The Principles of Fetal Doppler Imaging

The Doppler shift is proportional to velocity flow, and the Doppler equation permits estimation of flow velocity, which is defined as the speed and direction of red blood cells in the circulation as follows:

V= fd times c/2 (ft times cos Φ) where V = velocity, fd = Doppler shift, (c) = speed of sound propagation in tissue, ft = transmitted beam, and cos Φ = the angle of incidence between the ultrasound beam and the blood flow direction.

In order to determine velocity, the angle of the ultrasound beam and the incident RBCs must be known in addition to the Doppler shift. The transducer frequency and the speed of sound in tissues are constants. [Maulik, Dev. Physical Principles of Doppler Ultrasonography. In Doppler Ultrasound in Obstetrics and Gynecology. Maulik, D, ed. Springer-Verlag, Heidelberg, 2005. pp 1-17.]

IC2 Pulsed-wave Doppler Transducer

In using pulsed-wave Doppler, the ultrasound crystal acts as both a transmitting beam and a receiving beam. [Nicolaides, K, Rizzo, G, Hecker, K, Ximenes. Diploma in Fetal Medicine and ISUOG Educational Series, Doppler in Obstetrics, R. The Fetal Medicine Foundation 2002.]

 

Definitions:  Aliasing

Pulses are transmitted at a given sampling frequency. The maximum Doppler frequency that can be measured unambiguously is half the transmitted frequency (the pulse repetition frequency, PRF). If the blood velocity and beam/flow angle being measured combine to give the maximum Doppler frequency a value greater than half of the pulse repetition frequency, ambiguity or aliasing in the Doppler signal occurs.

maximum Doppler frequency that can be measured unambiguously

Above. This is an example of aliasing. The pulse repetition frequency (PRF) or the scale is set too low.

pulse repetition frequency (PRF)

Above. This is an improvement over the previous image, with the pulse repetition frequency (PRF) or the scale being set at a higher level.

 

Image Acquisition:  Umbilical Artery

Below is a summary of optimum methods for Doppler interrogation of fetal vessels:

1.  Use color Doppler to determine the vessel of interest.

2.  Adjust gain or PRF to prevent aliasing.

3.  For low velocities, low PRF or color scale should be used (aliasing will occur if high velocities are encountered).

4.  Define the area of interest and reduce the width and maximum depth of the color flow area under investigation.

5.  Place the focal point at the area of interest.

6.  Place a Doppler cursor within the vessel of interest.

7.  Adjust the gate size. Unless volume measures are being determined, for the umbilical artery, the gate may overlap the umbilical artery and vein.

8.  The beam/vessel angle should be 60° or less if velocity measurements are to be taken.

 

Other Fetal Imaging Considerations:

1.  Obtain mid-segment.

2.  Perform study during quiet fetal state.

3.  Use appropriate SVS (sample volume size [the same as “gate”]): usually 3 mm. midtrimester.

4.  Assess a minimum of 3 to 5 waveforms.


Obtaining Waveforms for Umbilical Artery:  Summary

Obtaining Waveforms for Umbilical Artery: Summary

1. Obtain mid-segment fetal image. When clear vessels are demonstrated in 2-D gray scale, place the Doppler cursor first.

Obtain mid-segment fetal image

2. When Doppler gate and appropriate angle corrections are made, rapid display of umbilical artery Doppler data can be achieved.

Doppler gate and appropriate angle corrections

3. Color flow Doppler demonstration of umbilical artery and normal Doppler waveform illustrates scale, region of interest, focal point, and Doppler result.

dop2.7

Above. Schematic of progressively abnormal umbilical artery waveforms. See Doppler Information section for more examples of abnormal umbilical artery waveforms.

 

Some Factors Affecting Umbilical Artery Indices

Sample site: affects Doppler indices

Factors Affecting Umbilical Artery Indices

For the umbilical artery, the highest resistance is near the fetal abdominal cord insertion site.

fetal abdominal cord insertion site

For the umbilical artery, the lowest resistance is near the placental cord insertion site. Select a mid-cord segment. There is no need to angle correctly, since it is the ratio between systolic and diastolic velocities that is of interest and not the absolute velocity value.

