Hi,

MRI safety manual says that whenever you are scanning a patient with a certain surface coil you should remove all unplugged coils from the bore during a scan. For example, if you have a head/neck + spine coil matrix with two coil ports and you are scanning only spine part then you should either connect the head part also or remove head/neck coil from the bore.
I was wondering what is the physical mechanism that can cause patient burns?
For example, if you scan patient's spine and leave the head coil unplugged.
Is it because the unplugged coil is a resonant system designed to absorb RF energy so that when the scan is on and a pulse sequence with alot of RF (T2 FSE for example) is being used the coil can absorb alot of RF energy thus heating up? If the coil were plugged then the system can "detune the coil" and not heat up?!
Another reason why unplugged coils should not be in the bore while scanning is the unplugged coil's coupling to patient which can cause increased noise, generate wrong flip angles etc.
Another thing I have noticed is that for example, if a patient is sweating alot during a spine scan for example, the image quality is alot more worse. For example Sag spine images are alot more darker than usual. Is it due to sweat that is being "coupled" with the coil thus possibly messing up the flip angles in the patient which results in dark images?

Thank you.

 

Have a look at this link http://www.mrisafety.com/safety_article.asp?subject=17

 

I have seen this webpage. There is too little physics in it and too much of do this and that.

 

I think my instructors skipped the MRI chapter in my electronics training courses

 

It's kind of hard to find coil specifications of a commercial MRI-system but if heating is the problem, then the only thing that comes to mind are eddy currents, induced in any conductor near a alternating magnetic field. (or a moving conductor through a static magnetic field.)

http://en.wikipedia.org/wiki/Eddy_current

Not exactly related, but talking about eddy currents, subsequent image distortion and design of gradient coils.
http://alexandria.tue.nl/extra2/200511789.pdf

 

I would recommend purchasing Electromagnetic Analysis and Design by Jianming Jin and chapter 5 gets into the physics of RF fields in biological objects.
Another quick link would be:
www.phy.olemiss.edu/Hoffmeister.ppt

These get into some of the physics behind it, but you're right about removing coils or making sure they are connected when scanning, they do absorb a substantial amount of RF energy and some coils need the return path through the system to dissipate that energy.

 

Apparently if the unused coils are left plugged in, there is a detuning circuit which prevents them from resonating under the influence of the active coils. If you leave them unplugged, they will resonate with the active RF coil. This causes potential heating issues (with no energy drain through the plug). They will also continue to resonate the picked-up signal after the transmit coil turns off, which would contribute significant noise to the received data.