JAMES U. MEINDL 
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DOPPLER SIGNAL 
EXTERNAL 
IMPLANTABLE PULSED ULTRASOMIC FLOWMETER-BLOCK DIAGRAM 
Figure 12. — Block diagram of implantable pulsed Doppler flowmeter. 
mathematical set of conditions than you can 
handle easily in a verbal discussion. 
Essentially, you have to take the integral of 
frequency times the spectral power density from 
plus to minus infinity. In other words, you take 
the first moment of the spectral power ■ density 
and that gives you the average Doppler frequency. 
Now in order for this to give you the average 
Doppler frequency, you have to assume that you 
insonify the cross section of the vessel with the 
ultrasonic energy. In other words, it's essentially 
the same condition you have in the electro- 
magnetic flowmeter ; that is, the magnetic field 
has to be uniform. If you insonify the entire 
cross-section of the vessel in the CW flovmieter 
and then do this calculation that I just described 
(instead of doing a simple zero crossing demod- 
ulating process) you can measure the average 
velocity over that cross section. If you know the 
average velocity, then assuming you know the 
lumen diameter, you can come up with a volume 
flow. 
Now, my estimation of the best technique 
(and I'm not saying at all that you can't get 
useful data on volume flow with the CW flow- 
meter) is to use the CW flowmeter to measure 
velocity in a very small volume. The best way 
to use the pulse flowmeter is to measure a 
velocity profile and a lumen diameter, and this 
gives you the basis for measurement of volume 
flow. I might point out that if you use the 
instruments this way, you can precalibrate them. 
The user does not have to calibrate them; the 
calibration is stable ; and the baseline is stable. 
Chairman : I have no objection to what you 
say at all. I think we agree with your point. But 
really the Pulse Doppler is the only volume 
flowmeter available at the moment. I guess the 
point I was getting to was perhaps the lesser 
one than you referred to. The point is that 
