Positron Emission Tomography (PET) provides functional medical images by detecting gamma photons produced when emitted positrons annihilate with electrons, enabling clinicians to study metabolic activity in tissues.

PET Scanner Physics

Radioactive decay and positron emission

PET imaging relies on radionuclides that decay through β⁺ emission, producing positrons with small kinetic energies suitable for medical imaging.

Once emitted from the decaying nucleus, the positron travels a short distance through tissue. This distance, called the positron range, depends on the isotope’s energy and affects image resolution. Understanding this behaviour helps explain why PET images reveal function rather than just structure. After slowing down through interactions with surrounding electrons, the positron eventually encounters an electron.

Positron–electron annihilation

When a positron collides with an electron, they undergo annihilation, a process where their mass is converted entirely into energy. Two gamma photons are produced, each with an energy of 511 keV, and they travel in almost exactly opposite directions (approximately 180° apart). This predictable geometry is essential for reconstructing accurate PET images.

Between releasing the annihilation radiation and their detection by the PET scanner, the photons may undergo interactions such as Compton scattering. These interactions can degrade image quality, so scanners use systems of detectors and reconstruction algorithms to identify genuine photon pairs.

Coincidence detection

A defining feature of PET imaging is coincidence detection, a method used to ensure that only photon pairs originating from the same annihilation event are registered.

This diagram shows the principle of coincidence detection in PET: two gamma photons from a single annihilation event travel in opposite directions and are recorded simultaneously by two detectors. The line joining these detectors represents the line of response used in reconstructing the tracer distribution. Some labels are in German and include extra geometric detail beyond the OCR syllabus, but they do not affect the core physical idea. Source.

• Each detector ring contains many individual detectors made from materials with high gamma absorption efficiency.

• When two opposing detectors record gamma photons within a very short time window (typically a few nanoseconds), this simultaneous detection is termed a coincidence event.

• The line joining the two detectors is defined as the line of response (LOR).

• Repeated detection of many LORs originating from the distribution of annihilation events allows reconstruction of a 3D image.

This schematic illustrates how a PET scanner detects pairs of gamma photons from annihilation events, forms lines of response, and reconstructs an image on a computer. It highlights the detector ring, coincidence processing unit, and image reconstruction stage. The specific scanner hardware and computer model shown are illustrative and not required by the OCR syllabus. Source.

This method avoids the need for physical collimators and increases efficiency compared with systems such as gamma cameras.

Components of a PET Scanner

Detector rings and scintillation crystals

PET scanners are built from circular or cylindrical rings of detectors containing scintillation crystals such as lutetium oxyorthosilicate (LSO) or bismuth germanate (BGO). These materials efficiently absorb high-energy gamma photons and emit light pulses in response.

The crystals are coupled to photodetectors, traditionally photomultiplier tubes (PMTs) but increasingly silicon photomultipliers (SiPMs), which convert the light into electrical signals. These signals provide precise timing information, allowing the system to determine when pairs of photons arrive almost simultaneously. Between this detection and the final display, several layers of processing refine the image and reduce noise.

Time-of-flight PET

Modern PET scanners often incorporate time-of-flight (TOF) information, which measures the tiny difference in arrival time between the two photons. This allows more accurate localisation of the annihilation event along the LOR.

• TOF reduces background noise.

• It improves spatial resolution.

• It enhances image contrast, particularly useful for imaging larger patients.

Although TOF does not change the fundamental physical processes in PET, it greatly improves the final quality of diagnostic images.

PET Image Formation

Data acquisition and reconstruction

PET images are formed by combining thousands of coincidence events.

• Each coincidence defines an LOR.

• Many LORs crossing at the site of tracer concentration indicate high activity there.

• Reconstruction algorithms such as filtered back projection and iterative reconstruction convert this raw data into a detailed 3D image.

Following reconstruction, the scanner’s computer assigns colour scales or intensity values to represent areas of higher or lower tracer concentration, allowing clinicians to interpret metabolic or functional activity clearly.

Functional imaging and tracer distribution

Unlike structural imaging (e.g., X-rays or CAT scans), PET visualises biochemical processes. PET radiotracers—commonly those linked to glucose analogues—accumulate in tissues in proportion to their metabolic rates. Regions with higher uptake generate more annihilation events and therefore produce more coincident gamma detections along intersecting LORs.

This PET image shows a horizontal slice through the human brain, with red and yellow regions indicating high uptake of the radiotracer and blue areas indicating low uptake. It reinforces that PET images display metabolic activity rather than anatomy. The source page references the tracer ¹⁸F-FDG and specific scanner details, which are additional contextual information not required by the OCR syllabus. Source.

This distribution enables PET to detect abnormalities in physiology, such as altered metabolism in tumours, even before structural changes appear. Thus, PET offers clinicians a powerful tool for early detection and precise mapping of disease.

This combination of positron emission, annihilation physics, and coincidence detection is central to understanding PET scanner operation and the formation of diagnostic images.