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Re: Speed of Sound in Blood

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Fri, 15 Nov 2002 12:53:30 -0500

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 ```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 > To unsubscribe or search other topics on UVM Flownet link to: http://list.uvm.edu/archives/uvmflownet.html To unsubscribe or search other topics on UVM Flownet link to: http://list.uvm.edu/archives/uvmflownet.html ```