Physics Chapter 7 Flashcards
Doppler effect
change in frequency that occurs when there is relative motion between the transducer and the reflecting surface (RBCs) determines velocity of reflectors and direction of blood flow
What frequency are the transmit sounds and receive sounds
Transmit in Mhz and recieve and process into audible range KHz
Dopppler shift
doppler shift= reflected f - transmitted f this is the frequency of the moving blood cells
postive and negative shift
positive flow is towards transducer
negative flow is away from transducer
Insonation angle
Best doppler angle= 0 or 180
Vascular angle=60 most common range 30-60
echo= 0
90= no doppler shift
cos values
cos 0=1
cos 60=0.5
cos90=0
RBC Aggregation (Rouleau)
Low shear flow states BC form a rouleau formation like a roll of coins increasing the refelctibity often results in ability to visualize the blood (spontaneous contrast)
obtain a good angle
Beam sterring- signal phasing process allows doppler to be steered left or right
heel toe trasnducer- decrease angle to flow
natural vessel angle
angle correction
CW doppler
Continuous stream of sound waves, measures velocity of blood flow along a fixed line cant determine exact location, high sensitivity but limited range
PW doppler
Short pursed of sound waves to measure flow velocities, allow to determine exact location and velocity, lower sensitivity but wider range.
Nyquist limit
highest doppler freq accurately processed by pw ultrasound without aliasing= 1/2 PRF
PRF
Number of echoe cycles per second determined by depth of sample volume increase sv decreae in max prf
Aliasing
artifact that occurs when using pw doppler and the nyquist limit is exceeded. high velocitys reported in opposite direction they are in
Spectral doppler aliasing
Only in pw not cw
Bidirectional doppler intrument
distinguishes between pos and neg doppler shifts
can be pw or cw
directional doppler
speed and direction of rbcs use stereo system to determine pos or neg shift
unidirectional
presence of shift
inexpensive small units determine presence or absence of flow
non directional
only speed not flow simple speaker
spectral doppler
examines flow at one sire
detailed analysis of distribution of flow
good temp resolution
allow calc of velocity and indices
pos shift above baseline
neg shift below baseline
Colour doppler
overall view of flow in region
limite flow info
poor temp resolution
determines velocity, direction and flow pattern
Colour flow map
High PRF spectral doppler
used to measure high velocitys without aliasing exceed nyquist limit
power doppler
assigns values to doppler shift
power determined by concentration of moving scatters producing doppler shifts
independent of doppler shift, freq and angle
b mode imaging
Helps image diagnose vascular disease
image moving particles
flow plus b mode creates image
vector flow imaging
uses multiple beams to create flow map assessment of flow direction velocity and patterns within blood vessels. enhances capabilities for conditions
spectral analysis
study or interpretaation of collection allows visualization of doppler signal and provides quantitative data to evaluate doppler shift
What happens to recieved echoes (very long 7 things)
-amplifier preforms amplification, mixer preforms doppler shift detection
-wall filter-selectively decreases dynamic range by eliminating high amplitude low velocity echoes
-gain- addition gain after dr reduce so weak signals are amplified
-audio- stereo signal and one speaker for forward and one speaker for reverse flow
-adc- start to prepare signal for visualization
-fft
-post processing- compression, reject, gray scale
FFT fast fourier transform
mathmatical process that analyses returning doppler signals and sieplays on spectral display
x axis time y axis doppler shift f an amplitude
Quadrature phase detection
samples are separated into forward and reverse shifts to determine flow direction
seperated by 90 one quadrant to detect flow
spectral window
clear area under spectrum
freq range narrow- large clear window
freq range wide- window filled with echoes
waveform magnitude or brightness
amplitude of signal
changed by scanning technique of sonographer
main factors affecting spectral doppler
power- transmitting power into tissue
gain-overall sensitivity to flow signals (amplification of doppler signal recieved)
velocity scale/prf- high prf high velocity low prf low velocity
gate size
beam steering- can allow improved beam flow angle for better accuracy of v calc
live duplex/ triplex-scanning modes spectral resolution constrained by need for b mode/ color pulses
wall filters and sweep speed
wf-eliminate unwanted blood flow signals high amplitude low freq signals
ss- faster speed easier to measure
slower more cardiac cycles
Color doppler
superimpose colour code map of doppler shifts on b mode image
pulses fewer and shorter
mean velocities shown
Packet
color of pulses per scan line of color
bmode= 1
color 3-20 common 8-20
high number robust colour lower frame rate improved sensitivity to weak signals
low number weak color oposite of high
Autocorrelation provides
-flow direction
-mean freq shift
-power amplitude
-variances
colour frame rate variables
pulses per line packet size
density scan lines
color box width
prf
color persistence (smoothing)
averages frames over time improves signal to noise ratio, allows colour to remain on image longer
Directional color power doppler
combines power (amplitude) of signal with direction info to encode direction and variation in blood flow
HPRF
aliasing eliminated PW switched to HPRF increase number of active sample volume when operator wants to measure blood flow at certain depth where aliasing occurs with pw
color box orientation
orientate color box so that its orientation aligns with orientation of blood vessels
