Positron Emission Tomography

Positron Emission Tomography (PET) is a highly advanced nuclear imaging technique used in medicine and biomedical research to visualise metabolic processes and physiological functions inside the human body. By detecting pairs of gamma rays emitted indirectly by a positron-emitting radionuclide (tracer), PET provides detailed, three-dimensional images of functional activity at the cellular and molecular levels.
Unlike traditional imaging methods such as X-rays or CT scans, which show anatomical structures, PET scans reveal how organs and tissues are working, making it invaluable for diagnosing and monitoring diseases such as cancer, heart disease, and neurological disorders.

Principle of PET

PET imaging is based on the detection of positron emissions from radioactive substances introduced into the body. The process involves three main steps:

1. Radiotracer Injection: A biologically active molecule (such as glucose) is labelled with a positron-emitting isotope (e.g., Fluorine-18, Carbon-11, Oxygen-15, or Nitrogen-13). The most common tracer is Fluorodeoxyglucose (FDG), which mimics glucose metabolism.
2. Positron Emission and Annihilation: When the radionuclide decays, it emits a positron (β⁺ particle). Upon encountering an electron in nearby tissue, the positron annihilates, producing two gamma photons (each of 511 keV energy) that travel in nearly opposite directions (at 180°).
3. Detection and Image Reconstruction: A ring-shaped PET scanner detects these simultaneous gamma photons. Using coincidence detection and complex mathematical algorithms, the system reconstructs an image showing the distribution and concentration of the tracer, thereby mapping the organ’s metabolic activity.

Components of a PET Scanner

A PET imaging system comprises several essential components:

• Radiopharmaceuticals (Tracers): Radioactive compounds that participate in biological processes.
• Detector Ring: Surrounds the patient and detects the gamma rays produced by annihilation events.
• Photomultiplier Tubes or Solid-State Detectors: Convert detected photons into electrical signals.
• Computer System: Processes signals to reconstruct high-resolution images using algorithms such as filtered back-projection or iterative reconstruction.
• Patient Table: Mechanically moves through the gantry for whole-body imaging.

Modern scanners often combine PET with other imaging modalities—especially Computed Tomography (CT) or Magnetic Resonance Imaging (MRI)—to produce hybrid systems like PET-CT and PET-MRI, offering both anatomical and functional data in a single session.

Common Radioisotopes and Tracers

Because of their short half-lives, most isotopes are produced on-site using a cyclotron and immediately synthesised into radiopharmaceuticals for patient use.

Procedure of PET Scan

1. Preparation: The patient fasts for several hours before the scan to stabilise glucose levels. Diabetic or cardiac patients may require special preparation.
2. Tracer Administration: The selected radiotracer is injected intravenously. The patient then rests quietly for 30–60 minutes to allow the tracer to distribute and accumulate in target tissues.
3. Scanning: The patient lies on the scanning table, which passes slowly through the gantry. The scanner detects gamma photons and collects data over 20–45 minutes, depending on the study type.
4. Image Reconstruction: The raw data are processed by computers to generate cross-sectional, 3D, and fused PET-CT images for clinical interpretation.
5. Post-Procedure: Patients are advised to drink plenty of fluids to flush out residual radioactive material.

Applications of PET

1. Oncology (Cancer Diagnosis and Management):

• Detects malignant tumours based on increased glucose metabolism.
• Differentiates between benign and malignant lesions.
• Assesses cancer staging, recurrence, and response to therapy.
• Commonly used for cancers of the lung, breast, colon, brain, and lymphatic system.

2. Cardiology:

• Evaluates myocardial perfusion (blood flow) and viability of heart tissue after infarction.
• Helps determine if revascularisation procedures (like bypass surgery) are beneficial.

3. Neurology:

• Assesses brain metabolism and neurotransmitter function.
• Aids in the diagnosis of Alzheimer’s disease, Parkinson’s disease, epilepsy, and brain tumours.
• Evaluates brain function in psychiatric disorders and cognitive research.

4. Research and Pharmacology:

• Used in drug development to study pharmacokinetics and receptor binding.
• Helps explore brain functions and mapping of metabolic pathways.

Advantages of PET

• Functional Imaging: Reveals biochemical changes before structural alterations appear, allowing early disease detection.
• Quantitative Analysis: Measures physiological parameters such as glucose metabolism, oxygen consumption, and blood flow.
• High Sensitivity: Capable of detecting minute concentrations of radiotracers.
• Combination Imaging: PET-CT and PET-MRI provide complementary structural and metabolic information.
• Non-invasive: Requires only a small injection of tracer with minimal discomfort.

Limitations

• High Cost: PET scans are expensive due to the need for radiopharmaceutical production and advanced equipment.
• Limited Availability: Requires on-site or nearby cyclotron facilities because of the short half-lives of isotopes.
• Radiation Exposure: Although low, the patient is exposed to ionising radiation.
• Lower Spatial Resolution: Compared to CT or MRI, PET images have lower anatomical detail (though this is compensated when combined with CT/MRI).
• Short Shelf-Life of Tracers: Limits scheduling flexibility and geographical reach.

Safety Considerations

• PET scans are generally safe, with radioactive exposure comparable to that from other diagnostic procedures.
• Pregnant and breastfeeding women are usually advised against PET scans unless absolutely necessary.
• Allergic reactions to tracers are extremely rare.
• Medical personnel follow strict radiation safety protocols.

Recent Advances

• PET-CT and PET-MRI Fusion Imaging: Combining metabolic and anatomical data for enhanced diagnostic accuracy.
• Time-of-Flight (TOF) Technology: Improves image resolution and reduces noise by measuring the arrival time difference of photon pairs.
• Total-Body PET Scanners: Capture images of the entire body simultaneously, reducing scan time and radiation dose.
• New Radiotracers: Development of tracers targeting specific biological processes (e.g., amyloid plaques in Alzheimer’s disease, hypoxia in tumours).