Introduction to Technetium
Technetium, represented by the symbol Tc, is a chemical element with an atomic number of 43. It holds a unique position in the periodic table as the lightest element for which all isotopes are radioactive; no stable isotopes exist. Its discovery marked a significant milestone, as it was the first element to be artificially synthesized rather than being first found in nature.
Natural Occurrence and Production
Scarcity on Earth
Technetium is exceedingly rare on Earth. It occurs in trace quantities as a spontaneous fission product within uranium and thorium ores, where heavy atomic nuclei naturally decay and split. These amounts are negligible for any practical extraction. Outside of Earth, Technetium has been spectroscopically identified in certain types of stars, particularly red giant stars, indicating nuclear synthesis processes within these celestial bodies.
Industrial Production
For practical applications, Technetium is not extracted from natural sources but is primarily produced artificially in nuclear reactors. The most significant route involves the fission of uranium-235 targets, which yields various fission products, including Molybdenum-99 (Mo-99). Mo-99 has a half-life of 66 hours and decays to Technetium-99m (Tc-99m), an excited nuclear isomer of Technetium-99.
Hospitals and radiopharmacies typically receive Mo-99 in the form of a technetium generator, often referred to as a “Tc-99m generator” or “molybdenum cow.” Tc-99m is then “milked” or eluted from this generator at the point of use. India’s Department of Atomic Energy (DAE) and its units, such as the Board of Radiation and Isotope Technology (BRIT), play a crucial role in the production and supply of medical radioisotopes like Mo-99, supporting diagnostic nuclear medicine procedures across the country.
Significant Applications of Technetium-99m (Tc-99m)
Technetium-99m is the most widely used medical radioisotope globally, accounting for approximately 80% of all diagnostic nuclear medicine procedures. Its short half-life of 6 hours and its emission of gamma rays (suitable for detection by external cameras) while emitting minimal particulate radiation make it ideal for diagnostic imaging, allowing for clear images with relatively low radiation exposure to the patient. While not directly handled by individuals in daily life, these medical applications are routine and profoundly impact patient care.
Bone Scans
Tc-99m is commonly incorporated into phosphate compounds which are then injected into the patient. These compounds accumulate in areas of high bone turnover. Bone scans using Tc-99m are critical for detecting fractures, bone infections (osteomyelitis), primary bone tumours, and metastases (spread of cancer to bone).
Cardiac Stress Tests
In cardiology, Tc-99m is tagged to specific agents (e.g., sestamibi or tetrofosmin) to assess blood flow to the heart muscle. During cardiac stress tests, imaging is performed both at rest and after exercise or pharmacological stress. This helps in diagnosing coronary artery disease by identifying areas of reduced blood flow or myocardial infarction (heart attack).
Thyroid Gland Imaging
Tc-99m pertechnetate is used for imaging the thyroid gland. The thyroid gland actively takes up pertechnetate ions, mimicking iodide uptake. This imaging helps in evaluating thyroid function, identifying thyroid nodules, goitre, and assessing conditions like hyperthyroidism or hypothyroidism.
Kidney Function Assessment
Tc-99m can be complexed with agents like DTPA (diethylenetriaminepentaacetic acid) or MAG3 (mercaptoacetyltriglycine) to assess kidney function. These radiopharmaceuticals are rapidly cleared by the kidneys, allowing for the measurement of glomerular filtration rate (GFR) or effective renal plasma flow (ERPF), and detection of urinary tract obstructions or renal artery stenosis.
Sentinel Lymph Node Mapping
In oncology, Tc-99m-labelled colloids are injected around a tumour. These particles drain into the lymphatic system, allowing surgeons to identify the “sentinel lymph node(s),” which are the first lymph nodes to which cancer cells are likely to spread from the primary tumour. Biopsy of these specific nodes helps in cancer staging, particularly in breast cancer and melanoma.
Other Specialized Applications
Corrosion Inhibition
Technetium, particularly in the form of pertechnetate (TcO4-), has been investigated and used as a highly effective corrosion inhibitor for steel. Its use is predominantly limited to closed systems, such as nuclear reactor cooling circuits, due to its radioactive nature and associated costs and safety considerations.
Catalytic Research
Some research has explored the catalytic properties of technetium and its compounds. It exhibits catalytic activity in certain chemical reactions, but its radioactivity and scarcity prevent widespread industrial application in this field.