Radionuclide Imaging

Radionuclide imaging is a branch of nuclear medicine that uses radioactive atoms — called radionuclides or radioisotopes — introduced into the body to produce images of organs and tissues from the inside. Unlike X-ray or MRI, which show anatomy (what structures look like), radionuclide imaging reveals physiology and metabolism: how organs are actually functioning at the cellular level. A cancerous tumour, for example, often consumes glucose far faster than healthy tissue — radionuclide imaging can detect that difference long before the tumour is large enough to see on a structural scan. The fundamental principle is straightforward: a radioactive tracer is attached to a biologically active molecule that accumulates in a specific target organ or tissue type. As the radionuclide decays, it emits gamma rays — high-energy photons — that pass out of the body and are detected externally by a gamma camera or a ring of detectors. A computer reconstructs the pattern of detected photons into a two- or three-dimensional image showing where the tracer has collected. The brighter the signal, the more metabolic activity in that region. There are two main techniques. Single Photon Emission Computed Tomography (SPECT) uses a rotating gamma camera to capture multiple 2D projections that are reconstructed into 3D slices. Positron Emission Tomography (PET) uses radionuclides that emit positrons; each positron annihilates with a nearby electron to produce two gamma photons travelling in exactly opposite directions, which are detected simultaneously — a process called coincidence detection. PET offers higher spatial resolution and sensitivity than SPECT and is particularly powerful in oncology, cardiology, and neurology.

Key takeaways

Choosing the Tracer
Administering and Waiting
Detection — Gamma Camera (SPECT)
Detection — PET Coincidence Detection
Image Reconstruction and Interpretation

Choosing the Tracer

A radiopharmaceutical is designed by attaching a short-lived radionuclide to a carrier molecule. For PET brain and cancer scans, fluorine-18 (¹⁸F) is attached to deoxyglucose to make FDG (fluorodeoxyglucose). Cells that consume lots of glucose — like tumour cells or active neurons — absorb FDG but cannot metabolise it further, so it accumulates. For bone scans (SPECT), technetium-99m (⁹⁹ᵐTc) labelled to a phosphate compound is used because bone-forming cells absorb phosphate.

Administering and Waiting

The tracer is injected intravenously (occasionally inhaled or swallowed). The patient waits 30–90 minutes for the tracer to distribute and accumulate where needed. During this uptake phase, vigorous activity is avoided so muscles don't compete with the target tissue for glucose. The radiation dose is carefully calculated — typical effective doses are 4–10 mSv for a PET-CT scan, comparable to a few years of background radiation.

Detection — Gamma Camera (SPECT)

A gamma camera contains a large sodium iodide (NaI) crystal backed by an array of photomultiplier tubes (PMTs). Gamma photons from the patient cause the crystal to scintillate (emit flashes of visible light). The PMTs convert these flashes into electrical signals whose relative intensities pinpoint the photon's origin on the crystal face. A lead collimator — a grid of narrow parallel channels — ensures only photons travelling perpendicular to the detector are counted, giving spatial information. The camera rotates around the patient to collect projections from many angles for 3D SPECT reconstruction.

Detection — PET Coincidence Detection

In PET, the radionuclide (e.g. ¹⁸F) decays by emitting a positron (β⁺). The positron travels only 1–2 mm before annihilating with an electron, producing two 511 keV gamma photons emitted at exactly 180° to each other. A ring of detectors surrounds the patient; when two detectors fire within a narrow time window (~10 ns), the system records a coincidence event — the annihilation occurred somewhere along the line connecting the two detectors. Millions of such lines are mathematically back-projected to form the image. No collimator is needed, making PET inherently more sensitive than SPECT.

Image Reconstruction and Interpretation

Raw coincidence or projection data is processed using algorithms such as filtered back-projection or iterative reconstruction (OSEM). The result is a colour-coded map of tracer concentration: 'hot spots' indicate high metabolic activity (possible tumour, infection, or seizure focus) while 'cold spots' may indicate dead tissue or poor perfusion. PET is almost always combined with CT (PET-CT) so that functional metabolic data is overlaid on precise anatomical structure, allowing clinicians to locate exactly which anatomical feature corresponds to the metabolic signal.

The Key Equation

A(t) = A_0 \, e^{-\lambda t}, \quad \lambda = \frac{\ln 2}{T_{1/2}}

A(t) — Activity at time t (Bq (becquerel)): Number of nuclear decays per second at time t; determines the photon emission rate and hence image quality.
A_0 — Initial activity (Bq): Activity of the tracer at the moment it is administered to the patient.
\lambda — Decay constant (s⁻¹): Probability per unit time that a given nucleus will decay; specific to each radionuclide.
T_{1/2} — Physical half-life (s (or min, h)): Time for half the nuclei to decay. ¹⁸F has T₁/₂ ≈ 110 min; ⁹⁹ᵐTc has T₁/₂ ≈ 6 h.
t — Elapsed time (s): Time since administration of the tracer.

Worked Example

Problem. A PET department receives a batch of fluorine-18 FDG with an initial activity of A₀ = 8.0 GBq at 08:00. The half-life of ¹⁸F is 110 minutes. A patient scan is scheduled for 10:30. What is the activity of the sample at scan time, and what fraction of the original activity remains?

A(150) = 8.0 \times 10^9 \cdot e^{-0.945} \approx 3.1 \text{ GBq}

Answer. The activity at scan time is approximately 3.1 GBq, which is 38.9% of the original sample.

Figure

Diagram

Origins · 1 minRosalyn Sussman Yalow and how this was discovered

Rosalyn Sussman Yalow

1921–2011 · American

Rosalyn Yalow co-developed Radioimmunoassay (RIA) in the 1950s alongside Solomon Berson — a revolutionary technique using radioactive tracers to measure vanishingly small concentrations of biological substances such as hormones in blood. RIA became the conceptual and technical foundation for using radiopharmaceuticals as tracers in modern nuclear medicine imaging. She was awarded the Nobel Prize in Physiology or Medicine in 1977. Yalow was the first woman to receive the Albert Lasker Award for Basic Medical Research (1976) and faced significant gender discrimination throughout her early career — she was initially denied a position because she was a woman, but this did not stop her from becoming one of the most influential medical physicists of the 20th century.

X-ray Imaging — X-rays produce structural anatomical images through differential absorption; comparing X-ray with PET highlights the distinction between anatomy and functional imaging.
MRI (Magnetic Resonance Imaging) — MRI also produces detailed soft-tissue images non-invasively; PET-MRI hybrid systems combine metabolic and anatomical information without ionising radiation for structural scans.
Radioactive Decay — The decay law A(t) = A₀e^(−λt) underpins all radionuclide imaging: choosing the right half-life balances adequate imaging time against radiation dose.
Radiation Dose and Safety — Understanding absorbed dose, effective dose, and ALARA (As Low As Reasonably Achievable) principles is essential for safe use of radiopharmaceuticals in patients.