Molecular Imaging in Cardiology

MI Basics & Tracers - Heart's Glow Show

• Visualizes cardiac physiology (perfusion, metabolism, viability) at molecular/cellular levels.
• SPECT: Single photon emission ($^{201}$Tl, $^{99m}$Tc).
• PET: Positron emission, higher resolution ($^{82}$Rb, $^{13}$N, $^{18}$F-FDG).

Key Cardiac Tracers:

⭐ $^{18}$F-FDG, a glucose analog, is crucial for assessing myocardial viability and detecting inflammation (e.g., sarcoidosis), not just ischemia.

Myocardial Perfusion - Heartbeat Maps

MPI assesses myocardial blood flow with radiotracers ($^{99m}$Tc-MIBI, $^{201}$Tl) via SPECT/PET at stress & rest.

• Heartbeat Maps (Bull's Eye Plot):

  • LV polar plot: apex (center), base (periphery).
  • LV wall perfusion (ant, inf, sept, lat).
  • Compares stress vs. rest.

• Perfusion Patterns:

  • Normal: Homogeneous uptake.
  • Ischemia (Reversible): ↓ stress, normal rest.
  • Infarct (Fixed): ↓ stress & rest.
  • 📌 Mnemonic: Reversible (Ischemia), Fixed (Infarct), Reverse (Rarer).

• Quantitative Scores:

  • SSS (Summed Stress Score): Severity. Normal < 4; Mild 4-8; Mod 9-13; Severe > 13.
  • SDS (Summed Difference Score): Ischemia extent.

⭐ A reversible perfusion defect (↓ stress uptake, normal rest uptake) on MPI indicates myocardial ischemia.

Viability & Inflammation - Cellular Detectives

Evaluates myocardial viability and detects cardiac inflammation/infection.

• Myocardial Viability ($^{ ext{18}} ext{F-FDG PET}$):

  • Hibernating: ↓ Perfusion, Normal/↑ $^{ ext{18}} ext{F-FDG}$ (Mismatch). Viable.

    ⭐ The 'perfusion-metabolism mismatch' (preserved $^{ ext{18}} ext{F-FDG}$ uptake in a region of reduced perfusion) is a key indicator of hibernating, viable myocardium.

  • Scar: ↓ Perfusion, ↓ $^{ ext{18}} ext{F-FDG}$ (Match). Non-viable.
  • Viability Prep: Glucose load.

• Inflammation & Infection Imaging:

  • $^{ ext{18}} ext{F-FDG PET}$: ↑ uptake in inflammatory cells.
    • Inflammation Prep: Prolonged fast/High-Fat Low-Carb Diet (HFLCD) (suppress normal myocardium). 📌 Sarcoid Starves!
  • Other tracers: $^{ ext{67}} ext{Ga-citrate}$, $^{ ext{99m}} ext{Tc-HMPAO}$ WBCs.

• FDG-PET Findings in Cardiac Inflammation:

  Condition	FDG Uptake Pattern
  Myocarditis	Focal/diffuse, non-coronary.
  Cardiac Sarcoidosis	Patchy/focal, basal septum/lateral wall, LNs.
  Endocarditis	Focal at valves/devices.

  

• Cardiac Sarcoidosis FDG-PET Pathway:

Innervation & Plaques - Nerves & Nasties

• Cardiac Innervation Imaging (Sympathetic)
  • Tracer: $^{123}$I-MIBG (SPECT), NE analog.
    • Assesses sympathetic integrity.
  • Key Metrics:
    • Heart-to-Mediastinum (H/M) ratio: Early & delayed; delayed < 1.6 abnormal.
    • Washout Rate (WR): ↑WR = poor prognosis.
  • Applications: HF (prognosis, arrhythmia risk), IHD, diabetic neuropathy.
  • PET: $^{11}$C-HED.
• Atherosclerotic Plaque Imaging
  • Identifies vulnerable plaques (inflammation, µCalcification).
  • Tracers:
    • $^{18}$F-FDG (PET): Targets inflammation.
    • $^{18}$F-NaF (PET): Targets µCalcification.
    • 📌 FDG: Fiery (inflamed) plaques; NaF: Nasty (calcified) formations.
  • Applications: CAD risk, therapy monitoring.
  • Challenges: Resolution, motion.
  • 

⭐ Reduced cardiac MIBG uptake (low H/M ratio) strongly predicts mortality & arrhythmic events in heart failure.

High‑Yield Points - ⚡ Biggest Takeaways

• Myocardial Perfusion Imaging (MPI) with SPECT (^99mTc agents) or PET (⁸²Rb, ¹³N-Ammonia) detects ischemia/infarction.
• PET MPI offers superior resolution & quantification vs SPECT.
• ¹⁸F-FDG PET is key for myocardial viability assessment (hibernating myocardium).
• ¹⁸F-FDG PET/CT detects cardiac inflammation (sarcoidosis, myocarditis).
• Cardiac amyloidosis shows uptake with ^99mTc-PYP/DPD.
• ¹²³I-MIBG assesses cardiac sympathetic innervation for HF prognosis.
• MUGA/RNV accurately measures LVEF, vital for monitoring cardiotoxicity.