Question

What are the differences between MRI spectra of body tissue against that of an organic compounds ?

Answer 1

NMR spectroscopy and magnetic resonance imaging both techniques require that the sample is typically placed in a strong magnetic field. In the former, the compound of interest is dissolved in a deuterated solvent and placed in a 5 mm sample tube, which is introduced vertically into the bore of a powerful magnet. In magnetic resonance imaging, the 'sample' is a living human body (or a part of it), which is introduced horizontally into a wide-bore magnet in the MRI scanner. Once inside the magnet, the 'sample' is subjected to radiofrequency radiation. Radiowaves and magnetic fields penetrate the biological tissues of the human body as readily as they permeate the inanimate samples used in high-resolution NMR spectroscopy. Since water (H₂O) accounts for some 60-90 per cent of the soft tissues in our bodies, it is primarily the hydrogen nucleus in water that is excited by the radiofrequency and which therefore contributes most to the signal of an 'MRI scan'. Indeed, the medical version of nuclear magnetic resonance imaging was originally referred to as NMRI, though the 'nuclear' component of the acronym has been quietly dropped, presumably in deference to the concerns of the patient.

The excitation which occurs in MRI, like high-resolution NMR spectroscopy, can be described by the fundamental equation:

= B₀/2

ie the frequency at which a spinning nucleus, such as hydrogen, absorbs radiofrequency radiation () is directly proportional to the strength of the magnetic field (B₀) which that nucleus is experiencing. Because the water molecule contains only one chemical environment for hydrogen, its ¹H-nmr spectrum consists of a single peak.