2.1 Radioactive Materials, Isotopes, and Decay Modes

Radioactive Materials, Isotopes, and Decay Modes

Quick Answer: Nuclides used in nuclear medicine are unstable nuclei that transform by α, β−, β+, electron capture, isomeric transition, or gamma emission. Know each mode’s product and the exam’s flagship examples: Tc-99m (IT/140 keV γ, T½ ≈ 6.02 h), F-18 (β+/PET, T½ ≈ 110 min), I-131 (β− + γ, therapy/imaging), and Ra-223 (α, bone therapy). Convert activity with 1 mCi = 37 MBq and 1 Ci = 3.7 × 10¹⁰ Bq.

Atoms, Nuclides, and Isotopes

An atom is defined by its atomic number (Z) — the number of protons — and its mass number (A) — protons plus neutrons. A nuclide is a specific nuclear species with a given Z and A (for example, ⁹⁹ᵐTc or ¹⁸F). Isotopes of an element share the same Z but differ in neutron number (and therefore A). Technetium has many isotopes; only a few (notably Tc-99m and Tc-99) matter in nuclear medicine technology (NMT).

Stable nuclides do not change spontaneously. Radioactive nuclides (radionuclides) transform until they reach a more stable configuration. That spontaneous transformation is radioactive decay, and the rate is a nuclear property independent of chemical form, temperature, or pressure under ordinary clinical conditions. The daughter may itself be radioactive (decay chain) or stable.

Half-Life and Decay Energy

Physical half-life (T½) is the time required for activity to fall to half its initial value. Half-lives range from seconds (e.g., Rb-82 ≈ 75 s) to days (I-131 ≈ 8.0 d) to years (Co-57 ≈ 272 d for camera QC sources). On the CNMT exam and in the clinic you memorize the workhorses:

Decay energy is the energy released per transformation (keV or MeV). Photon energy determines collimator choice, window settings, and penetration. Particle energy determines range in tissue and therapeutic effect. You do not need to memorize every MeV value, but you must link mode → product radiation for the common agents.

Modes of Decay

1. Alpha (α) decay. The nucleus ejects a helium nucleus (2 protons + 2 neutrons). α particles are heavy, highly ionizing, and travel only tens of micrometers in tissue — excellent for therapy when localized, poor for external imaging. Ra-223 dichloride (Xofigo) is the classic NMT α therapy example; its daughters also emit radiation, so radiation-safety handling differs from pure γ emitters.

2. Beta-minus (β−, negatron) decay. A neutron converts to a proton; an electron and antineutrino are emitted. Z increases by 1. Continuous energy spectrum up to E_max. I-131 is the primary β− therapeutic example; its accompanying γ allows post-therapy imaging and requires shielding and distance rules different from pure β emitters.

3. Beta-plus (β+, positron) decay. A proton converts to a neutron; a positron and neutrino are emitted. Z decreases by 1. The positron annihilates with an electron, producing two 511 keV photons ~180° apart — the basis of PET coincidence imaging. F-18 is the flagship PET radionuclide; others include C-11, N-13, O-15, Ga-68, and Rb-82.

4. Electron capture (EC). An orbital electron is captured by the nucleus; a proton becomes a neutron. Z decreases by 1. Characteristic x-rays and Auger electrons follow. Many NMT nuclides mix EC with other modes (e.g., I-123 primarily EC/γ; Tl-201 primarily EC).

5. Isomeric transition (IT) and gamma emission. An excited nucleus drops to a lower energy state by emitting a γ photon (or by internal conversion). When the excited state is long-lived enough to be treated as a separate species, it is metastable — marked with m, as in Tc-99m. Tc-99m → Tc-99 by IT, emitting primarily a 140 keV photon (high internal-conversion fraction also produces conversion electrons and characteristic x-rays). Metastable generators (Mo-99 → Tc-99m) exploit this.

6. Internal conversion. Instead of emitting a γ, the nucleus transfers energy to an orbital electron that is ejected. Important for dosimetry and for understanding that “γ emitters” still produce local electron dose.

Activity Units

Activity (A) is the number of decays per unit time.

Clinic shorthand: a typical adult FDG dose might be ~10 mCi ≈ 370 MBq; a bone scan dose might be ~20–25 mCi Tc-99m MDP. Always convert carefully when reading European literature (MBq) versus U.S. labels (mCi).

Why Mode Matters Clinically

• Imaging modality: γ → Anger camera/SPECT; β+ → PET; α → therapy only (no diagnostic image from α itself).
• Shielding: high-energy γ (I-131 364 keV, PET 511 keV) needs thicker lead/tungsten than Tc-99m 140 keV; β needs low-Z (plastic) first to limit Bremsstrahlung, then lead as needed.
• Biological effect: α and β deposit energy locally (therapy); γ deposits energy over longer paths and is better for external detection.

Master the table of workhorse nuclides, the six decay modes, and unit conversions — Domain I repeatedly tests exactly these links.