PET Scanning (Copy)

1. Understanding Tracers in PET Scanning

• A tracer is a substance that contains radioactive nuclei.
• It is introduced into the body (often via injection) and accumulates in specific tissues or organs.
• The tracer emits radiation that can be detected externally to provide functional imaging (unlike X-rays or CT which are structural).
• In PET (Positron Emission Tomography), the tracer used undergoes β⁺ decay.

2. β⁺ Decay and PET Scanning

• In β⁺ decay, a proton is converted into a neutron, releasing a positron (e⁺) and a neutrino (νₑ):
  • p → n + e⁺ + νₑ
• The positron is the antiparticle of the electron and is short-lived.
• When the emitted positron collides with an electron in the tissue, they annihilate.

3. Annihilation Process and Conservation Laws

• Annihilation occurs when a particle and its antiparticle collide and are destroyed.
• In this process:
  • Their mass is converted into energy (via E = mc²).
  • Two gamma-ray photons are produced.
• These photons travel in opposite directions (180° apart) due to conservation of momentum.
• Both mass–energy and momentum are conserved:
  • Total energy before = Total energy after
  • Total momentum before = Total momentum after

4. PET Mechanism Summary

• A β⁺ emitting radionuclide (e.g. fluorine-18) is attached to a biologically active molecule (e.g. glucose analogue).
• This compound is called FDG (fluorodeoxyglucose) and is commonly used in PET.
• The emitted positron travels a short distance before meeting an electron.
• On annihilation, two gamma-ray photons (each of energy 511 keV) are emitted in opposite directions.
• Detectors placed around the patient detect these photon pairs.

5. Calculating the Energy of Gamma-Ray Photons

• From E = mc², the mass of electron or positron is approximately 9.11 × 10⁻³¹ kg.
• Energy of each photon is:
  • E = (9.11 × 10⁻³¹ kg) × (3.00 × 10⁸ m/s)²
  • E ≈ 8.19 × 10⁻¹⁴ J
• In electronvolts:
  • 1 eV = 1.60 × 10⁻¹⁹ J
  • E ≈ 511 × 10³ eV = 511 keV per photon
• Thus, total energy released = 2 × 511 keV = 1.022 MeV

6. Image Formation in PET Scanning

• The two gamma-ray photons from each annihilation event are detected by a ring of detectors around the patient.
• The arrival times and detection positions of the photon pair are used to triangulate the origin of annihilation.
• By processing millions of such events, a 3D image of tracer concentration is built.
• This provides functional imaging, useful for:
  • Cancer detection
  • Brain activity
  • Heart blood flow
  • Metabolic activity

Advantages of PET Scanning

• Detects biochemical changes before structural changes occur.
• Non-invasive method for monitoring disease progression.
• Used in combination with CT or MRI for better resolution.

Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change A2 Level Physics Full Scale Course