26.2 Positron Emission Tomography (PET/CT) & Unsealed Radiation Source Safety

26.2 Positron Emission Tomography (PET/CT) & Unsealed Radiation Source Safety

Positron Emission Tomography (PET) Physics & Annihilation Events

Positron Emission Tomography (PET) is a quantitative molecular imaging modality that detects pair production annihilation photons emitted following positron decay. PET provides unmatched functional evaluation of tissue metabolism, blood flow, and receptor density.

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|                    POSITRON ANNIHILATION EVENT                          |  
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|  1. Proton-Rich Nucleus (e.g., F-18) decays via Beta-Plus (b+)          |
|                                                                         |
|  2. Emitted Positron (e+) travels short distance (Positron Range 0.5-2mm)|
|     losing kinetic energy in soft tissue.                               |
|                                                                         |
|  3. Positron collides with a free Tissue Electron (e-).                 |
|                                                                         |
|  4. ANNIHILATION: Mass of e+ and e- converted into pure energy (E=mc^2) |
|                                                                         |
|        511 keV Gamma Photon <======== (180°) ========> 511 keV Gamma    |
|        (Photons travel simultaneously in exactly opposite directions)   |
+-------------------------------------------------------------------------+

1. The Annihilation Event

1. Positron Emission ($\beta^+$ Decay): A proton-rich radionuclide (such as Fluorine-18, Carbon-11, Nitrogen-13, or Oxygen-15) undergoes $\beta^+$ decay, converting a nuclear proton into a neutron while emitting a positron ($\text{e}^+$) and a neutrino ($\nu$).
2. Positron Range: The emitted positron travels a short distance ($0.5 \text{ to } 2.0 \text{ mm}$ in tissue, known as the positron range) while dissipating its kinetic energy through ionizations and collisions with tissue atoms.
3. Annihilation Reaction: Once the positron reaches thermal energy levels, it interacts with a free orbital electron ($\text{e}^-$) in surrounding soft tissue. The combined mass of both particles is completely annihilated and converted into energy according to Einstein's mass-energy equivalence equation ($E = mc^2$).
4. Annihilation Photons: Annihilation produces two $511 \ \text{keV}$ gamma-ray photons that are emitted simultaneously ($180^\circ \pm 0.25^\circ$ apart) in opposite directions along a straight line.

2. Electronic Coincidence Detection

Unlike single-photon gamma cameras that rely on lead collimators to restrict incoming photon angles, PET scanners utilize electronic coincidence detection.

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|                    ELECTRONIC COINCIDENCE DETECTION                     |
+-------------------------------------------------------------------------+
|                    /--- Detector Crystal A ---\                         |
|                   /                            \                        |
|                  /        Line of Response      \                       |
|  Detector Ring  | <=========== (LOR) ===========> | Detector Ring        |
|                 |           [ ANNIHILATION ]    |                       |
|                  \                             /                        |
|                   \                           /                         |
|                    \--- Detector Crystal B ---/                         |
|                                                                         |
|  Coincidence Timing Circuit: Both 511 keV photons must strike A & B     |
|  within Coincidence Time Window (3 - 12 nanoseconds) to record LOR.     |
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• Detector Ring Architecture: The PET gantry contains a complete $360^\circ$ circular array of thousands of small scintillation crystals (such as Lutetium Oxyorthosilicate [LSO] or Lutetium-Yttrium Oxyorthosilicate [LYSO]) coupled to photomultiplier tubes or silicon photomultipliers (SiPMs).
• Line of Response (LOR): When two opposing $511 \ \text{keV}$ photons strike pair detectors on opposite sides of the ring within a strict coincidence timing window ($3 \text{ to } 12 \ \text{nanoseconds}$), the event is recorded as a valid annihilation occurring somewhere along the straight line connecting those two detectors (Line of Response, LOR).
• Electronic Collimation: Because spatial geometry is determined by timing coincidence, PET requires no physical lead collimator septa. Removing lead collimators increases geometric detection efficiency and sensitivity by $100\times$ compared to conventional SPECT gamma cameras.
• Coincidence Event Types:
  1. True Coincidences: Both $511 \ \text{keV}$ photons from a single annihilation event reach opposing detectors unscattered within the timing window (desired diagnostic signal).
  2. Scatter Coincidences: One or both photons undergo Compton scattering in patient tissue before detection, shifting the calculated LOR outside the true origin (degrades image contrast).
  3. Random (Accidental) Coincidences: Two separate, unrelated annihilation events occur almost simultaneously, emitting photons that coincidentally hit opposing detectors within the timing window (adds noise background).
• Time-of-Flight (TOF) PET: Advanced PET systems measure the infinitesimal time difference ($\Delta t$, on the order of picoseconds) between the arrival of the two annihilation photons at opposing detectors. Measuring $\Delta t$ pinpoints the exact position of the annihilation along the LOR, significantly increasing signal-to-noise ratio (SNR) and image quality, especially in obese patients.

