[Ultrasound Physics] DOPPLER

[Contents]

1. Doppler shift

2. Doppler effect

3. Doppler angle

4. Continuous Wave Doppler

5. Pulsed Wave Doppler

6.Color Flow Doppler

7. Power Doppler (Energy mode, Color Angio)

8. Spectral Analysis

1. Doppler shift

 

1) Doppler effect is used non-invasively to detect blood flow & the motion of body structures

 

2) Frequency of the reflected beam will be different than the initial frequency

- doppler shift = difference between the transmitted and received frequencies [★★]

 

3) Equations

: fd = fr- ft

fd= Doppler shift

fr= reflected frequency

ft= transmitted frequency

2. Doppler effect

 

1) A change in reflected frequency caused by reflector motion [Hz]

- Positive (toward) / Negative (away)

3. Doppler angle

 

 

1) The angle that the ultrasound beam makes with the direction of flow

 

2) Doppler Shift signal is largest when the blood flow is directly toward or away from transducer

- Parallel transducer orientation is not possible

- Ultrasound beam will be at angle with respect to the vessel

 

3) Doppler Equation [★★★★★]

: Δf = 2 • F_T • V • cosϴ / C

Δf = Doppler shift

F_T = transmitted frequency (doppler frequency)

               ↑ doppler frequency → ↑ doppler shift

V = reflector/RBC speed (velocity of the interface)

               ↓ reflector speed → ↓ doppler shift

ϴ = doppler angle of incidence

               cosϴ: smaller the angle, larger the Δf (↑ doppler angle → ↓ doppler shift)

               : cosine changes rapidly at large angles

                              - angles of 60⁰ or less are recommended to reduce error

C = propagating speed (velocity of the medium)

 

4) Velocity estimation in doppler US

- based on measurement of doppler angle of incidence

- angle correct cursor should be adjusted parallel to the vessel wall

 

* system control on doppler that adjusts PRF: spectral velocity scale

 

5) Angle should be between 30 -60

 

- Above 60⁰: too little f shift

- Below 30⁰: increase beam attenuation due to longer path lengths

- Image is best obtained at 90⁰

               : cannot detect frequency shift at incident angle of 90⁰ (perpendicular)

- Doppler is most accurate at 0⁰

               : maximum frequency shift will be obtained at 0⁰

4. Continuous Wave Doppler [★]

 

 

1) continuously transmitting and receiving an ultrasound signal

 

2) Transducer

- 2 crystal elements, one transmits and one receives

: One crystal in continuously transmitting

: The other is continuously receiving

- Advantage

: High velocities are accurately measured

: crystals overlap to produce a region of maximum sensitivity

→ most accurate Doppler shift info

: higher sensitivity and ease in detecting small doppler shifts

- Disadvantage: range ambiguity

- need to change angle with change in frequency (increase your f = greater beam attenuation)

- no damping is applied

 

3) Receiver

- detects the differences between f when there is a reflector motion (Doppler shift)

- demodulation of the signal

- Phase quadrature detection determines the direction of the shift

: Bi directional systems: determine motion and flow

: Uni or non-directional systems: only detect motion

- Threshold function eliminates noise and weaker signals

- CW detects flow anywhere within the sensitive region, regardless of depth

→ can be confusing with 2 vessels at once

 

4) CW doppler does not provide Range resolution

- range resolution

: ability to determine depth from which an echo has arrived

: sound must be pulsed

                              - echo arrival time from each pulse can be measured

5. Pulsed Wave Doppler

 

 

 

1) designed to overcome the lack of range resolution in continuous wave

 

2) Transducer

- Number of Crystals: One crystal, alternates between sending and receiving

: low quality factor, low sensitivity, wide bandwidth

- Advantage

: Echoes arise only from the area of interrogation (sample volume or Gate)

: greatest advantage being able to select the exact location

- Disadvantage

: Aliasing, error in measuring high velocities

 

