"Bruce Sterling - Magnetic Vision" - читать интересную книгу автора (Sterling Bruce)

healthy and perfectly normal -- anatomically at least. (For a science
fiction writer this news is something of a letdown.)

The discovery of X-rays in 1895, by Wilhelm Roentgen, led to the
first technology that made human flesh transparent. Nowadays, X-rays
can pierce the body through many different angles to produce a
graphic three-dimensional image. This 3-D technique, "Computerized
Axial Tomography" or the CAT-scan, won a Nobel Prize in 1979 for its
originators, Godfrey Hounsfield and Allan Cormack.

Sonography uses ultrasound to study human tissue through its
reflection of high-frequency vibration: sonography is a sonic window.

Magnetic resonance imaging, however, is a more sophisticated
window yet. It is rivalled only by the lesser-known and still rather
experimental PET-scan, or Positron Emission Tomography. PET-
scanning requires an injection of radioactive isotopes into the body so
that their decay can be tracked within human tissues. Magnetic
resonance, though it is sometimes known as Nuclear Magnetic
Resonance, does not involve radioactivity.

The phenomenon of "nuclear magnetic resonance" was
discovered in 1946 by Edward Purcell of Harvard, and Felix Block of
Stanford. Purcell and Block were working separately, but published
their findings within a month of one another. In 1952, Purcell and
Block won a joint Nobel Prize for their discovery.

If an atom has an odd number of protons and neutrons, it will
have what is known as a "magnetic moment:" it will spin, and its axis
will tilt in a certain direction. When that tilted nucleus is put into a
magnetic field, the axis of the tilt will change, and the nucleus will also
wobble at a certain speed. If radio waves are then beamed at the
wobbling nucleus at just the proper wavelength, they will cause the
wobbling to intensify -- this is the "magnetic resonance" phenomenon.
The resonant frequency is known as the Larmor frequency, and the
Larmor frequencies vary for different atoms.

Hydrogen, for instance, has a Larmor frequency of 42.58
megahertz. Hydrogen, which is a major constituent of water and of
carbohydrates such as fat, is very common in the human body. If radio
waves at this Larmor frequency are beamed into magnetized hydrogen
atoms, the hydrogen nuclei will absorb the resonant energy until they
reach a state of excitation. When the beam goes off, the hydrogen
nuclei will relax again, each nucleus emitting a tiny burst of radio
energy as it returns to its original state. The nuclei will also relax at
slightly different rates, depending on the chemical circumstances
around the hydrogen atom. Hydrogen behaves differently in different
kinds of human tissue. Those relaxation bursts can be detected, and
timed, and mapped.