Skip to main content
. 2013 Sep 30;168(2):1220–1228. doi: 10.1016/j.ijcard.2012.11.059

Fig. 2.

Fig. 2

Isospeed hemisphere (outer curve) versus isovelocity urchinoid (inner curve). The consequence of the difference between speed (Fig. 1A) and vertical Doppler velocity component (Fig. 1B) is that when flow convergences hemispherically, Doppler imaging correctly shows not a hemisphere but a more completely round shape. For blood moving directly away from the ultrasound probe, i.e. vertically on this figure, (a), speed and velocity are practically synonymous, so the isospeed hemisphere (grey) is at the same position as the Doppler isovelocity surface (blue). For blood moving at 45° (b) the vertical velocity is reduced to cos 45° × speed, i.e. is substantially smaller. Therefore only closer to the orifice (label b on right), where the speed is even higher, is the Doppler velocity component high enough to match its value at (a). For blood moving almost perpendicularly to the ultrasound beam (c), the vertical Doppler velocity component is very much smaller than the speed for example by a factor of ~ 10 at 84° (cos 84° ≈ 0.10). Therefore the Doppler isovelocity surface is ~ 10 times closer to the orifice at that angle.