Physics #3 exam Flashcards
autocorrelation defined… used to measure what? what does it correlate? comparison to FFT?
it is a digital technique used to ANALYZE COLOR FLOW DOPPLER (used w/color bc huge amount of data)
Correlates data from multiple sample sites and analyzes it for mean frequency, amplitude, and variance or turbulence
Somewhat less accurate but faster than FFT
Color bar… no color means what degree
90 degrees!
velocity mode measures a ______ velocity
mean
nyquist limit = ______ PRF
half the PRF
color scale AKA… how should you set the color scale?
AKA PRF (important control as it increases or decreases the max velocity that can be displayed without aliasing)
the scale should be chosen to accommodate the range of velocities thought to be present
Too low a scale setting will cause aliasing and too high will result in lack of color filling in the vessel
PRF should be ideally 1/2 of the expected velocity of blood flow in region of interest
Think of “lower” as in zooming in… as if you were on google maps and you were getting lower to the map everything would appear bigger
PRF or color flow should be ideally set at half the expected velocity of blood flow… example if the carotid artery blood flow is appx 70-80 cm/s in a normal adult, what should the PRF be set to?
between 30-40 cm/s
Just as in spectral Doppler waveforms, the color signals in Color Doppler will do what?
“alias” if the speed of the blood exceeds 1/2 of the PRF scale
- red will wrap into blue and vice versa depending on flow direction
eliminate aliasing… how to in 5 quick steps
- adjust PRF / scale - nyquist limit
- shift baseline
- increase angle of approach
- decrease transducer frequency
- change depth - get closer to vessel
Color flow Doppler… complete the sentence
Flow reversal AKA turbulence AKA bidirectional flow,
Aliasing
main difference between turbulence and aliaing in color Doppler flow?
ALIASED DISPLAY WILL NEVER DISPLAY BLACK!
which provides more information… velocity mode or variance mode?
variance mode
vascular probe …. head
feet
the only transducers that display a square shaped color box which steers from left to right to provide Doppler angle is a
linear array transducer; all others display a sector shaped color box
TO REITERATE… in color Doppler flow, the color dropout exists WHY???
NOT BECAUSE THERE IS AN ABSENCE OF FLOW BUT BECAUSE THE INSONIFICATION ANGLE IS… CLOSE TO 90 DEGREES!!!
color doppler, single or multiple sample gates?
multiple
what needs a longer transmit pulse time… b-mode or color doppler?
color
to register a mean Doppler shift color must transmit what?
multiple acoustic lines … referred to as color packets or color ensembles
the more pulses in the packet, the more _________ the velocity measurement
accurate
worst temporal resolution of all the modes
color doppler
in color doppler, if u choose a larger packet what will happen… something good and something bad
it will produce smoother color with less noise but sacrifice temporal rez
between _________ is a normal packet size for color doppler
4-12
one of most important misinterpretations of MIRROR ARTIFACT in color doppler is a misdiagnosis of what? how to avoid?
DISSECTION!
to avoid this, change the scaling plane and manipulate the incident angle of the sound beam
Twinkling artifact looks like… caused by? can help identify this particular pathology
looks like a rapidly changing mix of colors… gives the appearance of turbulent blood flow
caused by the presence of small strongly reflective objects within the Doppler study.
used to identify kidney stones!
Doppler artifact - BLOSSOMING!
used to refer to a signal that is essentially over gained… signal bleeds or blossoms into neighboring regions of spectrum
Color Doppler artifact - FLASH!
spurious appearance of blood flow - can be due to motion of patient’s body, motion of probe, motion of anatomic structure due to external force
use prevention (motion discriminator function found on most US machines)
Power Doppler disadvantages
- no measurement of velocity or direction
- lower frame rate (reduced temporal rez, than normal Color Flow Doppler)
- susceptible to motion which may produce a burst of color / flash artifact
- more sensitive TO MOTION than color… uncooperative patients… byeeeeeee
- can provide false data when used at low prf/scale
intensity of power Doppler (energy mode) depends on number of
RBCs within sample volume
whats the point of power doppler?
it emphasizes the quantity of blood flow…. used whenever EXTREME SENSITIVITY TO LOW/SLOW FLOW IS NEEDED
power doppler has aliasing?
NO, measures amount of flow, there is no frequency change
power doppler angle matters?
no, unless at exactly 90 degrees
appx how many times more sensitive is power doppler to color doppler?
3x - it eliminates color artifacts
in color doppler, a low color priority allows what? a high color priority? (understanding is in the name)
low color priority allows valid grey scale info to override Doppler info and a high color priority allows Doppler info to override grey scale info
ex… a low color priority in carotid artery results in poor filling of vessel lumen
also called temporal averaging/compounding
color persistence or persistence… a temporal smoothing technique used to reduce noise
combines frames from previous frames - averages them, enhances low velocity flow images
improves signal to noise
when color signal is weak we use this tool
PERSISTENCE aka color persistence!
this is the amplification applied to Doppler component of the image during signal processing
color gain
when trying to detect low velocity flows with color, you must make sure to ________. if you do not… what will happen?
lower the color scale…
if you do not Lowe the color scales, the integrated wall filters will be too high, and will eliminate the very signals you are trying to detect
saturation is
how much white is added compared to original color
brightness is
how intense the color is; related to amplitude of signal
