Doppler ultrasound basics.
Doppler effect consists of the variation produced in the frequency of the ultrasound emitted when the object in which the wave reflexed,
The deviation of the frequency follows the formula shown in the image (2).
Fig. 2: The formula in the image shows why the diference between the frequency recived (Fr) and transmited (Ft) changes depending on the angle with the vessel.
Power Doppler shows the “power” of the signal detected but not velocities and direction of flow as Colour Doppler.
Power Doppler has advantages (3):
Fig. 3: The image on the top shows the portal vein with Colour-Doppler, in the bottom one Power-Doppler was used.
- No aliasing.
- Less angle dependence.
- More sensitivity to detect flow.
- Less noise: noise is homogeneous in the base of the signal; Power Doppler can eliminate it but not Doppler Colour (4).
Fig. 4: The noise is the base of both signals. You can eliminate it in Power-Doppler (left) without affecting the flow detection because its scale is from the top to the bottom (“Power” arrow), but in Colour-Doppler (right) if you suppress the base you will affect the information of the flow direction, because the scale is placed from the left to the right ( “- to +” arrow).
- What determines a pixel colour? (5)
- Pulse Repetition Frequency.
- Colour map chosen.
- The frequency deviation (determined by the vessel velocity and the angle).
Fig. 5: Those are the factors which affects the result of a pixel. Note the first one is the frequency deviation but not the velocity of the flow because of the influence of other factors in the final frequency deviation (see the formula shown in the figure 1).
2. How to acquire a fine adjustment of several parameters:
- Scanning window and transducer selection (6):
About the transducer selection we must know that more frequency means more sensitivity but less penetration.
if we want to study the abdominal organs we need penetration (low frequencies),
but if we want to evaluate the abdominal wall,
a higher frequency is the better option.
A correct window selection is important when flow direction and velocities are studied,
taking into account the vessel being studied.
a subcostal oblique position with cranial angulation of the transducer is needed for imaging the hepatic veins converging into the vena cava,
but to study the Portal vein an intercostal lateral approach must be the choice in order to capture the flux coming into the transducer.
Fig. 6: More frequency means more sensibility but less penetration.
It is the angle between the transducer and the vessel,
it should be between 45° and 60°.
Remembering the formula of Doppler effect,
if the cosine results 0 (that is the case of 90°),
no flow will be detected. Besides that,
from 60° the change to 0 is fast.
That is the reason why an insonation angle under 60° is needed.
Fig. 7: If we want to see the wall of the vessel, the optimal aproach is perpendicular to it, but to evaluate the flow an insonation angle under 60° is needed.
- Pulse repetition frequency and aliasing (8):
The Doppler frequency is sampled with a defined rate,
known as pulse repetition frequency (PRF).
PRF depends on the sound velocity and on the tissue depth.
The deeper the tissue,
the longer time the echoes needs to come back.
When the sampling is too low for the frequency sampled it falls outside the selected range and aliasing appears. Aliasing is one of the artefacts secondary to an inappropriate adjustment and usually occurs with high velocities; specifically it happens with flow velocities which correspond to Doppler shifts above the Nyquist frequency.
Nyquist frequency is a frequency twice the PRF.
The velocity sampled is placed into the opposite flow portion.
Fig. 8: The PRF is the number of pulses (send and listen cycles) of ultrasound sent out by the transducer per second. PRF depends on the sound velocity and on the tissue depth. The deeper the tissue is, the longer time the echoes need to come back.
If aliasing appears during the examination,
the options are:
- Baseline: Up or down,
depending on the case.
- Velocity scale: increase.
- Angle of insonation.
- Transducer: use lower frequency.
The velocity scale corresponds to the range of velocities depicted in colour or spectral Doppler.
If we sample a velocity outside the scale,
aliasing will occur.
Aliasing can be avoided by expanding the scale enough to include the velocity sampled.
But a higher velocity scale means higher PRF: the higher velocity depends on the number of pulses allowed to be sampled,
Fig. 9: If we are studying a high flow and we aply a low PRF (means a low scale), aliasing will appear. To avoid it with high flows we need to use a high PRF (a high scale). On the other hand, if we have a slow flow, we will no detect it with a high PRF (high scale), so we will have to reduce the PRF (low scale).
