Positron Emission Tomography (PET)

Positron Emission Tomography (PET)

Imaging modality that detects positron-emitting radiopharmaceutical distribution in the body

Fluorine-18 (F-18)

Common positron-emitting radionuclide used in PET imaging

Fluorodeoxyglucose (FDG)

a glucose analog used for differentiating malignant neoplasms from benign lesions, staging malignant neoplasms

FDG Clinical Use

Differentiates malignant from benign lesions and evaluates myocardial viability

Positron Interaction

Positrons lose energy through ionization and excitation in matter

Annihilation

Process where positron and electron combine and convert mass into energy

511-keV Photons

Two photons produced during annihilation emitted in opposite directions

Positrons travel a very short distance

before annihilation in tissue

Annihilation Coincidence Detection (ACD)

Simultaneous detection of two 511-keV photons to localize events

Line of Response (LOR)

Line connecting two detectors where annihilation event is assumed to occur

True Coincidence

is the simultaneous interaction of emissions resulting from a single-nuclear transformation.

Random Coincidence

mimics a true coincidence, occurs when emissions from different nuclear transformations interact simultaneously with the detectors.

Another term for Random Coincedence

accidental or chance

Scatter Coincidence

occurs when one or both of the photons from a single annihilation are scattered, but both are detected

A scatter coincidence is true coincidence

because both interactions result from a single positron annihilation

Random coincidences and scatter coincidences result in misplaced coincidences

because they are assigned to lines of response that do not intersect the actual locations of the annihilations

PET Detectors

Use scintillation crystals coupled to photomultiplier tubes

Scintillation Crystal

Material that emits light when struck by gamma photons

Photomultiplier Tube (PMT)

Device that converts light into electrical signals

Early PET Design

Each crystal coupled to a single PMT determining spatial resolution

Modern PET Design

Multiple PMTs coupled to larger crystals for improved positioning

Spatial Resolution (PET)

Determined by size and number of detector crystals

Scintillation Requirement

Material must emit light quickly to detect coincident events

Dead Time

Loss of counts at high interaction rates due to detector limitations

Bismuth Germanate (BGO)

Common PET crystal with high density and detection efficiency

BGO Light Output

Approximately 12–14% of NaI:Tl light output

BGO Advantage

High atomic number and density improve 511-keV photon detection efficiency

Gas and Semiconductor Detectors

Not suitable for PET due to low efficiency for 511-keV photons