Nuclear medicine foundations

In short: Nuclear medicine uses small amounts of radioactive compounds to image how the body functions, and in some cases to treat disease from the inside.

Function, not just structure

CT and MRI mainly show anatomy. Nuclear medicine shows physiology: where a compound goes, how fast, and how much accumulates. A bone scan shows bone turnover, an FDG PET scan shows glucose uptake, and a perfusion scan shows blood flow.

The tracer principle

The radioactive compound is given in such a small mass that it does not disturb the process it measures. It simply follows the biology and, because it emits radiation, can be detected from outside the body. This idea, developed in the early twentieth century, is the foundation of the field.

Imaging and therapy

The same principle can deliver radiation where it is needed. Radioiodine therapy for thyroid disease is the classic example; radioligand therapies that target tumour receptors are newer ones.

In short: How radionuclides decay, what half-life means, and where medical radionuclides come from.

Ways to decay

• Positron (β+) emission: the positron meets an electron and annihilates, producing two 511 keV photons travelling in nearly opposite directions. PET detects these pairs.
• Gamma emission: a photon leaves the nucleus. Technetium-99m emits a gamma ray of about 140 keV, well suited to gamma cameras.
• Beta-minus (β−) emission: an electron with a range of millimetres in tissue, used in therapy (for example lutetium-177, yttrium-90, iodine-131).
• Alpha emission: a heavy particle with a range of tens of micrometres and very dense energy deposition, used in targeted alpha therapy research and in radium-223.
• Electron capture and Auger electrons: very short-range electrons, studied for therapy at cellular scales.

Half-life

The physical half-life is the time for half of the atoms to decay. It ranges from about two minutes for oxygen-15 to about eight days for iodine-131 among common medical radionuclides. Short half-lives limit how far a product can travel; long ones affect handling and waste.

Where they come from

Reactors produce radionuclides such as molybdenum-99 (the parent of technetium-99m), lutetium-177 and iodine-131. Cyclotrons produce fluorine-18, carbon-11, copper-64 and zirconium-89. Generators hold a longer-lived parent that decays into a short-lived daughter which can be eluted on site, as with molybdenum-99/technetium-99m and germanium-68/gallium-68.

Takeaways

• Half-life values quoted in RadioGx are approximate reference values; the IAEA LiveChart is the authoritative source.

In short: The physics that makes a radionuclide good for imaging is different from the physics that makes it good for therapy.

For imaging: photons that escape

Imaging needs radiation that leaves the body and reaches a detector: gamma rays or annihilation photons. The aim is the useful image at the lowest practical radiation exposure, so short half-lives and minimal particle emission are preferred.

For therapy: energy that stays

Therapy needs radiation that deposits energy close to where the compound accumulates. Beta particles travel millimetres, alpha particles tens of micrometres. Alpha particles have high linear energy transfer, causing dense damage along a short track.

Both at once

Some therapeutic radionuclides also emit photons. Lutetium-177 and iodine-131 can be imaged after treatment, which lets teams see where the therapy went. This is part of what makes theranostics possible.

In short: How PET uses coincidence detection, how SPECT uses collimators and rotation, and what planar imaging still does well.

PET

A ring of detectors records photon pairs that arrive at almost the same time. Each pair defines a line of response through the body; millions of lines are reconstructed into a 3D image. Time-of-flight PET also uses the tiny difference in arrival times to localise each event more precisely along its line.

SPECT and the gamma camera

Single-photon emitters such as technetium-99m are imaged with a gamma camera. A collimator, a thick plate of holes, accepts only photons travelling in certain directions, which forms an image but discards most photons. SPECT rotates the camera around the patient to reconstruct a 3D volume.

Planar imaging

A single static or whole-body view from a gamma camera is planar scintigraphy. Bone scans and thyroid scans are often planar, sometimes with SPECT added for selected regions.

In short: PET/CT, SPECT/CT and PET/MR pair function with anatomy. Radiation protection principles keep exposures as low as reasonably achievable.

Why combine scanners

CT supplies anatomy and a map of how tissue attenuates photons, which is used to correct the nuclear medicine image so that quantification is more accurate. MRI offers soft-tissue contrast without additional ionising radiation, at the cost of more complex attenuation correction.

Radiation protection, briefly

Facilities apply three principles: justification (the exam should do more good than harm), optimisation (as low as reasonably achievable) and dose limits for workers and the public. Time, distance and shielding are the everyday tools. National regulators and radiation safety officers set and enforce the rules.