Zen and the Art of the Internet: A Beginner's Guide by Brendan P. Kehoe (1992) Prentice Hall. Brief but useful Internet guide with plenty of good advice on useful machines to paw over for data. Mr Kehoe's guide bears the singularly wonderful distinction of being available in electronic form free of charge. I'm doing the same with all my F&SF Science articles, including, of course, this one. My own Internet address is bruces@well.sf.ca.us.
"Magnetic Vision"
Here on my desk I have something that can only be described as
miraculous. It's a big cardboard envelope with nine thick sheets of
black plastic inside, and on these sheets are pictures of my own brain.
These images are "MRI scans" -- magnetic resonance imagery from
a medical scanner.
These are magnetic windows into the lightless realm inside my
skull. The meat, bone, and various gristles within my head glow gently
in crisp black-and-white detail. There's little of the foggy ghostliness
one sees with, say, dental x-rays. Held up against a bright light, or
placed on a diagnostic light table, the dark plastic sheets reveal veins,
arteries, various odd fluid-stuffed ventricles, and the spongey wrinkles
of my cerebellum. In various shots, I can see the pulp within my own
teeth, the roots of my tongue, the boney caverns of my sinuses, and the
nicely spherical jellies that are my two eyeballs. I can see that the
human brain really does come in two lobes and in three sections, and
that it has gray matter and white matter. The brain is a big whopping
gland, basically, and it fills my skull just like the meat of a walnut.
It's an odd experience to look long and hard at one's own brain.
Though it's quite a privilege to witness this, it's also a form of
narcissism without much historical parallel. Frankly, I don't think I
ever really believed in my own brain until I saw these images. At least,
I never truly comprehended my brain as a tangible physical organ, like
a knuckle or a kneecap. And yet here is the evidence, laid out
irrefutably before me, pixel by monochrome pixel, in a large variety of
angles and in exquisite detail. And I'm told that my brain is quite
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.
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