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OCR for page 22
C. Satan
Rigl
a'
~ patient lies quietly
CAT scan's x-ray tube
As from hundreds of
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..~;~nce 1895) when
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passing through the patient.
With this data, the CAT scan
computer generates an image
showing the structure in that
cross section of her head.
A young man's vertebrae,
broken in a motorcycle
accident, appear almost
lifelike in ~ three-dimensional
image created from dozens of I
CAT scans. CAT scans are
particularly good for showing
the fine detail of bone
structure. I
22
1
roenigenogram, x-ray pictures
have been clouded by shad-
ows of bones, teeth, and tissue
piled together on one piece
of film. But the marriage of
modern computers to this old
technology has given birth to
a machine that produces clear, ~
detailed images of things i
inside the body.
The machine is called a CAT scan, for
computerized axial tomography scanner. The
wedding of computers to other diagnostic
techniques has subsequently led to ultra-
sound imaging, magnetic resonance imaging
(MRI), and other new procedures that peer
inside the body without resorting to surgery.
Indeed, many doctors believe that more
progress has been made in medical diagnos-
tics since the CAT scan was introduced than
in all previous medical history.
CAT has so far had the widest impact of
the new imaging technologies. It has saved
untold numbers of lives by quickly finding
tumors, infections, bleeding, and blood clots
that would have been found too late or
missed altogether by regular x-rays or other
diagnostic techniques. A normal series of 30
scans is performed in about 20 minutes and
usually without injections. CAT scans have
reduced the need for some diagnostic I
techniques that require uncomfortable
injections and a stay in the hospital.
The procedure has also revealed to
researchers brain abnormalities in some
persons suffering from schizophrenia,
alcoholism, manic-depressive illness, and
Alzheimer's disease. CAT scans locate brain
tumors precisely for radiation treatment and
are used to build three-dimensional models
for reconstructive surgery. CAT is especially
good at finding internal injuries because it
easily distinguishes between blood and
E N G ~ N E E R ~ N G A N D T H E A D VA N C E M E N T O F H U M A N W E ~ FA R E
OCR for page 23
l
tissue. Some hospitals now have a CAT
scanner in the emergency room, and the U.S.
Army is developing a rugged, lightweight
one that can be transported by truck or
helicopter to medical stations in combat
zones.
CAT-often shortened to CT, for
computed tomography-doesn't make a flat
picture on photographic film, like a normal
x-ray. Instead, it constructs a television image
C AT 5 C A ~ I
A cross on this three-
dimensional image generated
from a series of CAT scans
marks the location of a tumor
near the center of the
patient's head. Color coding
shows sensitive areas, such as
the eyes, spinal cord, and
lungs, that should be
protected from unnecessary
radiation when the tumor is
treated.
23
OCR for page 24
An enhanced MRI scan shows
a tumor growing in the spinal
cord of a 4-year-old girl, who
lost her ability to walk. A
doctor quickly removed the
tumor, and the girl eventually
recovered the use of her legs.
24
based on density measurements of a "slice"
of the patient. The process is something like
seeing a slice of salami without cutting the
meat.
To measure density, the CAT scan's
x-ray tube shoots a thin beam edge-to-edge
through a slice while circling the patient.
Detectors circling opposite the tube measure
the strength of the emerging beam-and
therefore the density of the slice from
hundreds of points around the patient. With
this information, the CAT scan computer
calculates the density for each of more than
250,000 tiny bits of tissue in the slice. It then
displays these values in a television picture
showing density patterns that reveal the fine
structure of the slice.
The practical CAT scanner emerged
from the confluence of several new technolo-
gies. The most important developments were
low-cost minicomputers and, later, parallel-
array computers that could perform many
calculations at the same time. These high-
speed computers and the mathematically
complex programs to operate them made it
possible to run millions of calculations for
each slice. In 1979 the Nobel Prize in physiol-
ogy or medicine was awarded to British
research engineer Godfrey Hounsfield for
pioneering the CAT scan and to U.S. physi-
cist Allan Cormack for work on the mathe-
matics behind it. The first CAT scanners were
E N G ~ N E E R e N G A N D T H E A D VA N C E M E N T O F ~ U M A N W E L FA R E
OCR for page 25
installed in Great Britain in 1971 and in the
United States two years later. Today they are
used in more than half of U.S. hospitals and
in hundreds of clinics.
Very small x-ray detectors and ultrasen-
sitive signal amplifiers have greatly increased
the speed of CAT scanning. An early scan
took about 4~/: minutes. Today it takes less
than 2 seconds as a scanner's 1,000 detectors
each record 2,000 separate measurements.
The machine's sensitivity has improved, too.
Now, for example, CAT can detect a liver
tumor of almost the same density as normal
tissue. The difference would not show up on
a regular x-ray. Tomorrow's scanner will
produce even more detailed images as scan
slices become progressively thinner and scan
times decrease even more. The patient will
move continuously through the scanner, like
a car on an assembly line, completing a 40-
slice series in about a minute. And it may
soon become routine for a treating doctor to
beam the scan image via satellite to a
consulting specialist thousands of miles
away.
Small, powerful computers have been
applied to other diagnostic imaging tech-
niques since the CAT scan. Each uses a
different means of probing the patient, but
the image is constructed by computer. Low-
level radioactive isotopes are injected into the
blood during positron emission tomography
(PET) and single-photon emission computed
tomography (SPECT). With digital subtrac-
tion angiography (DSA), a computer sub-
tracts an earlier x-ray image from another
one taken after a contrast substance has been
injected into the blood, leaving the clear
contrast outline of blood vessels on the
second image. Ultrasound devices bounce
high-frequency sound waves off internal
surfaces to indicate their position and
motion.
The most promising computer-assisted
technique, though, uses magnetic fields and
radio waves to coax information from the
nuclei of atoms in body chemicals. This
nuclear magnetic resonance imaging, or
MRI, is particularly good for examining the
brain and spinal cord, whose surrounding
bone is invisible to it.
CAT culminates the development of
Roentgen's technology. And although
traditional x-ray pictures still account for 80
percent of medical diagnostic imaging, they
may one day be completely replaced by
computer-generated video images. The
. ~
a_
continued development of CAT, ultrasound
imaging, and especially MRI should further
reduce the need for exploratory surgery or
invasive imaging techniques such as those
involving injection of contrast agents that
may occasionally cause allergic reactions. In
fact, in 10 or 15 years, MRI may be able to
perform an entire diagnostic chemical
analysis of a patient, without breaking the
skin.
CAT SCAN I
Doctors discuss features
revealed by MRI images.
MRI employs radio waves and
magnetic fields to produce
images in which bones are
invisible. It is therefore very
useful for looking at soft
tissue inside the skull and
spinal column.
25
Representative terms from entire chapter:
cat scans
