1
A
C L O S E R
LOOK
Medical Imaging: Illuminating the Body
Until 50 years ago, the
magical
but
murky
X ray was the only nonsurgical
means to extract information from within
a living body. Produced by directing
X rays
, electromagnetic waves of very
short wavelength, at the body, an
X ray
or
radiograph
is essentially a
shadowy
negative image
of internal structures.
Dense structures absorb the X rays most
and so appear as light areas. Hollow air-
containing organs and fat, which absorb
the X rays less, show up as dark areas.
What X rays do best is visualize hard, bony
structures and locate abnormally dense
structures (tumors, tuberculosis nodules)
in the lungs.
The 1950s saw the advent of
ultrasound and of nuclear medicine, which
uses radioisotopes to scan the body to not
only reveal the structure of our “insides”
but also wring out information about the
hidden workings of their molecules.
Computed tomography
(
CT
, formerly
called
computerized axial tomography
,
CAT
) uses a refined version of X-ray
equipment. As the patient is slowly moved
through the doughnut-shaped CT machine,
its X-ray tube rotates around the body and
sends beams from all directions to a specific
level of the patient’s body. Because at any
moment its beam is confined to a “slice” of
the body about as thick as a dime, CT ends
the confusion resulting from overlapping
structures seen in conventional X rays. The
device’s computer translates this information
into a detailed, cross-sectional picture of
each body region scanned. CT scans are at
the forefront for evaluating most problems
that affect the brain and abdomen. Their
clarity, illustrated in photo (a), has all but
eliminated exploratory surgery.
Xenon CT
is a CT brain scan enhanced
with radioactive xenon gas to quickly trace
blood flow. Inhaled xenon rapidly enters the
bloodstream and distributes to different body
tissues in proportion to their blood flow.
Absence of xenon from part of the brain
indicates that a stroke is occurring there,
information that aids treatment.
Dynamic spatial reconstruction (DSR)
uses ultrafast CT scanners to provide three-
dimensional images of body organs from any
angle, and scrutinize their movements and
changes in their internal volumes at normal
speed, in slow motion, and at a specific
moment. DSR’s greatest value has been to
visualize the heart beating and blood flowing
through blood vessels. This information
allows clinicians to evaluate heart defects,
constricted or blocked blood vessels, and the
status of coronary bypass grafts.
Another computer-assisted X-ray
technique,
digital subtraction
angiography (DSA
)
(
angiography
5
vessel pictures), provides an unobstructed
view of small arteries. Conventional
radiographs are taken before and after a
contrast medium is injected into an artery.
The computer subtracts the “before”
image from the “after” image, eliminating
all traces of body structures that obscure
the vessel. DSA is often used to identify
blockages in the arteries that supply the
heart wall, as in photo (b), and in the brain.
Just as the X ray spawned related
technologies, so too did nuclear medicine
in the form of
positron emission
tomography
(PET)
. PET excels in observing
metabolic processes
. The patient is given
an injection of radioisotopes tagged to
biological molecules (such as glucose) and
is then positioned in the PET scanner. As
the radioisotopes are absorbed by the most
active brain cells, high-energy gamma rays
are produced. The computer analyzes the
gamma-ray emission and produces a
live-action picture of the brain’s biochemical
activity in vivid colors. PET’s greatest value has
been its ability to provide insights into brain
activity in people affected by mental illness,
stroke, Alzheimer’s disease, and epilepsy.
One of its most exciting uses has been to
determine which areas of the healthy brain
Left
Liver
Vertebra
Left kidney
Spleen
Pancreas
Right
(a) A CT scan through the superior abdomen.
are most active during certain tasks (e.g.,
speaking, listening to music, or figuring
out a mathematical problem), providing
direct evidence of the functions of specific
brain regions. Currently PET can reveal
signs of trouble in those with undiagnosed
Alzheimer’s disease (AD) because regions
of beta-amyloid accumulation (a defining
characteristic of AD) show up in brilliant red
and yellow, as in photo (c). PET scans can
also help to predict who may develop AD in
the future by identifying areas of decreased
metabolism in crucial memory areas of the
brain.
Sonography
, or
ultrasound imaging
,
has some distinct advantages over the
approaches examined so far. The equipment
is inexpensive, and the ultrasound used as
its energy source seems to be safer than
the ionizing forms of radiation used in
nuclear medicine. The body is probed with
pulses of sound waves that cause echoes
when reflected and scattered by body
tissues. A computer analyzes these echoes
to construct somewhat blurry outlines of
body organs. A single easy-to-use handheld
device emits the sound and picks up the
echoes, so sections can be scanned from
many different body planes.
Because of its safety, ultrasound is the
imaging technique of choice in obstetrics
for determining fetal age and position and
locating the placenta. However, sound
waves have low penetrating power and
rapidly dissipate in air, so sonography
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