CHEM 125A: FRESHMAN ORGANIC CHEMISTRY II - Lecture 22 - Professor J. Michael McBride
Medical MRI and Chemical NMR
Magnetic resonance imaging (MRI) requires gradients in the applied
magnetic field, while chemical nuclear magnetic resonance (NMR) requires
a highly uniform field. When protons in different parts of the body can
be driven to broadcast different frequencies, tomography allows
reconstructing a three-dimensional image showing water location.
Dependence of the signal intensity on relaxation allows BOLD functional
MRI that shows brain activity. When the applied magnetic field is
sufficiently uniform, chemical NMR spectra differentiate proton signals
according to local field variations within molecules. Modern research in
a chemical laboratory like Yale's depends on the availability of many
magnetic resonance spectrometers. Peak integrals show the relative
number of protons in different molecular environments, while peak
frequencies or "chemical shifts" show the bonding environment of groups
of protons. Often downfield (deshielded) or upfield (shielded) shifts
are correlated with local electron density.
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