The human body contains a great many hydrogen atoms, mainly in molecules of the body's soft tissue. The atomic nuclei of these hydrogen atoms are protons, spinning in random orientation. Approximately 0.0003% of these protons can be made to align their natural spin when an intense magnetic field is applied about a patient for the imaging of the body's interior. The oriented spinning protons behave as synchronized (resonant) microscopic magnetic dipole fields, which precess together with a period determined by the applied magnetic field (much like the familiar spinning-top precession in the

FIGURE 2.18 ► Magnetic Resonance Imaging (MRI) represents the fields from the synchronous spinning protons in the body aligned by an intense magnetic field, detected by magnetometers, and analyzed by computers.

pull of a gravitational field). The applied magnetic field and the nuclear spin alignment are perfectly harmless to living organisms. A brief radio frequency (r.f.) electromagnetic signal pulse is next introduced to purposely disturb the proton-aligned precession. A large number of receiver magnetometer coils then measure the time for the protons to realign their spin precession and release energy obtained from that r.f. pulse. Each group of similar body cells has its own density of hydrogen atoms and characteristic strength of the precession response. With instantaneous computer modeling (called tomographic analysis) of the magnetometer received signals, live pictures of the functioning human body interior are obtained (Figure 2.18). Physicists call this process nuclear magnetic resonance (NMR) scanning or imaging. Hospitals call the procedure MR Imaging because, occasionally, apprehensive individuals wrongly associate the word "nuclear" with radioactivity. The harmless MRI scans can reconstruct excellent patient soft-tissue pictures at almost any desired artificial body slice.

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