341
Positron Emission Tomography
causing the 18 O(p, n) 18 F reaction. FDG can consequently be recovered as an aqueous
solution of 18 fluoride and can then be extracted by ion-exchange chromatography.
One hour of cyclotron irradiation can produce approximately 800 mCi of 18 F. The
nomenclatures of related chemicals involved in the production of 18 F are H 18 F, F 18 F,
and 18 F − .
It is important to note that 18 F can also be produced as a radioactive gas through
the deuteron bombardment of neon ( 20 Ne) to produce 18 F and an alpha particle. The
gaseous state is currently not a popular mechanism and is overall considered a less
preferred method.
Now that we have briefly discussed the production of the radioisotopes, next we
discuss the degeneration of these substances.
17.2.2 DEGENERATION PROCESS
The engineered radiopharmaceuticals used in PET have unstable nuclei that degenerate to lower energy states. The decay process is demarcated by a time constant of
the decay called the radioactive half-life. The end product of the decay process is
generally a stable element that emits no radiation.
The half-life of a radioactive isotope is based on the fact that not all radioactive
nuclei decay simultaneously. The decay process is more a probability-determined
process, in which the exact isotope that decays cannot be identified with complete
certainty. However, the probability of any number of isotopes decaying within a
certain interval of time is well known and can be established by empirical methods. The fact that the decay process is irreversible means that the quantity of radioactive isotopes will continuously decline over time. Certain radionucleotides are
more stable than others and therefore will take a longer time to produce one single
decay event.
The radioactive half-life is formally defined as the time interval in which the
atomic count of the isotope has dropped to half the initial quantity as a result of
radioactive decay. The half-lives of the main radioisotopes used in PET imaging
span a range of almost 100 s to almost 100 min. Specifically, for 18 F, as found in an
FDG, the half-life is 109.7 min. A shorter half-life is that of the carbon isotope, 11 C,
which is only 20 min. The other two main isotopes are the nitrogen isotope, 13 N, with
a half-life of 10 min and the oxygen isotope, 15 O, with the half-life of only 124 s.
Out of the four main radioisotopes used in PET scans, the nucleotide fluoride,
18 F, is the most commonly used. The fluoride isotope is almost always incorporated
into FDG, which is essentially a radioactive equivalent of glucose. Because of its
similarity to glucose, FDG is used to measure the glucose metabolic rate in a number
of body organ systems. Monitoring the consumption of glucose is therefore measuring the metabolism of the cells. This is why PET is a functional imaging. As shown
earlier, the fluoride isotope has a relatively slow decay, which means that it can be
shipped after production in the cyclotron and still have ample useable molecular
weight left at the time it reaches the hospital that can be as far away as 100 km. This
is the main reason for the popularity of FDG.
The next issue that needs to be addressed is the detection of the radionucleotides.
Positron Emission Tomography
causing the 18 O(p, n) 18 F reaction. FDG can consequently be recovered as an aqueous
solution of 18 fluoride and can then be extracted by ion-exchange chromatography.
One hour of cyclotron irradiation can produce approximately 800 mCi of 18 F. The
nomenclatures of related chemicals involved in the production of 18 F are H 18 F, F 18 F,
and 18 F − .
It is important to note that 18 F can also be produced as a radioactive gas through
the deuteron bombardment of neon ( 20 Ne) to produce 18 F and an alpha particle. The
gaseous state is currently not a popular mechanism and is overall considered a less
preferred method.
Now that we have briefly discussed the production of the radioisotopes, next we
discuss the degeneration of these substances.
17.2.2 DEGENERATION PROCESS
The engineered radiopharmaceuticals used in PET have unstable nuclei that degenerate to lower energy states. The decay process is demarcated by a time constant of
the decay called the radioactive half-life. The end product of the decay process is
generally a stable element that emits no radiation.
The half-life of a radioactive isotope is based on the fact that not all radioactive
nuclei decay simultaneously. The decay process is more a probability-determined
process, in which the exact isotope that decays cannot be identified with complete
certainty. However, the probability of any number of isotopes decaying within a
certain interval of time is well known and can be established by empirical methods. The fact that the decay process is irreversible means that the quantity of radioactive isotopes will continuously decline over time. Certain radionucleotides are
more stable than others and therefore will take a longer time to produce one single
decay event.
The radioactive half-life is formally defined as the time interval in which the
atomic count of the isotope has dropped to half the initial quantity as a result of
radioactive decay. The half-lives of the main radioisotopes used in PET imaging
span a range of almost 100 s to almost 100 min. Specifically, for 18 F, as found in an
FDG, the half-life is 109.7 min. A shorter half-life is that of the carbon isotope, 11 C,
which is only 20 min. The other two main isotopes are the nitrogen isotope, 13 N, with
a half-life of 10 min and the oxygen isotope, 15 O, with the half-life of only 124 s.
Out of the four main radioisotopes used in PET scans, the nucleotide fluoride,
18 F, is the most commonly used. The fluoride isotope is almost always incorporated
into FDG, which is essentially a radioactive equivalent of glucose. Because of its
similarity to glucose, FDG is used to measure the glucose metabolic rate in a number
of body organ systems. Monitoring the consumption of glucose is therefore measuring the metabolism of the cells. This is why PET is a functional imaging. As shown
earlier, the fluoride isotope has a relatively slow decay, which means that it can be
shipped after production in the cyclotron and still have ample useable molecular
weight left at the time it reaches the hospital that can be as far away as 100 km. This
is the main reason for the popularity of FDG.
The next issue that needs to be addressed is the detection of the radionucleotides.