Cardiac CT Techniques

Fundamentals & Prep - Heart Scan Essentials

• Cardiac CT (CCT): Non-invasive X-ray imaging for detailed cardiac & vascular anatomy.
• Key Applications: Coronary Artery Disease (CAD) assessment via CCTA, calcium scoring, TAVI/TMVR planning, congenital heart defects.
• Patient Preparation:
  • Heart Rate Control: Target <60-65 bpm (beta-blockers).
  • Coronary Vasodilation: Sublingual nitroglycerin (if no contraindications).
  • IV Access: 18-20G cannula, preferably right antecubital fossa.
  • Fasting: 4-6 hours prior.
  • Key Contraindications: Severe renal dysfunction (eGFR <30 mL/min/1.73m²), prior severe contrast reaction, pregnancy.
  • Breath-hold instructions & practice.

⭐ A Calcium Score (Agatston) of 0 indicates a very low risk of significant coronary artery disease and future cardiovascular events.

ECG Gating - Freezing Motion

• Principle: Synchronizes CT data acquisition with the patient's ECG to minimize cardiac motion artifacts, effectively "freezing" heart motion.
• Crucial for visualizing coronary arteries and cardiac structures without blur.
• Types of Gating:
  • Prospective Gating (Sequential or Step-and-Shoot):
    • X-ray tube activated only during a specific, predefined quiescent phase of the cardiac cycle (typically mid-diastole).
    • Significantly ↓ radiation dose.
    • Requires stable, regular heart rate (ideally < 65-70 bpm).
  • Retrospective Gating (Helical or Spiral):
    • Continuous data acquisition throughout the cardiac cycle with simultaneous ECG recording.
    • Images can be reconstructed at any phase of the R-R interval.
    • Allows for functional cardiac assessment (e.g., ejection fraction, wall motion).
    • Higher radiation dose, though ECG-controlled dose modulation can reduce it.

⭐ Retrospective gating is preferred when functional cardiac information or assessment of valvular motion is required, despite its higher radiation dose compared to prospective gating.

• Prerequisites: Optimal heart rate control (e.g., using beta-blockers) and regular sinus rhythm are key for successful ECG gating. 📌 Remember: Gating Goes with Good Grhythm!

CCTA & Calcium Score - Plaque & Score

• Coronary CT Angiography (CCTA):
  • IV contrast, ECG-gated; visualizes coronary lumen & wall.
  • Stenosis severity: mild (<50%), moderate (50-70%), severe (>70%).
  • Plaque characterization:
    • Calcified: Dense, often stable.
    • Non-calcified (soft): Low attenuation, ↑ ACS risk.
    • Mixed: Components of both.
  • CAD-RADS: Standardized reporting & management.
• Coronary Artery Calcium (CAC) Score:
  • Non-contrast CT quantifies CAC (atherosclerotic burden).
  • Agatston Score: $\Sigma (Lesion \ Area \times Density \ Factor)$.
    • Density factors: 1 (130-199 HU), 2 (200-299 HU), 3 (300-399 HU), 4 (≥400 HU).
  • Agatston Score & Risk:
    • 0: Very Low Risk (excellent prognosis)
    • 1-10: Minimal Risk
    • 11-100: Mild Risk (statins considered)
    • 101-400: Moderate Risk (statins recommended)
    • >400: High Risk (aggressive Rx)
  • Independent MACE predictor.

⭐ A CAC score of 0 has a very high negative predictive value (NPV) for obstructive Coronary Artery Disease (CAD), often termed the "power of zero," implying a <1% chance of significant stenosis.

Recon & Dose Safety - Images & Safety

• Image Reconstruction:
  • FBP (Filtered Back Projection): Faster, ↑image noise.
  • IR (Iterative Reconstruction): Significantly ↓noise, ↓dose (e.g., ASiR, iDose).
  • Recon Kernels: Affect sharpness vs. noise (e.g., stent vs. soft tissue).
  • Slice Thickness: 0.5-0.75 mm for high Z-axis resolution.
• Radiation Dose Metrics:
  • CTDIvol (mGy): Scanner output.
  • DLP (mGy·cm): CTDIvol × scan length; estimates patient stochastic risk.
  • Effective Dose (E, mSv): DLP × k-factor (chest k ≈ 0.014 mSv/mGy·cm). Whole-body risk estimate.
• Dose Reduction (ALARA Principle):
  • Prospective ECG-gating ("step-and-shoot"): Lowest dose.
  • Retrospective ECG-gating + ECG-pulsed dose modulation.
  • ↓kVp (e.g., 80-100 kVp, even 70 kVp): ↓dose, ↑contrast.
  • Tube Current Modulation (mA).
  • Iterative Reconstruction (IR).
  • Minimized scan length (z-axis).

⭐ Prospective ECG-gated axial scans can achieve <1 mSv doses, significantly reducing patient radiation burden.

• Typical Effective Dose: 1-15 mSv (protocol/scanner dependent).

High‑Yield Points - ⚡ Biggest Takeaways

• Coronary Artery Calcium (CAC) scoring uses Agatston score for risk stratification; non-contrast.
• CT Coronary Angiography (CTCA) assesses coronary stenosis, plaques, and anomalies; requires IV contrast.
• ECG-gating is crucial for motion artifact reduction; prospective gating means lower radiation dose.
• Beta-blockers optimize heart rate (<60-65 bpm); nitroglycerin dilates vessels for CTCA.
• High temporal resolution is essential to freeze cardiac motion for diagnostic images.
• Iterative reconstruction techniques are key for radiation dose reduction in cardiac CT.