Gamma Camera & Positron Emission Tomography (PET) Study Notes

Indirect Detection: Human senses cannot detect radioactive decay directly, requiring specialized instruments to measure ionization or light emission resulting from radiation.
Personnel Safety: Monitoring is essential for individuals working with radiation, such as hospital staff in nuclear medicine departments, to manage cumulative exposure.
Regulation: National protection agencies utilize dose monitoring (e.g., annual dose of workers) to ensure compliance with safety standards and prevent biological damage.

Film Badge Detectors: These use the blackening effect of photographic film to record cumulative exposure. They incorporate various filters (different materials and thicknesses) to estimate the energy and type of radiation encountered.
Geiger-Muller (GM) Tubes: A gas-filled tube with a high-voltage electrode. When radiation enters, it triggers an ionization avalanche, creating an intense current pulse. The process concludes with quenching to stop the discharge, resulting in a dead time of approximately $100-500 \text{ ms}$ .
Photomultiplier Tubes (PMTs): These are light detectors paired with scintillators (like $NaI$ ). They convert visible light photons into photoelectrons and use dynodes to amplify the signal through electron cascades, allowing for the determination of the original photon energy.

Structure:
- Collimator: A thick lead sheet with precise holes that limit the angle of incoming photons to create an accurate spatial map.
- Scintillation Crystal: A large Sodium Iodide ( $NaI$ ) crystal, approximately $\approx 45 \text{ cm}$ in diameter and $1 \text{ cm}$ thick.
- PMT Array: A hexagonal arrangement of photomultiplier tubes that detect flashes from the crystal to localize events.
Applications: Gamma cameras are used to create 2-D images of organ function by plotting the distribution and intensity of a radiopharmaceutical within the body.

Half-Life: Must have short radioactive and biological half-lives to ensure the dose leaves the body quickly.
Energy Emission: Should emit low-energy $\gamma$ -rays (e.g., $140 \text{ keV}$ ) to provide high-quality data with minimal patient dosage.
Toxicity: Must be non-toxic and chemically suitable for biological targeting (e.g., $^{99m}\text{Tc}$ ).

Step 1: The patient is orally administered a dose of $8 \text{ mCi}$ of radioactive iodine ( $123\text{I}$ ).
Step 2: The uptake by the thyroid gland is measured after a period of $24 \text{ hours}$ .
Step 3: The measurement is compared to a standard value to determine the percentage uptake, which indicates the health of the thyroid.

Annihilation: A positron ( $e^+$ ) emitted by a tracer encounters an electron ( $e^-$ ). Both are annihilated, converting mass into energy.
Photon Pair: The result is two identical $511 \text{ keV}$ $\gamma$ -rays traveling in opposite directions ( $180^{\circ}$ ).
Coincidence Detection: By sensing two photons at the exact same moment on a ring of detectors, the system identifies the emission source along a line of coincidence, forming a 3-D metabolic image.

Detection Rings: Arrays of scintillators and PMTs designed to catch coincident photons.
Time-of-Flight (TOF): Advanced logic that calculates the arrival time difference between photons to increase spatial accuracy.
Isotope Production: Many PET isotopes like $^{11}\text{C}$ , $^{13}\text{N}$ , $^{15}\text{O}$ , and $^{18}\text{F}$ require a cyclotron for production due to their short half-lives.
Multimodal Scanners: PET is frequently integrated with CT to provide co-registered metabolic and anatomical images.