11.3 Echocardiography, Cardiac MRI & Non-Invasive Cardiac Imaging

Echocardiographic Modalities: TTE versus TEE

Echocardiography utilizes high-frequency ultrasound waves to evaluate cardiac structural anatomy, ventricular wall motion, ejection fraction, valve morphology, and continuous blood flow velocity profiles.

Transthoracic Echocardiography (TTE)

• Modality Overview: Non-invasive bedside test using an external transducer positioned on the anterior chest wall (parasternal, apical, subcostal, and suprasternal views).
• Primary Indications: Frontline diagnostic evaluation of unexplained dyspnea, heart failure, heart murmurs, suspected pericardial effusion/tamponade, and routine ejection fraction quantification.
• Limitations: Image resolution can be significantly degraded by body habitus (obesity), severe chronic obstructive pulmonary disease (COPD / hyperinflated lungs), or chest wall deformities.

Transesophageal Echocardiography (TEE)

• Modality Overview: Semi-invasive procedure where a specialized high-frequency ultrasound probe is inserted into the esophagus and stomach, placing the transducer in direct anatomical proximity to the posterior atrium and aorta without intervening lung or bone attenuation.
• Primary Indications:
  1. Detection of Left Atrial Appendage (LAA) Thrombus prior to cardioversion or catheter ablation in Atrial Fibrillation.
  2. Diagnosis of Infective Endocarditis (identifying subtle valvular vegetations <2-5 mm, ring abscesses, or valvular perforation).
  3. Evaluation of Prosthetic Heart Valve Dysfunction (paravalvular leaks, prosthetic thrombosis).
  4. Assessment of Acute Aortic Syndromes (Aortic Dissection, intramural hematoma).
  5. Intraoperative monitoring during cardiac surgery or structural heart interventions (TAVR, MitraClip).
• Nursing Priorities: NPO for ≥ 6 hours pre-procedure; verify absence of esophageal strictures or varices; monitor conscious sedation (midazolam/fentanyl); apply topical pharyngeal anesthesia (cetacaine/lidocaine spray); monitor airway and pulse oximetry; maintain NPO post-procedure until pharyngeal gag reflex fully returns.

Ventricular Function & Valvular Doppler Hemodynamics

Echocardiography provides quantitative measurement of both systolic and diastolic performance as well as valve orifice metrics.

Systolic Function: Simpson's Biplane Rule

• Method of Discs (Simpson's Biplane Rule): Recommended quantitative echocardiographic technique for measuring Left Ventricular Ejection Fraction (LVEF). Left ventricular endocardial borders are traced in orthogonal apical 4-chamber and 2-chamber views at end-diastole and end-systole. The software divides the LV lumen into a stack of 20 elliptical discs to calculate End-Diastolic Volume (EDV) and End-Systolic Volume (ESV): $LVEF\ (\%) = \frac{EDV - ESV}{EDV} \times 100$
• LVEF Categorization: Normal (≥ 50%), Mildly Reduced (41–49%), Moderately Reduced (30–40%), Severely Reduced (<30%).

Diastolic Function Assessment

Diastolic dysfunction reflects impaired LV relaxation and increased passive chamber stiffness, leading to elevated filling pressures:

1. Pulsed-Wave (PW) Doppler Mitral Inflow: Measures blood flow velocity across the mitral valve during diastole:
   • E Wave: Peak velocity during early rapid diastolic filling.
   • A Wave: Peak velocity during late diastolic filling produced by atrial contraction.
   • E/A Ratio: Normal value is 0.8 to 2.0. An E/A ≤ 0.8 indicates impaired early relaxation; an E/A > 2.0 with rapid deceleration time indicates severe restrictive filling.
2. Tissue Doppler Imaging (TDI) e' Wave: Measures intrinsic velocity of mitral annular tissue movement during early diastole (e' wave).
3. E/e' Ratio: Dividing peak early blood inflow velocity (E) by peak early annular tissue velocity (e') cancels out the effect of LV relaxation, yielding a direct index of left ventricular end-diastolic filling pressure:
   • An E/e' Ratio > 14 (average of septal and lateral annulus) is a highly specific marker of pathologically elevated LV filling pressures (PAOP > 15 mmHg).

