PHYSICS OF COLOR DOPPLER IMAGING

DOPPLER EFFECT:
When the receiver is stationary the frequency of sound emitted (Fo) and that reflected(Fr) from the receiver are the same.
When the receiver of the sound is moving the frequency of emitted sound (from the source) shows change. If the receiver is moving towards the source the waves get closer and net frequency is increased (positive change). If the receiver is moving away, the net frequency is decreased.(negative change).





When bloodflow & the ultrasound beam are moving towards each other (opposite direction) cos theta is positive and frequency shift is displayed as spectral waveform above the baseline.

When bloodflow & the ultrasound beam are moving in the same direction cos theta is negative and frequency shift is displayed as spectral waveform below the baseline.

In Doppler ultrasound this concept is used to detect velocity of blood flow.(detected or displayed blood flow velocity.).
Frequency change (n) = 2 x Fo x V x cosq
C
Fo -Frequency of sound originally transmitted.
C---velocity of sound in human body (constant)-1540m/s
V= blood flow velocity.

EFFECT OF BEAM ANGLE:
This is the angle of insonation between the direction of ultrasound beam and that of blood flow. Normally it should range between 45 to 60 degrees to prevent erroneous calculation of velocities.

Whenq is 90o then cosq = 0 and whenq =0 cosq=1.Thus maximum frequency shift is detected when blood flow (vessel) is parallel to beam (that is beam vessel angle is 0) and no frequency shift is detected when beam is perpendicular to blood flow (vessel) . No frequency shift means no flow is detected.

This effect is opposite to a normal 2 D scan where maximum resolution is when the beam is perpendicular.
V= n x c
2 Fo cos q

WITH HIGHER ANGLE VELOCITY OF FLOW DETECTED WILL HOWEVER INCREASE AS IS DEPICTED BY ABOVE FORMULA WHICH SHOWS THE LOWER THE VALUES OF Fo AND cos q THE HIGHER IS VELOCITY DUE TO INVERSE RELATION. THUS TOO HIGH INSONATION ANGLE WILL PRODUCE FALSELY HIGH VALUES OF VELOCITY CALCULATION.
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PERPENDICULAR TO BEAM NO FLOW SEEN

In Doppler ultrasound this concept is used to detect velocity of blood flow.(detected or displayed blood flow velocity.).
Frequency change (n) = 2 x Fo x V x cosq
C
Fo -Frequency of sound originally transmitted.
C---velocity of sound in human body (constant)-1540m/s
V= blood flow velocity.

EFFECT OF BEAM ANGLE:
This is the angle of insonation between the direction of ultrasound beam and that of blood flow. Normally it should range between 45 to 60 degrees to prevent erroneous calculation of velocities.

Whenq is 90o then cosq = 0 and whenq =0 cosq=1.Thus maximum frequency shift is detected when blood flow (vessel) is parallel to beam (that is beam vessel angle is 0) and no frequency shift is detected when beam is perpendicular to blood flow (vessel) . No frequency shift means no flow is detected.

This effect is opposite to a normal 2 D scan where maximum resolution is when the beam is perpendicular.
V= n x c
2 Fo cos q

WITH HIGHER ANGLE VELOCITY OF FLOW DETECTED WILL HOWEVER INCREASE AS IS DEPICTED BY ABOVE FORMULA WHICH SHOWS THE LOWER THE VALUES OF Fo AND cos q THE HIGHER IS VELOCITY DUE TO INVERSE RELATION. THUS TOO HIGH INSONATION ANGLE WILL PRODUCE FALSELY HIGH VALUES OF VELOCITY CALCULATION.
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PERPENDICULAR TO BEAM NO FLOW SEEN

If we go on increasing the frame rate the smooth quality of Image will decrease. The reason being that every machine probe can scan only a certain number of lines per second. Thus, if we increase the frame rate the number of lines per frame will decrease and this will decrease resolution. The advantage of higher frame rate is that movements are picked up better. During routine 2D scanning it is advisable to keep a low frame rate so as to get a smooth sharp image. However when required the frame rate should be increased for e.g. When looking for cardiac pulsations in an early fetal node or when doing echo.







FOCAL ZONE—

Every machine has focal zone. This zone is adjusted to bring it close to the area of interest so that visualization is good. Focal zone can be single or multiple. With multi- focal zones multiple areas at different depths can be brought at focus producing a much smoother image than that achieved with single zone. However the problem with multi-focal zones is that the frame rate is significantly decreased producing slow images. Thus in routine scanning number of focal zones used should be regulated by depth. When scanning very deep only a single focal zone may be afforded while when scanning at low depths multiple focal zones may be used without affecting the frame rate significantly.

