PET Physics
Annihilation
PET imaging relies on a radionuclide that decays by releasing a positron. That positron travels a short distance through the body until it collides with an electron, in a process called annihilation. The collision releases two gamma photons, each with an energy of 511 keV, which travel outward in exactly opposite directions.
PET scanner design
A PET scanner is built from gadolinium oxyorthosilicate scintillation blocks arranged in a ring around the gantry. Each block connects to four photomultiplier tubes (PMTs), and the blocks themselves are divided into individual detector elements separated by reflective material.
Forming an image
Because each annihilation event produces two gamma photons traveling in opposite directions, the scanner only records a pair of photons as valid data when both are detected by two separate detectors along the same line of response within one nanosecond of each other. This "coincidence detection" is what allows the scanner to localize where the annihilation occurred.
Data acquisition
Dimensions
- 2D acquisition — uses a collimator to accept photons only from a specific slice of tissue.
- 3D acquisition — skips the collimator, allowing the scanner to image a larger volume of tissue at once.
Filtering unwanted coincidences
- Scatter coincidence — occurs when a photon from a single annihilation event is deflected before detection, distorting its apparent line of response.
- Random coincidence — occurs when two photons from entirely different annihilation events are mistakenly detected as a pair.
Data correction
| Correction | What it accounts for |
|---|---|
| Attenuation correction | Adjusts for photons absorbed or scattered within the body, either by assuming a cross-sectional shape and uniform tissue attenuation, or by scanning with a radioactive rod source (such as gallium-68) both with and without the patient present to calculate the correction directly |
| Normalisation | Uses a rod source to calculate a correction factor for individual detector elements, accounting for differences between separate lines of response |
| Dead time correction | Accounts for the brief period after a detection when a detector can't register further incoming photons, adjusting counts accordingly |
| Radioactive decay correction | Corrects for the tracer's ongoing decay as the scanner moves along the length of the patient during the scan |
Data reconstruction
Once collected and corrected, the raw coincidence data is turned into an image using either filtered back projection or iterative reconstruction.
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