Megabecquerel to Microcurie

Diagnostic imaging doses fall neatly in the MBq range — a PET scan uses 185–370 MBq, a bone scan 500–800 MBq. Using becquerels would mean writing hundreds of millions; using gigabecquerels would mean awkward decimals like 0.37 GBq. MBq is the Goldilocks unit for the hospital pharmacy: large enough to avoid scientific notation, small enough to express a single patient dose as a tidy number on a syringe label.

That depends entirely on the isotope. Technetium-99m, the workhorse of diagnostic imaging, has a 6-hour half-life — so a 740 MBq injection drops to 370 MBq in 6 hours, 185 MBq in 12, and becomes negligible within 2 days. Fluorine-18 (used in PET) has a 110-minute half-life and is essentially gone in a day. Iodine-131 (used in therapy) lingers for about 8 days per half-life. Hospitals choose isotopes partly based on how fast they want the activity to vanish.

Most diagnostic isotopes (Tc-99m, F-18) have half-lives under a day, so hospitals simply store waste in shielded bins and let it decay. After 10 half-lives — about 3 days for Tc-99m — the activity is down to less than 0.1% of the original and can be disposed of as normal clinical waste. Longer-lived therapeutic isotopes like I-131 require weeks of decay storage. The vast majority of nuclear medicine waste is never shipped to a radioactive disposal site; it just sits in a locked closet until physics solves the problem.

A patient injected with 370 MBq of F-18 for a PET scan emits gamma rays at a dose rate of roughly 5–6 µSv/hr at one meter. That means sitting next to them for two hours gives you about 10–12 µSv — less than a chest X-ray. Staff handle dozens of patients daily so they follow time-and-distance protocols, but for family members the exposure from a single visit is trivially small. The activity halves every 110 minutes, so by evening the patient is barely distinguishable from background.

Molybdenum-99, which decays into the technetium-99m used in 30+ million scans per year worldwide, can only be produced in a handful of aging research reactors. It has a 66-hour half-life so it cannot be stockpiled — you have to make it, ship it, and use it within days. When a reactor goes down for maintenance (as happened in 2009 when both the Canadian NRU and Dutch HFR shut down simultaneously), hospitals worldwide face scan cancellations within a week. New production methods using particle accelerators and LEU targets are slowly diversifying supply.

Because microcuries are the threshold where regulatory accountability begins for most isotopes. A lab ordering 250 µCi of P-32 must log the receipt, track usage, survey for contamination weekly, monitor personnel doses, and account for every fraction disposed of or decayed. Multiply that by dozens of labs across a campus, each using different isotopes with different rules, and you get a full-time radiation safety program. The obsession is not about the hazard of any single vial — it is about preventing the slow accumulation of untracked material that eventually leads to a contamination incident or regulatory violation.

It depends on what the isotope emits. A 100 µCi tritium source needs no shielding at all — the beta particles cannot penetrate a sheet of paper. A 100 µCi phosphorus-32 source (high-energy beta) needs about 1 cm of acrylic to stop the betas, but acrylic is preferred over lead because lead produces bremsstrahlung X-rays from energetic betas. A 100 µCi caesium-137 source (gamma emitter) needs a thin lead container. At microcurie levels the shielding is lightweight and portable — nothing like the heavy lead pigs used for millicurie medical sources.

Most check sources contain 0.1–1 µCi of caesium-137, chosen because Cs-137 has a convenient 662 keV gamma ray and a 30-year half-life — long enough that the source maintains predictable activity for decades without frequent recalibration. The activity is high enough to produce a clear above-background reading (several hundred counts per minute) but low enough to be exempt from most transport regulations. Technicians hold the check source near the detector before each use to verify the instrument is responding. If the reading is off by more than 10–20% from the expected value, the instrument goes back for calibration.

Not from external exposure — the dose rates are far too low. At 1 meter from a 500 µCi unshielded Cs-137 source, the dose rate is about 1.6 µSv/hr, which is only a few times background. The danger from microcurie quantities comes from internal exposure: inhaling or ingesting even micrograms of an alpha emitter like polonium-210 or americium-241 can deliver a concentrated dose to lung or gut tissue. Alexander Litvinenko was killed by roughly 26 µCi of Po-210 dissolved in tea — a quantity invisible to the eye.

Autoradiography uses radioactive decay to make an image — you label DNA or protein with P-32, separate the molecules on a gel, press the gel against X-ray film or a phosphor screen, and the beta particles expose the film wherever your target molecule sits. A typical experiment uses 50–250 µCi, which gives a visible image in hours to overnight. P-32 is favored because its high-energy beta (1.7 MeV) produces sharp, high-contrast bands without the weeks-long exposure times that weaker emitters like S-35 or C-14 require.