Radiopharmaceuticals and Medical Imaging

Introduction

Nuclear medicine turns radiochemistry into a window on the living body. A tiny quantity of a radioactive compound, usually nanomoles or less, is injected; it concentrates in a tissue according to its chemistry and biology, and its emissions are imaged from outside. The images show function rather than anatomy: how fast a tumour consumes glucose, whether heart muscle is perfused, how bone is remodelling. This page explains the physics of the two main techniques and the chemistry that attaches a radionuclide to a useful molecule.

Core explanation

Choosing a radionuclide. A diagnostic radionuclide should emit radiation that escapes the body and can be detected, deliver as little dose as possible, and have a half-life matched to the biological process — long enough for synthesis, delivery and imaging, short enough to limit dose. Alpha and beta-minus emitters deposit their energy locally, so they are unsuitable for imaging but useful for therapy.

PET. A positron emitter such as fluorine-18 (t½ ≈ 110 min) emits a positron, which travels about a millimetre in tissue, slows and annihilates with an electron. Two 511 keV photons fly out in almost opposite directions. A ring of detectors registers them in coincidence , and each pair defines a line along which the decay occurred. Reconstructing millions of lines gives a three-dimensional map of tracer concentration. Other PET nuclides include carbon-11 (20 min), nitrogen-13 (10 min), oxygen-15 (2 min) and gallium-68 (68 min).

SPECT. A gamma emitter such as technetium-99m (t½ ≈ 6.0 h, 140 keV gamma) is imaged by a gamma camera with a lead collimator that accepts only photons travelling in particular directions. Rotating the camera around the patient gives tomographic slices. The 140 keV photon is ideal: energetic enough to leave the body, low enough to be collimated and detected efficiently. Technetium-99m is a metastable isomer that decays by isomeric transition with essentially no particle emission, keeping the dose low.

The technetium generator. Molybdenum-99 (t½ ≈ 66 h) is adsorbed on an alumina column, where it decays to technetium-99m. Because the parent lives longer than the daughter, transient equilibrium develops. Saline washed through the column elutes pertechnetate, TcO₄⁻, while molybdate stays bound. The daughter regrows between elutions, so a hospital has a supply for about a week.

Labelling chemistry. Fluorine-18 is produced as aqueous fluoride and is a poor nucleophile in water because it is strongly hydrated. It is dried and activated with a phase-transfer agent (for example a cryptand with potassium carbonate), then substituted onto a precursor bearing a leaving group such as triflate. For [¹⁸F]FDG, a glucose analogue, the fluoride displaces triflate by S N2 attack, and protecting groups are then removed. The whole synthesis must be complete, purified and quality-controlled within about one or two half-lives, which is why it is automated. Technetium-99m labelling is coordination chemistry: pertechnetate (Tc(VII)) is reduced, typically with tin(II) chloride, in the presence of a ligand supplied in a freeze-dried "kit", giving a complex whose charge, lipophilicity and shape determine where it goes in the body.

Therapy and theranostics. Beta-minus emitters such as iodine-131 and lutetium-177, or alpha emitters such as radium-223, destroy nearby cells. Pairing ⁶⁸Ga for imaging with ¹⁷⁷Lu for therapy on the same peptide lets clinicians image a tumour and then treat it with the same targeting molecule.

Formulae

Positron annihilation: e⁺ + e⁻ → 2γ, each photon 511 keV (m ec²).

Effective half-life: 1/T eff = 1/T phys + 1/T biol.

Step-by-step reasoning

To assess whether a radionuclide suits imaging:

1. Identify the emission: positron or gamma for imaging; alpha or beta-minus for therapy. 2. Check the photon energy (about 100–200 keV for SPECT; 511 keV for PET). 3. Compare the half-life with the synthesis time and the biological process. 4. Consider the chemistry available for attaching it to a targeting molecule. 5. Consider production: generator, cyclotron or reactor, and distance to the patient.

Visual explanation

Imagine a ring of detectors around a patient. Each positron annihilation fires two detectors on opposite sides at the same instant, and a straight line is drawn between them. Where thousands of lines cross, the tracer is concentrated — like finding a lamp in a dark room from many pairs of light rays.

Real-world analogy

A technetium generator is like a cow that is milked each day: the parent keeps producing the daughter, the milk is drawn off when needed, and it builds up again ready for the next milking. Hospital staff indeed call it a "moly cow".

Real-world example

[¹⁸F]FDG is taken up by cells in proportion to glucose use and is phosphorylated but not metabolised further, so it becomes trapped. Many cancers consume glucose rapidly and appear as bright spots on FDG-PET scans, which are used to stage tumours and monitor treatment response.

Why?

Why are PET nuclides so short-lived? Positron emitters are proton-rich light nuclides with large decay energies and short half-lives. Short half-lives give high activity from few atoms and low overall dose, but they force production close to the hospital and rapid, automated chemistry.

Common misconception

"A patient becomes highly radioactive for a long time after a scan." The activities used are chosen to minimise dose, and the combination of physical decay and biological excretion reduces activity quickly; for technetium-99m and fluorine-18 most activity is gone within a day.

Worked example

Question: A dose of [¹⁸F]FDG has an activity of 400 MBq at the end of synthesis. What is its activity 220 minutes later? (t½ = 110 min)

Reasoning: 220 minutes is two half-lives, so A = 400 × (1/2)² = 100 MBq.

Answer: 100 MBq — showing why transport time matters.

Quick check

1. Why is technetium-99m well suited to SPECT imaging? Answer: It emits a single 140 keV gamma with little particle radiation and has a six-hour half-life that fits imaging while limiting dose.

Exam focus

Be able to explain positron annihilation and coincidence detection in PET, the operation of the ⁹⁹Mo/⁹⁹ᵐTc generator in terms of transient equilibrium, and the reasons for choosing a given nuclide. Use the effective half-life equation and simple decay calculations with care over units of time.

Advanced insight

The chemistry of fluorine-18 labelling happens under unusual conditions: the fluoride is present at nanomolar concentration while the precursor is in huge excess, so reactions are pseudo-first-order in fluoride and side products derived from the precursor dominate purification. Newer methods, such as copper-mediated fluorination of arylboronic esters, allow ¹⁸F to be placed on electron-rich aromatic rings that resist classical nucleophilic substitution.

Summary

Radiopharmaceuticals combine a radionuclide with a targeting molecule. PET detects pairs of 511 keV photons from positron annihilation; SPECT images single gamma photons, most often from technetium-99m supplied by a molybdenum generator in transient equilibrium. Fluorine-18 is attached by rapid nucleophilic substitution; technetium-99m by reduction and coordination. Beta-minus and alpha emitters are used for therapy.

Practice questions

1. Why are the two annihilation photons emitted in almost opposite directions? Answer: Momentum is conserved; the positron and electron are nearly at rest, so the two equal-energy photons must travel in opposite directions. 2. Explain why aqueous fluoride-18 must be dried and activated before labelling. Answer: In water, fluoride is strongly hydrogen-bonded and a poor nucleophile; removing water and using a phase-transfer agent makes it reactive. 3. A radiopharmaceutical has physical half-life 6.0 h and biological half-life 12 h. Find the effective half-life. Answer: 1/T eff = 1/6 + 1/12 = 3/12, so T eff = 4.0 h. 4. Why is an alpha emitter used for therapy but not for imaging? Answer: Alpha particles travel only micrometres in tissue, so they cannot be detected outside the body but deposit intense local damage in target cells.