1
17
is of little value for looking at air-filled
structures (the lungs) or those surrounded
by bone (the brain and spinal cord).
Magnetic resonance imaging (MRI)
produces high-contrast images of our
soft tissues, an area in which X rays and
CT scans are weak. As initially developed,
MRI primarily maps the body’s content of
hydrogen, most of which is in water. The
technique subjects the body to magnetic
fields up to 60,000 times stronger than
that of the earth to pry information from
the body’s molecules. The patient lies in a
chamber within a huge magnet. Hydrogen
molecules act like tiny magnets, spinning like
tops in the magnetic field. Their energy is
further enhanced by radio waves, and when
the radio waves are turned off, the energy
released is translated into a visual image.
MRI distinguishes body tissues based on
their water content, so it can differentiate
between the fatty white matter and the
more watery gray matter of the brain.
Because dense structures do not show up
at all in MRI, it peers easily into the skull and
vertebral column, enabling the delicate nerve
fibers of the spinal cord to be seen. MRI is
also particularly good at detecting tumors
and degenerative disease. Multiple sclerosis
plaques do not show up well in CT scans,
but are dazzlingly clear in MRI scans. MRI
can also tune in on metabolic reactions, such
as processes that generate energy-rich ATP
molecules.
Until recently, trying to diagnose asthma
and other lung problems has been off limits
to MRI scans because the lungs have a
low water content. However, an alternate
tack—filling the lungs with a gas that can
be magnetized (hyperpolarized helium-3 or
xenon-129)—has yielded spectacular pictures
of the lungs in just the few seconds it takes
the patient to inhale, hold the breath briefly,
and then exhale. This technique offers a
distinct improvement over the hours required
for conventional MRI and it has the additional
advantage of using a magnetic field as
little as one-tenth that of the conventional
MRI. Lately, MRI scans have become the
“diagnostic darling” in emergency rooms
for their ability to accurately diagnose heart
attacks or ischemic strokes—conditions
that require rapid treatment to prevent fatal
consequences. Also exciting are the so-called
bloodless MRIs
currently being tested in
animals, which measure water flow instead
of blood flow.
Newer variations of MRI include
magnetic resonance spectroscopy
(MRS)
, which maps the distribution of
elements other than hydrogen to reveal
more about how disease changes body
chemistry. Other advances in computer
techniques display MRI scans in three
dimensions to guide laser surgery.
The
functional MRI
tracks blood flow
into the brain in real time. Prior to the 1990s,
matching thoughts, deeds, and disease to
brain activity was the sole domain of PET.
Because functional MRI does not require
injections of tracers and can pinpoint much
smaller brain areas than PET, it may provide
a desirable alternative and has transformed
neuroscience. Clinical studies are also using
functional MRI to determine if a patient in
the vegetative state has conscious thought.
However, some researchers worry that the
brightly colored images explain little about
the mechanisms of human thought—that
is, it is not possible to assume a particular
mental state from the activation of a
particular brain region.
Despite its advantages, the powerful
magnets of the clanging, claustrophobia-
inducing MRI present some thorny
problems. For example, they can “suck”
metal objects, such as implanted
pacemakers and loose tooth fillings,
through the body. Moreover, although
such strong magnetic fields are currently
considered safe, there is no convincing
evidence that they are risk free.
Although stunning, medical images
other than straight X rays are abstractions
assembled within the “mind” of a
computer. They are artificially enhanced
for sharpness and artificially colored
to increase contrast (all their colors are
“phony”). The images are several steps
removed from direct observation.
As you can see, medical science offers
remarkable diagnostic tools. Consider the
M2A Swallowable Imaging Capsule
,
a tiny camera that a patient swallows like
a pill, and then excretes normally 8–72
hours later. As the M2A travels through the
digestive tract, it photographs the small
intestine and beams the color images to a
small video data recorder worn on a belt
or harness. A study found the device to
be 60% effective at detecting intestinal
problems, compared to a 35% success rate
with other imaging techniques. At present
the M2A can provide images only of the
small intestine because the battery gives out
before it enters the large intestine.
New imaging technologies also make
long-distance surgery possible. Visual
images of a diseased organ travel via
fiber-optic cable to surgeons at another
location (even a different country), who
manipulate delicate robotic instruments to
remove the organ.
A CLOSER LOOK
(continued)
Artery
supplying
heart
Narrowing
of artery
(b) A DSA image of the arteries that supply the heart.
(c) In a PET scan, regions of beta-amyloid accumulation “light up” (red-
yellow) in an Alzheimer’s patient (
left
) but not in a healthy person (
right
).
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