Doppler Ultrasound

Question 1

Doppler shift

Answer 1

Change in frequency = Fr - Ft

• depends on velocity and direction of blood flow
• dependent on angle

Question 2

Doppler Equation

Answer 2

Delta F = V Cos(theta) 2 Ft / C

• delta F —> Fr - Ft
• V —> velocity of blood flow
• theta —> angel between blood flow and u/s beam
• Ft —> transmitted frequency
• C —> speed of u/s in soft tissue (1540 m/s)

machine assumes theta is close to 0 therefore will underestimate velocities if angle is too large [Cos 0 = 1 ; Cos 90 = 0]

Question 3

Continuous Wave Doppler

Answer 3

2 crystals — one continuously sends and the other continuously listens for u/s

Very high PRF
•1/2 PRF = Nyquist limit (max doppler shift without aliasing) therefore can measure very high velocities

Measures velocities all along beam therefore range ambiguity

Question 4

Pulsed Wave Doppler

Answer 4

Measures flow velocity at a specific point at the area of interest

Emits a pulse and waits for return echo

Benefit of range resolution but limited by max doppler shift — high velocities appear negative

Question 5

How to reduce aliasing

Answer 5

Increase PRF by moving sample gate closer to the probe (decrease depth)

Use a lower transmitted frequency (decrease delta F)

Optimize the baseline

Use CWD

Question 6

Color Flow Doppler

Answer 6

Codes velocities as colors
Sends out packets of pulses and measures the doppler shifts within those packets
Codes velocities as mean velocities
BART = blue away, red towards

Question 7

Wall filters

Answer 7

Function to filter out part of Doppler signals

Low pass —> filters out HIGH velocities
•TDI

High pass —> filters out LOW velocities
•blood flow
•PWD measures high velocity, low amplitude signals with high pass filter on and low pass filter off
•if too high will remove all signals near the baseline and prevent detection of the onset and termination of a flow signal

Question 8

5 functions of the receiver

Answer 8

Amplification

Compensation

Compression

Demodulation

Rejection

Question 9

Rejection

Answer 9

Rejects low amplitude 2D signals

•NOT for doppler

Question 10

Power Doppler

Answer 10

Only signifies the presence of a doppler shift
•no direction and no velocity information
•advantage: increased sensitivity to low flow, unaffected by angle (unless exactly 90 degrees), unaffected by aliasing
•disadvantage: lower frame rates, no velocity or direction, susceptible to outside motion

Question 11

Parameters determined by sound source only

Answer 11

Time = seconds
•period
•frequency

Strength
•power
•amplitude
•intensity

Question 12

Parameters determined by medium only

Answer 12

Velocity — determined by:
•stiffness - proportional
•density - inversely proportional

Question 13

Speed of u/s in tissues

Answer 13

Air < lung < fat < soft tissue < bone

Question 14

Parameters determined by both sound source and medium

Answer 14

Length
•wavelength
•spatial pulse length

Question 15

Wavelength equation

Answer 15

Lambda = velocity / frequency

Wavelength of probe = (1.54 mm/microsecond) / MHz of probe

Question 16

Pressure Half Time

Answer 16

Time it takes to go from max pressure gradient to 1/2 max pressure gradient

• shorter PHT —> faster equalization of pressures —> big hole
• longer PHT —> slower equalization of pressures —> small hole
• used to determine degree of regurgitation or stenosis [ MS and AR ]

Question 17

Factors affecting MV stenosis PHT

Answer 17

AI: decreases PHT —> overestimates MVA —> underestimates MS

Decreased LV compliance: rapid equilibration of pressures —> overestimates MVA —> underestimates MS

Impaired LV relaxation: slower equilibration of pressures —> underestimates MVA —> overestimates MS