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PET Radiopharmaceuticals & F-18 FDG Kinetics

1. Fluorine-18 Fluorodeoxyglucose ($^{18}\text{F-FDG}$)

$^{18}\text{F-FDG}$ is the primary radiopharmaceutical utilized in clinical PET oncology, neurology, and cardiology, accounting for over $90%$ of all PET examinations.

• Physical Properties of Fluorine-18 ($^{18}\text{F}$):
  • Physical half-life $T_{1/2} = 109.8 \ \text{minutes} \ (\approx 110 \ \text{min})$.
  • Produced on site or regionally in a medical cyclotron by proton bombardment of Oxygen-18 enriched water via the nuclear reaction: $^{18}\text{O}(p,n)^{18}\text{F}$.

+-------------------------------------------------------------------------+
|                  18F-FDG METABOLIC TRAPPING MECHANISM                   |
+-------------------------------------------------------------------------+
|  Extracellular Fluid                          Intracellular Space        |
|                                                                         |
|  [ 18F-FDG Molecule ] -- (GLUT-1 / GLUT-4) --> [ 18F-FDG in Cytosol ]   |
|                                                          |              |
|                                                      Hexokinase         |
|                                                          |              |
|                                                          v              |
|                                              [ 18F-FDG-6-Phosphate ]    |
|                                                          |              |
|                                              * METABOLIC TRAPPING *     |
|                                              (Cannot enter Glycolysis;  |
|                                               Cannot exit Cell Membrane)|
+-------------------------------------------------------------------------+

• Mechanism of Accumulation & Metabolic Trapping:
  1. $^{18}\text{F-FDG}$ is a glucose analog in which a hydroxyl group ($\text{-OH}$) at the C-2 position of the glucose molecule is replaced by radioactive Fluorine-18.
  2. $^{18}\text{F-FDG}$ is transported across cell membranes into the cytoplasm via glucose transporter proteins (GLUT-1 and GLUT-4), which are overexpressed on malignant cancer cells.
  3. Inside the cell cytoplasm, the enzyme hexokinase phosphorylates $^{18}\text{F-FDG}$ into $^{18}\text{F-FDG-6-Phosphate}$.
  4. Because $^{18}\text{F-FDG-6-phosphate}$ lacks a 2'-hydroxyl group, it cannot be recognized by glucose-6-phosphate isomerase and cannot undergo further glycolysis. Furthermore, because it is negatively charged, it cannot diffuse backward out of the cell membrane.
  5. This phenomenon—known as metabolic trapping—causes $^{18}\text{F-FDG-6-phosphate}$ to accumulate inside hypermetabolic cells in direct proportion to their glucose metabolic rate (Warburg Effect in cancer cells).
• Normal Physiological Biodistribution:
  • Intense uptake: Brain cortex (high baseline glucose metabolism), myocardium.
  • Moderate uptake: Liver, spleen, tonsils, GI tract, bone marrow.
  • Excretion: Excreted renal system (intense activity in kidneys, ureters, and urinary bladder).
  • Clinical preparation: Patient must fast for $4 \text{ to } 6 \ \text{hours}$ prior to $^{18}\text{F-FDG}$ injection to minimize serum glucose competition. Serum glucose must be verified ($<150 \text{ to } 200 \ \text{mg/dL}$) prior to administration.

