DOPPLER PRINCIPLES

Christian Johann Doppler described the effect of motion of sound sources and its effect on the frequency of the sound to the observer. In medical applications we find that the frequency of the reflected signal is modified by the velocity and direction of blood flow. If blood cells are moving towards the transducer, they increase the frequency of the returning signal. As cells move away from the transducer, the frequency of the returning signal decreases.


The mathematical formula is:




The frequency difference is equal to the reflected frequency minus the originating frequency. If the resulting frequency is higher then there is a positive Doppler shift and the object is moving toward the transducer and if the resulting frequency is lower, there is a negative Doppler shift and it is moving away from the transducer.The angle theta, cos D component is the angle of incidence of the beam upon the object. For the most accurate determination of flow, the beam should be parallel to the flow of blood where the angle theta is zero. If the angle of incidence is greater, the results are less reliable. It is generally accepted that results from the Doppler shift where the angle theta is greater than 20 degrees is not used for calculation.

Doppler Instrumentation:

Doppler techniques are dependent on the transducers used. The transducer operating in continuous wave mode utilizes one half of the element(s) and are continuously sending sound energy while the other half is continuously receiving the reflected signals.

If the transducer is being used in a pulsed wave mode, the whole transducer is used to send and then receive the returning signals.

Comparing the two modes of Doppler techniques describes the advantages and disadvantages.

Pulsed wave techniques have proven to be very valuable in blood flow studies. The technique allows the accurate measurement of blood flow at a specific area in the heart and detection of both velocity and direction. Measurement is performed by timing the reception of the returning signals giving a view of flows at specific depths. The region where flow velocities are measured is called the sample volume. Errors in the accuracy of the information arise if the velocities exceed a certain speed. The highest velocity accurately measured is called the Nyquist limit.

Nyquist Limit - defined as ½ the Pulse Repetition Frequency (the number of pulses per second.) If the velocity of flow exceeds the Nyquist limit, the direction and velocity are inaccurately displayed and, in fact, appear to change direction. Color flow Doppler capitalizes on this effect allowing us to detect flow disturbances from laminar to turbulent flow.

High PRF - a Doppler technique that attempts to overcome the effects of the Nyquist limit. This technique may be seen as a compromise of pulsed wave and continuous wave properties and involves the use of multiple sample volumes thereby increasing the accuracy of velocity measurements at the cost of range ambiguity.

Doppler Displays:

The display of Doppler velocity data is the Doppler frequency shifts versus time. Included in the display are the Doppler settings such as frequency, calibration, range, and timing markers.

Doppler Controls:

Controls used during the Doppler examination are dependent on manufacturers specifications and the modes available. Controls for the cursor, sample volume length and depth, angle correction, gain, filters, and spectral averaging are typically included.

Color Flow Imaging:

Sampling methods - CFI is based on pulsed Doppler technology where multiple sample volumes among multiple planes are detected and displayed utilizing color mapping for direction and velocity flow data. Common mapping formats are BART (Blue Away, Red Towards ) or RABT, and enhanced or variance flow maps where saturations and intensities indicate higher velocities and turbulence or acceleration, respectively. Some maps utilize a third color, green, to indicate accelerating velocities and turbulence.

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Artifacts - aliasing of the data displayed in pulsed wave technology is utilized as a benefit in determining transitions from laminar to turbulent flow. Other artifacts associated with CFI and spectral Doppler are artifacts due to gain set too high, "ghosting" from improperly set wall filters (low frequency), mirroring, crying/talking artifacts, and signal loss from data sharing.


Limitations of CFI - CFI is a "qualitative" examination and has not yet been "quantified", that is, results cannot be measured to give discrete numbers for diagnosis. Qualitative assessment gives comment on the overall view or quality of the results as in flow conditions and jet direction, velocity, and pattern. Quantitative results are those measured and given discrete numbers used in calculations. There are some current semi-quantitative results given as ratios of jet length by jet width to determine the degree of regurgitation given as mild, moderate or severe.