Radiochemistry of Technetium-99m

Sodium (Tc-99m) Pertechnetate

• Chemical element with atomic number 43, a transition metal in group VIIA with Mn, Tc, and Re
• Formula: Na^99mTcO₄
• Structure: Contains the pertechnetate anion, TcO₄^-
• TcO₄^- is a large monovalent oxyanion with technetium (Tc) in the highest oxidation state of +7
• Oxidation state of Tc in TcO₄ is +7; +4 in absence of chelating agents
• Other oxidation states include +5, +3, and +1, often found in presence of chelating agents.
• ^99mTc(VII) is readily reduced by reducing agents, most commonly SnCl₂·2H₂O (stannous chloride dihydrate)
• Readily bound by chelating agents in reduced states
• Forms the basis of many radiopharmaceuticals
• ^99mTcO₄^- is hydrophilic/lipophobic
• Electron configuration of Tc (Z = 43): [Kr]4d⁵5s²
• Molecular weight of TcO₄^- anion is approximately 163 Daltons; the full molecular weight of sodium pertechnetate (NaTcO₄) is slightly higher, accounting for the sodium cation
• Classified as a simple salt
• Prepared by eluting from a ⁹⁹Mo/^99mTc generator using 0.9% normal saline as an eluent
• Sterile, non-pyrogenic, physiologically compatible solution
• Concentration ~ 10^-9M ^99mTcO₄^-

Physical Characteristics

• ^99mTc decays to ⁹⁹Tc via isomeric transition
• ^99mTc is a metastable nuclear isomer of technetium-99, "m" indicating it's in an excited state compared to its stable isotope, ⁹⁹Tc
• During isomeric transition, ^99mTc releases energy in the form of gamma radiation (140 keV photon) for medical imaging.
• Half-life of ^99mTc is approximately 6.02 hours
• ⁹⁹Tc, a different form of technetium has a significantly longer half-life (2.12 x 10⁵ years).
• ⁹⁹Tc decays via beta emission to form ⁹⁹Ru (ruthenium)
• Half-life of ⁹⁹Mo is 67 hours (2.8 days)

Primary Radiation

• Primary radiation is gamma (γ) at an energy of 140 keV
• Silver-gray metal
• Insoluble in hydrochloric acid

Quality Control

• Visual inspection: Eluate should be clear, colorless, and free from visible foreign particulate matter
• Radiochemical purity (RP) is ≥ 95% ^99mTcO₄^- using ITLC-SG/saline
• ⁹⁹Mo content is < 0.15 kBq ⁹⁹Mo per 1.0 MBq ^99mTc at the time of administration
• Al³⁺ content is < 10 µg/mL (calorimetric test)
• pH: 4.5 to 7.5
• Radionuclidic purity will be less than HPB limits at the time of administration
• Expiry is 8 to 18 hours after elution
• Sterility and pyrogenicity should be retrospectively checked

Pharmacokinetics

• Routes of administration: intravenous (primary) and oral
• Pertechnetate ion rapidly diffuses from the gastrointestinal tract into the circulatory system, based on concentration gradients.

Biodistribution

• Tissue and blood distribution
• Binding to plasma proteins
• Rapidly equilibrates with extracellular space
• Equilibrates with red blood cells
• Excreted by the kidneys
• Localizes in: thyroid gland (active transport into follicular cells), gastric mucosa (actively concentrated by mucoid cells), salivary glands (actively concentrated by salivary gland ductal epithelial cells), choroid plexus, mucous membranes, sweat glands, brain lesions (simple diffusion across compromised BBB)

Elimination

• Excreted via kidneys (urine) and intestines/feaces

Test your knowledge on the radiochemistry of technetium-99m, including its modes of decay, chemical properties, and applications. This quiz covers essential facts about its half-life, oxidation states, and common reducing agents relevant to its use in medical imaging. Perfect for students and professionals in the field of radiopharmacy.