In hospitals or on
hospital dramas on TV, you've probably seen patients
undergoing radiation therapy for cancer, and
doctors ordering PET scans to diagnose patients. These are
part of the medical specialty called nuclear medicine.
Nuclear medicine uses radioactive
substances to image the body
and treat disease. Nuclear medicine looks at both the
physiology (functioning) and the anatomy of the body in
establishing diagnosis and treatment.
In this edition of How Stuff
Works, we will explain some of the techniques and
terms used in nuclear medicine. You'll learn how radiation
helps doctors see deeper inside the human body than they ever
Imaging in Nuclear Medicine One problem with
the human body is that it is opaque, and looking inside is
generally painful. In the past, exploratory surgery was one
common way to look inside the body, but today doctors can use
a huge array of non-invasive techniques. Some of these
techniques include things like X-rays, MRI scanners,
CAT scans, ultrasound and so on. Each of these techniques has
advantages and disadvantages that make them useful for
different conditions and different parts of the body.
Nuclear medicine imaging techniques give doctors
another way to look inside the human body. The techniques
combine the use of computers,
detectors, and radioactive substances. These techniques
Positron emission tomography (PET)
Single photon emission computed tomography (SPECT)
irregular or inadequate blood flow to various tissues
blood cell disorders and inadequate functioning of
organs, such as thyroid and pulmonary function deficiencies.
The use of any specific test, or combination of
tests, depends upon the patient's symptoms and the disease
Positron Emission Tomography
(PET) PET produces images of the body by
detecting the radiation emitted from radioactive substances.
These substances are injected into the body, and are usually
tagged with a radioactive
atom, such as Carbon-11, Fluorine-18, Oxygen-15, or
Nitrogen-13, that has a short decay
time. These radioactive atoms are formed by bombarding
normal chemicals with neutrons to create short-lived radioactive
isotopes. PET detects the gamma rays given off at the site
where a positron emitted from the radioactive substance
collides with an electron in the tissue (Figure 1).
In a PET scan, the patient is injected with a radioactive
substance and placed on a flat table that moves in increments
through a "donut" shaped housing. This housing contains the
circular gamma ray detector array (Figure 2), which has
a series of scintillation crystals, each connected to a
photomultiplier tube. The crystals convert the gamma rays,
emitted from the patient, to photons of light, and
the photomultiplier tubes convert and amplify the photons to
electrical signals. These electrical signals are then
processed by the computer to generate images. The table is
then moved, and the process is repeated, resulting in a series
of thin slice images of the body over the region of interest
(e.g. brain, breast, liver). These thin slice images can be
assembled into a three dimensional representation of the
PET provides images of blood flow or other biochemical
functions, depending upon the type of molecule that is
radioactively tagged. For example, PET can show images of
glucose metabolism in the brain, or rapid changes in activity
in various areas of the body. However, there are few PET
centers in the country because they must be located near a
particle accelerator device that produces the short-lived
radioisotopes used in the technique.
Single Photon Emission Computed Tomography
(SPECT) SPECT is a technique similar to PET.
But the radioactive substances used in SPECT (Xenon-133,
Technetium-99, Iodine-123) have longer decay times than those
used in PET, and emit single instead of double gamma rays.
SPECT can provide information about blood flow and the
distribution of radioactive substances in the body. Its images
have less sensitivity and are less detailed than PET images,
but the SPECT technique is less expensive than PET. Also,
SPECT centers are more accessible than PET centers because
they do not have to be located near a particle accelerator.
Cardiovascular Imaging Cardiovascular
imaging techniques use radioactive substances to chart the
flow of blood through the heart and blood
vessels. One example of a cardiovascular imaging technique is
a stress thallium test, in which the patient is
injected with a radioactive thallium compound, exercised on a
treadmill, and imaged with a gamma ray camera. After a period
of rest, the study is repeated without the exercise. The
images before and after exercising are compared to reveal
changes in blood flow to the working heart. These techniques
are useful in detecting blocked arteries or arterioles in the
heart and other tissues.
Bone Scanning Bone scanning detects
radiation from a radioactive substance (technetium-pp
methyldiphosphate) that, when injected into the body, collects
in bone tissue. Bone tissue is good at accumulating phosphorus
compounds. The substance accumulates in areas of high
metabolic activity, and so the image shows "bright spots" of
high activity and "dark spots" of low activity. Bone scanning
is useful for detecting tumors, which generally have high
Treatment in Nuclear Medicine Nuclear
materials are used to create radioactive tracers that can be
ingested or injected into the bloodstream. One form of tracer
flows in the blood, and allows the structure of the blood
vessels to be viewed. This form of observation allows clots
and other blood vessel abnormalities to be easily detected.
Also, certain organs in the body concentrate certain types of
chemicals -- the thyroid gland concentrates iodine, so by
ingesting a radioactive iodine (pill, liquid), certain thyroid
tumors can be detected and treated. Similarly, cancerous
tumors concentrate phosphates. By injecting the radioactive
phosphorus-32 isotope into the bloodstream, tumors can be
detected by their increased radioactivity.
In nuclear medicine imaging tests and treatments, ingested
or injected radioactive substances do not harm the body. The
radioisotopes used in nuclear medicine decay quickly, in
minutes to hours, have lower radiation levels than a typical
or CT scan, and are eliminated in the urine or bowel movement.
In contrast, radiation therapy, which is different
from nuclear medicine treatment, takes advantage of the fact
that some cells are severely affected by ionizing radiation --
alpha, beta, gamma and X-rays. Cells multiply at different
rates, and the quickly multiplying cells are affected more
strongly than standard cells because of two properties:
Cells have a mechanism that is able to repair damaged DNA.
If a cell detects that its DNA is damaged while it is
dividing, it will self-destruct.
cells have less time for the repair mechanism to detect and
fix DNA errors before they divide, so they are more likely to
self-destruct when corrupted by nuclear radiation.
Since many forms of cancer are
characterized by rapidly dividing cells, they can sometimes be
treated with radiation therapy. Typically, radioactive wires
or vials are placed near or around the tumor. For deep tumors,
or tumors in inoperable places, high-intensity X-rays are
focused on the tumor.
The problem with this sort of treatment is that normal
cells that happen to reproduce quickly can be affected along
with the abnormal cells. Hair cells, cells lining the stomach
and intestines, skin cells and blood cells all happen to
reproduce quickly, so they are strongly affected by radiation.
This helps explain why people undergoing treatment for cancer
frequently suffer hair loss and nausea.
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