3) Receiver

- detects the Doppler shift

- Range Gating

: allows to get depth information

: receives velocity from small regions along ULS beam

6.Color Flow Doppler [★★]

1) Doppler shifts are coded into colors and superimposed on the existing B-mode image

- color and B-mode images are formed from separate pulses

               : frequency used for color doppler is generally lower than B-mode image

- color threshold (priority control) [★★★]

: controls gray-scale brightness at which color will be displayed

: ↓ color threshold - color will overwrite vessel or cardiac wall

: spatial resolution (axial & latera) resolution in color doppler: always poorer than in B-mode

- Determining spatial resolution of color image: frequency & line density

- frame rate decrease when color doppler is activated

               : more pulses are fired on each line of sight

- once color doppler is selected, the system automatically turns off all but 1-2 focal zones

               : 1-2 pulses per scan line are used to create the underlying B-mode image

 

2) Color doppler is based on PW: subject to range resolution and aliasing

 

3) Velocity Mode: displays average values

 

 

 

- Colors = flow direction [★★★]

: Black = no Doppler shift (flow perpendicular to sound beam)

: Above black region (red) = flow toward the transducer (+ doppler shift)

: Below black region (blue) = flow away from the transducer (- doppler shift)

: highest positive doppler shift (yellow/orange)

 

4) Variance Mode (Variance) [★★]

 

 

 

- Velocity info

: + shift = colors in top half

: -shift = colors in bottom half

- Distinguishes laminar from turbulent flow

- Variance maps display different colors from side to side

: Left side = laminar flow

: Right side = turbulent flow (yellow: turbulent toward, green: turbulent away)

 

5) Packet size (= ensemble length, shots per line, dwell time) [★★]

- # of pulse (listen cycles) per acoustic scan line

               : for color doppler, each line of sight must be pulsed multiple times.

- ↑ packet size → ↓ frame rate, improved signal-to-noise ratio

 

6) Doppler signal spectral display

- depicts relative signal power (amplitude) at each frequency in the doppler signal

(depicts the frequency bandwidth, range of amplitude in reflected signal)

- z-axis (brightness) on doppler spectrum = amplitude

               : adjust gain to increase amplification

 

9) Color sample gate

- parameter to describe axial length of sampling volume for a color pixel

7. Power Doppler (Energy mode, Color Angio)

 

 

1) Doppler shift colorized without consideration of direction or speed

- non-directional, not angle dependent

- will only show that flow is present

- colorized amplitude of reflected doppler signal

 

2) Advantage

- Increased sensitivity to low flows (Venous flow, Flow in small vessels)

- Not affected by doppler angles

- No aliasing

 

3) Limitations

- No measurements of velocity or direction

- Lower FR, (reduced Temporal resolution) when compared to conventional color flow doppler

- Motion sensitive

8. Spectral Analysis

 

1) determine the distribution and magnitude of frequency shifts in the reflected doppler signal

 

2) Current Methods

- For PW or CW doppler = Fast Fourier Transform (FFT)

- For color doppler = Auto-correlation function

 

3) Fast Fourier Transform (FFT)

- processes PW & CW Doppler

- very accurate

- display all individual velocities

- distinguishes laminar (similar velocities) from turbulent flow (chaotic)

 

4) Auto-correlation function

- analyzes color flow

- faster than FFT, not as accurate as FFT

- used when larger amounts of data need to be processed

 

5) spectral window

 

 

- area underneath the systolic peak on the doppler waveform that is absent of echoes

-filled in when

: the doppler sample volume size is large compared to the size of the vessel

: turbulent flow is present

: doppler gain is set too high

: position of doppler sample volume is not centered within the vessel

 

Reference

 

* Davies Ultrasound Physics review

* https://sites.google.com/site/lindadmsportfolio/ultrasound-physics/

* https://sites.google.com/site/nataljasultrasoundphysics/

* https://sites.google.com/site/ektasphysicseportfolio/doppler