Chemistry Reference
In-Depth Information
1.3
WhAt Is sIngle Photon eMIssIon coMPuted toMogrAPhy (sPect)?
SPEcT is another nuclear imaging technique for imaging molecules, metabolisms, and biochemical functions of organs and
cells, and like PET, the use of radioisotopes is required. As its name suggests, it involves the emission of a single
γ
-ray per nuclear
disintegration, which is measured directly, unlike in PET, where the positrons are emitted to produce the
γ
-rays. numerous single
γ
-rays are detected by rotating gamma cameras to reconstruct an image of the origin of the
γ
-rays, which identifies the location
of the radioisotope. Thus, specific radio-ligands are used to incorporate typical radioisotopes such as
99m
Tc to target to areas of
interest [11]. An example of a radiopharmaceutical commonly used in cardiac imaging is
9m
Tc-tetrofosmin, also known as
'Myoview' (Figure 1.7) [12].
1.3.1
Basic Principles
Gamma rays from radioactive nuclides, apart from radioactive decay, also produce other forms of radiation such as alpha and
beta. A gamma ray results from the relaxation of an excited daughter nucleus to a lower energy state after a nucleus emits an
α or β particle [13]. An example of this is technetium-99, which is a commonly used radioisotope in radiopharmaceuticals
produced from molybdenum-99. As shown in Figure 1.8, the excited nuclear state is more stable than average excited states
after a β-decay where a β particle is released, thus the daughter nucleus forms a metastable excited state resulting in the
Technetium (
99m
Tc) tetrofosmin
O
O
O
O
O
P
P
99m
Tc
P
P
O
O
O
O
O
FIgure 1.7
Myoview: A typical contrast agent used in SPEcT.
99
Mo
β decay
τ
1/2
= 66 h
99m
Tc
γ ray
isomeric transition,
τ
1/2
=6.01 h
99
Tc
β decay
τ
1/2
= 211100 y
99
Ru
FIgure 1.8
A schematic for the formation of
99m
Tc.
