Chemistry Reference
In-Depth Information
taBle 5.1 relevant properties of selected pet Imaging Isotopes, ec = electron
capture; some low abundance positrons have been omitted for clarity [10, 11].
e
+
(keV)
Isotope
t
1/2
(h)
decay Mode
Production Method
60
Cu
0.4
β
+
(93%)
EC (7%)
3920, 3000
2000
cyclotron,
60
ni(p,n)
60
Cu
61
Cu
3.3
β
+
(62%)
EC (38%)
1220, 1150
940, 560
cyclotron,
61
ni(p,n)
61
Cu
62
Cu
0.16
β
+
(98%)
EC (2%)
2910
62
Zn/
62
Cu generator
64
Cu
12.7
β
+
(19%)
EC (41%)
β
-
(40%)
656
cyclotron,
64
ni(p,n)
64
Cu
66
Ga
9.5
β
+
(60%)
EC (10%)
4150, 935
cyclotron,
63
Cu(α,nγ)
66
Ga
68
Ga
1.1
β
+
(90%)
EC (10%)
1880
68
Ge/
68
Ga generator
86
Y
14.7
β
+
(33%)
EC (66%)
1221
cyclotron,
86
sr(p,n)
86
Y
89
Zr
78.5
β
+
(23%)
EC (77%)
897
cyclotron,
89
Y(p,n)
89
Zr
124
I
100.2
β
+
(23%)
EC (77%)
2138, 1535
cyclotron,
124
Te(p,n)
124
I
positrons travel a larger radius and therefore decrease spatial resolution. In general, lower energy β
+
and γ emissions provide
better image quality. The nuclides
86
Y,
89
Zr, and
124
I emit a large amount of γ rays relative to the amount of positrons (poor
branching ratios). These additional γ emissions can both complicate PET imaging by interfering with the detection of coin-
cident 511 keV γ rays that originate from β
+
emission/annihilation and increase the radioactive dose accumulated by patients
[8, 9]. despite these shortcomings, the PET nuclides discussed here have a multitude of chemical and physical properties
that make them attractive for imaging purposes. Radionuclides are typically produced by proton (p,n) or deuteron (d,2n)
bombardment via a cyclotron, neutron bombardment via a nuclear reactor (n,xp), or elution from a generator system (the
parent nuclide in generators must be produced via cyclotron or reactor). The most common production methods of various
isotopes are displayed in Table 5.1 [10, 11]. several of the most promising inorganic PET nuclides (excluding
18
F, Chapter 3)
will be discussed in this section; however, there are many other unconventional PET nuclides that have received less attention
and have been discussed elsewhere [12]. As recently noted, there are many discrepancies in nuclear decay properties
reported in the literature, thus the half-lives, branching ratios, and positron energies reported here should be considered
approximate [12, 13].
The preparation of imaging agents based on
18
F and radioiodine typically involve complex radiolabelling syntheses and
are technically demanding (see Chapter 3). With the short half-life of
18
F (110 minutes), this provides significant logistical
challenges. On the contrary, radiopharmaceutical preparations based on radiometals can be very simple to radiolabel, because
they are often suitable for making kit formulations. These kit formulations simply require the addition of a pure and high
specific activity radiometal to a buffered solution of BFC-biovector conjugate, incubation at an appropriate temperature to
allow for quantitative radiometallation, and finally purification. This is possible because the majority of synthesis work is
accomplished in the laboratory before radiolabelling is performed, thus these kit formulations are designed with a simple
“shake and bake” preparation style that requires minimal processing and training for clinical deployment. The obvious draw-
back to BFC-radiometal preparations is that the radiometal-chelate complex is relatively large and often charged, which
means that it may not be compatible with traditional agents such as small molecule drugs and neurotransmitters. The
small molecule medicinal agents can have a hydrogen atom (typically aromatic) substituted by
18
F or a radioiodine iso-
tope with minimal biological impact, allowing for imaging of the molecule's native and essentially unadulterated function.
Radiohalogens can also be labelled onto a prosthetic group, which is typically comprised of an aromatic moiety that
undergoes facile radiohalogenation and an appropriate bioconjugation tether for attachment to a biovector. The use of pros-
thetic groups to utilise radioiodine is a comparable modality to radiometal-based BFC systems, where the radioactive moiety
is a separate chemical entity and is conjugated to a biovector.
Gallium(III), yttrium(III), and zirconium(IV) ions are resistant to redox reactions under biological conditions (aqueous,
~ph 7.4), which is important for complex stability; however, they are sensitive to hydrolysis. Changing the oxidation state
