Radiopharmacy Basics
On this page
Direct answer
Radiopharmacy prepares, dispenses and controls radioactive diagnostic and therapeutic agents, built around technetium-99m: a six-hour half-life, a 140 kilo-electron-volt gamma ideal for cameras, no beta particle, eluted each morning ("milking") from a molybdenum-99 generator — an alumina column holding the 66-hour parent, replaced roughly weekly. Lyophilised kits (MDP bone, DTPA and MAG3 renal, MAA lung perfusion, sestamibi cardiac, HMPAO brain) are reconstituted with the eluate, stannous chloride reducing pertechnetate so it binds the ligand; radiochemical purity must reach about 95 per cent by instant thin-layer chromatography, and the eluate is controlled for molybdenum breakthrough (not more than 0.15 microcurie per millicurie at administration) and aluminium (not more than 10 micrograms per millilitre). Activity runs in becquerels (one disintegration per second) or the legacy curie (3.7 × 10¹⁰ becquerels), and clearance combines with decay in the effective half-life, 1/Te = 1/Tp + 1/Tb. In India, the Atomic Energy Regulatory Board licenses radiopharmacies: lead shielding for gamma emitters, plastic for beta emitters, dose calibrators, ALARA discipline and decay-in-storage.
What you must remember
- Why Tc-99m is near-ideal: six-hour half-life long enough to image, short enough to limit dose; pure 140 keV gamma matched to cameras; versatile chemistry; no beta dose.
- Generator physics: Mo-99 (66-hour half-life) decays to Tc-99m on an alumina column; daily saline elution ("milking") yields sodium pertechnetate; elution efficiency should exceed about 90 per cent; generators are replaced roughly weekly.
- Kit chemistry: freeze-dried ligand plus stannous chloride (Sn²⁺) reducer; pertechnetate binds the chelator on reconstitution; post-reconstitution expiry is hours.
- Radiochemical purity control: ITLC separates free pertechnetate and hydrolysed-reduced technetium; acceptance about 95 per cent or higher before patient use.
- Generator eluate limits: molybdenum-99 breakthrough not more than 0.15 microcurie per millicurie of Tc-99m at administration time; aluminium not more than 10 micrograms per millilitre — both quotable numbers.
- Other nuclides: iodine-131 (about 8 days, beta plus gamma — hyperthyroidism, thyroid cancer), F-18 fluorodeoxyglucose PET (about 110 minutes, 511 keV), lutetium-177 and radium-223 in therapy.
- Half-life arithmetic: effective half-life from 1/Te = 1/Tp + 1/Tb — a Tc-99m agent with 30-hour biological clearance has Te of about 5 hours.
- Safety and regulation: AERB licensing in India; lead for gamma, perspex for beta to limit bremsstrahlung; ALARA by time, distance and shielding; waste decay-stored conventionally ten half-lives.
From morning elution to injected patient
The day begins at the generator: elution into an evacuated vial, activity assayed in the dose calibrator, yield logged. Two quick tests protect the day's patients — the eluate assayed behind a lead shield (molybdenum's higher-energy photons penetrate where technetium's do not) confirms breakthrough within the 0.15 microcurie per millicurie limit, and an indicator strip checks aluminium against the 10 microgram standard. Only then are kits reconstituted: pertechnetate into an MDP vial, then a drop chromatographed on ITLC — free pertechnetate runs with the solvent front, the bound complex stays put — and radiochemical purity, needing about 95 per cent, is signed off.
Dispensing follows the nuclear physician's prescription: an adult bone-scan dose of several hundred megabecquerels drawn shielded, assayed, labelled with radionuclide, activity, time and expiry, then released with its records. Behind it sits decay arithmetic — Tc-99m halves every six hours, so injection-time activity is calculated from the assay clock, and the effective half-life governs imaging and dosimetry. Staff work behind lead barriers with extremity dosimeters, and spent vials go to the decay store for about ten half-lives before disposal, all under AERB licence conditions.
Where students slip
Half-life arithmetic is the reliable separator: candidates average the half-lives instead of adding reciprocals — with Tp 6 hours and Tb 30 hours, 1/6 + 1/30 = 0.2 gives Te = 5 hours, not 18. Second, shielding logic is inverted: high-energy beta emitters get plastic, not lead, because dense metals convert beta energy into bremsstrahlung; lead is for gamma emitters. Third, "milking" means saline elution of daughter technetium from the molybdenum-loaded alumina column, with elution efficiency a measured parameter. Fourth, the molybdenum limit of 0.15 microcurie per millicurie applies at administration time — hence re-testing of decaying eluate. Finally, radiochemical purity (ITLC — the chemical form of technetium) differs from radionuclidic purity (the shielded assay — which radionuclide is present).
Frequently asked questions
Why is Tc-99m the workhorse of nuclear medicine?
Its six-hour half-life, pure 140 keV gamma emission matched to gamma cameras, negligible beta radiation and rich labelling chemistry suit most diagnostic scans while keeping patient dose low.
What is "milking" a generator?
Daily saline elution of daughter Tc-99m from the alumina column holding its 66-hour Mo-99 parent into an evacuated vial; efficiency above about 90 per cent is expected.
Why must macroaggregated albumin never be given in right-to-left shunts?
MAA particles of 10-100 micrometres lodge in pulmonary arterioles by design; a shunt lets them enter systemic circulation and embolise brain and coronaries.
What are the Mo-99 and aluminium limits for generator eluate?
Molybdenum-99 breakthrough not more than 0.15 microcurie per millicurie of technetium-99m at administration time, and aluminium not more than 10 micrograms per millilitre.
How is effective half-life calculated?
By reciprocal addition, 1/Te = 1/Tp + 1/Tb: a technetium agent with 6-hour physical and 30-hour biological half-lives has an effective half-life of 5 hours — never an average.