BioPHYSICAL chemistry

of only 110 min and so the isotope must be generated at a cyclotron

immediately prior to use.

PET

The technique of PET is an imaging technique that makes use of positron-

emitting isotopes as probes of specific biochemical processes in vivo

(Figure 19.10). The positron is not stable and will combine with an elec-

tron to produce energy that has a value equal to twice the electron mass,

2 mec^2 , or twice 511 keV. The energy takes the form of two photons that,

to conserve momentum, are directed along equal and opposite directions

from the point where the positron and electron came together, with each

photon having an energy of 511 keV. The detection of the two simultan-

eously emitted 511-keV photons occurs simultaneously and the common

point of origin can be found by following the line of detection. For meas-

urements of a patient, millions of pair combinations are recorded from

many different angles around the subject. The combinations of all of these

measurements provide reconstruction of the concentrations of the probe

at different locations to generate the tomographic images.

In PET, the subject is injected with a molecule containing a label

with an isotope that will emit positrons (Figure 19.11). One probe is 2-

[^18 F]fluoro-2-deoxy-D-glucose, which is used to locate glycolysis processes

in the body. For these measurements, the end-product accumulation

is proportional to the rate of glycolysis. Another probe is 3′-deoxy-3′-

[^18 F]fluorothymidine. This probe is a deoxy analog of thymidine. The probe

can be transported and phosphorylated but the resulting modified probe is

not dephosphorylated by phosphohydrolases and accumulates in the cell.

Thus, both probes show where active transport and phosphorylation are

416 PART 3 UNDERSTANDING BIOLOGICAL SYSTEMS USING PHYSICAL CHEMISTRY

OH

180 



O

511 keV photon

E  mc^2

511 keV photon

H

HO

H

H

OH H

H

CH 2 OH

2-[^18 F]Fluoro-2-deoxy-D-glucose (FDG)

F

Figure 19.10A schematic representation of PET. The positron-emitting radioisotope is injected
intravenously into the body. When the^18 F decays a positron is emitted and rapidly annihilates
with an electron releasing, two photons in opposite directions, which are detected. Modified from
Phelps (2002).