Radiochemistry of Technetium-99m

Study Notes

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^-

^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)

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

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

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)