Organic Chemistry in "Real Life"
Be warned, this weeks' Real Life page is a little racy (see below)!
NMR spectroscopy relies on the fact that different frequencies of radiation are required to flip the spins in magnetic nuclei that experience different overall magnetic fields. Different nuclei in a molecule experience slightly different magnetic fields depending upon their location in the molecule that are a consequence of the external field of the magnet and the local fields due to proximity to electronegative groups, unsaturation etc. These differences in magnetic field are small, and so in the NMR spectrometer great care has to be taken to ensure that the external magnetic field is very homogeneous,. otherwise differences in the external field may wash out diffrences in the local field. This turns out to be important, so let's say it again, differences in the external field from the magnet will translate into different frequencies for protons in a sample. So, if a magnetic field gradent were used deliberately, we could in principle determine where in the sample a proton was from its frequency. For example, see the schematic diagram below, which shows some arbitrary protons (H, assumed to be the same) in different physical locations in a large sample. Protons towards the left hand edge of the sample resonate at higher frequency than those in the middle, which resonante higher than those at the right edge etc. The position of the protons in the sample can thus be determined.
If the protons in this case were those in water, and the sample was you, and the water in your organs was not moving around (very much), then the positions of the various water molecules in your various organs could be located in space. If there was some additional way in which the water molecules in your various organs would look different by NMR (i.e. in addition to their absorbance frequencies) then you would have the ability to do 3D imaging. Well, there is! When the nuclear spins are flipped to the high (opposing field) energy state, they eventually relax towards the (thermal) distribution between low and high energy states they had before being irradiated. This "relaxation time" is quite different in different media, fast for water in solution, slower for water in soft tissue, even slower for water in harder tissue, and quite slow for water in solids such as bones. MRI uses this combination of different absorption frequencies and relaxation times to build a 3D image of the tissues inside your body. Its important different from the oldeer CAT scan is that it uses radio waves, whereas the CAT scan uses X-rays, which are obviously potentially more harmful. MRI also gives complimentary information to CAT scans.
MRI is obviously a very important medical tool. There are lots of sites that describe the use of MRI, such as this one and this one.
MRI is an application of NMR, it really should be called NMRI (Nuclear Magnetic Resonance Imaging), but the "N" is dropped because some people thought it was something to do with nuclear radiation.
OK, here is the racy bit. If you are at all of a sensitive disposition, hit the return button on your browser now. If you are a consenting adult, then click here to proceed!