Fig. 10: In spectral Doppler, reduce the scale allows a better valoration of the wave. Compare the first picture to the second one and note the wave is represented bigger but has the same velocity (arrows). The colour image does not change because we have modifyed the spectral scale but not the Colour scale.
The baseline defines which flow is going to be positive and which negative,
allowing us to alter the velocity.
The colour baseline (a black line on the colour bar) can be modified to emphasize the flow towards the transducer (lowering the base line) or the flow away from the transducer (elevating the baseline).
Note that the range of velocities shown will not change but the flow will be emphasized.
Fig. 11: The yellow circle is sourronding the baseline. The picture shows an example of what happens if we move down the baseline: the scale is the same but a higher proportion of the velocities are now considered towards the transducer, so we are emphasizing these flows and reducing their aliasing. But also we are reducing the scale dedicated to the flow away from the transducer.
The baseline will be important to reduce aliasing and characterize flow: if we have aliasing and a flow away from the transducer,
elevating the baseline could fix it.
Fig. 12: An example of what happens with a correct portal vein approach (image at the top) if we use a too low baseline (left image) and reduce the scale (right image).
The spectral baseline is separated from the colour baseline.
Modifying it brings the waveform down onto the baseline preventing wraparounds.
Fig. 13: With spectral Doppler move down the baseline allows avoiding aliasing (arrow). Besides that in the second image the scale was reduced. Do not be confused with the position of the baseline in the colour bar: spectral and colour baseline are different during the examination.
Wall filters are used to eliminate low-frequency noise coming from vessel wall motion. Typically the options are high,
medium and low wall filter.
We can choose a high wall filter if we are studying big vessels,
but if we want to study,
the perfusion of a testicle,
a low wall filter is needed to avoid flows.
On colour Doppler,
filters are indicated as black areas on both sides of baseline.
The higher the filter is,
the wider the black band will be.
Fig. 14: The yellow circle surround the baseline. The second colour bar shows a thicker baseline, that means a high wall filter used.
On spectral Doppler,
the filter application made the flows disappear just above and under the baseline,
a higher filter implies a higher band without flow surrounding the baseline.
The operator must remember the wall filters should be adjusted independently for colour and spectral Doppler.
Fig. 15: The wall filter in spectral Doppler clear the slow velocities surrounding the baseline. Note the black area above the baseline in the first image was bigger than the one in the second image, so a higher wall filter was used.
This angle has its application in spectral Doppler,
to calibrate the velocity of the flow sampled.
It is important to explain the difference between Doppler angel correction and angle of insonation: the last one is the angle between vessel and transducer but with the Doppler angle correction we specify to the computer the true angle with the vessel flow.
Fig. 16: The insonation angle was the angle between the transducer and the vessel, but the Doppler angle correction consist in specify to the computer the true angle with the vessel flow.
Under correction of the Doppler angle means unreal lower velocities and vice versa,
that is the importance of the Doppler angle.
Fig. 17: The images show the changes in the velocities (arrow) depending on the correction of the Doppler angle.
Gain consists on the amplification of the information to improve the depiction of the obtained data.
Gain only increase the data sampled,
not facilitate the detection of slow flow,
but if gain is too low the flow could not be seen in the monitor.
A too high gain will produce noise making the individualization of the real signal difficult.
Fig. 18: The gain allows us to amplify the signal received. The image on the right shows an artifact which appears when the gain is too high and colour goes out the vessel limits. This artefact is called “blooming”.
The colour box is an adjustable area within the image in which colour Doppler information is displayed.
It is possible to adjust the size,
and location of the box,
allowing us to determine the volume of tissue from which data are acquired.
The increase of the box size or width means a decrease in the frame rate (the rate per second at which images are produced),
increasing the required processing power and time and affecting to image resolution and quality.
In spectral Doppler is the same,
the gate which determines the area sampled should be as small as possible to exclude adjacent vessels but a too smaller gate could be not able to detect the flow.
