Week 7 spectral CW and PW

Question 1

Doppler shift defn

Answer 1

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)

Question 2

The Doppler Eqn

Answer 2

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)

Question 3

Should the transmit freq (fo) be high or low

Answer 3

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

Question 4

Doppler angle (Cosθ) - General versus Cardiac scanning

Answer 4

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.

Question 5

What happens if general sonographer estimates angle correct inaccurately?

Answer 5

Increased angle correct error and/or increased θ (Doppler angle) causes increased error in calculated v

Question 6

FFT

Answer 6

FFT is Fast Fourier Transform
= complex math process that sorts a large volume of freq data being received

for SPECTRAL Doppler only

Question 7

Spectral graph: x, y, and Z axis

Answer 7

x is time
y is velocity in cm/s
z is intensity or brightness (# RBCs)

Question 8

Spectral envelope

Answer 8

the range of frequencies (velocities) displayed

Question 9

Transmit power button

Answer 9

do not adjust, default

if incr, incr sensitivity of Doppler signals but also incr exposure to patient

Question 10

Gain

Answer 10

incr to amplify the received signals without incr exposure to patient

Question 11

volume

Answer 11

turn up to hear different Doppler frequencies and recognize when you are in the center of a jet

Question 12

spectral invert

Answer 12

reverses the direction of forward and reverse flow (y axis)

not used in cardiac

Question 13

wall filter

Answer 13

= 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

Question 14

Range gate or sample volume

Answer 14

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

Question 15

sweep speed

Answer 15

changes how fast the x axis moves

adjust to see 2-3 traces

Question 16

baseline

Answer 16

adjust to see the right flow - above baseline? below? or both?

Question 17

scale

Answer 17

trace should fill 2/3 of screen

Question 18

sample site or focus zone

Answer 18

PW - location of sample vol

CW - ball is focal zone

Question 19

Acoustic power: Doppler vs 2D

Answer 19

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

Question 20

Explain CW doppler

Answer 20

• 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

Question 21

Duplex vs PEDOF probe

Answer 21

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

Question 22

Explain PW Dopper

Answer 22

• 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

Question 23

What is high PRF mode?

Answer 23

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

Question 24

PRF

Answer 24

Pulse Repetition Freq is the number of pulse trains emitted per second

limited by depth for PW

Question 25

pulse train

Answer 25

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

Question 26

What is Aliasing

Answer 26

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

Question 27

Nyquist limit - eqn and defn

Answer 27

Nyquist limit = PRF/2 = the highest velocity that can be measured accurately on PW at that particular depth

Question 28

What are 4 ways to eliminate aliasing?

Answer 28

1. Incr scale = incr PRF (until you reach Nyquist limit) 2. adjust baseline (to optimize for flow above or below, PRF does not change) 3. Decr transmit freq (in reality, leave at default) 4. Incr Doppler angle (general only)