Radionuclide Imaging
Every imaging method covered so far in this module works the same way in principle: send something into the body from outside — X-rays, ultrasound, a magnetic field and radio pulses — and detect what comes back out. Radionuclide imaging turns that idea inside out. Instead of probing the body from the outside, it puts a small radioactive source inside the patient and detects the radiation it emits from within.
The patient as the source
A radionuclide (a radioactive isotope chosen for medical use) is introduced into the patient's body, usually by injection, and allowed to travel to and accumulate at a particular target organ or tissue. Rather than shining radiation through the body and measuring transmission, absorption or reflection, radionuclide imaging detects the gamma radiation the tracer itself emits from inside the body, using an external detector. Wherever the tracer has accumulated in greatest concentration, the emitted radiation is most intense — so mapping the intensity of radiation arriving at the detector, position by position, builds up an image showing where the tracer has gathered, which in turn reveals the structure or function of the target organ.
This is a genuinely different physical strategy from every other technique in this module. A plain X-ray radiograph or a CT scan images differential attenuation of an external beam passing through different tissues; ultrasound images reflection of an external beam at tissue boundaries; MRI images the response of hydrogen nuclei to an external magnetic field and radio pulse. Radionuclide imaging instead makes the patient's own body the emitting source, and simply detects what comes out.
Three named radionuclides
Three particular radionuclides are used repeatedly in practice, each suited to a different clinical purpose because of their differing emission type, half-life and gamma energy.
Technetium-99m (Tc-99m)
| Property | Tc-99m |
|---|---|
| Emission type | Gamma only |
| Half-life | ≈ 6 hours |
| Gamma energy | Well matched to detection by a gamma camera |
Tc-99m is by far the most widely used radionuclide in medical imaging. It is a pure gamma emitter, so it delivers essentially no unnecessary particle dose to the patient's tissue, and its gamma photon energy is well suited to efficient detection by a gamma camera (the instrument covered in full in the next-but-one lesson). Its short half-life of around six hours means it decays away quickly once its clinical job is done, keeping the patient's radiation exposure low — but that same short half-life creates a supply problem, addressed below by the molybdenum–technetium generator.
Iodine-131 (I-131)
| Property | I-131 |
|---|---|
| Emission type | Beta and gamma |
| Half-life | ≈ 8 days |
| Gamma energy | Suitable for external detection (imaging use) |
I-131 emits both beta and gamma radiation, and has a considerably longer half-life than Tc-99m, around eight days. Because iodine is an element the body naturally takes up into the thyroid gland (to make thyroid hormones), I-131 is historically important for both imaging and treating the thyroid: its gamma emission allows the thyroid to be imaged externally, while its beta emission — which deposits its energy locally, over a short range — can be used therapeutically to damage overactive or cancerous thyroid tissue from within. This dual imaging/therapy role, both arising from the same natural uptake mechanism, makes I-131 a distinctive case among medical radionuclides.
Indium-111 (In-111)
| Property | In-111 |
|---|---|
| Emission type | Gamma only |
| Half-life | ≈ 2.8 days |
| Gamma energy | Suitable for gamma camera detection |
In-111 is also a gamma emitter, but with a half-life of around 2.8 days — much longer than Tc-99m's six hours, though shorter than I-131's eight days. This longer half-life makes In-111 useful for tracking processes in the body that unfold over a longer timescale than Tc-99m's rapid decay would allow, such as monitoring how a labelled compound distributes and migrates through the body over one or more days.
Labelling to target organs
A radionuclide on its own has no way of "knowing" which organ to travel to — a free technetium or iodine atom, injected alone, would simply distribute through the body in whatever way its own chemistry dictates, which is rarely the organ a clinician actually wants to image. The solution is labelling: chemically attaching the radionuclide to a carrier molecule or compound that the body naturally directs to a specific target organ or tissue.
- A compound naturally taken up by bone can be labelled with a suitable radionuclide to image the skeleton and detect abnormal bone activity.
- A compound naturally taken up by the thyroid (exploiting the body's own iodine uptake pathway) can be labelled to image thyroid structure and function.
- A compound preferentially taken up by a particular type of tumour cell can be labelled to reveal the location and extent of that tumour.