Fig. 19: Volume sample in Colour Doppler is defined with the box (left image) and in spectral Doppler correspond to the space between the two bars (right images).
- In relation with spectral Doppler are important:
- Resistance index (20): Vessels with low resistance index will have higher telediastolic velocities; high resistance index means telediastolic velocities near 0.
Fig. 20: A resistance index above 0,7 is considered high and below that limit is considered low. The image shows some examples of each one.
- Spectral broadening (21): It is a widening of the spectral line,
reducing the clear black zone between the spectral line and the baseline.
The causes could be:
+ Artificial: Volume and gain.
+ Physiologic: Small vessels and turbulences.
+ Pathologic: Compression,
Fig. 21: The picture on the right shows a spectral broadening in spectral Doppler because of a renal artery stenosis. Compare with the normal renal artery spectral Doppler on the left.
- Wave´s Phasicity (22):
+ Pulsatile flow (arteries).
+ Phasic flow (veins).
+ Nonphasic flow (diseased veins).
+ Aphasic flow (diseased vessels without flow).
Fig. 22: Types of wave´s phasicity in spectral Doppler.
Common abdominal and pelvic pathologies where Doppler ultrasound is of choice are shown:
- Renal artery stenosis (23,24,25):
Direct and indirect signs of renal artery stenosis are described.
Indirect signs are useful when the renal artery is not accessible because of the characteristics of the patient (obesity,
Fig. 23: The figure shows the direct signs of the renal artery stenosis. The image is a normal renal artery spectral Doppler. RI= resistance index; PSV= peak systolic velocity; RAR: renal aortic ratio.
Fig. 24: When the direct signs are not posible to evaluate, the indirect signs could gave us the diagnosis. The image is a normal interlobar artery spectral Doppler. AT: Aceleration time; IR: Resistance index.
Fig. 25: The spectral Doppler image showed a tardus-parvus flow and an aceleration under 300 cm/s2. A left renal artery stenosis was confirmed at MRI (arrow).
- Portal hypertension (26,
The normal portal vein has a hepatopetal,
low and ondulatory flow.
The causes of a portal hypertension are shown in the figure (30).
Fig. 26: The figure explains the Colour and spectral Doppler characteristics of the normal portal vein.
Fig. 27: The figure explains the Colour and spectral Doppler characteristics of the normal portal vein.
Fig. 28: The suprahepatic veins are different to portal vein because they receive the atrial pressure changes.
Fig. 29: Causes of the different appearances of the portal vein. HF: Heart failure.
Fig. 30: Ultrasound findings of Portal Hypertension (PHT).
- Testicular (31,
33) and ovarian torsion (34,
In these pathologies Doppler is essential,
but in ovarian torsions we must not rule out the diagnosis because of the absence of the specific signs.
Fig. 31: Ultrasound findings of testicular torsion. Remember always compare with the contralateral testicle with the same parameters adjustment.
Fig. 32: Flow was detected in the right testicle (up-right) but not in the left one (up-left). The examination should be with low filter and scale, a low frequency transducer and the radiologist must compare both testicles with the same adjustment.
Fig. 33: This figure shows a patient with a testicular torsion treated by manoeuvres. The distorted testicle (right one) presents an increased flow compare with the contralateral. That is the normal finding in testicular detorsion.
Fig. 34: Ultrasound findings of ovarian torsion. A torsed pedicle and a decrease of venous flow are highly specific signs but its absence does not exclude the possibility of torsion. The reason why it is possible the absence of venous flow with arterial preservation is the double arterial irrigation because of the uterine artery. The ovarian torsion (yellow arrow) compromise organ vessels and later, because of the lack of venous drainage, the arterial flow is also missed.
Fig. 35: The right ovary (first image) was bigger than the contralateral and its vascularization was decreased, suggesting an ovarian torsion. The possible cause was a cystic lesion adjacent to the ovary (*).
It is necessary to do the examination also with the patient standing up and with Valsalva manoeuvre.
There are multiples scales and they usually include the time between the start of the manoeuvre and the start of the flow in the vein.
