1180 
MONITORING 
Figure 11. — Photomicrograph of command receiver chip. 
pared with the lumen diameter, with a given 
channel of demodulator circuitry, and we es- 
sentially make the same kind of frequency to 
voltage conversion in that channel. You can 
have a number of channels. We happen to have 
eight, but you could have twelve or sixteen. We 
weight the output from each of these channels, 
depending on the range which corresponds to 
the radius of that set of targets, then sum that 
information, and we come up with a single 
voltage, which is proportional to the volume of 
flow. This number depends on a direct measure- 
ment of the velocity profile and the lumen 
diameter. 
Chairman : May I take the privilege of the 
Chairman in disagreeing with you, at least in 
part, Jim, with regard to the spectrum? Now 
I can't and I don't think anybody can mathe- 
matically justify the fact that a zero crossing 
detector is usable for converting the wide band 
frequency information to average velocity. Yet, 
empiricallj^ we've done this repeatedly only 
under the conditions where there is a very high 
signal-to-noise ratio. Under these conditions, 
we found empirically that there is, indeed, a 
linear relationship between volume flow and 
the output of the zero crossing detector. 
Dr. Meindl : We don't disagree. I think your 
point is well taken. We have made (and as 
yet it is unpublished) a very rigorous analysis 
of the Doppler flowmeters. I think you've seen 
a preliminary version of it. The analysis shows 
very clearly that a linear relationship exists 
between volume flow and the output of a zero 
crossing detector if the blood velocity profile 
is relatively flat or blunt. In addition, if you 
want to measure volume flow when the blood 
velocity profile changes rapidly with position 
throughout the lumen the analysis shows what 
conditions are necessary to achieve that. I could 
describe these conditions, although it's a more 
