Gamma Camera & PET Scanning

A gamma camera is a nuclear medicine device that detects gamma photons emitted by a radioactive tracer inside a patient's body and converts them into a two-dimensional image. Unlike X-ray imaging, which uses radiation transmitted through the body, gamma imaging relies on radiation emitted from within.

The patient is injected with a radiopharmaceutical — a radioactive isotope attached to a biologically active molecule. The tracer accumulates in specific tissues depending on its chemical properties. For example, technetium-99m is the most commonly used tracer, with a half-life of 6 hours and a gamma emission energy of 140 keV.

Gamma Camera Components

1. Collimator — A lead or tungsten plate containing thousands of narrow parallel channels. Since gamma photons cannot be focused by lenses, the collimator acts as a physical filter, allowing only photons travelling in near-parallel directions to pass through. This preserves spatial information. Narrower holes give higher resolution but reduce sensitivity; wider holes increase count rate at the cost of image sharpness.

2. Scintillator — A large crystal of thallium-doped sodium iodide, NaI(Tl). When a gamma photon enters the crystal, it interacts (via the photoelectric effect or Compton scattering) and produces a flash of visible light called a scintillation. The crystal must be thick enough to absorb most incoming gamma photons but not so thick that lateral light spread blurs the image.

3. Photomultiplier tubes (PMTs) — An array of PMTs sits behind the scintillator. Each PMT converts the faint light flash into an electrical signal and amplifies it enormously using a series of dynodes at increasing voltages. A single electron from the photocathode triggers a cascade producing millions of electrons. Because the light from one scintillation spreads across several PMTs, the relative signal strengths from each tube allow the system to calculate the position of the original gamma interaction.

4. Computer — Receives signals from the PMT array and processes them. It calculates the X- and Y-coordinates of each gamma event (using Anger logic), applies energy discrimination to reject scattered photons, and accumulates thousands of events into a position-intensity matrix that forms the image.

5. Display — The processed data is rendered as a 2D image, typically in greyscale. Brighter regions indicate greater tracer uptake (higher gamma activity); darker regions suggest reduced or absent uptake.

Gamma cameras are used to diagnose a wide range of conditions by revealing the distribution and concentration of a radiopharmaceutical within the body. Because the tracer follows specific biological pathways, the resulting image shows function as well as structure.

Common applications include:

• Bone scans — Tc-99m labelled phosphate compounds accumulate in areas of increased bone turnover, revealing fractures, infections, or metastases.
• Lung scans (V/Q) — Detect blood clots (pulmonary embolisms) by comparing ventilation (airflow) and perfusion (blood flow) images.
• Renal (kidney) scans — Assess kidney function by tracking tracer filtration and excretion over time.
• Thyroid scans — Iodine-123 or Tc-99m pertechnetate is taken up by thyroid tissue; "hot" nodules indicate overactivity, "cold" nodules may be suspicious.
• Cardiac imaging — Tc-99m sestamibi or tetrofosmin is taken up by healthy heart muscle; areas of reduced uptake indicate ischaemia or infarction.

Advantages of gamma cameras: functional imaging, relatively low radiation dose, non-invasive, wide availability.

Limitations: limited spatial resolution compared to CT/MRI, images take time to accumulate (minutes to hours), patient must remain still, collimator reduces sensitivity significantly.

Positron emission tomography (PET) is a nuclear imaging technique that detects pairs of gamma photons produced when a positron annihilates with an electron inside the body. It produces 3D images of tracer concentration, providing exceptionally detailed functional information.

Positron–Electron Annihilation

PET uses radionuclides that decay via β⁺ emission, releasing a positron (the electron's antiparticle — same mass, opposite charge). After emission, the positron travels a short distance (typically 1–2 mm) through tissue before encountering an electron.

When the positron and electron meet, they undergo annihilation: their combined rest mass is converted entirely into energy as two gamma photons. By conservation of momentum and mass–energy:

The two gamma photons are emitted in almost exactly opposite directions (approximately 180° apart). This predictable back-to-back geometry is the key physical principle that makes PET imaging possible.

Coincidence Detection and Image Formation

The PET scanner consists of a ring of scintillation detectors (using crystals such as BGO or LSO) coupled to photomultiplier tubes. When two opposing detectors register gamma photons within a very short time window (typically 5–15 ns), the system records a coincidence event.

The line joining the two detectors is called the line of response (LOR). The annihilation must have occurred somewhere along this line. By recording many thousands of coincidence events from all angles, the computer reconstructs a 3D map of tracer distribution using mathematical techniques (filtered back-projection or iterative reconstruction).

Why coincidence detection is powerful:

• No physical collimator is needed — electronic timing replaces lead filtering, so sensitivity is much higher than a gamma camera.
• Only photon pairs travelling in exactly opposite directions are accepted, naturally rejecting scattered photons (which change direction).
• The 511 keV energy is higher than most Compton-scattered photons, so energy discrimination further reduces noise.

The most commonly used PET tracer is fluorodeoxyglucose (FDG), a glucose analogue labelled with fluorine-18 (half-life ≈ 110 minutes). Because many cancers have abnormally high glucose metabolism, FDG-PET is extremely effective at detecting tumours and metastases.

Key diagnostic applications:

• Oncology — Detecting primary tumours, staging cancer, assessing treatment response, and identifying recurrence. Malignant tissue shows increased FDG uptake ("hot spots").
• Neurology — Diagnosing Alzheimer's disease (reduced glucose metabolism in temporal/parietal lobes), Parkinson's disease, and epilepsy (localising seizure foci).
• Cardiology — Identifying viable heart muscle after a heart attack; tissue that is alive but not contracting will still take up FDG.

Advantages of PET:

• No collimator → much higher sensitivity than gamma camera
• 3D imaging with excellent functional detail
• Can detect disease at very early stages (before structural changes)
• Quantitative — standardised uptake values (SUVs) allow numerical comparison

Limitations and considerations:

• Extremely expensive to purchase and operate (millions of pounds)
• Requires a cyclotron nearby (or reliable isotope delivery) because F-18 has a short half-life
• Patient receives a radiation dose, though typically comparable to a CT scan
• Patient must lie still for the duration of the scan
• Limited anatomical detail — usually combined with CT (PET/CT) for structural context