COLOR FLOW SCANNING:
Application of color to an image requires a number of extra pulses within a frame thus frame rate automatically dips when color is applied to an image. In order to achieve a reasonable frame rate following things can be used:
1.B mode gain is kept as low as possible so that enough pixels are available for color study.
2.Field of view of B mode image to kept as small as possible.
3.Depth to be kept as low as possible.
4.Only single focal zone is used
5.Mechanical probes have low frame rates and should be avoided in color flow studies.
6.Velocity range (PRF) should be sufficiently high. It should be remembered however that higher velocity range means lower color flow sensitivity.

PULSE REPITITION FREQUENCY (PRF)(VELOCITY RANGE)

A probe can emit certain number of pulses per second. The number of pulses emitted change with depth, less number of pulses are formed with increase in depth since time taken by the pulse to return is increased.
With low PRF the pulses travel slowly so slow flow is better detected. Frame rate is thus low.
Higher velocities require a higher PRF.
PULSE REPITITION FREQUENCY (PRF)(VELOCITY RANGE)

A probe can emit certain number of pulses per second. The number of pulses emitted change with depth, less number of pulses are formed with increase in depth since time taken by the pulse to return is increased.
With low PRF the pulses travel slowly so slow flow is better detected. Frame rate is thus low.
Higher velocities require a higher PRF.

COLOR FLOW
As a tradition flow direction is suggested in red & blue. Red suggests flow towards the transducer and blue suggests flow away from the transducer. Blood flow velocity is however suggested by intensity of color. Higher velocities are depicted by less intense (lighter) color while low velocity is depicted by more intense (darker) color. Flow in curved vessel may be red in a segment and blue in other depending on change in direction. Where change in direction occurs the beam is at 90o to blood flow and so frequency shift is 0 hence no flow is detected.
Color pattern can however be changed by machine button (inversion)


FLOW TOWARDS PROBE IN RED AND AWAY IN BLUE

TORTOUS VESSEL SHOWS DIFFERENT COLORS SINCE FLOW AT ONE MOMENT MAY BE TOWARDS PROBE AND AT OTHER IT MAY BE AWAY

FILM BENEATH SHOWS CENTRAL HIGH VELOCITY OF FLOW IN LIGHTER COLOR








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FILM BENEATH SHOWS DIFFERENT COLORS IN A STRAIGHT VESSEL AT DIFFERENT TIME. THIN ARROWS SHOW BLACK SUGGESTING NO SIGNAL (SINCE FREQ. SHIFT IS ZERO)WHEN BEAM IS PERPENDICULAR TO FLOW
CURVED ARROWS SHOW CHANGE IN COLOR WHEN THE BEAM IS NEARING 90 DEGREES.






COLOR BOX:

1.Size: The smaller the size the higher the frame rate. Since lesser number of pulses are used in the frame, more frames can be formed. The color box can be adjusted both laterally & along depth. Reduction of size in lateral direction is more important to increase the frame rate.

2.Angulation: This is possible with linear probe. Angulations can be used to provide best of flow.


THE UPPER FILM HAS POORLY ALIGNED COLOR BOX AND THUS HARDLY ANY FLOW IS SEEN

PROPER ALIGNMENT OF COLOR BOX SHOWS GOOD FLOW

SPECTRAL DOPPLER:

• Sample volume placed in the center of the vessel.
• Blood velocity calculated from frequency shift
• Velocity (calculated on the basis of frequency shift) is displayed against time.
• Wall filter - eliminates very slow blood flow near baseline that occurs due to movements of the vessel wall.


TOP LEFT FILM SHOWS SAMPLE VOLUME NEAR VESSEL WALL . THUS LOW VELOCITY BLOOD NEAR BASELINE IS SEEN. TOP RT. FILM SHOWS SPECTRAL IN MIDDLE OF VESSEL .THUS ONLY HIGH VELOCITY OF BLOOD SEEN WHILE LOW VELOCITY BLOOD NEAR WALLS IS NOT SEEN.
BOTTOM FILM SHOWS SAMPLE FROM FULL WIDTH AND THUS BOTH LOW AND HIGH BLOOD VELOCITIES ARE SEEN.



Types of Spectral Doppler:

Arteries have two types of flow:

BIPHASIC FLOW
• Central type arteries- low resistance pattern (biphasic pattern)
• Peripheral type - high resistance. (Triphasic pattern)
Veins have a monophasic continuous flow.

TRIPHASIC FLOW WITH ALAISING

VENOUS FLOW WITH RESPIRATORY VARIATIONS



SPECTRAL WINDOW

It is an area beneath a Doppler spectrum commonly below the systolic wave. Clear window indicates that slow flow components are not present. Spectral window is filled with low velocities if wall filter is adjusted to low (i.e. less of the filtration is done.) or if turbulence e.g. due to stenosis is present.

