|
|
 |
 |
 |
|
|
|||
|
||||
|
||||
Harmonic modeFrom: DuBose, Terry (DuboseTerryJ@exchange.uams.edu)Thu Dec 18 08:19:07 1997
Thanks, Jim. I'm going to share this with some other folks.Hope you don't mind my copying you. Peace, Terry J. DuBose, M.S., RDMS; Assistant Professor Director, Diagnostic Medical Sonography Program University of Arkansas for Medical Sciences, Fellow, AIUM; Secretary, SDMS DuBoseTerryJ@Exchange.uams.edu http://www.uams.edu/chrp/dmshome.htm http://www.io.com/~dubose/ http://www.obgyn.net/CORRESP/DUBOSE.HTM http://www.obgyn.net/us/us.htm http://www.obgyn.net/women/advisors/tdubose.htm VOICE: 501-686-6510 FAX: 501-686-5613 Now is the time for all good folks to come to the aid of the Earth. -----Original Message----- From: jimzag@nutronOSF.medphysics.wisc.edu [SMTP:jimzag@nutronOSF.medphysics.wisc.edu] Sent: Wednesday, December 17, 1997 6:37 PM To: DuBose, Terry Subject: Re: [soundadvice] vector versus curvi-linear Hi Terry Harmonic mode is just what you mentioned: you transmit say 2 MHz and you detect 4 MHz. Where does the 4 MHz come from? Well, if the process is being done as advertised, the 4 is actually generated by the tissue. The idea is that when you have what we term "linear acoustics", transmitting 2 MHz yields 2 MHz echoes. As you go to higher intensities, however, the linear approximation gets worse. The tissues begin to distort the wave, changing a nice sine wave, for example into more of a triangular wave. Mathematically and physically, this waveform contains not just the 2 MHz fundamental, but 4 MHz, 6 MHz, etc. The harmonic component gradually get weaker with increasing frequency. We know that these distortions occur for ultrasound in water. That's one of the major difficulties when manufacturers measure acoustical output in water, which is the required medium. The problem is, when they do so, they are required to have the power cranked way to max, so not too surpriasing, hell breaks loose and the waveform they detect with their measurement device (a hydrophone) looks crappy. The wave contains the fundamental, the first harmonic, the second harmonic, and so forth. We secretely hoped we would not have to deal with this problem with tissues, although the literature contains warnings about it at various places. (This is becasue the equations needed for non-linear acousticsa make college physics look like a breeze and sonographer physics look like kindergarden reading). However, ne of the major interesting things that has happened over the years is the development of harmonic imaging with contrast agents. These little bubbles compress so easily in the presence of a sound field that we were not surprised to see harmonic signals quite easily from agents within tissues. The bothersome thing about those early images was that you could also see what could be interpreted as harmonic signals from the tissues. (I secretely denied what I was seeing. Finally some folks started putting 2 and 2 together (I certainly was not one of these, even though I was privy to some of that early image data.) and came up with 8 (it's non-linear, remember?). In other words, they realized that tissues distort the wave sufficiently, especially if the amplitude is high enough, that you can detect the harmonic signals. Theory says that the harmonic from tissue should gradually build with increasing distance traveled. The neat application of this aspect of the principle is that your beam starts out say pure 2 MHz. The 4 MHz harmonic is not generated immediately at zero depth, but gets created with increasing depth. It actually strengthens with depth for awhile. Then at some point the rate of creation gets less than the rate this component is attenuated, and the 4 itself starts to weaken. The reason the noise is cleaned up in harmonic is that some of the noise is caused by reverberations, etc within the body wall, especially with larger patients. Well, the beam was mainly 2 MHz when launched, so the noise from the body wall also is mostly 2 MHz. Most of the harmonic signals are generated by the beam after it has traversed the body wall. Harmonic generation is very nonlinear, so the stronger the beam, the more harmonic you see. The 2 MHz primary beam is much stronger than the reverb noise, so its mainly the good part of the beam, not the acoustic noise that generates the harmonic stuff. Now if you only detect that signal, you should have a cleaner image. The absolute amazing thing, after all this gibberish, is that this seems to work!!! I wouldn't have believed it!!! Jim
|
|
Return to
|
Mail a New Message to the Forum: ultrasound@obgyn.net Forum Administrator: terry.dubose@obgyn.net Report Technical Problems: webmaster@obgyn.net Last Updated: Mon Nov 2 05:38:25 2009 |
The American Medical Association is no longer designating CME hours for AMA Category II CME credit. However, physicians themselves may self designate learning activities as Category II CME credit hours if they feel it is of sufficient educational merit and meets the formal definitions of continuing medical education. OBGYN.net believes these interaction in this forum meets these criteria. For further information see the AMA web site.