362 MHz for NMRI - Why?

Hello Bruce,

The required frequency to excite the protons is given by the larmor formula

f = B0*(Gamma), f [Hz], B0 = magnetic field [T], gamma = larmor constant, for hydrogen about 42.57 MHz/T So there are no specific frequencies that have to be used.

Your frequency probably belongs to a B0 field of 8.5 T. (85000 Gauss).

When you classify the NMR/MRI devices by their strength of the B0 field, the required RF frequency follows from the larmor constant. In MRI scanners for humans, the required volume is that large that it is technically/economically spoken not possible to generate very large magnetic fields. Therefore these devices use lower frequencies. Small volume NMR devices can reach up to about 20 T and therefore require 900 MHz to excite the nuclei.

Best regards,

Wim PA3DJS

without abc, PM will reach me

In general higher magnetic fields means bigger energy differeneces between the energy levels of the nucleus being probed. IIRR the energy difference is mostly appreciably less than kT (the local noise level at the temperature T characterstic of the substance being probed (usually us, at 37C or 310K) and more energy difference means a biggger difference between the population of the higher and lower energy levels involved, whence more signal.

-- Bill Sloman, Njmegen

Wimpie wrote: : On 7 mar, 11:56, snipped-for-privacy@optoplex.com (Bruce Condine) wrote: : > Would anyone happen to know why 362 MHz is a standard frequency for : > NMRI?

: gamma = larmor constant, for hydrogen about 42.57 MHz/T : So there are no specific frequencies that have to be used.

: Your frequency probably belongs to a B0 field of 8.5 T. (85000 : Gauss).

8.5 teslas sound quite high for a MRI system with a significantly sized magnet bore, like those made for medical imaging. An example of a device of such caliber is the french Neurospin facility . We happen to collaborate with them, although not in the high-field MRI stuff.

For small sample sizes 8.5T is approximately the highest field that can be obtained by simple Nb-Ti superconducting magnets operated at 4.2K . For higher fields you'd need more expensive and hard-to- manufacture materials and/or lower temperature. Maybe the 8.5T is adopted as some sort of a standard, eg. in MRI-based chemical analysis.

Regards, Mikko

NMR and FTMS use smaller bore magnets, which generally aren't used for imaging (although micro-imaging gets close to optical microscope resolution.) The NMR magnets have about a 2" room-temp bore and the highest field magnets have their hydrogen resonance around a GHz. A

21T FTMS magnet has a (roughly) 8" bore and costs around $8M.

I think that NMR s/n goes up with the square of field strength, so the high-field magnets can be worth it. FTMS is linear, so the payoff is less.

One application for small-bore high-field magnets is MRI of lab animals. One can feed a rat or a rabbit some drug and image them regularly, as opposed to lopping off their heads and dissecting them. An animal-care person is usually present when they are imaged.

This is neat stuff.

John

I had occasion to be the sample in a MRI device last year. As one of the servants of The Machine explained I'd hear a clack-clack-clack noise, I told them that was the field gradient coil.

When asked if I was familiar with the technology, I replied that when I had studied it, it was known as NMR, but I understood that term was no longer palatable.

The staff agreed -- the n-word is no longer used.

...but that doesn't change how the device works!

r

Hello,

With the "N" (nuclear) people think it is dangerous, without it, it is OK. I am almost sure you knew this allready...

Best regards,

Wim PA3DJS