Page 292 - Radiochemistry and nuclear chemistry

276 Radiochemistry rout Nuclear Chemistry

(competing with electron capture). Typical positron emitting nuclides used in PET are
included in Table 9.5. They can be tagged to a variety of compounds.
For studying brain metabolism, l lC-labeled glucose has been extensively used. The
procedure is as follows:
1) H311BO3 is irradiated by protons in an accelerator, yielding llco2.
2) Rapid automatic synthesis produces llC6Hl206 (glucose) or the methyl glucose
derivative.
3) The glucose solution is injected into a patient. Since it is easily metabolizexl, the
glucose goes to the parts of the brain with the highest metabolism, rather than to places
with no metabolism.
4) The I l C nuclide decays according to I 1 C .., I l B + e + followed by e + + e- --, 2-/. The
0.51 MeV -/coincidences are registered by the PET cameras at various positions.
PET is used especially for studies of brain, heart and lungs. Figure 9.16, left, shows a
brain investigation. Because glucose is the only source of energy the brain uses, the rate
of glucose metabolism can be assessed throughout the brain, which is an indicator of the
brain viability. In this case, the patient has been injected with l lC-methyl glucose and the
dark spot indicates an occlusion of the left carotid artery. The technique is quite fast: within
a few hours the effect of administered drugs can be revealed.
Although TCT has now superseded other techniques in locating brain tumors, SPECT,
gamma-camera and PET have provided dramatically new information of various forms of
mental illness, such as epilepsy, manic depression, and dementias such as Alzheimer's
disease. The development of new, selective radiopharmaceuticals will not only continue to
increase the importance of this diagnostic technique but also contribute to our understanding
of the functioning of the normal brain.
The use of positron emitters usually requires fast chemical separation and synthesis
techniques. Fast chemical separation techniques for producing pure radioisotopes are
described in Chapter 15, but for rapid chemical synthesis techniques, specialized texts
should be consulted.

9.5.4. Radiation therapy with internal radionuclides

Radiotracers are also used for therapy though to less extent than in diagnosis (C3b in
Table 9.3). The main application is the use of 131I for treatment of thyreotoxicos (Graves
disease with enlargement of the thyroid gland), thereby reducing the function of the thyroid.
Some data for 131I use: amount administered ca. 200 - 1000 MBq, organ dose to the thyroid
- 340 nGy/Bq (total dose to the glands > 500 Sv); whole body dose - 0.04 nGy/Bq. 32p
compounds are used in the bloodstream to destroy excess red blood cells in polycythemia.
Diphosphonate compounds localize in normal bone tissue, but concentrate in cancerous bone
at relatively higher levels (5 - 15 times higher). Patients receiving this type of
radiopharmaceutical experience swift lowering of pain levels.
One of the most fascinating aspects of using radionuclide labelled compounds in medical
therapy is to develop monoclonal antibodies, which will seek out particular types of cancer
cells and bind to them. If a/3-, or a-emitting radionuclide is attached to these antibodies,
they will deliver a large absorbed dose to the cancer cells without needlessly irradiating the
surrounding tissues. Much research is directed towards this goal of cancer therapy.