ALIASING

At very high velocities (when frequency shift is > 50 % of PRF) spectral waveform is cut and displayed on opposite side of spectrum.
Aliasing in color flow shows as color inversion in center of vessel with a mixture of light (less intense) colors

ALAISING IN COLOR IMAGING
COLOR ALAISING


SPECTRAL ALAISING

INCREASING VELOCITY RANGE REMOVES ALAISING IN AORTIC BRANCH BUT AT HIGH VELOCITY RANGE FLOW IN PORTAL VEIN IS SEEN POORLY








Diff. Between aliasing &flow reversal

1.Changed colors in aliasing are lighter (high freq. shifts) while in true flow reversal the changed color remains dark. (Low freq. shifts)

2.Change from one color to another in flow reversal is separated by black line (not in aliasing) which is area where beam is at 90 deg hence Doppler frequency shift is zero

Basis of aliasing is high velocities falling beyond the velocity range
E.g. 1.intrastenotic or post stenotic acceleration 2-when PRF (velocity range) is set low.

Steps for reducing aliasing:

1. Shift the baseline lower down this will increase the range of velocity detection.
Base line shift should be tried before increasing velocity range (PRF) since it
can remove aliasing without decreasing the color flow sensitivity (as occurs
with increase in velocity range).
2. Decrease the depth---This will increase PRF since lesser distance means more pulses can be formed.
3. Increase the Velocity Range.
4. Use lower frequency transducer.
5.Increase beam angle.
4&5 act by decreasing the frequency shift.

{Note: Frequency change (n) = 2 x Fo x V x cos q}
C
V= n x c
2 Fo cos q

VELOCITY OF FLOW DETECTED WILL HOWEVER INCREASE AS IS DEPICTED BY ABOVE FORMULA WHICH SHOWS THE LOWER THE VALUES OF Fo AND cos q THE HIGHER IS VELOCITY DUE TO INVERSE RELATION.
ALIASING IS PREVENTED SINCE THE THRESHOLD OF VELOCITY WHICH PREVENTS ALIASING IS INCREASED .
THRESHOLD VELOCITY ( V-T)WHICH CAN BE DECTED WITHOUT ALIASING IS GIVEN BY


V-T= 3000
D X Fo cos q
D IS MAXIMUM DEPTH OF IMAGING
OPTIMIZING IMAGES

First B- mode image is optimized followed by color flow and then spectral Doppler.

B-mode optimization

1-Vessel of interest is seen by scanning at oblique angle
2-B- mode gain is set as low as possible so that enough pixels are left for color flow.
3- Single focal zone is used.

Color flow optimization:

1. Keeping the probe as parallel to the vessel as possible. Further parallelization of beam is done by beam steering (when linear probe is used)
2. PRF (velocity range) is adjusted. Too high will produce poor flow (color voids )while too low will produce aliasing.
3. Color gain set followed by decrease in size of color box to keep a high frame rate.

Doppler spectrum optimization:

1.Sample volume should be less than 2/3 of vessel diameter
2. PW gain adjusted—with too low gain the spectrum may not be seen & with too high gain
background noise is seen and there is filling of spectral window.(filling of spectral window may also be seen in turbulent flow or when wall filter is too low)
3.If aliasing is present then it is corrected.

COMMON PROBLEMS

A. Poor color flow:
Rectification:
1.Bring probe as parallel to vessel as possible
2.Reduce B- mode gain
3.Increase color gain
4.Reduce velocity range (in veins & areas of slow flow)
5.Reduce color wall filter (in veins & areas of slow flow.)
6.The acoustic power may be increased.



REDUCTION OF B MODE GAIN AUTOMATICALLY IMPROVES BLOOD FLOW
B. Low velocities despite normal color image
Rectification: Bring the beam as parallel to vessel as possible


C. Spectral broadening
Doppler spectrum has a window. Loss of window is due to—
(a) Turbulent flow causing fill up by lower velocities.
(b) Sample volume is large (width>2/3 rd).
(c) Wall filter too low.
(d) Too much PW gain.

COLOR NOISE—ARTEFACTS:

They are more of a problem when assessing slow flow. They may be caused by tissue motion or improper machine settings.
This noise will usually appear first in anechoic areas due to the preference of the system to write color data where there are no gray scale echoes.
How to remove it?
1.Reduce color gain.
2.Increase the color wall filter: ---Color wall filter when low will improve detection of slow flow in small vessels but may produce color noise. Thus too low color filter should be avoided.
3.Change the patient’s position. This may help by moving unwanted area away from the area of interest. For e.g.: a peristaltic bowel may be shifted away.
4.Breath holding may reduce tissue motion artefacts.
5.Changing the position and size of the color box. This may help to remove unwanted moving artifacts from the field of view.­­­­6.Increasing velocity range (PRF). This will however reduce color flow sensitivity



INCREASING WALL FILTER HAS REMOVED COLOR NOISE

COLOR NOISE IS REMOVED BY DECREASING SIZE OF COLOR BOX AND COLOR GAIN AND INCREASING VELOCITY RANGE
COLOR NOISE OR SLOW FLOW? CONFUSION? WHAT TO DO?

