Hey all!

I'm a biomedical student and one of my projects is to figure out why implantable leads heat up in an MRI and what can be done about it. I actually in ME, so circuits and stuff doesn't really come naturally at all. But I think I understand the basics of what is going on: that it is the 64MHz (or 128MHz) RF alternating field which is basically being picked up by the lead wire acting like an antenna. But I wouldn't think any insulated wire would be THAT good a receiving antenna, which makes me think that somehow that little bit of picked up energy is somehow growing and amplifying enough to burn tissue on the exposed end, that maybe the lead becomes like a resonating beam under periodic oscillation, where the amplitude just keeps growing and growing, so the reaction forces on the ends (like the current) keeps getting bigger, too.

Does that sound like I am on the right track? It seems to make sense, but the problem I'm having is that apparently this overheating phenomenon can happen at many different lead lengths, and not just when the lead happens to be the right length compared to the wavelength of the RF. Could it really be that some random length of wire could be THAT good of an antenna to generate currents big enough to burn someone? That just doesn't seem possible... so can anyone tell me what am I missing?

 

Look at this thread: http://forum.allaboutcircuits.com/showthread.php?t=57185&highlight=MRI

The guy had a similar question and I was concluding that it has to do with "Eddy Currents". http://en.wikipedia.org/wiki/Eddy_current

 

I think it is sort of the same as in the lead heating problem - current being induced in other conductors within the RF field. But the coil inside an implanted lead is tiny... nothing like the RF generator coils in the MRI. The RF coils in the MRI are huge and tuned to radiate RF very efficiently, which would make them really good pick-ups, too.

One person suggested they need to be removed or stay plugged in because the plug allows excess picked-up energy to drain off to ground. Maybe the key with implanted leads is the same? So would the solution be to make sure the lead in the body "dumps" it's accumulated energy to ground?

It also seems that the other post supports my theory that it is not the antenna effect but the accumulation which causes overheating. If all you have to do to keep a giant RF generator coil from overheating is keep it plugged in, then it cannot be picking up THAT much energy on every cycle... right?

 

oops... I was wrong. Apparently if the unused coil is left plugged in, it is actively detuned by the system and doesn't resonate at all: http://www.freepatentsonline.com/6469509.html