24.4. Applications of Radioactivity http://www.ck12.org
metastable indicating a very short half-life). This isotope decays to Tc-99 by gamma emission. If a very low
dose of the isotope is administered, the radiation will be of a very low intensity, so cellular damage will be minimal.
Additionally, gamma radiation has a very high penetrating power, so most of it will reach the detector in the scanner.
The half-life of Tc-99m is about six hours, so it will remain in the body for some time.
Tc-99m is often used to look at cardiac damage. If there is less blood flow in the heart, there will be less of the
isotope concentrated in the heart muscle. Similar information can be obtained for blood flow in the brain.
There are presently over 25 different isotopes in use for diagnosis and medical treatment. A very partial list can be
seen inTable24.2.
TABLE24.2:Isotopes Used for Diagnosis and Medical Treatment
Isotope Half-Life Application
Cr-51 28 days labeling red blood cells
Fe-59 446 days study iron metabolism in spleen
Xe-133 5 days study lung function
Ho-166 26 hours cancer treatmentPET Scans
One of the more interesting and useful medical applications of radioisotopes is positron emission tomography (PET),
often referred to as aPET scan. This technique is especially useful for studying processes in the brain. Many
compounds do not enter the brain because of the blood-brain barrier, which is a particularly selective filter that
prevents many substances in the blood from being transported into brain tissue.
In order to get a good picture of what is happening in the brain, radiolabels (radioactive trackers) are attached to
different compounds that are known to enter the brain. Since the brain accounts for about 25% of the body’s glucose
consumption, this molecule is often labeled with a positron emitter, such as F-18 (half-life of 109.8 minutes), to
study brain function in general. Other radiolabels are attached to specific compounds that will localize in certain
areas of the brain to look at specific structures.
The PET scanner detects gamma emissions from the collision of a positron with an electron. A positron has the
same mass but opposite charge of an electron. As the positron is released from the nucleus of the atom, it will
collide with an electron. This meeting of matter (electron) with antimatter (positron) results in annihilation of both
particles and the release of two gamma photons that travel in exactly opposite directions. The scanning apparatus
detects these gamma rays and stores the data in a computer. From this information, a detailed picture of the brain
can be developed.
FIGURE 24.14
One useful application of PET scanning is in the diagnosis of Alzheimer’s disease. This debilitating condition
associated with memory loss primarily occurs in elderly individuals. A protein known as beta-amyloid gradually
forms deposits, or plaques, in the brain. Severe memory loss and impaired movement appear to be direct results of