|Fig. 1: This is the first whole body scanner. The diagram above shows a scan taken of the scanner.  (Courtesy of the U.S. Department of Energy)|
Radioisotopes are unstable atomic isotopes that give off radiation spontaneously. They can be measured by a suitable apparatus at amounts as small as one-billionth of a gram. Thus, they can be safely used in small amounts as in the body.  Scientists began using radioisotopes in biomedical experiments two decades before the atomic age, but their use remained on the small-scale until the nuclear bomb projects developed nuclear reactors.  Today, radioisotopes are used both diagnostically and therapeutically in medicine. Radioisotopes are used to tag molecules that are expected to act in certain ways. The body treats the tagged and untagged compound in the same way. For example: Cr-51 labeled blood cells are used to measure rate of blood flow from the heart. A recording device is used to count the amount of radioactivity appearing in the aorta as a function of time.  The exact amount of energy of each isotope can be measured and quantified, thus act as an effective biological tracer.
Optical imaging is an important field in cancer diagnostics because it allows noninvasive diagnosis of tumors. Optical imaging scans are used to detect cancer and monitor its progression and metastases. CT (computed tomography), MRI (magnetic resonance imaging), SPECT (single-photon emission computed tomography), and PET (positron emission tomography) imaging modalities are preferred methods of optical imaging for cancer detection because they are three-dimensional. However, all of these three-dimensional imaging modalities suffer from deficiencies in sensitivity and resolution that create difficulties in catching small numbers of cancer cells. Thus, contrast agents and radiotracers are useful methods of improving imaging sensitivity.  Radiotracers that target specific tumors display localized signal at that tissue, revealing location and size of the tumor. Several radiopharmaceuticals have been shown to have a high affinity for malignant tissue. However, the sensitivity of the tumor imaging procedure is dependent on the radiopharmaceutical and its target. Tumor-seeking radiopharmaceuticals may also nonspecifically bind to benign tumors and other infectious processes. 
Technetium-99m is referred to as the "workhorse of modern medical imaging", because it accounts for about 80% of the world's radioactive isotopes in nuclear medicine, 90% of which is used in diagnosis scans. It is an ideal element for nuclear medicine because it has a half life of 6 hours and emits gamma rays. Gamma rays are massless and easily detected outside of the body. In addition, Technetium-99m has a volatile chemistry that allows it to easily be incorporated in biologically active substances that target an organ or tissue of interest. Medical scans like SPECT then pick up the glow of the radioisotope and its localization. These tests are used to check how well blood is flowing to heart muscles, to spot whether cancers have spread through bones and to assess blood flow in the brain. 
The use of radiotracers in diagnostic medicine is an advancing field that bears promise. These methods could improve noninvasive methods of cancer detection and diagnoses.
© Meghana Golla. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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