Biomedical Engineering Reference
In-Depth Information
Examples of radiotracers labeled with
99m
Tc (SPECT) and
18
F (PET)
Tabl e 1
Name
Application
99m
Tc DTPA
Kidney, lung (aerosol)
99m
Tc DMSA
Kidney, tumors
99m
Tc HMPAO
Cerebral perfusion,
infection/inflammation
99m
Tc
FPCIT
Basal ganglia
99m
Tc Sestamibi
Parathyroid, myocardial perfusion, thyroid
tumors and breast cancer
99m
Tc
Technegas
Pulmonary ventilation
99m
Tc
MAG3
Kidney
99m
Tc
Tetrofosmina
Parathyroid, myocardial perfusion
18
F-Fluorodeoxyglucose (FDG)
Glucose metabolism
18
F-Fluorothymidine
Cell proliferation
18
F-Fluoromisonidazole
Hypoxia
2.2
Interaction of Radiation with Matter
The typical gamma photon energy used in nuclear imaging ranges from 70 to
511 keV. In fact, for PET technology, which makes use of positron emitters, all the
photons resulting from positron annihilation have energies equal to 511 keV. For this
range of energies, the two predominant effects of radiation with matter interaction
are the photoelectric and the Compton effect. It also occurs Rayleigh effect, even
though the occurrence probability is much smaller than the other two cases. Only for
higher energies (
>
1,022 keV) there is pair production, and photonuclear interaction
only becomes significant for energies above a few MeV.
When a photon interacts with an electron and there is a deviation of the photon
and simultaneously recoil of the electron, a process called Compton effect or
Compton scattering occurs.
Applying the law of conservation of energy and the principle of conservation of
momentum to the scattering, we get the relation between the energy of the incident
photon,
E
i
, and the energy of the scattered photon,
E
d
,whichis:
E
i
E
d
D
cos
˛/
;
(1)
E
i
1 C
m
e
c
2
.1
where
m
e
c
2
511 keV).
Despite the representation depicted in Fig.
1
may indicate that the deflection
angle is fixed or there is an equal probability of deflection, this is not what occurs.
In fact, the probability of a photon to be deflected into a certain angle is not constant
and is given by the differential cross section,
d =d
, which is the probability
of a photon being scattered into the solid angle,
d
. The relation that defines the
is the energy of the electron at rest (
D