2. Other Key PET Radiopharmaceuticals

• $^{18}\text{F-NaF}$ (Sodium Fluoride): High-resolution PET bone scanning agent; incorporates into hydroxyapatite.
• $^{18}\text{F-PSMA}$ / $^{68}\text{Ga-PSMA}$: Prostate-Specific Membrane Antigen ligands for staging and recurrent prostate carcinoma.
• $^{68}\text{Ga-DOTATATE}$ / $^{68}\text{Ga-DOTATOC}$: Somatostatin receptor analogs for neuroendocrine tumor (NET) evaluation.
• $^{11}\text{C-Choline}$: $T_{1/2} = 20.4 \ \text{min}$; cell membrane lipid synthesis tracer.
• $^{13}\text{N-Ammonia}$: $T_{1/2} = 9.96 \ \text{min}$; quantitative myocardial blood flow perfusion tracer.
• $^{15}\text{O-Water}$: $T_{1/2} = 122 \ \text{seconds}$; gold-standard blood flow tracer.

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Hybrid PET/CT Imaging & Attenuation Correction

Modern PET scanners are manufactured exclusively as hybrid PET/CT systems, combining a 3D PET detector ring and a multi-slice helical CT scanner aligned co-axially.

Advantages of Hybrid PET/CT

1. Anatomical Coregistration: Overlays precise anatomical CT structures onto functional PET metabolic activity, allowing accurate localization of small malignant tumor nodes, distinguishing physiological muscle/bowel uptake from true pathology.
2. CT-Based Attenuation Correction (CT-AC): $511 \ \text{keV}$ photons exiting deep abdominal tissues are attenuated by soft tissue and bone. In standalone PET, attenuation correction required external Germanium-68 ($^{68}\text{Ge}$) rod source transmission scans taking $15 \text{ to } 20 \ \text{minutes}$ per bed position. In PET/CT, a low-dose CT scan converts Hounsfield Units into linear attenuation coefficients ($\mu$) at $511 \ \text{keV}$, performing attenuation correction in less than 1 minute and reducing total scan time from 45 minutes to 15 minutes.

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Modality Comparison: PET vs. SPECT

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Radiation Safety for Unsealed Sources

Unlike sealed radiation sources (encapsulated radioactive seeds or external beams), unsealed radioactive sources (liquid solutions, gases, aerosols) present both external radiation hazards and severe internal contamination hazards (via ingestion, inhalation, skin absorption, or accidental injection).

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|                 HOT LAB RADIATION PROTECTION TOOLKIT                    |
+-------------------------------------------------------------------------+
|  [ Tungsten Syringe Shield ] -> Reduces finger hand dose >90% (PET/Tc)  |
|  [ Lead L-Block Shield ]     -> Leaded glass window protects face/body  |
|  [ Dose Calibrator ]         -> Gas ionization chamber assays mCi/MBq   |
|  [ GM Counter Survey Meter ] -> Detects low-level spills (mR/h or cpm)  |
|  [ NaI Well Counter ]        -> Assays removable wipe tests (dpm)       |
|  [ DIS Storage Room ]        -> Stores waste T1/2 < 90d for 10 half-lives|
+-------------------------------------------------------------------------+

