Biomedical Engineering Reference
In-Depth Information
1.2
Positron Emission Tomography
PET is a non-invasive molecular imaging technique used in medical diagnostics.
In contrast to morphological imaging modalities like CT, functional, i.e., metabolic,
processes can be visualized and quantified.
A radioactively labeled substance called
tracer
is administered to a patient.
A typical tracer is Fluorodeoxyglucose (
18
F-FDG) where glucose is radioactively
labeled using the
β
+
decay, is detected by a scanner and a volumetric image of the tracer concentration
can be reconstructed to visualize the metabolism.
During the
β
+
emitting isotope
18
F. The emission of radiation, based on
β
+
decay of a
18
F nucleus a proton
p
is converted into a neutron
n
while a positron
e
+
and an electron neutrino
ν
e
is emitted
e
+
+
ν
e
p
→
n
+
.
(1.1)
The positron travels through the nearby tissue while loosing energy (velocity) by
interacting with adjacent atoms. The positron then collides with a nearby electron
e
−
which is its antiparticle, having the same mass, but opposite charge. As a result
of this annihilation, two gamma photons
are produced with 511 keV which are
emitted approximately in opposite directions (angle of
γ
180
◦
)
∼
e
+
+
e
−
→
2
γ
.
(1.2)
This process is illustrated in Fig.
1.2
.
β
+
decay, the emitted positron
e
+
annihilates with a nearby electron
e
−
which results in two gamma photons
Fig. 1.2
As a result of
traveling in opposite direction. These photons can be
recorded by detectors, circularly arranged around the patient. The length of the path of the positron
to the electron (positron range, see Sect.
1.4.2
) is rather short with about 0
γ
.
1 mm Full Width Half
Maximum (FWHM) [
109
]
The architecture of most PET scanners is based on circularly arranged detector
blocks, see Fig.
1.2
. Each detector block consists of a certain number of single
detectors elements. To increase the Field Of View (FOV) and the sensitivity of the
scanner, several detector rings are usually stacked together.
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