Nuclear Magnetic Resonance
Understand chemical shift and spectral width
Understand signal averaging and signal-to-noise ratio
Observe Bo homogeneity (shim) and line width effects
Understand excitation flip angle
Make temperature measurements
Compute T1 and T2 from measurements
INTRODUCTION TO NUCLEAR MAGNETIC RESONANCE (NMR)
First discovered in 1946, the phenomenon of NMR forms the bedrock of magnetic resonance imaging (MRI) and spectroscopy (MRS). It exploits the fact that many atomic nuclei (for our purposes: hydrogen nuclei, otherwise referred to as 1H) possess a magnetic dipole moment. In the presence of an external magnetic field (Bo), individual magnetic moment vectors or spins (to learn more about spins go to http://www.cis.rit.edu/htbooks/mri/inside.htm ) in a sample tend to preferentially align parallel to the Bo field (unit is given by Tesla), creating a net bulk magnetization (M). When disturbed from its equilibrium position (parallel to the Bo field) the M vector precesses around the B0 field at an angular Larmor frequency n = (gB0/2p), where g is the gyromagnetic ratio and differs from one nucleus to another (e.g., 1H has a value of 42.58 MHz/Tesla). For 1H at a Bo field of 2 Tesla, n is approximately 85 MHz (i.e., in the radio frequency (RF) range). A precessing M vector in a sample can be detected by an appropriately tuned RF coil (i.e., much like a radio antenna) placed near the sample. The precession induces an alternating RF current in the coil, the amplitude of which is directly proportional to the amount of transverse magnetization (Mxy).
At equilibrium, M is always aligned with the Bo field. However, when a short burst of RF energy (in the form of a transverse magnetic field B1 rotating at the Larmor frequency) is applied to a sample, M is disturbed from its equilibrium position. It is tipped from its equilibrium position by an angle known as the excitation flip angle of the RF pulse. The transverse magnetization thus generated is detected using an RF coil in NMR experiments.
The chemical environment of nuclei tends to modify the magnetic field “seen” by them. It often shifts their Larmor frequency by an amount that is small but detectable. Such “chemical shifts” are usually quoted in parts per million, ppm (multiplication by the operating frequency, n, yields the Hz value). The chemical shift is the difference between the resonance frequency of the moiety (e.g., -CH vs. -OH) and a standard, given by d.
d = (n - nREFERENCE) ´ 106 / nREFERENCE
In NMR spectroscopy, this standard is often trimethylsilypropionate (TSP) or tetramethylsilane (TMS). To learn more about the study of molecule structure from NMR go to http://www.chem.ucla.edu/~webspectra/NMRsummary.html . A convenient way of visualizing the time domain NMR signal, which is known as the free induction decay (FID), is in the frequency domain by Fourier transform (to learn more about the use of Fourier transform (FT) in NMR go to http://www.cis.rit.edu/htbooks/mri/inside.htm ). The FT of an FID leads to a “spectrum” with peaks (with different chemical shifts) each of which has a line width that is partially dependent on the T2 of that moiety and Bo homogeneity (shim).