 

Fetal Activity and Breathing:  Affects Doppler Indices

Fetal Activity and Breathing: Affects Doppler Indices

Above. Note variations in diastolic velocities in the presence of fetal breathing and activity.

 

Maternal Activity and Breathing:  Affects Doppler Indices

Maternal Activity and Breathing: Affects Doppler Indices

Above. Note variations in diastolic velocities and overlap of waveforms in the presence of maternal breathing and activity. Also note the presence of aliasing, which can be adjusted by increasing the pulse repetition frequency (PRF) or scale.

 

Fetal Irregular Rhythm:  Affects Doppler Indices

Fetal Irregular Rhythm: Affects Doppler Indices

Above. Note variations in diastolic velocities in the presence of an irregular fetal cardiac rhythm.

 

Obtaining Waveforms of the Middle Cerebral Artery (MCA)

Waveforms of the Middle Cerebral Artery (MCA)

1. Obtain a transverse view of the fetal brain at the biparietal diameter level. Zoom image to see the entire length of the MCA.

transverse view of the fetal brain

biparietal diameter level

2. Use color flow to identify the circle of Willis and the MCA, which is the major lateral branch of the circle of Willis which courses anterolaterally.

Use color flow to identify the circle of Willis

3. Place the pulsed-Doppler sample just proximal to the circle of Willis and, if possible, acquire the anterior vessel in the circle.

pulsed-Doppler

4. Angle correctly to obtain an angle of insonation, which is in the range of 2-4°, but always less than 10°.

5. Avoid transducer-induced pressure to the fetal head since transducer pressure will alter the recordings.

Avoid transducer-induced pressure to the fetal head

Above. Normal waveforms of fetal middle cerebral artery with relatively high resistance (low diastolic velocities).

Normal waveforms of fetal middle cerebral artery

Above. Abnormal waveforms of middle cerebral artery with relatively low resistance (high diastolic velocities).

 

Obtaining Waveforms of the Ductus Venosus

  • Scan in the upper abdomen in a mid-sagittal longitudinal plane.
  • Scan to the left first, identifying the fetal stomach bubble.
  • In the same plane, activate color Doppler over the fetal liver.
  • Identify the umbilical vein and the ductus venosus (see typical color Doppler depiction of the vessels below).
  • Velocities in the DV may be higher than the umbilical vein, and gain may need to be adjusted.
  • Aliasing is a cue that the DV is being imaged.
  • Sample volume size may be in the 2 mm. range.
  • A typical S,D,A waveform is normal, and there is no reversal of flow in diastole (see example).
  • An absent or reversed A-wave is considered abnormal (see example).

Above. The umbilical vein enters the fetal abdomen in an ascending direction. It then travels more horizontally and to the right near the region of the right portal vein. The ductus venosus (DV) arises from the umbilical vein in this region and descends steeply toward the head to enter the inferior vena cava.

Above. To identify the DV, a sagittal scan for the fetal upper abdomen should be taken initially toward the left side to include an image of the fetal stomach. The transducer is then moved longitudinally toward the right side of the fetus. Using color Doppler, the umbilical vein and DV can be identified through a mid-sagittal longitudinal section of the fetal liver. The origin of the umbilical vein can be determined by an oblique transverse section through the upper abdomen, and color Doppler may indicate higher velocities of the DV compared to the umbilical vein. [2] The aliasing is a cue that the sample is in the DV (see above). The lumen of the DV is smaller than the umbilical vein, and a smaller sample volume size (2 mm. or less) may be needed to obtain adequate Doppler velocity waveforms.

As noted previously, the normal flow velocity waveform of the ductus venosus is described in 3 phases and reflects events in the fetal right ventricle and right atrium as follows:

• (S) right ventricular systole

• (D) early ventricular diastole

• (A) right atrial contraction

In addition, an absent or reversed A-wave in the ductus venosus is considered abnormal. The duration of persistent absent or reversed flow of the A-wave of the ductus venosus is a strong predictor of stillbirth, and this finding is independent of gestational age, while prolongation of pregnancy for greater than 1 week is unlikely under this setting.1

 

References

 

1.
Turan O., Turan S., Berg C., et al. Duration of persistent abnormal ductus venosus flow and its impact on perinatal outcome in fetal growth restriction. Ultrasound Obstet Gynecol 2011;38(3):295–302. [PubMed]