Unlike two-dimensional imaging which analyzes the transit time and intensity of a returning sound wave to identify a cardiac structure, Doppler interrogation analyzes the change in frequency of the transmitted ultrasound. At first, this was shown as the actual frequency shift presented in the kilohertz range and the magnitude of this frequency shift is represented in the kilohertz range. Doppler is used in many different formats. Through sharing computational resources, any of the Doppler methodologies can be performed together with anatomic two-dimensional imaging. Modern Doppler instruments are able to real-time 2D imaging plus Doppler interrogation.
Spectral Doppler Imaging
Continuous Doppler imaging, as the name implies, continuously transmits and receives the ultrasound signals. Here only the frequency shift of the returning signal is determined and the identification of the time of transit is lost. This is called the “range ambiguity.” This implies that the exact velocity of motion can be determined but not the location where that velocity occurred.
Pulsed Wave Doppler
Pulsed waved Doppler imaging, in contrast, interrogates the velocity of motion, through discrete ultrasound packets sent out at a predetermined rate, the pulse frequency of repetition. The timing and receipt of is monitored so the location where the frequency shift arose can be pinpointed from the time of transit. The sample volume can then be steered along the lateral and longitudinal axes. Since sampling is not continuous in pulsed wave Doppler imaging, there are limitations on maximal velocity which can be known.
Multigate Doppler Imaging
This is a variation of the pulsed wave Doppler imaging where multiple Doppler interrogation points are opened simultaneously. This increase the pulse repetition frequency and also the Nyquist limit. This maintains the ability of the technique to locate the site of maximal velocity and allows the recording of velocities.