Nuclear magnetic resonance and the brain

Summary

The first successful demonstrations of nuclear magnetic resonance (NMR) in bulk matter were reported in 1946 (Bloch, Hansen and Packard 1946; Purcell, Torrey and Pound 1946). Since then NMR has become a widespread technique for investigating matter of all kinds. In the 1970's NMR was applied to living systems, including man, in 2 distinct approaches. One application was in the production of images (Lauterbur 1973), called Magnetic Resonance Imaging or MRI, and the other in the production of NMR spectra (Moon and Richards 1973; Hoult et al. 1974), called Magnetic Resonance Spectroscopy or MRS. By appropriate manipulation of the NMR signal an NMR image may be generated. This can be a 2D image of a single slice, or a set of 2D images of parallel slices, or a 3D image. 2D images may be obtained directly in any orientation, axial, coronal, sagittal. The method uses no ionizing radiation and is inherently safe. It is non-invasive, although paramagnetic solutions may be injected intravenously to improve contrast. MRI images observed in normal clinical practice are maps of the NMR signals from water and fat in the tissues; they depend on proton density, but also significantly on the relaxation times T₁ and T₂. Images can be provided of flow (MR angiography) and diffusion (free, restricted or anistropic). Images are typically 512×512 pixels with spatial resolution of about 0.5mm. The images can be correlated with anatomical structures and indeed MRI is a primary source of such structures with localization precision of 0.5mm as in CT. Normal imaging times are about 5mins, but fast images of lower resolution can be obtained in 50ms, enabling real time movie images to be generated. Recording sessions are typically 1hr. The NMR spectrum from living tissue gives a non-invasive measure of the concentration of each molecular species. Such spectra (MRS) provide information concerning the biochemistry of the body's metabolism and associated pathology.³¹P spectra report concentrations of ATP, ADP, phosphocreatine, inorganic phosphate, other metabolites and also local pH.¹H spectra (with suppression of water and lipid responses) give spectra from lactate, NAA, choline, creatine and other components. Spectroscopic Imaging (SI) combines MRI and MRS to provide spectra simultaneously from an array of pixels or voxels, each usually several cm³ in size in an overall time of order 20 mins. This procedure provides a spatial map of the whole spectrum or individual maps of each molecular species. Two recent developments have demonstrated that NMR can provide functional mapping of the normal human brain, and map the response of the human cerebral cortex to physiological stimulation.