Valvular Doppler Hemodynamics Table

Cardiac Magnetic Resonance (CMR) Imaging

Cardiac Magnetic Resonance (CMR) provides unmatched spatial resolution and tissue characterization without ionizing radiation. It is the absolute clinical reference gold standard for quantifying Right Ventricular volumes, RV mass, and ejection fraction.

Late Gadolinium Enhancement (LGE) & Myocardial Viability

Intravenous gadolinium-based contrast agents distribute into extracellular space. Normal tightly packed, viable myocytes exclude gadolinium. In necrotic, fibrotic, or scarred myocardium, extracellular volume expands, causing delayed clearance (retention) of contrast imaged 10 to 15 minutes post-injection (Late Gadolinium Enhancement):

• Ischemic LGE Pattern: Extends from the subendocardium outward toward the epicardium in a specific coronary artery vascular distribution.

  • Viability Threshold: The depth of transmural LGE predicts functional recovery following revascularization:
    • LGE <50% Transmural Depth: Indicates viable hibernating myocardium with a high probability of functional recovery post-PCI/CABG.
    • LGE >50% Transmural Depth: Indicates non-viable, transmural scar tissue with minimal to no likelihood of functional recovery post-revascularization.

• Non-Ischemic LGE Patterns: Typically spare the subendocardium:

  • Acute Myocarditis: Patchy subepicardial or mid-wall enhancement, predominantly involving the inferolateral LV free wall.
  • Cardiac Amyloidosis: Diffuse, circumferential subendocardial LGE paired with rapid blood pool gadolinium clearance.
  • Hypertrophic Cardiomyopathy (HCM): Patchy mid-wall LGE restricted to the points of RV-LV anterior and posterior septal insertion.

Cardiac Computed Tomography (CT) and Nuclear Cardiology

Non-invasive imaging pathways incorporate advanced radiographic CT and nuclear radionuclide modalities for CAD risk evaluation and ischemia testing.

Cardiac CT & Coronary Artery Calcium (CAC) Scoring

• Coronary Artery Calcium Scoring (Agatston Score): Non-contrast, ECG-gated cardiac CT that quantifies calcified atherosclerotic plaque within epicardial coronary arteries:
  • Score 0: No identifiable calcification (extremely low 10-year cardiovascular risk).
  • Score 1 – 99: Mild calcification.
  • Score 100 – 399: Moderate plaque burden.
  • Score ≥ 400 (or >75th percentile for age/sex): Severe atherosclerotic burden, predicting high risk of future acute coronary events.
• Computed Tomography Coronary Angiography (CTCA): Contrast-enhanced ECG-gated CT providing 3D anatomical imaging of coronary lumens. High negative predictive value (>97%) makes CTCA ideal for ruling out CAD in low-to-intermediate risk emergency department chest pain patients.

Nuclear Cardiology: SPECT and PET Myocardial Perfusion Imaging

Myocardial Perfusion Imaging (MPI) utilizes radiotracers (Technetium-99m sestamibi/tetrofosmin for SPECT; Rubidium-82 or 18F-FDG for PET) injected at rest and during peak exercise or pharmacological stress (Adenosine, Regadenoson, Dobutamine):

• SPECT Reversible Ischemia vs. Fixed Scar:
  • Reversible Ischemia (Inducible Ischemia): Radiotracer uptake defect is present during stress imaging, but completely normalizes (fills in) on resting images. Indicates significant coronary arterial stenosis causing flow limitation under demand, which will benefit from revascularization.
  • Fixed Perfusion Defect (Infarted Scar): Radiotracer uptake defect is present on stress imaging and persists unchanged on resting images. Represents necrotic myocardial scar.
• PET Metabolic FDG Viability Scanning: Combines perfusion imaging with 18F-Fluorodeoxyglucose (FDG) metabolic imaging. Segments exhibiting reduced perfusion but preserved FDG glucose uptake ("Perfusion-Metabolism Mismatch") represent hibernating viable myocardium that will recover contractile function post-revascularization.