22.8.2 PET Scanner

What is a PET Scanner?

PET stands for Positron Emission Tomography. It is a nuclear medicine imaging technique that produces three-dimensional images of functional processes in the body — particularly metabolic activity — rather than just anatomical structure.

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How PET Scanning Works

Step 1 — Radiotracer Injection

A positron-emitting radioisotope (radiotracer) is introduced into the body, usually by injection. The most common tracer is Fluorine-18 (${}_9^{18}\text{F}$), which is attached to a glucose molecule to form FDG (Fluorodeoxyglucose).

Because metabolically active tissues (e.g., brain cells, cancer cells) consume more glucose, the FDG accumulates preferentially in those regions.

Step 2 — Positron Emission (β⁺ Decay)

The radioisotope undergoes beta-plus (β⁺) decay, emitting a positron ($e^{+}$):

${}_9^{18}\text{F}{\to }_8^{18}\text{O}+e^{+}+{\nu }_e$

The emitted positron travels only a very short distance (a few millimetres) through tissue before it encounters an electron.

Step 3 — Electron-Positron Annihilation

When the positron meets an electron in the surrounding tissue, annihilation occurs:

$e^{+}+e^{-}\to \gamma +\gamma$

The entire rest-mass energy of both particles is converted into two gamma-ray photons emitted in exactly opposite directions (180° apart). This back-to-back emission is required by the Law of Conservation of Momentum — since the total momentum of the nearly-at-rest pair is approximately zero, the two photons must have equal and opposite momenta.

Step 4 — Energy of the Gamma Photons

Each photon carries the rest-mass energy of one electron (or positron):

$E=m_0{c}^2=0.511\text{ MeV}$

The total energy released per annihilation event is:

$E_{\text{total}}=2m_0{c}^2=2\times 0.511=1.022\text{ MeV}$

This is consistent with conservation of mass-energy ($E=m{c}^2$).

Step 5 — Coincidence Detection

The patient lies inside a ring of scintillation detectors. The two 0.511 MeV gamma photons travel outward and strike detectors on opposite sides of the ring at virtually the same instant.

The scanner uses coincidence detection: it only records an event when two detectors fire simultaneously (within a nanosecond window). This allows the scanner to determine that the annihilation occurred somewhere along the line of response (LOR) connecting the two triggered detectors.

By collecting millions of such coincidence events from all angles, a computer reconstructs a 3D cross-sectional image (tomogram) of the metabolic activity inside the body.

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Detection of Gamma Rays Outside the Body

Gamma rays are used in PET scanning because:

• They are highly penetrating — they pass through body tissue and reach the external detectors without significant absorption.
• Alpha and beta particles are stopped within the body and cannot be detected externally.
• The 511 keV energy is specific and characteristic, allowing the detectors to distinguish true annihilation photons from background radiation.

The detectors are typically made of scintillator crystals (e.g., BGO or LSO) coupled to photomultiplier tubes or silicon photomultipliers, which convert the gamma-ray energy into an electrical signal.

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Summary Table

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Key Radioisotopes Used in PET