PET Scanning

This lesson covers: 

1. The use of positron-emitting radiotracers in PET scans
2. Electron-positron annihilation and gamma ray emission
3. PET scanner detection and image reconstruction
4. Relating radiotracer distribution to metabolic activity
5. Applications of PET scanning for medical diagnosis

Positron-emitting radiotracers

In a PET scan, the patient is injected with a radiotracer containing a positron-emitting radioisotope, such as:

• ¹³N
• ¹⁵O
• ¹⁸F

These have a short half-life. The radiotracer attaches to molecules used by the body like glucose. This allows it to concentrate in certain organs and tissues.

Positron-electron annihilation

The positrons emitted collide with electrons in the patient's tissues. This annihilation produces a pair of gamma rays propagating in opposite directions.

Annihilation process:

• Positron collides with electron
• Both are annihilated
• Two 511 keV gamma rays are emitted at 180° to each other

PET scanner detection and image reconstruction

The PET scanner contains gamma ray detectors in a ring around the patient. These detect the annihilation photons, and a computer uses this data to build a 3D map of radiotracer distribution.

Image reconstruction steps:

1. Record gamma ray detection events.
2. Localise origin along line between paired detectors.
3. Reconstruct image slice-by-slice.

Relating radiotracer distribution to metabolic activity

Areas of high radioactivity indicate increased metabolic activity. This results from greater radiotracer uptake:

• More radioactive molecules used
• Faster turnover
• Increased consumption

Cancer cells can display heightened metabolism.

Applications in medical diagnosis

By locating areas of abnormal metabolic activity, PET scans help diagnose:

• Cancer
• Heart disease
• Brain disorders

The short half-life minimises radiation exposure.