MRI -please help !

[eloise]

Hello everyone. i have a very limited knowledge about electromagnetic and microwaves, the only technology related to this field which im a bit familiar with is MRI (Magnetic Resonance Imaging) which i rarely use when i try to identify some chemicals.
i'm mostly into the pharmaceutical industry, design of reactors and pharmaceutical plants/processes. i am also involved in Monoclona antibodies and Tissue Engineering productions which is a very exciting field as well.
going back to MRI, i'd be grateful if someone could give me a simple explanation of the mechanism as i know how to use the device and get the chemical composition of unknown compounds, but i do not know how does the device creates these different magnetic fields shown on the histogram.

Thank you so much for your help.

Eloise

 

[IRstuff]

It's not MRI unless its imaging, which is what the medical community uses.

The original name is nuclear magnetic resonance (NMR), which, apparently, confuses patients.

did a websearch on NMR theory:

http://www.shu.ac.uk/schools/sci/chem/tutorials/molspec/nmr1.htm

http://www.ch.ic.ac.uk/local/organic/nmr.html

http://www.process-nmr.com/pdfs/NMR Overview.pdf

TTFN

 

[eloise]

Hello IRstuff and thanks for your input. you said "The original name is nuclear magnetic resonance (NMR), which, apparently, confuses patients" what did you mean by "confuses patients"? can you elaborate please.

Thanx again

 

[IRstuff]

The word "nuclear" has a specific meaning in NMR, referring to nuclear spin, while the typical patient thinks, "RADIATION!"

TTFN

 

[Higgler]

My take on NMR/MRI operation;
MRI's work by creating a very strong axial magnetic field inside a supercooled coil. This aligns electrons rotating in humans/materials along this axial direction. A second coil/antenna transmits a multi kilowatt pulse (of different frequency) to re-orient the rotating electrons to an axis 90 degrees (sideways) from the primary axis. When the multi-kw pulse ends, the electrons realign in the axial direction. The rotation from a sideways orientation to the axial direction creates an electromagnetic emission from the electrons changing direction. Essentially, the material becomes a radiating electromagnetic source. An antenna (the same one used to create the multi-kilowatt pulse) and sensitive receiver (spectrum analyzer) listens for the rf energy and records amplitude and phase of received signals. In humans, it takes up to 3 seconds for the electron spin axis to realign. Hence, the need for you to be very still while in an MRI machine, or you may blur the image.
The multi-kw pulse frequency can also be changed. You can impart a frequency of spin in one part of the body, such as the blood in the heart, and listen for that frequency for the next few seconds to track the flow of blood as it exits the heart. Pretty impressive.

Different materials re-align to the axial direction at different speeds and varying amplitudes. Hence, in humans, you can detect different speeds, and amplitude shift differentials. Sometimes data is taken at the start of spin realignment or at end of spin realignment. That way the fast moving materials are all settled while you listen for the slow moving materials and the MRI receiver can attribute different levels to different materials.
Similar technique for material analysis I'd suspect. But the field strength is much higher, primarily because the test sample are smaller than humans and the field strengths can be more concentrated. The frequency of operation is also much higher (humans use 63 Mhz usually, materials use ?300-1000 Mhz or so, they keep pushing state of the art upwards in frequency it seems). MRI's in hospitals often have TV channel 3 fuzz out their images if the door to the room is left open. EMI tight doors are required at MRI hospital facilities to quiet the environment so that the receiver doesn't "watch tv" or listen to radio while on the job.

kch

 

[probewizzard]

Hi all,

just to clarify some "details":

1.) NMR means "nuclear magnetic resonance" because this technique excites and detects changes in the spin states of the NUCLEI (and not the electrons). However its possible to excite the electron spin-states too, but this is done in the so called ESR "electron spin resonance (imaging)", not used for imaging of humans.

2.) The frequency range of NMR depends on the strength of the external magnetic field and spans the range from a couple of MHz to 1GHz (the latter corresponding to a magnet with a static field of approx. 23Tesla (there exists only one at the moment in a lab in tallahasse, us)

3.) for humans the proton frequency (the nucleus that is used to generate the "image") is as low as 63MHz and only for very small samples - much smaller that a human - bigger magnets can be used...

cheers
probewizzard

 

[hacksaw]

eloise

the histogram you refer to is the frequency shift of the various protons in the specimen. this shift relects the molecular field seen by those protons and therefore correlated with placement within the molecule. it is not considered an independent method for determining composition, but is used in conjunction with other methods.