Fig. 36: Ultrasound findings of testicular varicocele. It is important to use the Valsalva manouvre and the standing position during the examination.
Fig. 37: The figure shows a varicocele. At the top, images with the patient lied down without (left) and with (right) Valsalva. At the bottom, images with the patient standing up without (left) and with (right) Valsalva. The flow is increased with both Valsalva and standing position.
we introduce our initial experience with ultrasound microvascular imaging (MicroFlow Imaging),
a novel application recently approved by FDA.
Conventional colour Doppler Technique suffers from limitations in relation to visualization of fine vessels and low-velocity blood flow.
Contrast-enhanced ultrasound could be an option to improve the detection of these small vessels but it also has its drawbacks: it is not available everywhere,
not everybody knows how to use and interpret it and imply an additional cost on the patient.
Microvascular Imaging is a new Doppler mode carried out to detect slow and weak blood flow in tissues,
allowing to overcome classical barriers in the detection of small vessels with higher resolution and less artifacts.
Our experience with microvascular imaging: examples of its utility.
All the images shown were taken with Microflow,
the microvascular imaging application designs by Philips.
- Phlegmon vs abscess ( 38,
The detection of microvascular flow in the tissue could allow differentiating if inside an area of inflammation (phlegmon) exist zones of necrosis,
helping to diagnosis the abscess formation without contrast.
Fig. 38: The images show a postsurgical fluid collection in a patient with fistulizing Crohn disease. Microvascular imaging shows a small amount of internal fluid surrounded by inflammatory tissue (arrow: black zone inside the blue area), without clear abscess formation.
Fig. 39: Follow-up ultrasound in a patient with acute diverticulitis. Microvascular imaging (right) shows a 9 mm no-flow area adjacent to the antimesenteric border of the sigma. CEUS imaging (down) displays the same zone as no-enhanced spot. Both indicate this area correspond to a small fluid collection.
Fig. 40: This lesion in the upper pole at the right kidney is heterogeneous in B-mode, but with no internal flow in both microvascular imaging (top right) and CEUS (bottom left). A thin marginal ring is also visible in both images. The lesion, in an appropriated clinical context, is compatible with an abscess.
- Viability of the wall (41):
In appendicitis or cholecystitis,
the presence of flow in the wall usually is increase; the absence in some point is a sign of necrosis of the wall.
Microflow is more sensible than Colour-Doppler in its detection.
Fig. 41: A case of appendicitis in which microvascular flow is detected along the appendix, without evidence of wall disruptions. Colour Doppler shows also vascularization but did not define the wall as the microvascular imaging.
- Extension of tumoral masses (42):
In some places like a gallbladder with biliary sludge or a portal vein thrombosis,
the presence of flow inside those contents helps to detect tumoral invasion.
Fig. 42: A case of appendicitis in which microvascular flow is detected along the appendix, without evidence of wall disruptions. Colour Doppler shows also vascularization but did not define the wall as the microvascular imaging.
- Mass characterisation (43,
Vascularisation inside a mass or septa differentiates solid and cystic parts and could help in the detection of tumoral components.
Fig. 43: Cystic anexial mass in a patient with suspected tubo-ovarian abscess. The presence of vascularization define septa (white arrow) and the abscense differenciate no-solid parts (*)
Fig. 44: The image is from a patient with nephropathy who was detected an accidentally hepatic lesion. It is a solid and high vascularized mass, with both small and large vessels (arrow).
Microvascular image helps detecting areas of ischemia thanks to its sensitivity with small vessels.
Fig. 45: The images belong to a patient with an orchiepididymitis complicated with surgical wound infection. The ultrasound show hypoechogenic areas in the testicle with no microvascular flow (arrows), suggesting ischemia.
Fig. 46: Patient with severe scrotal pain and a postsurgical scrotal haematoma. The images at the top shows the left (on the left) and the right (on the right) testicle in both mode b and microvascular imaging. Compare both testicles is essential to avoid mistakes because of a wrong parameters adjustment. The ischemia of the whole right testicle is clear. At the bottom microvascular imaging overlap with mode B.