Comprehensive Introduction to Nuclear Medicine and Radiation Safety and Instrumentation

Key Performance and Imaging Parameters

Spatial Resolution:
- Defined as the ability of the camera to distinguish between adjacent points in an organ.
- It is measured by Full Width at Half Maximum ( $FWHM$ ).
- Regarding measurement, a smaller $FWHM$ value indicates better resolution.
Energy Resolution:
- The ability of the camera to distinguish between pulses of different energies.
- This allows the system to discriminate between various photon energies.
Temporal Resolution:
- Refers to the minimum time required for the acquisition of counts.
Sensitivity:
- Defined as the ability of the camera to detect photons of the radiopharmaceutical.
- Low Sensitivity: The camera detects only a small fraction of the generated photons.
- High Sensitivity: The camera detects a higher fraction of the generated photons.
- Structural Factors: High sensitivity is achieved using a smaller number of larger holes in the collimator separated by thin septa.
The Resolution-Sensitivity Trade-off:
- There is a constant trade-off between sensitivity and resolution.
- Greater sensitivity (where more counts or photons are accepted) means the collimator will accept more photons, which inherently results in decreased resolution.
Matrices:
- The digital grid used for imaging typically ranges from $64 \times 64$ to $256 \times 256$ .
- A larger matrix improves resolution because it provides more pixels for the image.
- However, larger matrices require more time and a higher number of counts to produce a usable image.
Stop Conditions:
- The point at which a scan acquisition terminates is determined by specific conditions.
- Time: Used for Dynamic and SPECT scans.
- Counts: Used for Static scans.
Scintillation:
- The process involving the release of visible light photons when radiation interacts with certain materials.

Common Nuclear Medicine (NM) Isotopes

$99mTc$ (Technetium-99m): Classified as low energy and is the most widely used isotope.
$I-131$ (Iodine-131): High energy.
$I-123$ (Iodine-123): Medium energy.
$Ga-67$ (Gallium-67): Medium energy.
$Tl-201$ (Thallium-201): Low energy.

Gamma Camera Instrumentation

Instrumentation Components and Signal Flow:
1. Collimator: This component defines the photon path by allowing only photons traveling in specific directions to pass through.
2. $NaI$ Scintillation Crystal (Sodium Iodide): This crystal converts incoming $\gamma$ photons (gamma radiation) into visible light.
3. Photomultiplier Tubes (PMTs): These tubes pick up the visible light and convert the light signal into an electrical signal.
4. Pulse Height Analyser (PHA): This final stage determines the energy and location of the event to create the image.

Quality Control (QC) Protocols

Definition of Quality Control: Inspecting equipment to ensure the quality of all aspects is satisfactory and adheres to specific performance criteria.
Camera Quality Control:
- Daily Routine:
  - Visual safety checks.
  - Background check.
  - Photopeak: Aligning the energy window to the specific isotope being used.
  - Uniformity Flood: Conducted to detect potential issues with the PMTs or the scintillation crystal.
- Uniformity Flood Types:
  - Intrinsic Flood: Performed with no collimator on the camera, using a point source.
  - Extrinsic Flood: Performed with the collimator on, using a sheet source.
- Monthly Routine:
  - Centre of Rotation ( $COR$ ).
  - Spatial Linearity and Resolution: Tested using Bar phantoms.
Dose Calibrator Quality Control:
- Constancy: Performed daily using a long-lived source. The result must be within $\pm 10\%$ .
- Linearity: Performed quarterly or annually. It tests the decay of a high-activity $Tc-99m$ source over time.
- Accuracy: Performed annually using a traceable standard.
- Geometry: Performed upon installation. It tests for volume changes and must be within $\pm 2\%$ .

Radiation Safety and ALARA

ALARA Principles (Pillars):
- Time: Minimise the time spent in the vicinity of the radiation source.
- Distance: Based on the Inverse Square Law: $1/d^2$ . Exposure decreases as distance increases.
- Protection/Shielding:
  - Use Lead shielding for $\gamma$ (gamma) radiation.
  - Use Perspex or Plastic for $\beta$ (beta) radiation to avoid Bremsstrahlung (braking radiation).
Radiation Monitoring:
- Luxel badges made of Aluminum oxide are used to measure exposure to X-rays, $\gamma$ , and $\beta$ radiation.
Waste Disposal:
- Solid Waste: Allowed to decay until it reaches background radiation levels.
- Liquid Waste: Disposed of via designated sewerage sinks.
Spill Procedures:
- Prioritise personal decontamination first.
- When cleaning a spill, wipe inward toward the center to avoid spreading the contamination.

Positioning and Image Acquisition Modes

Standard Views:
- ANT: Anterior view.
- POST: Posterior view.
- OBL: Oblique views (examples include RAO and LPO).
- Lateral: Side view.
Acquisition Modes:
- Static: A single-point image taken over a set time or count.
- Dynamic/Flow: Multiple frames acquired sequentially over time to see movement.
- SPECT: 3D reconstruction created from the camera rotating around the patient.
- Whole Body: A scan encompassing the patient from head to toe.
Uptake Terminology:
- Photopenic/Cold: Indicates decreased uptake.
- Intense/Hot: Indicates increased uptake.
- Focal: The uptake is localized to a specific spot.
- Diffuse: The uptake is spread out over an area.

Clinical Workflow and Communication

Workflow Method:
1. Evaluate patient history.
2. Review the order of acquisition.
3. Identify normal versus abnormal uptake.
4. Provide a diagnostic comment.
Patient Personnel Path:
- The patient travels from Reception $\rightarrow$ Nuclear Medicine Technologist $\rightarrow$ Physician.
Communication and Consent:
- Effective communication is vital for obtaining informed consent.
- It helps reduce patient anxiety, particularly when explaining time delays inherent in nuclear medicine procedures.