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was listed as one of the “Ten emerging technologies that will change the world” by
the MIT 2003 Technology Review [1].
Because nuclear medicine uses radioelements, this discipline is central to the devel-
opment of molecular imaging. The two main techniques used in nuclear medicine,
single-photon emission computerized tomography (SPECT) and positron emitter
tomography (PET) rely on the use of radiotracers able to interact with living organ-
isms to provide images. Coupled with X-ray computed tomography, the latter provide
high-resolution images useful as diagnostic or therapeutic follow-up tools. It is clear
that the progresses in PET imaging are related to available radiotracers. Thus, the
search for new efficient radiotracers is of tremendous importance and constitutes an
interesting playground for chemists [2].

7.2 NUCLEAR IMAGING AND RADIOTRACERS

A radiotracer can be defined as a biologically active compound associated with a
radioisotope with a short half-life. The radioisotope can be directly linked to the
active compound or complexed by means of a suitable complexing molecule. The
principle of imaging is to detect the decay of the radioisotope of the radiotracer.
Radiotracers useful in SPECT imaging are gamma-emitting radioelements like
technetium-99m ( 99m Tc), iodine-123 ( 123 I), indium-111 ( 111 In), or gallium-67 ( 67 Ga).
SPECT imaging is acquired with a gamma camera as multiple 2-D images from
several angles. A computed tomographic reconstruction is then carried to yield 3-D
datasets which are used to show thin slices of the body.
PET radiotracers incorporate one of the + -emitters like fluoride-18 ( 18 F), carbon-

11 ( 11 C), nitrogen-13 ( 13 N), oxygen-15 ( 15 O), copper-64 ( 64 Cu) or gallium-68 ( 68 Ga).
All these elements decay, producing a positron which encounters an electron in
tissues. This so-called annihilation reaction produces the two photons of high energy
(511 keV) which are detected by the camera allowing the acquisition of 3-D images
constructed by computer analysis. This three dimensional image construction is often
accomplished with the aid of CT X-ray scan performed at the same time.
Working with radioelements induces some constraints among which the half-life
(t 1/2 ) of the element is prominent. For these reasons only a few radioelements are
useful in imaging. For SPECT, 99m Tc t 1/2 6 hours, 123 It 1/2 13 hours, 111 In t 1/2 67 hours
and 67 Ga t 1/2 3.3 days are used in radiotracers. For PET, 18 Ft 1/2 109 minutes, 11 Ct 1/2
20.4 minutes, 68 Ga t 1/2 67.7 minutes, 13 Nt 1/2 <

10 minutes, 15 Ot 1/2 122.24 seconds,
64 Cu t 1/2 12.7 hours can be used. Because of radioelement physical characteristics,
(decay energy and t 1/2 ), only a few are routinely usable. 18 F, 11 C and more recently
64 C have found applications, 18 F being the most extensively used in clinics and in
research and development as well.
Until recently, only a few radiotracers were available in the clinics and 2-deoxy-
2-[ 18 F]fluoro-D-glucose (FDG) ( 1 ), the first radiotracer with approval, is still the
workhorse in PET imaging. This carbohydrate derivative is used for tumor imaging
and for a lot of applications. As glucose, FDG is taken up by cells using GLUT
transporters and accumulates in highly metabolic brain, kidney, and cancer cells