Medicine is a subspecialty of radiology
which uses a special camera to observe or
map radioactive medicine injected into the
human body. Typically, a small amount of
an appropriate radionuclide is combined or
tagged with a certain chemical so that when
the mixture is injected it will tend to go
to the part of the body we want to "photograph"
with our nuclear imaging camera. This might
be the heart, thyroid, bones, brain or other
body system. Different chemicals are used
to direct or cause the material to concentrate
in one organ vs. another. We can also introduce
the radioactive medicine into the body via
food (to study the digestive tract) or via
a special breathing mask (to study the lungs).
the radioactive medicine has had time to gather
at the body organ of interest (from several
seconds to several hours, depending on the
chosen drug or radiopharmaceutical) the patient
is positioned under the nuclear imaging camera.
Then either the camera is moved slowly along
the body (called a planar scan) or it moves
around the body (called a SPECT scan - Single
Photon Emission Computed Tomography). It stops
periodically to take a "picture" (which
consists of accurately detecting the radioactive
emissions from the body - rather like having
the shutter open for a few seconds). It then
moves on a few degrees or a few inches to
the next picture location and stops to take
another picture. The amount of time the patient
remains on the camera table for a typical
study varies but can last from 20 to 50 minutes.
pictures are combined to form a larger image.
In the case of the planar image, the image
combination largely consists of pasting consecutive
images together (all in the same 2-dimensional
plane). In the case of the SPECT images, a
more complex technique called tomography is
used to yield a 3-dimensional picture. This
same mathematical process is used in other
types of radiology imaging such as Magnetic
Resonance Imaging (MRI) and Computed Tomography
(CT) to generate the "slices"
of anatomy which we have all seen .
radioactive medicine will concentrate at the organ
of interest as body functions dictate (such as
cell activity, blood flow, metabolic rate, etc.).
Therefore, the nuclear imaging scan shows function
rather than anatomy. This differs from most diagnostic
imaging equipment (such as MRI, CT, and standard
X-ray) which show anatomy. This is an important
distinction as it allows detection of injuries
and some tumors (since they are metabolically
active) sooner than they might be seen anatomically.
amount of radioactive isotope in the body will
gradually decrease. The
two factors contributing to this reduction
are the natural half-life of the radioactive
material (usually 6-48 hrs to decrease by a
factor of 2) and the biological half-life (that
is, how rapidly the body expels the material).
These cause the amount of radioactive medicine
to return to near background levels in 1-5
days following injection.