The Doppler effect is a change in frequency of a wave when it interacts with a moving structure. Ultrasound pulses are waveform packets with well defined frequency characteristics. When those pulses encounter moving particles or structures- such as corpuscles bustling along, the frequency spectrum is shifted up or down (depending on the direction of blood flow) by an amount that is proportional to the velocity with which blood cells are moving. Measuring the frequency shift between pulses when they are launched and when their echoes are received provides a way to determine the velocity of the moving structure. Typically, frequency shifts are sampled about 2000 times per second during ultrasound imaging, leading to a very precise way of monitoring patterns of flow velocity.When the sampling is done across an imaging field, a map of blood flow is created - as in this technical breakthrough image of ours from 1991 of the heart and great vessels (in red) of this 8.5 week embryo. We also use this same technology to assess patterns of blood flow within organs (like the brain), the umbilical cord, and the placenta.
The original form of Doppler is to look at changes in velocity within a small "window". The right image is the Doppler pattern for a heart beat of a 1 mm embryo, 27 days after conception (left image). Doppler features tell us a lot about the pump function of the developing heart, including an indication of when a heart is beginning to fail mechanically. We have discovered that heart failure occurs in embryonic life and is the final event that precedes at least 60% of miscarriages. Using Doppler signs as early as a month after conception lets us identify with high reliability which pregnancies will continue and which ones will not.
