Hey Rob, didn't I just warn you about this ;-)?
Bill Beymer

Bill Beymer BS, RVT, RDMS
Section Manager
Diagnostic Imaging
373-4704 (office)
373-4724 (Fax)
>>> [log in to unmask] 11/15/02 12:27 PM >>>
True, and to find the location of the sample volume, you should use the
speed of sound in the 90% of the tissue.  And if that vessel is under
fat,
then the velocity for computing the location of the sample volume should
be 1.45 mm/us

Now let's look at the Doppler problem.  With Doppler (also with time
domain velocimetry), you "look" at the location of a cluster of RBCs at
one time and then you look again at 100 micorseconds (us) later (if your
PRF is 10 KHz).  You measure the change in the position of the RBC
cluster
between the two "looks".  In your example, you have 22 mm of superficial
tissue during the first look and 3 mm of blood (to place the sample
volume
at 25 mm deep = 2.5 cm).  At the second look, you still have 22 mm of
superficial tissue and 3.01 mm of blood.  if the RBC cluster moved away
from you in the 100 us between "looks".  3.01 mm - 3 mm = 0.01 mm =
10 um, so the RBC cluster has moved away (in blood) by 10 um/100 us =
0.1 um/us = 0.1 m/s = 10 cm/s.  If the Doppler angle is at 45
degrees, then you have to divide by the cosine of 45 degrees 0.707 to
adjust for the angle giving you 14 cm/s as the blood velocity.

The change occurred in blood, so that is where the speed of ultrasound
is
important.  The ultrasound system can't measure distance, only time, so
when the cluster is 10 um further away, that means that the echoes from
the blood arrive later.  The ultrasound had to travel 10 um further to
the
cells and 10 um further back, both in blood.  20 um = 0.02 mm  The extra
time is 0.02 mm / 1.57 mm/us = 0.01274 us.  If we used the speed in
liver
0.02 mm / 1.54 mm/us = 0.01299 us.

This is only a 2% difference.  Only a petty person like me would bring
it up.

Kirk


On Fri, 15 Nov 2002 [log in to unmask] wrote:

> Hey Kirk,
> Just out of cursiosity, why do you say manufacturers should use the
speed of
> sound in blood instead of average speed in soft tissue?  Scenario:  a
6 mm
> vessel at 2.5 cm depth with pulsed wave Doppler sample gate placed in
the
> middle of the vessel, seems like about 90% of that sound wave path
length is
> through tissue.
> Rob Daigle
>

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