Biomedical Engineering Reference
In-Depth Information
the atoms present in all the basic building blocks proteins, fat, and carbohydrates.
Further, their
t
1/2
matches reasonably well the biological
t
1/2
of many processes of
interest. The radionuclides used in PET can be classified into three categories:
1. Direct imaging where the visualization and intensity of a radiolabelled
probe is directly related to its interaction with the target molecule (which
can be a protein or RNA or DNA). The expression of cell-surface receptors
(described in Chapter 7) is imaged using radiolabelled ligands or receptor
antagonists. An example is the imaging of vascular endothelial growth fac-
tor (VEGF) using anti-VEGF monoclonal antibodies.
2. Indirect imaging, which involves multiple components. These strategies are
adapted from imaging techniques used at the single cell level. An example is
the herpes simplex virus type 1 thymidine kinase (HSV1-tk) system where
9-(4-fluoro-3-[hydroxymethyl] butyl) guanine (FHBG) is used as a reporter
tracer. FHBG stays inside the cells after phosphorylated by the thymidine
kinase in HSV1-tk. Hence, PET imaging identifies sites of thymidine kinase
activity.
3. Surrogate imaging, which refl ects an event that occurs downstream of a
particular process. For example,
18
F-2-fl uoro-2-deoxy-D-glucose (
18
F-
FDG), a glucose analog utilized to image functional changes in a glucose
metabolism.
8.5.2.1 Theory of PET
When the radionucleide emits a positron from the nucleus, the positron (
e
+
) inter-
acts with an electron (
e
−
) in the tissue and the masses are annihilated to two high-
energy photons that travel in opposite directions,
+−
+→ +
ee
γγ
(8.15)
1
2
The high-energy gamma rays have an average path length of about 10 cm in the
tissue before they are scattered or absorbed; thus, the rays have a high probability
of escaping the body. The detection of the emitted positron annihilation radiation
allows functional and structural imaging. Hence, PET gives images of the distribu-
tion of an injected positron-emitting radionuclide. Gamma or annihilation photon
detectors record the activity to produce analog signals. An electron-positron an-
nihilation is possible only if the momentum and energy conservation laws are met
(discussed in Chapter 4). Photons have momentum just as massive particles do
although their momentum is not calculated by mv as usual, which would be zero,
but rather by hf/
V
light
.
Consider an electron and a positron with
M
1
and
M
2
momenta. The net mo-
mentum
M
is the sum of the two momenta, that is,
MM M m
+==
2
υ
(8.16)
1
2
