What is it called when source and receiver are approaching each other?

Is the reflected frequency higher or lower than transmitted frequency?

Positive change or shift

Higher

What is it called when source and receiver are moving away from each other?

Is the reflected frequency higher or lower than transmitted frequency?

Negative change or frequency shift

Lower

What are the typical values of frequency shift in sonography?

20 Hz - 20 kHz, audible.

We still use 2 MHz to 10 MHz transducers to perform a Doppler ultrasound study, but the Doppler shift (which is a change) ranges from 20-20,000 Hz.

Δf=2f_oV(cosθ)/c

Δf = Doppler shift (kHz)

f_o = Transmitted frequency (MHz)

V = Velocity of blood flow (cm/s)

cosθ = cosine angle between US and blood flow direction

 c = speed of sound in soft tissue (1540 m/s)

velocity

speed

velocity

direction

cosine of the angle

Two crystals in the transducer:

one crystal is constantly transmitting

the other is continuously receiving

Aliasing is the most common error associated with Doppler ultrasound. An alias is a false identity. The false identity in PW Doppler is that very high velocities in one direction are incorrectly displayed as going in the opposite direction. 

Flow above the baseline is going toward the transducer, whereas flow below the baseline is going away from the transducer. With aliasing, the high velocity flow reaches the very top of the spectral display then wraps around and reappears at the bottom. 

Either:

• reduce the Doppler shift (reduce the speed)
• raise the Nyquist limit (raise the speed limit)

• aliasing occurs only with pulsed Doppler
• aliasing occurs when the Doppler sampling rate is too low in comparison to the measured blood velocities

• Higher frequency transducers create more aliasing.
• Doppler shift (Δf) is (=) directly related to the transducer's frequency (f_o). The Doppler shift is percentage of the transducer frequency.
• At a particular velocity, higher frequency transducers create higher Doppler shifts and lower frequency transducers create lower Doppler shifts.
• Smaller Doppler shifts from lower frequency transducers are less likely to exceed the Nyquist limit. Therefore, aliasing artifacts are less common with lower frequency transducers.

• The colors provide information on flow direction and velocity.
• Colors in the top half represent blood cells moving toward the transducer, while colors in the bottom half represent away from the transducer.

• Similar to velocity mode maps, the colors in a variance mode map indicate flow direction. Colors in the top half represent blood cells moving toward the transducer, while colors in the bottom half represent away from the transducer.
• In addition, the variance maps display different colors side to side. Colors on the left side of the map indicate laminar flow.
• Colors on the right side of the map indicate turbulent flow.
• Remember L--left, laminar

Packets composed of a larger number of pulses (larger packets or longer ensemble lengths) have two advantages:

• more accurate velocity measurement
• increased sensitivity to low flow

Packets with more pulses have these disadvantages:

• more time needed to acquire data
• reduced frame rate
• decreased temporal resolution

Power mode only identifies the presence of a Doppler shift. It does not evaluate speed or direction. Power mode is non-directional color Doppler.

Power Doppler is also called energy mode or color angio.

• increased sensitivity to low flow or velocity, such as venous flow or flow in small vessels
• unaffected by Doppler angles, unless the angle is exactly 90°
• no aliasing, since the velocity information is ignored

• no measurement of velocity or direction
• lower frame rates (reduced temporal resolution) than conventional color flow Doppler
• susceptible to motion of the transducer, patient, or soft tissues, which may result in a burst of color, or flash artifact

• Doppler shifts generally arise from moving blood. However, slowly moving anatomy, such as heart muscle or pulsating vessel walls, may create very low frequency Doppler shifts.
• On spectral displays, these low frequency Doppler shift artifacts are called clutter. 
• With color Doppler, they are called ghosting artifact.

• A wall filter is used to eliminate low frequency Doppler shifts from moving anatomy rather than from moving blood cells. In essence, wall filters serve as a "reject" for Doppler. 
• Wall filters are also called, high pass filter

• Crosstalk is a special form of "mirror image" artifact, arised only with spectral Doppler. 
• Crosstalk appears as an identical Doppler spectrum both above and below the baseline. It appears as bidirectional flow when in reality it is unidirectional.

• Doppler gain is set too high (electronic crosstalk)
• the incident angle is near 90° between the sound beam and the flow direction, when flow is at the beam's focus.

• Performed to extract or identify the individual frequencies making up the complex signal. It is used to interpret individual velocities in the signal.

Two methods of spectral analysis are currently used:

• For PW or CW Doppler: Fast Fourier Transform (FFT)
• For color flow Doppler: autocorrelation or correlation fuction

A digital (computer-based) technique, is used to process both PW and CW Doppler signals. FFT have two advantages

• exceedingly accurate
• displays all individual velocity components that make up the complex reflected signal

[image]

 In color Doppler image A, why does the vessel appear without color?

[image]

Why does vessels B and C have color and A does not?

Why is vessel B blue and C red?

• In vessel A, the angle of incidence is 90° which makes the cosine θ zero so there will be no Doppler shift.
• In vessel B, the color box has been steered to the left, creating an oblique angle (>90°); thus, the cosine θ is negative. Therefore, the flow is away from the transducer's beam.
• In vessel C, the color box has been steered to the right, creating an acute angle (<90°); thus, the cosine θ is positive. Therefore, the flow is towards the transducer's beam. 

[image]

In color Doppler image A, why is there no color within the vessel?

[image]

What will make color appear in vessel A?

[image]

What single control on an ultrasound system can make color appear throughout the box  as shown in image B?

Image B was created by setting the color gain too a level that is too high. 

When the color gain is set to an inappropriately high level, color appears throughout the color box. A term that effectively describes this is color confetti.

[image]What alteration was responsible for the change in the appearance of the spectrum from image A to image C?

Aliasing is present in the color Doppler image in image A.

Image B is a zoomed image of the flow in the artery (upper vessel). The central flow stream is blue, indicating flow toward the transducer, or from right to left (blue arrow).

The flow streams closer to the vessel walls are red, indicating flow away from the transducer, or from left to right (red arrow). It is unlikely that there is flow in different directions within a single vessel. Aliasing should be considered as a possible source of this peculiar color variation, whenever the color changes directly from red to blue or vice versa.

To answer the question, look at the colors that are adjacent to the blue area. The transition of colors is from red to black to blue (yellow arrow). On the color map, red to black to blue forces us to move through the center of the map. This color transition indicates flow reversal. 

Aliasing on the image would appear as green adjacent to yellow.