Week 7 spectral CW and PW Flashcards
Doppler shift defn
Doppler shift is the difference between the original transmitted freq and received freq; it only occurs if the source and/or the receiver is moving.
transducer is receiver (stationary)
RBC is source (moving)
fd = Fr - Fo
= audible (20-20000 Hz)
The Doppler Eqn
fd = [2fov*Cosθ]/c
fd is Doppler shift or freq
fo is transmitted freq
v is reflector (RBC) velocity
Cosθ is doppler angle (machine assumes θ is 0)
c is 1540 m/s in soft tissue (machine assumes this)
to increase fd, increase fo, v, or Cosθ (more parallel to flow)
Should the transmit freq (fo) be high or low
use low freq for Doppler
- low freq beams are less sensitive to low vel, low amplitude signals (RBCs are Rayleigh scatterers, I proportional to f^4)
BUT
- less attenuation
- alias less quickly
therefore, better option
Doppler angle (Cosθ) - General versus Cardiac scanning
θ is angle btw incident beam and blood flow
In cardiac, must be parallel to flow (machine assumes this, Cos(0) is 1, otherwise we will underestimate v and disease severity
In general, can use angle correct and enter an estimate for θ into the machine. Only angles less than 60° should be used. Cos(60) is 0.5 and Cos(90) is 0.
What happens if general sonographer estimates angle correct inaccurately?
Increased angle correct error and/or increased θ (Doppler angle) causes increased error in calculated v
FFT
FFT is Fast Fourier Transform
= complex math process that sorts a large volume of freq data being received
for SPECTRAL Doppler only
Spectral graph: x, y, and Z axis
x is time
y is velocity in cm/s
z is intensity or brightness (# RBCs)
Spectral envelope
the range of frequencies (velocities) displayed
Transmit power button
do not adjust, default
if incr, incr sensitivity of Doppler signals but also incr exposure to patient
Gain
incr to amplify the received signals without incr exposure to patient
volume
turn up to hear different Doppler frequencies and recognize when you are in the center of a jet
spectral invert
reverses the direction of forward and reverse flow (y axis)
not used in cardiac
wall filter
= black by baseline; want to adjust to get rid of noise but not signal
filters out low freq high intensity signals that are caused by tissue motion
Range gate or sample volume
determine the size of the area to investigate in PW
Cardiac - don’t adjust
General - adjust sample size to 2/3 the width of the vessel
sweep speed
changes how fast the x axis moves
adjust to see 2-3 traces
baseline
adjust to see the right flow - above baseline? below? or both?
scale
trace should fill 2/3 of screen
sample site or focus zone
PW - location of sample vol
CW - ball is focal zone
Acoustic power: Doppler vs 2D
Doppler has higher acoustic exposure than 2D
bc energy is focused on one area for a length of time
therefore, strict guidelines for Doppler on OB patients
Explain CW doppler
- 2 crystals, 1 to transmit continuously and 1 to receive continuously
- displays all velocities along line = range ambiguity and no spectral window
- use to get peak velocity (no Nyquist limit)
- Duplex or PEDOF probe
Duplex vs PEDOF probe
Duplex probe is the regular probe
- both imaging and spectral doppler (get thumbnail image and graph)
PEDOF is Pulsed Echo DOppler Flow
- name is misleading bc its used only for CW now
- blind CW probe (no 2D)
- old and simple, very small so fits btw ribs - 1 crystal to transmit and 1 to receive
Explain PW Dopper
- 1 crystal transmits then receives (only listens at time corresponding to desired depth)
- get velocity of blood at a certain location (sample gate) = has spectral envelope
- cannot analyze blood that exceeds Nyquist limit
- only Duplex probe bc need to see spl gate
- watch out for high PRF mode
What is high PRF mode?
high PRF mode is when multiple gates open up during PW scanning. This increases the PRF but now you have range ambiguity.
Watch out!! on old machines, high PRF mode turns on automatically when you turn up the scale and exceed the Nyquist limit
PRF
Pulse Repetition Freq is the number of pulse trains emitted per second
limited by depth for PW
pulse train
pulse train is the number of cycles in a single pulse
the more cycles in a pulse, the better the signal quality BUT this requires more time
What is Aliasing
Need to spl at least twice per wave cycle in order for analyzer to get accurate fd; otherwise get alaising (wrap around)
…if incr v, the fd increases and you get alaising if the PRF (ie spl rate) is not high enough
…if incr depth, the PRF decr and you can get aliasing if the v is high
Nyquist limit - eqn and defn
Nyquist limit = PRF/2
= the highest velocity that can be measured accurately on PW at that particular depth
What are 4 ways to eliminate aliasing?
- Incr scale = incr PRF (until you reach Nyquist limit)
- adjust baseline (to optimize for flow above or below, PRF does not change)
- Decr transmit freq (in reality, leave at default)
- Incr Doppler angle (general only)
