Biomedical Engineering Reference
In-Depth Information
8.5.2
Positron Emission Tomography (PET)
PET relies on the interactions of radioactive substance (also referred as radionu-
clides) that decay via positron emission. The positron (
e
+
) is the antiparticle of the
electron (
e
−
) and behaves nearly the same as electrons. PET was originally devel-
oped for research purposes but has found clinical applications. In PET, radionu-
clides in tracer amounts (hence they are referred as tracers) are used to image and
measure biological processes. The radiation dose is usually less than the dose that
individuals receive from their natural environment each year. A patient ingests (or
is injected with) the positron-emitting radionuclide (Figure 8.8) and depending on
the biodistribution properties of the radionuclide, it is taken up by different organs
and/or tissue types. The regional concentration of the labeled compound (in mCi/
cc) is imaged as a function of time. For example, oxygen and glucose accumulate in
brain areas that are metabolically active.
Commonly used radioisotopes are
18
fluorine (
t
1/2
=
109.8 minutes),
11
carbon
(
t
1/2
=
2.07
minutes). These radionuclides have short half-lives (e.g., 2 to 109 minutes) and are
produced in cyclotrons or reactors. Radionuclides are selected by first identifying
the process to be studied and then synthesizing a molecule with a radioisotope
through which the assessment is performed.
11
C is a natural choice since it is an iso-
tope of an atom that is present in organic molecules, while many of the
18
F charac-
teristics are similar to those of hydrogen.
13
N and
15
O have the advantage of being
20.4 minutes),
13
nitrogen (
t
1/2
=
9.96 minutes), and
15
oxygen (
t
1/2
=
Figure 8.8
Schematic showing the mechanism of PET along with the interactions. In addition,
components of the camera are also shown.
