Cardiac Nuclear Medicine

Radiopharmaceuticals - Heart's Glow‑Getters

• SPECT Perfusion Agents:
  • $^{201}$Tl (Thallium): K$^+$ analog; redistribution. T½ 73 hrs.
  • $^{99m}$Tc-Sestamibi (MIBI): No significant redistribution. T½ 6 hrs. 📌 MIBI = "Myocardial Ischemia But Imaging".
  • $^{99m}$Tc-Tetrofosmin: Similar to MIBI.
• PET Perfusion Agents:
  • $^{82}$Rb (Rubidium): K$^+$ analog. T½ 76 sec (generator-produced).
  • $^{13}$N-Ammonia: Diffusible tracer. T½ 10 min (cyclotron-produced).
• Viability Assessment:
  • $^{18}$F-FDG (Fluorodeoxyglucose): Glucose metabolism. T½ 110 min.

⭐ $^{99m}$Tc agents (Sestamibi, Tetrofosmin) offer superior image quality and lower radiation dose than $^{201}$Tl for Myocardial Perfusion Imaging (MPI). oka

Myocardial Perfusion Imaging (MPI) - Stress & See Defects

• Goal: Detect myocardial ischemia & assess viability by comparing blood flow at stress vs. rest.
• Radiotracers:
  • $^{201}Tl$ (Thallium chloride): Redistributes.
  • $^{99m}Tc$-Sestamibi (MIBI) / Tetrofosmin: No significant redistribution; separate rest/stress injections.
• Stress Induction:
  • Exercise: Treadmill (preferred).
  • Pharmacological:
    • Vasodilators: Adenosine, Dipyridamole, Regadenoson. 📌 "RegAdenoDip" (Regadenoson, Adenosine, Dipyridamole)
    • Inotrope: Dobutamine (if vasodilators contraindicated, e.g., severe asthma).
• Imaging: Tracer injected at peak stress, then imaged. Separate rest injection & imaging for $^{99m}Tc$ agents.
• Interpretation:
  • Normal: Uniform tracer uptake (stress & rest).
  • Ischemia: ↓ uptake on stress, normal on rest (Reversible Defect).
  • Infarction: ↓ uptake on stress & rest (Fixed Defect).

⭐ Reversible perfusion defects on MPI indicate myocardial ischemia, while fixed defects suggest prior infarction.

Ventricular Function & Viability - Pump Power & Life Signs

• Ventricular Function (Gated SPECT/PET):
  • LVEF (Left Ventricular Ejection Fraction): Key systolic function index. Normal >50-55%. Severe dysfunction <30%.
  • Assesses Regional Wall Motion (RWMA: hypokinesia, akinesia, dyskinesia), LV volumes & synchrony.
• Myocardial Viability (Ischemic but alive?):
  • Goal: Identify hibernating myocardium (dysfunctional but viable, recovers post-revascularization) vs. scar (non-viable).
  • Key Methods & Findings:
    • $^{\text{201}}\text{Tl}$ (Thallium): Rest-redistribution. Uptake = perfusion & Na/K pump integrity. Viable if uptake.
    • $^{\text{99m}}\text{Tc}$-agents (MIBI/Tetro): Rest ± NTG. Uptake = perfusion & mitochondrial integrity.
    • $^{\text{18}}\text{F-FDG}$ PET (Gold Standard): Glucose metabolism. Compare with perfusion (PET/SPECT).

      ⭐ FDG PET Perfusion-Metabolism Mismatch: ↓Perfusion (e.g., on perfusion scan) + Preserved/↑FDG uptake = Hibernating Myocardium (Viable).

  • 📌 Mnemonic: "FDG feeds the living heart" (glucose uptake = viability).

Advanced Cardiac NM - Beyond Ischemia Probes

• Inflammation/Infection Imaging:
  • $^{18}$F-FDG PET/CT: Sarcoidosis, myocarditis, device infections.
  • $^{111}$In-WBC / $^{99m}$Tc-HMPAO-WBC SPECT/CT: Endocarditis.
• Cardiac Amyloidosis Imaging:
  • $^{99m}$Tc-PYP/DPD/HMDP SPECT/CT: For ATTR amyloidosis (vs. AL).
  • PET Tracers ($^{18}$F-florbetapir, etc.): Emerging role.
• Innervation Imaging:
  • $^{123}$I-MIBG SPECT: Assesses sympathetic integrity; heart failure prognosis, arrhythmias.
  • $^{11}$C-HED PET: Research tool for sympathetic function.

⭐ $^{99m}$Tc-PYP myocardial uptake greater than or equal to rib uptake (Perugini grade 2 or 3) strongly suggests ATTR cardiac amyloidosis.

High‑Yield Points - ⚡ Biggest Takeaways

• Myocardial Perfusion Imaging (MPI) is key for ischemia detection using Thallium-201 or Tc-99m agents (Sestamibi).
• Stress MPI shows reversible defects (ischemia); rest MPI shows fixed defects (infarction).
• Hibernating myocardium: ↓perfusion but preserved/↑FDG uptake on PET, indicating viability.
• Stunned myocardium: post-ischemic dysfunction with normal perfusion.
• Radionuclide Ventriculography (MUGA) accurately measures LVEF.
• Cardiac PET with 18F-FDG is superior for myocardial viability assessment.