Neutrons have a magnetic dipole moment and can undergo spin flips by absorbing electromagnetic radiation. Why, then, are protons rather than neutrons used in MRI of body tissues? (See Fig. 43.1.)

Fig.43.1

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(a) Random spin of |hydrogen protons A brief radio signal causes the spins to flip orientation. As the protons realign with the B field, they emit radio waves that are picked up by sensitive Protons, the nuclei of hydrogen atoms in the tissue under study, normally have random spin orientations. In the presence of a strong magnetic field, the spins become aligned with a component parallel to B. detectors. (b) Since B has a different value at different (c) An electromagnet used for MRI Main coil supplies uniform B field. x coil varies B field from left to right. locations in the tissue, the radio waves from different locations have different frequencies. This makes it possible to construct an image. z coil varies B field from head to toe. y coil varies B field from top to bottom. - Transceiver sends and receives signals that create image.