[Nuclear magnetic resonance imaging].

Hydrogen is present (IbundanLly in biological tis~uc. Protons posse~s two fundamental physical properlie~, (i) spin (L) LInd (ii) a small magnetic movement ( 11). Therefore the ,Homic nuclei or hydrogcn behave like tiny spi nn ing magnets. Our eart h is al~o (l n examplt: of such a magnet ic spinning top. When they a re placed in a sIalic magnetic Ilcld (Fig. I), the nudcl align themselves parallel to Ihe applied magnetic field. Because they spin. the prolons expericnce a couple like a gyro~cope and ['Hecess (spin like a lap) aboul the dirl:cLion of lhe magnetic field B a indic;\ted in Fig. 2. j lISI as a spin.ni ng top precesse a bout the vert ical gravitat.ional field. This precession occurs at a definite rcquency proportional to the magnetic field intcn.~ity B. This frequency is a resonant frequency. known as the Larmor frequency which can be calculaled by the following equation. f := 'Y B ( I )