Principles of Doppler echocardiography
- Ayan Patel, MD
Ayan Patel, MD
- Professor of Medicine, Tufts University School of Medicine
- Director, Cardiovascular Imaging & Hemodynamic Laboratory, Tufts Medical Center
While M-mode and two-dimensional (2D) echocardiography allow for creation of anatomic images of the heart, Doppler echocardiography utilizes ultrasound to record blood flow within the cardiovascular system. Doppler echocardiography is based upon the changes in frequency of the backscatter signal from small moving structures (ie, red blood cells) intercepted by the ultrasound beam.
The principles of Doppler echocardiography will be reviewed here. The principles of other echocardiographic techniques, as well as the normal views and protocol for an echocardiogram, are discussed elsewhere. (See "Echocardiography essentials: Physics and instrumentation" and "Tissue Doppler echocardiography", section on 'Technical aspects' and "Transthoracic echocardiography: Normal cardiac anatomy and tomographic views".)
A moving target will backscatter an ultrasound beam to the transducer so that the frequency observed when the target is moving toward the transducer is higher and the frequency observed when the target is moving away from the transducer is lower than the original transmitter frequency (figure 1). This Doppler phenomenon is familiar to us as the sound of a train whistle as it moves toward (higher frequency) or away (lower frequency) from the observer. This difference in frequency between the transmitted frequency (F[t]) and received frequency (F[r]) is the Doppler shift:
Doppler shift (F[d]) = F[r] - F[t]
Blood flow velocity (V) is related to the Doppler shift by the speed of sound in blood (C) and ø, the intercept angle between the ultrasound beam and the direction of blood flow. A factor of 2 is used to correct for the "round-trip" transit time to and from the transducer.