Radiation Imaging - Introduction to Biomedical Engineering - page 1011

Progress in emission scanning and nuclear medicine has been linked to the availability of
radionuclides that could be used in human subjects in appropriate chemical forms. The
choice of a radionuclide depends on its physical characteristics in relation to diagnostic
and therapeutic applications and the possibility of incorporating it into an appropriate
chemical compound suitable for biomedical investigation.
A radiopharmaceutical contains a specific chemical compound labeled with a radionu-
clide. This may be a simple inorganic salt or a complex organic molecule. It is well recog-
nized that the chemical properties of an element reflect some of the possible biological
behaviors, and the behavior of several elements within the same group of the periodic table
appears similar. For example, strontium 85 has been used to represent calcium metabolism
in bone. Table 15.1 summarizes some of the important radionuclides that are currently used
in nuclear medicine.

15.2.5 Measurement of Radiation: Units
It should now be apparent that all radioactive substances decay and in the process emit
various types of radiation (alpha, beta, and/or gamma) during this process. However, since
they usually occur in various combinations, it is difficult to measure each type of radiation
separately. As a result, measuring radioactivity is usually accomplished by (1) counting the
number of disintegrations that occur per second in a radioactive material, (2) noting how
effective this radiation is in producing atoms (ions) possessing a net positive or negative

TABLE 15.1 Commonly Used Radionuclides in Nuclear Medicine

Radionuclide Half-life Transition Production Chemical Form

b þ Carbon 11 20.38 min Cyclotron 3- N -methylspiperone

b þ

Flourine 18 109.77 min Cyclotron Flourodeoxyglucose

b Phosphorus 31 14.29 days Reactor Phosphates

Chromium 51 27.704 days EC Reactor Sodium chromate

Cobalt 57 270.9 days EC Cyclotron Cyanocobalamin

Gallium 67 78.26 h EC Cyclotron Citrate complex

b

Reactor Molybdate in column

Molybdenum 99 66.0 h

Technetium 99m 6.02 h IT Generator TcO 4 and complexes

Indium 111 2.83 days EC Cyclotron DTPA and oxine

Iodine 123 13.2 h EC Cyclotron Mainly iodide

Iodine 123 60.14 days EC Reactor Diverse proteins

b Iodine 131 8.04 days Reactor Diverse compounds

b

Xenon 133 5.245 days Reactor Gas

Thallium 201 3.044 days EC Cyclotron Thallous chloride