Once labelled, the radionuclide is carried to and concentrates at the target site by the carrier compound's own natural biological behaviour, and it is only there that its emitted radiation reveals anything useful once detected externally. The choice of carrier compound, not the radionuclide alone, is what determines which organ or tissue actually gets imaged.
A radionuclide by itself only tells you that radioactive decay is happening somewhere in the body. It's the labelling compound that turns this into useful medical information, by ensuring the radionuclide accumulates specifically at the organ or tissue of clinical interest — the radiation detected externally then reveals the structure or function of that target, not just "radioactivity in general".
The molybdenum–technetium generator
Tc-99m's six-hour half-life is exactly what makes it clinically attractive (low patient dose, quick decay) — but that same short half-life makes it impossible to manufacture centrally and transport to hospitals in the ordinary way, since a useful fraction would decay away before it ever reached a patient. The practical solution is to produce Tc-99m on-site, at the hospital, exactly when it's needed, using a molybdenum–technetium generator.
Molybdenum-99 (Mo-99), the parent isotope, has a much longer half-life (around 66 hours) and continually decays into Tc-99m inside a sealed generator unit. Because Mo-99's half-life is so much longer than Tc-99m's, a generator can keep producing fresh Tc-99m for well over a week before it needs replacing. Periodically, hospital staff "milk" the generator — extracting the Tc-99m that has accumulated, using a simple chemical elution process — for immediate clinical use, while the Mo-99 parent remaining in the sealed unit continues decaying and producing more Tc-99m ready for the next extraction.
This generator system elegantly solves the short-half-life supply problem: Tc-99m never has to be transported any real distance while radioactive, since it's generated fresh at the point of use, but the hospital only ever needs to be supplied with the far more manageable, longer-lived Mo-99.
PET: positron emission tomography
Positron emission tomography (PET) uses a fundamentally different detection principle from the single-gamma-photon imaging described above, built around a different type of radioactive decay. Certain radionuclides decay by positron emission (beta-plus decay), releasing a positron — the electron's antiparticle. This positron travels only a very short distance through tissue before it encounters an ordinary electron, and the two undergo annihilation: their combined mass-energy converts entirely into two gamma-ray photons, emitted in almost exactly opposite directions to conserve momentum.
A ring of detectors surrounds the patient during a PET scan, and electronics record coincidence events — pairs of gamma photons detected at two roughly opposite points on the ring at essentially the same instant. Because the two photons from a single annihilation event travel in (very nearly) exactly opposite directions, the straight line joining the two detectors that registered a coincident pair must have passed through the point where the annihilation occurred. A single coincidence event only narrows the annihilation point down to somewhere along that line, but combining very many coincidence events, from many annihilation events occurring throughout the scan, allows the computer to reconstruct the full three-dimensional distribution of tracer accumulation in the body.
This coincidence principle — inferring position from a line connecting two simultaneous detections, rather than from the direction a single detected photon happened to travel through a collimator — is what makes PET a genuinely distinct radionuclide imaging technique from gamma-camera imaging of a single-gamma emitter such as Tc-99m.
Key takeaways
- Radionuclide imaging introduces a radioactive tracer into the body and detects the radiation it emits from inside, rather than passing an external beam through the body.
- Tc-99m: gamma emitter, half-life ≈ 6 hours, gamma energy well suited to gamma camera detection — the most widely used medical radionuclide.
- I-131: beta and gamma emitter, half-life ≈ 8 days, historically used for both imaging and (via its beta emission) therapy of the thyroid.
- In-111: gamma emitter, half-life ≈ 2.8 days, useful for tracking processes over longer periods than Tc-99m allows.
- Labelling attaches a radionuclide to a carrier compound naturally taken up by a specific target organ or tissue, so the radionuclide accumulates where it's clinically useful.
- The molybdenum–technetium generator produces short-lived Tc-99m on-site from longer-lived Mo-99, solving the supply problem created by Tc-99m's short half-life.
- PET relies on positron emission and electron-positron annihilation, producing two oppositely-directed gamma photons detected in coincidence by a detector ring.
- Combining many coincidence events lets a PET scanner reconstruct the 3D location of tracer accumulation — a distinct detection principle from single-photon gamma imaging.