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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 ca

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 sca

industry.

By the end of the 1980s, medical MRI sca

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-sca

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 sca

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 ca

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

sca

giant sca

power-surge protectors, line conditioners, and backup power supplies.

Fluorescent lights, which produce radio-frequency noise pollution, are

forbidden around MRIs. MRIs are also very bothered by passing CB

radios, paging systems, and ambulance transmissions. It is generally