Saturday, 7 May 2011
Technical priciples of doppler US (the basics)
Alternating voltage -> ceramic elements (crystals) -> the elements change Shape and Size -> pressure waves (ultrasound)
Conversly: Sonund waves returning as echos cause crystals to vibrate ->induce voltage -> processed -> image is produced
For determining velocity and direction of blood flow.
The doppler effect:
When a sound source and a reflector are moving towards each other, the sound waves are place closer together (higher frequency, high pitch sound).
Freq. shift = freq of echo (FE) - freq of sound originally emitted (F0)
EFFECT OF BEAM ANGLE:
The sound waves travel in human tissue at a constant speed of about 1540 m/s.
The frequency shift measured strongly depends on the cosine of the beam-vessel angle (alpha angle).
- In the least favourable case the cosine angle is =90` to the vessel, the frequency shift equals to zero, therefore no signal is detected.
- The most favourable case with the most accurate measurement is when the alpha angle is <60`, but <45` is even better.
CONTINUOUS WAVE (CW)
- The sound is continuously emitted from one piezoelectric crystal and is recieved by another crystal.
- Advantage: can detect very higy velocities
- Disadvantage: can`t tell the depth of echo, range ambiguity
- Note: when using CW you cant use the B mode or CDS in most machine, you will be using a frozen-out image to locate the sample gate.
POWER WAVE (PW)
The sound is alternately transmitted and recieved by the sam crystal. The TE-delay time to recieve the echo can be converted to distance. This is needed to construct a 2D color duplex image. The smaller the color box, the faster the operation and the higher the temporal resolution.
PULSE REPETITION FREQUENCY (PRF)
The PRF can only be increased to 1/TE ( can take samples quicker than the echo returns, you have to wait for the echo). PRF decreases as the scanning depth increases (more time needed for echo to return). 1/TE sets the upper limit to the flow velocities that can be accurately measured with PW doppler.
Vessels with high flow velocities => high PRF used
Vessels with lower flow velocities (veins, searching for DVT) => small PRF used
COLOR FLOW - COLOR FLOW DOPPLER SCAN (CDS)
Flow towar the transducer = RED
Flow away from the transducer = BLUE
Fast flow = BRIGHT RED/BLUE
Slow flow = DARK RED/BLUE
THE TURNING VESSEL:
Flow in a curved vessel maybe red in one segment and blue in the other.
! Color brightness also changes in accordance to the alpha angle! So the accuracy of measurment in a curved vessel changes along its curve due to the change of the alpha angle of the vessel !
The 90` alpha angle at the junction of the red-blue segments results in a color void which should not be misinterpreted as a thrombus.
Don`t interpret flow direction after you have used the invert button, it confuses you and can result in many mistakes.
Beam stearing is an option in linear transducer can be used to change the angle of doppler waves.
Angling the color box: can correct a bad alpha angle (>60`). This can make vessel segments that initially show poor flow easier to evaluate.
Manually angling the transducer: if still the alpha angle is not ideal.
INTERPRETING SPECTRAL WAVE FORMS:
Can be aqcuired in PW mode, by using the trackball to position the sample volume (SV) in the area of interest. The insonation angle (alpha) must be determined before the frequency shift data can be converted into a true flow velocity.
The scanner then displays the velocity distribution consisting of a slower and faster flow components.
The time to peak (TTP), is the time it takes for the blood to reach its peak velocity in systole (D->S in the image), the TTP proximal to a stenosis is increased, and the curve becomes less sharp. Normally the TTP almost appears as a vertical upstroke in systole.
Renal artery stenosis PROXIMAL to the sampling site i.e. before the sampling site, note the increased TTP
Severe stenosis of the innominate artery. PW Doppler spectral image of the right CCA shows a tardus-parvus waveform, which is suggestive of a severe stenosis proximal to the point of sampling. A severe stenosis of the innominate artery was subsequently demonstrated at angiography. EDV = end-diastolic velocity.
BASIC WAVE FORMS IN CENTRAL ARTERIES
Near the heart arteries pump blood against a relatively low peripheral resistance resulting in a biphasic wave pattern.
The smaller end-diastolic velocity peak is separated from the systolic peak by a small notch caused by the closure of the AO valve.
