How do transit time ultrasound measurements differ from Doppler measurements?
With the Doppler principle, the ultrasound is reflected off the moving
particles, generally red blood cells, and is received by the same transducer. The ultrasound that is reflected off the moving cells undergoes a shift in frequency that is
proportional to the velocity of the red blood cell. Doppler tells you the speeds of blood, but not how much flow.
With the transit time principle, the
ultrasound travels a fixed distance and is reflected off of a stationary reflector plate. The difference between upstream and downstream transit times is directly
proportional to volume flow, independent of the vessel diameter. Transit time ultrasound gives a direct measurement of volume flow in the graft.
Why does the reflector have to be attached to the probe?
The geometry of the probe is critical to
obtaining accurate and quantitative measurements of blood flow. The ultrasound transit time probes works by measuring the transit time of the ultrasound over a fixed
distance - from one transducer to the reflector and to the opposite transducer and back again. The flowmeter processes these signals and calculates the volume flow from
the difference between upstream and downstream transit times.
How do transit time ultrasound blood flow measurements differ from
electromagnetic flow measurements?
Transit time ultrasound technology has
eliminated the problems of electrical interference and baseline drift encountered with electromagnetic flowprobes. With transit time ultrasound flowprobes, you do not need
to have a precise fit on the vessel as you do with EM probes. You just need to have a couplant medium, such as sterile gel between the vessel and the probe. Becasue you
need electrical contact between an EM probe and the vessel, the probe has to constrict the vessel by as much as 20%.
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