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Date: Wed, 19 Mar 2003 11:23:47 -0800 (PST)
From: [log in to unmask]
To: UVM Flownet <[log in to unmask]> Subject:
That is a great answer. Thanks
The maximum PRF is the (speed of ultrasound)divided by (2 * depth) so for a 15 cm depth,(1.5 mm/us)/(2 * 150 mm) = (1.5 M/ms)/(0.3 M) = 5 pulses per millisecond = 5 KHz. To allow higher PRF than the formula predicts requires violating the rules. You can go to a higher PRF, but then a new, undesired sample volume appears closer to the scanhead along the ultrasound beam, that you don't want. If that is located outside of an artery, it gives no Doppler signal so no problem (except that it does contribute to wall motion so you may need to increase your wall filter.
The higher PRF is called "High PRF" and might be the source of the message below. The message might be suggesting that you select "High PRF". So, it might not be an error message, but a suggestion. When I've used "High PRF", the additional "shallow" sample volume, is marked on the Doppler line so that you can be sure that it is not in a vessel.
For those who really LOVE ultrasound physics, every time that you use pulsed Doppler, you always have multiple sample volumes along your beam line. You "select" the shallowest one. The others are spaced at C/(2*PRF) where C is the speed of ultrasound. You don't get a signal from them because the echoes from that depth are attenuated by ... the attenuation of tissue. Of course, when imaging, you also have images coming from those depths too. They are also not seen because the echoes are attenuated. If you image a target in non-attenuating water, the water surface, which is reflected from the bottom of the tank and is much deeper than the depth of your image, will appear in your image. If you image through a HUGE urinary bladder, setting the depth to shallow, you might see the deep bladder wall, artificaually at a shallow depth in your image.
On Wed, 19 Mar 2003, Cindy Owen wrote: > Hi Neil, > > The maximum Doppler PRF that you can obtain is determined by the system > manufacturer (some systems have much higher pulsed wave PRF's than others). > But for any given system, the maximum PRF that it can obtain is determined > by depth. In general, the shorter the distance to the sample volume, the > higher the PRF that can be obtained (you may notice that the PRF changes as > you move the sample volume up and down- as you increase SV depth the PRF > will go down but may jump back up at certain depths). If you are using > triplex (live color and Doppler) the maximum PRF may also be reduced (this > is only true for some systems). By using a lower frequency Doppler, you can > reduce the frequency shift for any given velocity. This would allow you to > show higher velocities at the same PRF without aliasing. > > So, to maximize the velocities seen without aliasing, try the following- > - if your system cannot obtain its' m!
aximum PRF during triplex, turn > triplex off (instead, use an update mode to "toggle" between Doppler and > color) > - use the lowest Doppler frequency to reduce the frequency shift from > the flow > - minimize depth to the vessel of interest (change your acoustic > window/approach or push in a bit more) > > When shopping for new equipment, the maximum PRF attainable is one important > thing to evaluate. Many systems are limited to about 18- 21 kHz, whereas > other systems have PRF's up to 30 kHz. Remember to try the Doppler at > different depths, triplex on and off. Having higher PRF's makes it easier to > get the information needed when you have high velocities. > > Hope this helps, > Cindy Owen
> > ----- Original Message ----- > From: "Neil" <[log in to unmask]> > To: <[log in to unmask]> > Sent: Tuesday, March 18, 2003 7:34 PM > Subject: Re: Mesenterics > > > > We have been having a problem getting started on these. One of my > > coworkers complained that on both the Acuson 128s and the ATL UM9s he > > is getting an error when trying to spectral doppler deep in the > > abdomen. (And all he gets is aliasing) The message is something about > > high PRF. Any tips on how you can get a higher PRF, deep? I'm not > > sure what he is doing wrong... (or not doing right) >
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