With special techniques, known collectively as magnetic resonance imaging (MRI), it is possible to obtain NMR spectra of entire organisms. A key to MRI is the application of a magnetic field that varies linearly across the specimen. Consider a flask of water held in a field that varies in the z-direction according to B₀+G_zz, where G_z is the field gradient along the zdirection. Then the water protons will be resonant at the frequencies

(Similar equations may be written for gradients along the x- and y-directions.) Application of a 90° radiofrequency pulse with ν=ν_L(z) will result in a signal with an intensity that is proportional to the numbers of protons at the position z. Now suppose a uniform disk-shaped organ is in a linear field gradient, and that the MRI signal is proportional to the number of protons in a slice of width δz at each horizontal distance z from the centre of the disk. Sketch the shape of the absorption intensity for the MRI image of the disk before any computer manipulation has been carried out.

Transcribed Image Text:

VL(z)=- ΥΝ 2π (Bo+Gzz)