week 8 CDI Flashcards
CDI defn
Colour Doppler Imaging = shows ave Doppler shifts over a large area
CDI explain how scanning works. Whats the minimum packet size?
put colour box on area of interest
A single line is repeatedly scanned to create a colour packet. Min packet size is 3 (today up to 16)
Then the packet is scanned across the next line…
Note: Each scan line has multiple gates - a sample gate at each pixel…therefore, each pixel represents mean velocity for flow in that area
What happens to accuracy and FR when you increase the colour packet size?
Incr packet size = incr accuracy and decr FR
PW uses __ to collect data
CDI uses __ to collect data
PW uses FFT
CDI uses Autocorrelation bc faster (large amount of data)
Autocorrelation - how does it work?
collects large amount of data
looks at the phase shifts of all the scan lines within a packet
differences in phase between received signals = movement
Ave. velocity is displayed
**correlating data from diff scan lines within a colour packet
Stationary echo cancelers
Removes echos that don’t change phase, which are assumed to be stationary tissue
High pass filters
removes high amplitude signals that are assumed to be from fast moving tissues and valves (cardiac)
Similar to a wall filter in PW
Exposure - CDI vs Spectral
CDI is less exposure than spectral PW/CW bc it covers a large area quickly with a low number of pulse trains, therefore lower intensity at any one area
colour box shape: cardiac vs general
cardiac - vector shaped
general - rectangular (linear array probe) and steerable
Colour maps: Saturation
Saturation map - only 2 colours and the amt of saturation changes to show velocity
Colour maps: Hue map
Hue map - the hue (colour) actually changes to show velocity
Colour maps: Variance map
Variance map - Adds a 3rd colour, usually green which means there is a lot of variance in the sample and therefore turbulent flow (pathology?)
Colour gain
Turn up until speckles… then turn down until speckles first disappear
Colour baseline
Leave in the middle…. only adjust in one case: PISA for AI or MR
Colour scale
Use the default, except lower when assessing for ASD
Colour comparison or simultaneous
Have both 2D image and 2D with colour side by side on the same screen.
OK, but you must take a separate 2D cine also bc colour lowers the FR
Colour wall filter
Cardiac - not adjusted
General - decr colour wall filter if assessing very low flows
Colour flow priority
Greyscale bar next to colour bar
Can adjust the amount of greyscale vs colour for a particular pixel
used in vascular scanning for flow detection in small arteries
Nyquist limit
Highest velocity we can accurately sample
Aliasing
Occurs when we scan deeper structures (decr PRF) or higher vel (incr Fd) bc can’t sample 2x per wave
Velocities that exceed Nyquist limit will wrap around… this is not a bad thing bc it draws our eye to fast flow (vs. PW where aliasing is bad)
3 ways to improve FR in CDI
- decr SLD = incr FR…. but each pixel is bigger and image will look pixelate therefore we don’t do this
- decr colour sector width = decr number of lines needed to create image…. always use the narrowest box possible but still include area of interest
- decr colour sector depth/height = incr PRF (bc decr the wait time) = incr the Nyquist limit
What is colour m-mode good for?
colour m-mode = put colour on m-mode trace
good for showing the timing of events (eg. exact duration of valvular regurg)
Power doppler Defn
Pro and con
Flow or not?
Used in general if low flow and only need to determine presence/absence of flow (started with CDI and no flow picked up?)
higher amplitude signals = mores RBCs moving = brighter colour displayed
eg. calf vein, breast, thyroid, transplant organ
Pro - sensitive to slow moving blood (low amplitude, low intensity signal)
Con - motion artifacts if the patient moves
TDI
Used in cardiac to show the motion of tissues
