Nuclear Medicine: Unsealed Sources (Concise Notes)

Nuclear Medicine: Unsealed Sources – Concise Notes

What is Nuclear Medicine

• Use of unsealed radioactive materials for functional medical imaging and treatment.
• Radiopharmaceuticals administered to the patient to localise in target organs.

Key Concepts

• Radiopharmaceutical: Radiophore (radioisotope) bound to a biological compound (carrier) to target specific tissues.
• Radionuclide: Radioactive isotope; undergoes decay emitting radiation (alpha, beta, gamma).
• Imaging vs Therapy: Gamma emissions enable imaging; alpha/beta emissions used for targeted therapy; radiopharmaceuticals concentrate in organs.
• Biological vs physical processes affect dose and timing (biological half-life Tb vs physical half-life Tp).

Radioactivity Refresher

• SI unit of radioactivity: $ext{Bq} = ext{decays per second}$.
• Decay processes: Alpha (α), Beta minus (β−), Beta plus (β+), Gamma (γ).
• Decay changes:
  • Alpha: Z → Z−2, A → A−4.
  • Beta−: Z → Z+1, A unchanged.
  • Beta+: Z → Z−1, A unchanged.
  • Gamma: No change in Z or A, energy emission only.
• Relationship to activity: A(t) = A0 e^{- abla t} ext{ with } abla = rac{ ext{ln} 2}{t{1/2}}

Important Decay Equations

• Exponential decay: A = A0 e^{-rac{ ext{ln}(2)}{t{1/2}} t}
• Physical half-life: t_{1/2} = rac{ ext{ln}(2)}{
  abla}
• Effective half-life (Te) when considering biological elimination: rac{1}{Te} = rac{1}{Tb} + rac{1}{T_p}
• Activity vs time in radiopharmaceuticals: use above relationships for planning dose and timing.

Radionuclides in Nuclear Medicine (General Rules)

• Ideal radionuclide properties:
  • Readily incorporable into a pharmaceutical without altering biochemical behaviour.
  • Safe, sterile, non-toxic, available on site when possible.
• Imaging radionuclides:
  • Pure gamma emitter (no alpha/beta emissions) to minimise dose and allow exit from body.
  • Optimum energy for gamma cameras: 50–500 keV, ideally around $E_ ext{γ} \,\approx \,150\text{ keV}$.
  • Optimum physical half-life similar to test duration (minimises dose).
• Therapeutic radionuclides:
  • Ability to concentrate in the organ of interest; half-life long enough to deliver dose.

Technetium-99m (99mTc) – The Workhorse

• Most common radionuclide in NM; accounts for ~80–85% of procedures.
• Why it’s ideal:
  • Energy: $E_γ \approx 140\ \text{keV}$ (easily detected, leaves body).
  • Half-life: $t_{1/2} \approx 6\ \text{h}$ (limits dose).
  • Gamma emitter with minimal beta emission.
• On-site production via 99Mo/99mTc generator: 99Mo decays to 99mTc; 99mTc has short half-life (~6 h) allowing local use.
• Versatility: can be bound to many biologically active substances to target tissues.
• On-site production efficiency and shielding allow safe hospital use.

Gamma Camera – Imaging Modality

• In vivo detection: gamma rays are detected by a gamma camera for functional imaging.
• Core components:
  • Collimator (project image from patient to detector)
  • NaI(Tl) scintillation crystal
  • Photomultiplier tubes (PMTs)
  • Analog-to-digital converters (ADCs) and digital processing
• Collimator basics:
  • Purpose: project the distribution onto the crystal; no gamma-ray lens exists.
  • Parallel-hole collimator: only photons normal to the surface pass; defines field of view and spatial resolution/sensitivity.
• New solid-state alternatives:
  • CZT detectors, CsI with photodiodes; trades off price, precision, and efficiency.

Radiopharmaceuticals and Bonding

• Radiopharmaceuticals consist of a radiophilic isotope bound to a biological compound (carrier) enabling localisation to target cells or tissues.
• Conceptual model:
  • Radiopharmaceutical → targets tissue via biochemical interaction; Radiophosphate emits radiation for detection or therapy.
• Uptake principle:
  • Abnormal metabolic activity shows increased radiopharmaceutical uptake (e.g., tumors vs normal tissue).

