Doppler Modalities Flashcards
what are the three standard Doppler Modalities
CW
PW
Colour
what does spectral refer to
fact that the Doppler frequency shifts are separated and displayed as a “spectrum.”
Doppler angle
The angular effect on the Doppler shift can be determined from the unit circle and the cosine function.
reflection from red blood cells
Rayleigh scattering
what is there a tradeoff between
There is a tradeoff between increased reflectivity and increased absorption with a shorter wavelength from higher frequency operation. Notice that as the depth increases beyond 3 cm, the transmit frequency really should be set as low as possible for optimal sensitivity because of the dominance of absorption.
what is red blood cell aggregation called
rouleau effect
what is the rouleau effect
With low shear flow states, blood cells form a rouleau formation (like a roll of coins), increasing the reflectivity. This increase in reflectivity often results in the ability to visualize the blood, referred to as spontaneous contrast. In this example, the rouleau formation is formed in the common femoral vein. (As also visualized in video clip presented in the next slide.)
what is the doppler insonifcation angle meaured between
the direction of the flow and the line of observation (Doppler steered line). The following slides illustrate how the Doppler angle is specified for varying cases.
doppler insonifcaton angle of 0
The flow is directly toward the transducer, so the angle between the flow direction and the steered line is 0 degrees. The resulting Doppler shift is positive.
Doppler insonfication angle 180
The flow is directly away from the transducer, so the angle between the flow direction and the steered line is 180 degrees. The resulting Doppler shift is negative.
with a doppler insonifcation angle of 90
will have no frequency shift
what do parameters of the transmitted signal chRCERTISTIC depend on
which type of Doppler is being performed how the user has set the transmit power control the sample (gate) volume in PW the gate depth in PW, the steered line, etc.
why is doppler a spectrum
Unlike the train model, with blood flow, there are many simultaneous frequency shifts (a spectrum) corresponding to the various blood velocities and angles to flow.
amplification
When the reflected echoes are received, the amplitude of all the echoes is very low
The system automatically amplifies the signals before we are able to manipulate them (Pre-processing)
Mixers: Doppler shift detection
The returning signal in Doppler is a combination of the transmitted frequency and the spectrum of frequency shifts as depicted in the last slide.
The Doppler system uses a process called “mixing to baseband” to remove the higher frequency transmitted signal component, so as to detect the lower frequency (baseband) Doppler shifted signals.
Demodulation
The signals reflected from stationary structures are still at the same frequency as the transmit frequency – therefore they are subtracted from the combined signal leaving only shifted frequencies
wall filters
high pass filters that pass signals with frequency shifts higher than the wall filter corner frequency and diminish signals with frequency shifts below the wall filter corner frequency
Clutter signals
Specular reflectors cause extremely high amplitude signals relative to reflection from blood cells (Rayleigh scattering).
These signals may be as much as 160 dB stronger than the desired blood signals.
after wall filter
Signal dynmic range is gretaly reduced
different studies
require different wall filters
venous stduies
lower wall filters because venous flow is typically at lower speeds
arterial wall filters
higherwall filters
typical wall fiolters settings
< 25 – 50 Hz Venous
50 – 100 Hz Arterial
200 – 600 Hz Adult Echo
600 – 800 Hz Pediatric Echo
correct wall filter frequnecy to use
depends on the operating frequency of the transducer as well as the velocity of the desired signal and the undesired clutter.
wall filter saturation
When the clutter signals are not adequately reduced by the wall filters, the circuit saturates. Saturation appears on the spectrum as a bright white signal relatively symmetric about the baseline. In the following spectrum, the circuit saturates as the specular reflection from the valve is evident for each cardiac cycle.
variable gain
Once demodulation occurs more amplification can occur
This is called Doppler Gain
The Doppler gain is logarithmic
Operator control to amplify the spectral Doppler tracing
Fast fournier technqiue
The full range of frequency shifts is divided into individual frequency bins
what does each bin correspond to (FFT
Each bin corresponds to a narrow range of frequency shifts which by the Doppler equation can be converted into a calculated velocity
last bin
corresponds to the maximum decectable velcoity
first bin
The first bins signals are zero because of the wall filter, if the wall filter were increased more bins would have zero frequency shifts
Grayscale compression and rejection
The Doppler signals span a greater dynamic range than is visible by the human eye (how many shades of gray can we detect?)
We can manually change the dynamic range on the machine while the Doppler is on or post-processing after image acquisition
axes of spectral doppler presentation
three
Frequnecy and amplitude are
unrelated!!!
Frequnecy shift
The frequency shift is determined by all of the factors in the Doppler equation. Higher velocities produce higher frequency shifts.
Amplitude
The amplitude is determined by the parameters affecting the power, the type and amount of reflection and the attenuation.
Continuous wave sample volume
CW operation detects flow that occurs anywhere within the intersection of the transmitting and receiving beams of the transducer
The sample volume is also the overlapping region of the transmitting and receiving beams
Because of the large sample volume, the spectral tracing/Doppler can give complex presentations if multiple flows are found between the beams transmitting and receiving – no range ambiguity
