70
UNIT ONE
P
hysicians use two techniques—comput-
erized tomography (CT) scanning and
positron emission tomography (PET)
imaging—to paint portraits of anatomy and
physiology, respectively.
In CT scanning, an X-ray-emitting device is
positioned around the region of the body being
examined. At the same time, an X-ray detector
is moved in the opposite direction on the other
side of the body. As these parts move, an X-ray
beam passes through the body from hundreds
of different angles. Tissues and organs of vary-
ing composition absorb X rays diF
erently, so the
intensity of X rays reaching the detector varies
from position to position. A computer records the
measurements made by the X-ray detector and
combines them mathematically. This creates on a
viewing screen a sectional image of the internal
body parts (±
g. 2D).
Ordinary X-ray techniques produce two-
dimensional images known as radiographs, X rays,
or films. A CT scan provides three-dimensional
information. The CT scan can also clearly diF
er-
entiate between soft tissues of slightly diF
erent
densities, such as the liver and kidneys, which
cannot be seen in a conventional X-ray image. In
this way, a CT scan can detect abnormal tissue,
such as a tumor. ²or example, a CT scan can tell
whether a sinus headache that does not respond
to antibiotic therapy is caused by a drug-resistant
infection or by a tumor.
PET imaging uses radioactive isotopes
that naturally emit positrons, atypical positively
charged electrons, to detect biochemical activ-
ity in a specific body part. Useful isotopes in
PET imaging include carbon-11, nitrogen-13,
oxygen-15, and fluorine-18. When one of these
isotopes releases a positron, it interacts with a
nearby negatively charged electron. The two
particles destroy each other in an event called
annihilation. At the moment of destruction, two
gamma rays form
and move apart. Special equip-
ment detects the gamma radiation.
To produce a PET image of biochemically
active tissue, a person is injected with a meta-
bolically active compound that includes a bound
positron-emitting isotope. To study the brain, for
example, a person is injected with glucose con-
taining fluorine-18. After the brain takes up the
isotope-tagged compound, the person rests the
head within a circular array of radiation detectors. A
device records each time two gamma rays are emit-
ted simultaneously and travel in opposite directions
(the result of annihilation). A computer collects and
combines the data and generates a cross-sectional
image. The image indicates the location and rela-
tive concentration of the radioactive isotope in dif-
ferent regions of the brain and can be used to study
those parts metabolizing glucose.
PET images reveal the parts of the brain
affected in such disorders as Huntington dis-
ease, Parkinson disease, epilepsy, and Alzheimer
disease, and they are used to study blood flow
in vessels supplying the brain and heart. The
technology is invaluable for detecting the physi-
ological bases of poorly understood behavioral
disorders, such as obsessive-compulsive disorder.
In this condition, a person repeatedly performs a
certain behavior, such as washing hands, show-
ering, locking doors, or checking to see that the
stove is turned off. PET images of people with
this disorder reveal intense activity in two parts
2.3
FROM SCIENCE TO TECHNOLOGY
CT Scanning and PET Imaging
(a)
(b)
FIGURE 2D
CT scans of (
a
) the head and (
b
) the abdomen.
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