Administration Routes (Examples)

• IV: e.g., lung perfusion (blood flow)
• Oral: reflux testing
• Ingestion: gastric emptying/gastric function
• Inhalation: aerosols/gases for lung ventilation and distribution

Common Radiopharmaceuticals (Representative Examples)

• 99mTc-based:
  • Pertechnetate (for thyroid, salivary glands)
  • MDP (HDP) for bone imaging
  • MIBI (sestamibi) for myocardial perfusion and tumors
  • DTPA for renal function (vasculature/clearance)
  • DMSA for renal cortex/scarring
  • HMPAO for brain perfusion
  • Meckel’s diverticulum imaging
  • Sestamibi/Dimercaptosuccinic acid variants for assorted scans
  • 99mTc-labeled WBC scanning for infection/localisation
  • 99mTc dimers for pulmonary ventilation (lung imaging)
• Other radionuclides:
  • 18F-FDG (PET) – oncology, neurology, cardiology
  • 111In-labeled WBC for infection
  • 67Ga for infection/localisation and some tumor imaging
  • 81mKr, 133Xe for lung ventilation studies
• Iodine-131 (I-131): used for thyroid disease treatment and imaging; see full details below.

Detection of Radiopharmaceuticals

• In vitro (non-imaging): measure radioactivity in fluids; counts via detectors.
• In vivo imaging: gamma camera detects emitted gamma rays to form images.

The Gamma Camera – Practical Aspects

• Gamma rays: high-energy EM radiation; can scatter/absorb; cannot be focused; require collimation for imaging.
• Image formation relies on the distribution of radiotracer in the body and the camera’s detection of gamma photons.

Iodine-131 (I-131) – Key Therapeutic Radiopharmaceutical

• Half-life: $t_{1/2} = 8.04\ \text{days}$.
• Emissions: beta energies with max $E_β^{\text{max}} = 606\ \text{keV}$ (89%), average $\approx 192\ \text{keV}$; gamma emissions include 364 keV (≈82%), plus other energies.
• Use: thyroid tissue treatment and hyperthyroidism/thyroid cancer management.
• Dose delivery: approx 90% of dose delivered by beta radiation; energy deposit largely within ~1 mm.

Treatment for Thyroid Disease (I-131 Therapy)

• Administration: oral capsule or solution; typical dose ranges:
  • Hyperthyroidism: $350\text{–}500\ \text{MBq}$
  • Cancerous/thyroid remnant ablation: $3700\text{–}7400\ \text{MBq}$
• Post-treatment isolation and monitoring:
  • Patients with high activities (>~800 MBq) may require inpatient shielding; lower activities allow outpatient management with precautions.
  • Precautions depend on remaining activity; typical guidance covers contact with others and travel.
• Patient safety and public guidance:
  • Isolation room during hospitalization; risk reduction for family and public exposure.
  • Pregnancy/breastfeeding avoidance for ~6 months; men advised to avoid fathering a child for ~4 months.
  • Lifestyle precautions: sleep alone for about 10 days; avoid crowded places; maintain hydration to aid elimination via sweat/urine.
  • Aftercare: information cards, specific instructions on hygiene (e.g., separate towels and careful handling of contaminated items).
• Post-treatment measurements:
  • Daily activity monitoring with Geiger counters to determine safe return times and visitor allowances.

Practical Points on Half-Lives and Exposure

• Biological half-life Tb: time for activity to halve due to body excretion.
• Effective half-life Te combines Tb and Tp: $\frac{1}{T<em>e} = \frac{1}{T</em>b} + \frac{1}{T_p}$.
• Example: 99mTc-based tracers have short Tb and short Te relative to Tp, balancing image quality with patient dose.

Quick Reference – Key Numbers

• 99mTc gamma energy: $E<em>γ \approx 140\ \text{keV}$; half-life: $t</em>{1/2} \approx 6\ \text{h}$.
• I-131: $t_{1/2} = 8.04\ \text{days}$; major beta energy up to $606\ \text{keV}$; major gamma at 364 keV.
• Common practical uses: bone imaging, myocardia perfusion, brain perfusion, renal function, sentinel node imaging, lung ventilation, infection imaging, PET with FDG.

Notes on Practice and Safety

• Always consider both physical and biological half-lives when planning imaging/therapy doses.
• I-131 therapy requires strict hygiene and isolation guidelines to protect others from radiation exposure.
• Modern detectors may use solid-state technologies to improve resolution and reduce cost, but traditional NaI(Tl) detectors remain common.

Reflective prompts (for quick recall)

• What makes a radionuclide ideal for nuclear medicine imaging? Answer: detectable gamma energy, appropriate half-life, ability to bind to a carrier without altering biology, on-site availability.
• How does the gamma camera image form from radiopharmaceuticals in tissue? Answer: radiopharmaceutical accumulates in tissue → gamma photons emitted → detected by gamma camera after collimation → image represents functional distribution.
• What are typical dose ranges for thyroid I-131 therapy and what precautions follow? Answer: Hyperthyroidism ~$350 ext{–}500\ MBq$; ablation ~$3700 ext{–}7400\ MBq$; inpatient/outpatient care, isolation, and lifestyle restrictions based on activity.