors
autocorrelation is used w color doppler cause of the enormous amount of doppler info that requires processing. It is ____ _____ but ______ than FFT
less accurate
faster
anremia and doppler
we can always successfully perform a doppler exam regardless of the patients hematocrit
RBC closest to the trans will create the highest
amplitude
no dppler shift is measured at a _____ angle
90*
at what engle will the doppler shift be at max
0*
T or F
only pulsed wave dopppler has ample vomules
T
T or F
the higher the emitted freq the more likely the pulsed wave signal is to alias
T
pulsed wave doppler has a rcv gate that is
adjusted by sonographer
nyquist limit -
1/2 PRF
errors in imaging
artifacts
artifacts are
not real missing reflectors iimproper brightness improper shape improper size improper position
causes of artifacts
violation of assumptions (acoustic artifacts)
equipment malfunction or design
physics of ultrasound
interpreter error (anatomic pitfalls)
operator error
