Nuclear Medicine Comprehensive Study Notes

Thomas Leitha
- Former Head of the Department of Diagnostic and Therapeutic Nuclear Medicine – Danube Hospital, Vienna
- Expert in hybrid imaging and the clinical application of tracers.
- Contact: thomas.leitha@gmx.at

Definition: Nuclear medicine is a medical specialty involving the application of radioactive substances in the diagnosis and treatment of disease. Unlike radiology, which focuses on anatomy, nuclear medicine focuses on physiology (functional imaging).
Diagnosis: Non-invasive detection of disease states based on biological function. Common modalities include:
- Scintigraphy: 2D imaging using gamma cameras.
- SPECT (Single Photon Emission Computed Tomography): 3D cross-sectional imaging.
- PET (Positron Emission Tomography): Highly sensitive imaging of metabolic processes.
Therapy: Target-specific treatment using radiopharmaceuticals that emit alpha or beta particles (e.g., $^{131}I$ for thyroid cancer).

Anatomy vs. Function:
- Radiology (CT/MRI): High spatial resolution; visualizes "what it looks like."
- Nuclear Medicine: High sensitivity; visualizes "how it works."
Transmission and Emission:
- Transmission: Radiation source is outside the patient (e.g., X-ray tube). The body attenuates the beam.
- Emission: Radiation source (tracer) is inside the patient. The body emits radiation that the detector captures.
Contrast Media vs. Tracer:
- Contrast Media: Pharmacological doses used to change the physical properties of tissues (e.g., iodine in CT).
- Tracers (Radiopharmaceuticals): Sub-pharmacological (nanomolar) doses that do not interfere with physiological processes.
Hot Spot and Cold Spot:
- Hot Spot: Increased uptake of tracer (e.g., increased bone turnover in metastases).
- Cold Spot: Decreased or absent uptake (e.g., a cyst in the liver or an infarct in the heart).

Harald Juhnke (1929-2005) Philosophy: Pathological processes follow a sequence: functional changes occur first, followed by biochemical changes, and finally morphological changes visible on standard imaging.
- High Sensitivity: Allows for early screening (pre-morphological stage).
- Low Specificity: A hot spot can often mean many things (infection, trauma, tumor). This necessitates Hybrid Imaging (PET/CT, SPECT/CT) to correlate function with specific anatomy.

Osteosarcoma: Primary malignant bone tumor. A three-phase bone scan ( $^{99m}Tc-MDP$ ) is used to assess vascularity (phase 1), soft tissue pooling (phase 2), and metabolic bone turnover (phase 3).
Osteomyelitis: Bone infection. Nuclear medicine can differentiate it from cellulitis by showing focal uptake in the delayed "bone" phase.

The Emission Process Chain:
- Source: The patient injected with a radionuclide (e.g., $^{99m}Tc$ , $^{18}F$ ).
- Object: The physiological distribution within the organ of interest.
- Detector: The Gamma Camera or PET detector ring.
- Processing: Computer algorithms perform filtered back-projection or iterative reconstruction.
- Reporter: Clinical interpretation by a physician.

Isotope Metabolism: Chemically identical isotopes (e.g., $^{123}I$ and $^{131}I$ ) behave the same way biologically but differ in physical half-life and decay type.
Radionuclides: Examples include:
- Technetium-99m ( $^{99m}Tc$ ): Most common; 6-hour half-life; 140 keV gamma peak.
- Fluorine-18 ( $^{18}F$ ): Common PET tracer; 110-minute half-life.
György Hevesy: Known as the "father of nuclear medicine."
- 1943 Nobel Prize: For the use of isotopes as tracers in the study of chemical processes.

Targeting Systems:
- Monoclonal Antibodies: Immunoscintigraphy targeting specific tumor antigens.
- Metabolite Substrates: Example: $^{18}F-FDG$ (glucose analog) to detect hypermetabolic tumors.
- Hormone Analogs: Somatostatin receptor imaging for neuroendocrine tumors.
- Carrier Particles: Nanoparticles for lymphoscintigraphy or microbubbles for blood flow mapping.

Resolution: Influenced by the collimator (standard vs. high-resolution). High-res collimators produce clearer images but require longer scanning times (lower sensitivity).
Signal-to-Noise Ratio (SNR): Essential for distinguishing small lesions from background activity.
SUV (Standardized Uptake Value): An absolute quantification metric in PET used to measure the intensity of tracer concentration: $SUV = \frac{\text{Activity in Tissue}}{\text{Injected Dose} / \text{Body Weight}}$ .

Photon Interactions:
1. Photoelectric Effect: Complete absorption of a photon; vital for image contrast.
2. Compton Scatter: Photon deflection; reduces image quality and increases background noise.
3. Pair Production: Occurs at high energies (> 1.022 \text{ MeV}); relevant in PET imaging where positrons annihilate with electrons.

Linear No Threshold (LNT) Model: Assumes the risk of stochastic effects (like cancer) is proportional to the dose, with no safe "threshold" dose.
Hormesis Theory: A controversial hypothesis suggesting that very low doses of radiation might stimulate repair mechanisms and be beneficial.
Risk Communication: Comparing the low dose of a thyroid scan to background radiation or a cross-Atlantic flight.

Diagnostics:
- Uptake Measurement: Using $^{131}I$ or $^{123}I$ to measure the percentage of iodine trapped by the thyroid.
- Scintigraphy: Using $^{99m}Tc-Permeantate$ to map functional vs. non-functional (cold) nodules.
Radioiodine Therapy (RIT):
- Mechanism: Use of $^{131}I$ which emits beta-minus ( $\beta^-$ ) particles that travel a short distance ( $1-3 \text{ mm}$ ), destroying local thyroid tissue.
- Conditions treated: Graves' disease, toxic multinodular goiter, and thyroid carcinoma.
- Vs. Surgery: RIT is non-invasive and target-specific, whereas surgery offers immediate removal but carries risks of laryngeal nerve damage or hypoparathyroidism.