PRINCIPLES OF MAGNETIC-RESONANCE-IMAGING AND MICROSCOPY

Magnetic resonance (MR) imaging is based upon the Nuclear Magnetic Res onance (NMR) experiment. The patient or sample is placed in a strong m agnetic field which polarizes the populations of hydrogen nuclei so th at a bulk magnetization appears. This magnetization, flipped by a radi ofrequency (RF) pulse returns to equilibrium along the direction of th e main magnetic field, inducing in the detection probe a signal whose frequency is proportional to the magnetic field intensity. In order to form an image, the spatial origin of the signal must be precisely loc alized. Such a localization is achieved by means of magnetic field gra dients, which establish linear relations between resonance frequency a nd distance. The spatial resolution is proportional to the product of the gradient strength by the latter's application duration. So that, i n theory, there should not be any limit in spatial resolution. In fact , the latter is limited by the signal to noise ratio, resonance freque ncy line width and diffusion of the hydrogen nuclei. The spatial resol ution actually achieved within these constraints is limited to only a few microns. However, magnetic resonance enables non-destructive measu rements of physical and chemical characteristics in living samples.