The lower the resistance the higher the end-diastolic flow, therefore if you see high diastolic flow then this might be indicating a higher perfusion demand by distal tissues (excercise, or in ischemia)
TRIPHASIC WAVE FORM IN PERIPHERAL ARTERIES
At greater distances from the heart the arteries loose their winkessel function while the the peripheral resistance increases. As a result the peripheral arteries show a triphasic waveform.
General feature: A rapid upstroke in early systole, which then falls off rapidly with a small notch marking the closure of the AO valve.
Due to high resistance in the peripheral arteries early diastole is normally marked by a period of reverse flow directed back to the heart (second phase), this appears as a deflection below the baseline. This is followed by a small upstroke that flows towards the periphery (3rd phase).
These are not distorted by improper angle selection. They are good in evaluating small arteries.
Pulsatility index (PI) = (Vpeak-Vdiast)/Vmean
Resistance index (RI) = (Vpeak - Vdiast)/Vpeak
The measured frequency shift exceeds the Nyquist limit of the (PRF)/2 at very high flow velocities, the wave will be cut off and displayed on the opposite side of the spectrum. This phenomenon is analogous to the wagon wheel effect in western movies, in which the wheels appear to be rotating in the opposite direction.
In doppler aliasing can be corrected by increasing the PRF shifting the zero baseline.
Aliasing in color flow image is manifested in color reversal.
What to do to avoid aliasing?
- increase PRF
- decrease penetration depth
- shift the baseline
- Use a lower frequency transducer (linear -> convex)
- Increase the alpha angle (to increase the range of error)
SIGNS OF STENOSIS IN THE DOPPLER SPECTRUM:
Proximal to stenosis: little or no change
Immediately before stenosis: backwash pattern
The areas under the curve in both the positive and negative range are the same in a case of total occlusion, all the blood pumped is returned back.
Intrastenotic waveform shows a very definite increase in flow velocity. The smaller the residual lumen of the artery the faster the blood must travel through it (Vmax increase, wave becomes more peaky).
The intra and post stenotic jet of high velocity blood flow after the stenosis falls off quickly with increasing distance from the stenosis.
In the intrastenotic wave there is filling of the area under the wave (filling of the spectral window), this is a sign of turbulance.
Far past the stenosis: delayed upstroke, and prolonged TTP, there may be relative elevation of the diastolic velocities.
Signs of proximal stenosis:
1- Post stenotic decrease in PSV
2- Post stenotic lengthening of TTP
3- Post stenotic elevation of diastolic waveform (depends on distal ischemia)
- Normal PSV
- Norma TTP
- Normal or slight increase in diastolic flow
- Clear spectral window
- Increased PSV
- Normal TTP
- Normal or increased diastolic flow depending on degree of stenosis & ischemia
- Spectral window maybe filled in.
- Slightly increased PSV
- Increased TTP
- Increased diastolic flow
- Spectral window is filled in due to turbulance
- Decreased PSV
- Increased TTP
- Increased diastolic flow
- Clear spectral window - no more turbulance
HOW TO OPTIMISE THE IMAGE QUALITY:
1- Angle the probe relative to the vessel axis
2- Place the focal zone to the center of the vessel
3- Set the B-mode gain to a low level
Color flow image (CFM):
4- Use beam stearing to improve the beam-vessel angle
5- Adjust the PRF to the prevailing flow velocity
6- Increase the color gain until blooming occures, the lower the signal until color is only seen intraluminally
7- Place the SV at the center of the lumen, an adjust its size to be 1/2 - 2/3 of the luminal diameter
8- Adjust the baseline level for spectral components to be above or below the baseline to avoid cutoff at the top or bottom
9- Adjust the velocity range (PRF-in PW), if aliasing still occures:
- Doppler trace too short => decrease PRF => to expand trace vertically
- Doppler trace too high => increase PRF => to compress the trace vertically
10- Adjust the PW gain to obtain good contrast-to-noise ratio:
try to get a dark background without noise pixels, but dont set the gain too low
11- Remember to watch out for the alpha angle
Other related images:
**** Very good for carotid examination :http://radiographics.rsna.org/content/25/6/1561.full
(Text main source: Thieme Teaching manual of CDS )