Duplex Doppler Imaging: A tool for finding the best Spectral Doppler signal.
©Walter Rasmussen, R.D.C.S.
Current ultrasound machines can utilize Duplex Doppler imaging, consisting of a two-dimensional image as a reference for guiding the Doppler cursor combined with a spectral Doppler strip beneath the image.
Image on left shows orientation two-dimensional image with color Doppler and PW Doppler sample volume above the spectral Doppler recording.
There are two types of duplex imaging: Live and frozen.
Live Duplex Doppler imaging is active when both the two-dimensional image and the spectral Doppler are moving. This mode noticeably degrades both modalities because the ultrasound machine must share resources within the limitations of its processing capabilities and the physics of ultrasound. For the most part, live duplex Doppler is used only to help locate the site to perform spectral Doppler recordings. Once a promising site is found, the sonographer switches to the frozen duplex mode, whereby the two-dimension image is frozen and the spectral Doppler is active.
Frozen Duplex Imaging is active when either the two-dimensional image or the spectral Doppler image is frozen. This is typically done with a button that toggles from live to frozen is pressed.