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.
The enormously powerful magnetic field within an MRI machine
can permeate the human body; but the resonant Larmor frequency is
beamed through the body in thin, precise slices. The resulting images
are neat cross-sections through the body. Unlike X-rays, magnetic
resonance doesn't ionize and possibly damage human cells. Instead, it
gently coaxes information from many different types of tissue, causing
them to emit tell-tale signals about their chemical makeup. Blood, fat,
bones, tendons, all emit their own characteristics, which a computer
then reassembles as a graphic image on a computer screen, or prints
out on emulsion-coated plastic sheets.
An X-ray is a marvelous technology, and a CAT-scan more
marvelous yet. But an X-ray does have limits. Bones cast shadows in X-
radiation, making certain body areas opaque or difficult to read. And X-
ray images are rather stark and anatomical; an X-ray image cannot
even show if the patient is alive or dead. An MRI scan, on the other
hand, will reveal a great deal about the composition and the health of
living tissue. For instance, tumor cells handle their fluids differently
than normal tissue, giving rise to a slightly different set of signals. The
MRI machine itself was originally invented as a cancer detector.
After the 1946 discovery of magnetic resonance, MRI techniques
were used for thirty years to study small chemical samples. However, a
cancer researcher, Dr. Raymond Damadian, was the first to build an MRI
machine large enough and sophisticated enough to scan an entire
human body, and then produce images from that scan. Many scientists,
most of them even, believed and said that such a technology was decades
away, or even technically impossible. Damadian had a tough,
prolonged struggle to find funding for for his visionary technique, and
he was often dismissed as a zealot, a crackpot, or worse. Damadian's
struggle and eventual triumph is entertainingly detailed in his 1985
biography, A MACHINE CALLED INDOMITABLE.
Damadian was not much helped by his bitter and public rivalry
with his foremost competitor in the field, Paul Lauterbur. Lauterbur,
an industrial chemist, was the first to produce an actual magnetic-
resonance image, in 1973. But Damadian was the more technologically
ambitious of the two. His machine, "Indomitable," (now in the
Smithsonian Museum) produced the first scan of a human torso, in 1977.
(As it happens, it was Damadian's own torso.) Once this proof-of-
concept had been thrust before a doubting world, Damadian founded a
production company, and became the father of the MRI scanner
industry.
By the end of the 1980s, medical MRI scanning had become a
major enterprise, and Damadian had won the National Medal of
Technology, along with many other honors. As MRI machines spread
worldwide, the market for CAT-scanning began to slump in comparison.
Today, MRI is a two-billion dollar industry, and Dr Damadian and his
company, Fonar Corporation, have reaped the fruits of success. (Some
of those fruits are less sweet than others: today Damadian and Fonar
Corp. are suing Hitachi and General Electric in federal court, for
alleged infringement of Damadian's patents.)
MRIs are marvelous machines -- perhaps, according to critics, a
little too marvelous. The magnetic fields emitted by MRIs are extremely
strong, strong enough to tug wheelchairs across the hospital floor, to
wipe the data off the magnetic strips in credit cards, and to whip a
wrench or screwdriver out of one's grip and send it hurtling across the
room. If the patient has any metal imbedded in his skin -- welders and
machinists, in particular, often do have tiny painless particles of
shrapnel in them -- then these bits of metal will be wrenched out of the
patient's flesh, producing a sharp bee-sting sensation. And in the
invisible grip of giant magnets, heart pacemakers can simply stop.
MRI machines can weigh ten, twenty, even one hundred tons.
And they're big -- the scanning cavity, in which the patient is inserted,
is about the size and shape of a sewer pipe, but the huge plastic hull
surrounding that cavity is taller than a man and longer than a plush
limo. A machine of that enormous size and weight cannot be moved
through hospital doors; instead, it has to be delivered by crane, and its
shelter constructed around it. That shelter must not have any iron
construction rods in it or beneath its floor, for obvious reasons. And yet
that floor had better be very solid indeed.
Superconductive MRIs present their own unique hazards. The
superconductive coils are supercooled with liquid helium.
Unfortunately there's an odd phenomenon known as "quenching," in
which a superconductive magnet, for reasons rather poorly understood,
will suddenly become merely-conductive. When a "quench" occurs, an
enormous amount of electrical energy suddenly flashes into heat,
which makes the liquid helium boil violently. The MRI's technicians
might be smothered or frozen by boiling helium, so it has to be vented
out the roof, requiring the installation of specialized vent-stacks.
Helium leaks, too, so it must be resupplied frequently, at considerable
expense.
The MRI complex also requires expensive graphic-processing
computers, CRT screens, and photographic hard-copy devices. Some