Place spectral doppler sample and if flow can be detected that is not color noise.

COLOR PRIORITY

This facility is available in many of the scanners. Here adjustment is made to give priority to either the display of color or B-mode image.
High color priority is useful when:
1.Vessel is too small e.g.: In thyroid & testicular examn.
2. When artifacts of grey scale echoes fill the vessel lumen.e.g. :when scanning at a greater depth.
FILMS SHOW HOW INCREASE OF COLOR PRIORITY HAS SHOWN FLOW WHERE NONE WAS SEEN

POWER DOPPLER (COLOR AMPLITUDE IMAGING)

Here the signal reflected from RBC S is displayed .The higher the amplitude the more powerful is the signal. Amplitude depends on
1.The number of RBCS within the sample volume.(The greater the number ,more is the amplitude)
2. Size of the vessel: (The larger the size the better the signal)
3.Beam attenuation: (lesser the attenuation more is the signal)

Advantages of power Doppler:
1.No aliasing since it is not based on frequency shift.
2.Less angle dependent than conventional color imaging thus eliminates multiple angle artifacts.
Multiple angle artifacts:
Seen in conventional color imaging due to the beam striking the different portions of the same vessel at different angles. There may be a variety of color shades displayed within the same vessel. This problem is little with linear probe (since incident angle is more or less constant) but much more with convex probe.

Why smaller vessels are seen better with power Doppler? This is by increase in the color priority by:
1.Increased persistence of flow signals (higher ensemble length)
2.No velocity & direction information

How to provide a good picture by power Doppler?
1.Use low velocity range (PRF). This will increase color sensitivity. Ailasing is not a problem (which occurs with conventional color imaging and spectral Doppler when too low PRF is used.) since direction information is not provided.
2.Movements of the probe should be slow since frame rates are low due to low velocity range used.
3.Within the velocity range being used a better frame rate should be achieved by using as small a color box (specially in lateral dimension) as possible.
4.A higher color gain maybe used (since signal to noise ratio is higher here than conventional color imaging and thus increase of gain will increase the signal more than noise.) till the time
color artifacts are not much of a problem.

PROBLEMS OCCURRING IN POWER DOPPLER

1- Color artifacts – They are more problematic than in conventional color imaging since power uses higher persistence and thus these artifacts remain for a longer period of time.Due to this power imaging cannot be used where movement is marked eg. In echocardiography.
The method employed for reduction of color artifacts in conventional color can be used here except increasing wall filter (which acts by removing low frequency shifts) cannot be used here since power imaging is not based on frequency shift.
2-Scanning deep areas is difficult since beam attenuation is more with power as compared to conventional color.
3-Gives no information on direction or velocity of blood flow .

Features available in conventional color imaging but of no use in power Doppler
1-wall filter-already described
2-Baseline shift-It is used to remove alaising within same velocity range. Not useful in power since no alaising occurs in it.
3-Color inversion
Ensemble length(also called color sensitivity or color quality )—It is the number of pulses used to form a line , the greater the number of pulses used more is the ensemble length and better is the color flow sensitivity. However frame rate decreases when ensemble length increases.

RESOLUTION

• Spatial resolution is the ability to differentiate two structures as separate. It can be axial or lateral.
• Temporal resolution or frame rate is the ability to track down moving areas over time. It is high in M-mode scanning.

Range Resolution

• With pulsed wave Doppler information in a particular region can be achieved .This provides range resolution.
• Continuous wave Doppler provides spectra throughout its path and thus range resolution is not possible. This feature referred to as range ambiguity.
• Range ambiguity can also occur with pulse doppler.
• Here signals are received not only from the region of sample volume but also from twice or three times the depth of sample volume.


HIGH PRF DOPPLER MODE

• This utilizes the range ambiguity to increase the maximum velocity that can be measured with pulse doppler.

• Signals are captured not only from sample area but also from twice the depth and further multiples of original distance.(2x,3x,4x……).

• Under normal conditions the range ambiguity can be ignored since the signal strength is low
• However, with high PRF mode the signal strength of these harmonics is increased to provide useful information.
• Just like continuous doppler the origin of signal of interest is based on supportive data.

How does this technique provide high PRF?
• For eg: If the sampling is done at 50% 0f the distance of the area of interest PRF is increased since PRF is dependent on the distance the waves travel.
• If sampling is done at 25% distance PRF is increased even further.

BEAM WIDTH

• Effects the doppler signal also and results in superimposition of adjacent flow signals at deeper regions.

MIRROR IMAGE ARTEFACT

• Symmetrical signal having slightly less intensity than the actual signal is seen in direction opposite to the actual signal.
• D/D—Sampling of an area near to rt. angle.