Chemistry Reference
In-Depth Information
The production of a PET tracer is initiated with the preparation of a simple precursor molecule in a particle accelerator
called a cyclotron. The cyclotron directs a beam of high-energy protons or deuterons into a target chamber that contains the
target material suitable for required radioisotope production (Table 4.1).
This simple radiolabelled product is then transferred from the cyclotron target to a 'hotcell' for further chemical reactions
to convert it into the desired radiolabelled product. Nowadays there is a greater degree of computer automation for tracer pro-
duction in order to reduce the risk of radiation exposure to the user to improve good manufacturing processes (gMP). Many
sophisticated systems have now been developed for tracer synthesis that integrate computer controlled valves, pumps, heaters,
and robotics to process and manipulate radioactive samples safely within the confines of the hotcell (Figure 4.1a) whilst rapid
preparative HPLC techniques are used as standard for purification and analysis prior to PET scanning (Figure 4.1b and 4.1c).
in spite of the challenges, a combination of innovative chemistry and engineering has enabled the routine production of
a wide range PET tracers and radiolabelling precursors. One key example is the production of the important C-11 reagent
[
11
C]methyl iodide (
11
CH
3
i), which can be produced rapidly in reaction times of <20 min. from cyclotron-generated
11
CO
2
via a gas phase catalytic hydrogenation reaction to give
11
CH
4
, followed by a free radical iodination. As outlined below,
11
CH
3
i is currently used to label a wide range of PET tracers.
tabLe 4.1
characteristics of the major short-lived pet radioisotopes c-11, n-13, and o-15.
radioisotope
Half-life, t
1/2
(min)
Nuclear reaction
Target
Major Precursor
Decay Product
11
C
20.4
14
N(
p
,
α
)
11
C
N
2
(+O
2
)
N
2
(+H
2
)
[
11
C]CO
2
[
11
C]CH
4
11
B
13
N
9.97
16
O(
p
,
α
)
13
N
H
2
O
H
2
O + EtOH
[
13
N]NO
x
[
13
N]NH
3
13
C
15
O
2.04
15
N(
d,n
)
15
O
N
2
(+O
2
)
[
15
O]O
2
15
N
(a)
(b)
(c)
FIgure 4.1
(a) A typical lead shielded hotcell used to perform radiolabelling reactions. (b) radiochemistry quality control laboratory.
(c) Combined PET-CT scanner.