1. Hot Lab Handling & Dose Preparation

• L-Block Shield: Handling radiopharmaceuticals must take place behind a lead L-block shield equipped with a $2 \text{ to } 4 \ \text{inch}$ thick lead glass window to shield the technologist's torso and eyes.
• Syringe Shields: High-density tungsten or leaded glass syringe shields must be used during dose draw and patient administration. For $511 \ \text{keV}$ PET photons, high-density tungsten syringe shields ($6 \text{ to } 9 \ \text{mm}$ thick) reduce extremity hand dose by $90% \text{ to } 99%$.
• Dose Calibrator (Ionization Chamber): A gas-filled well-type ionization chamber used to assay the absolute activity of radiopharmaceutical doses in millicuries ($\text{mCi}$) or Megabecquerels ($\text{MBq}$) before patient injection.
  • Mandatory Quality Control Tests:
    1. Constancy: Tested daily using a long-lived sealed source (e.g., Cesium-137 or Cobalt-57); must be within $\pm 10%$ of expected value.
    2. Linearity: Tested quarterly over the entire decay range from maximum patient dose down to $30 \ \mu\text{Ci}$; verifies accurate reading across decay levels.
    3. Accuracy: Tested annually using NIST-traceable standards (e.g., Co-57, Ba-133, Cs-137); must be within $\pm 10%$.
    4. Geometry: Tested at installation and after repair; evaluates reading changes caused by varying sample volumes ($1 \ \text{mL} \text{ to } 10 \ \text{mL}$) or container configurations (syringe vs vial).

2. Survey Instruments & Wipe Testing

• Geiger-Müller (GM) Counter: High-sensitivity gas-filled detector used to perform routine daily end-of-day radiation surveys of work surfaces and detect low-level radioactive contamination (measures in $\text{mR/hr}$ or counts per minute, $\text{cpm}$).
• Cutie Pie (Ionization Chamber Survey Meter): Used for measuring high exposure rates (in $\text{R/hr}$) around patients receiving therapeutic doses (e.g., Iodine-131 therapy patients) without experiencing instrument saturation.
• Wipe Test (Smear Test): Mandatory weekly monitoring for removable radioactive contamination. An absorbent filter paper is wiped over a $100 \ \text{cm}^2$ surface area and counted in a Sodium Iodide (NaI) Well Counter.
  • Action Threshold: Contamination exceeding $2000 \ \text{dpm} / 100 \ \text{cm}^2$ (disintegrations per minute) for beta/gamma emitters requires immediate decontamination and re-surveying.

3. Radioactive Waste Disposal & Decay-in-Storage (DIS)

• Decay-in-Storage (DIS): Applicable to unsealed radionuclides with a physical half-life of less than 90 days ($T_{1/2} < 90 \ \text{days}$, e.g., $^{99m}\text{Tc}, ^{18}\text{F}, ^{123}\text{I}, ^{131}\text{I}, ^{111}\text{In}$).
  • Waste is segregated in lead-lined decay storage containers for a minimum of 10 physical half-lives.
  • After 10 half-lives (when activity decays to $<0.1%$ of initial value), waste radiation levels are surveyed with a GM counter in a low-background area. If radiation equals background levels, radioactive hazard labels are removed or defaced, and waste is disposed of as non-radioactive medical waste.

4. Emergency Radioactive Spill Protocol (SWIMS)

When an unsealed radioactive spill occurs, the technologist must immediately execute the SWIMS emergency protocol:

1. S - Stop the Spill: Upright the spilled container immediately using tongs or gloved hands to prevent further liquid spread.
2. W - Warn Others: Immediately notify all personnel in the area of the spill and instruct non-essential persons to leave the room.
3. I - Isolate the Area: Close all doors, lock access, and post radioactive hazard warning signs to prevent inadvertent entry.
4. M - Minimize Exposure & Contamination: Wear protective clothing (double gloves, lab coat, disposable shoe covers). Avoid stepping in liquid.
5. S - Survey & Decontaminate: Clean up liquid using absorbent paper starting from the outer perimeter working inward toward the center. Monitor personnel, clothing, and skin with a GM counter. Report the spill immediately to the Radiation Safety Officer (RSO).

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Unsealed Source Safety & Monitoring Summary