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Noninvasive Cardiac Imaging: Echocardiography, Nuclear Cardiology, and Magnetic Resonance/ ComputedTomography Imaging

Chapter 248 | Part 6: Disorders of the Cardiovascular System · Part 6 – Cardiovascular Disorders · Chapter 248

Key Clinical Points

  1. The Bernoulli equation (p = 4v^2) is used to calculate pressure gradients across valves based on blood flow velocity.
  2. Left ventricular ejection fraction (LVEF) of 55% or greater is generally considered normal; 50–55% is low-normal.
  3. Coronary artery calcium (CAC) scoring: minimal (0–10), mild (10–100), moderate (100–400), severe (>400).
  4. The McConnell sign (preservation of basal/apical RV motion with mid-free wall dyskinesis) is highly specific for acute pulmonary embolism.
  5. E/E' ratio correlates with left ventricular filling pressures; E' is mitral annular relaxation velocity.
  6. PET imaging provides improved spatial resolution and absolute measures of myocardial perfusion compared to SPECT.
  7. Transesophageal echocardiography (TEE) is the test of choice for small lesions (vegetations, thrombi) and congenital abnormalities.
  8. T1 mapping in CMR is used to scale myocardial inflammation or fibrosis; T2 mapping for edema.
  9. Point-of-care ultrasound (POCUS) devices are gaining full diagnostic capabilities and are excellent screening tools.
  10. Myocardial strain imaging is a more robust measure of contractile function than volumetric-based ejection fraction.

DEFINITION & OVERVIEW

• Overview: Noninvasive cardiac imaging (Echocardiography, Nuclear Cardiology, MRI, CT) complements history, physical exam, lab tests, and exercise testing. • Echocardiography: Uses high-frequency sound waves to generate images of structure and function. ◦ M-mode: Displays a single ultrasound beam over time; used for motion assessment. ◦ 2D Imaging: Provides spatial information on cardiac structure. ◦ 3D Echocardiography: Removes geometric assumptions for volume/EF calculation (requires high expertise). • Doppler Principles: ◦ Principle: Difference in frequency of reflected ultrasound from moving red blood cells determines flow velocity. ◦ Bernoulli Equation: p = 4v^2 (where p = pressure gradient, v = velocity in m/s) used to calculate gradients across valves. ◦ Spectral Doppler: ◦ Pulsed Wave (PW): Determines flow at a specific location. ◦ Continuous Wave (CW): Assesses high-velocity flow but lacks depth specificity. ◦ Color Flow Doppler: Real-time color encoding of blood flow over 2D grayscale images. • Transesophageal Echocardiography (TEE): ◦ Mechanism: Transducer in the esophagus; allows higher frequency, closer proximity to posterior structures. ◦ Clinical Use: Test of choice for small lesions (vegetations, thrombi) and congenital abnormalities. • Stress Echocardiography: ◦ Purpose: Assess cardiac function during exercise or pharmacologic stress (e.g., dobutamine). ◦ Utility: Identifies myocardial ischemia and evaluates valvular function under stress.

EPIDEMIOLOGY

• Coronary Artery Calcium (CAC) Scoring: ◦ Clinical Value: High prognostic value in asymptomatic cohorts. ◦ Radiation Dose: Low (≈ 1-2 mSv). ◦ Classification: ◦ Minimal: 0–10 ◦ Mild: 10–100 ◦ Moderate: 100–400 ◦ Severe: >400 ◦ Reporting: Scores are normalized by age and gender as percentiles.

ETIOLOGY & PATHOPHYSIOLOGY

• Structural Assessment: ◦ Echocardiography: Primary method for chamber size and wall thickness. ◦ CMR/CT: More accurate for volumetric assessment as they do not require geometric assumptions or are less affected by foreshortening. ◦ Dilatation: Indicates remodeling; regional dysfunction in a coronary distribution suggests ischemic etiology. • Myocardial Perfusion & Viability: ◦ Nuclear Imaging (SPECT/PET): Used for CAD diagnosis, risk stratification, and viability assessment. ◦ Viability Assessment: Combination of perfusion imaging with metabolic imaging (e.g., FDG-PET). ◦ Specific Conditions: ◦ Amyloidosis: Use bone-seeking 99mTc radiotracers (PYP, HMDP, DPD) for diagnosis. ◦ Infection/Inflammation: FDG-PET used for myocardial/vascular inflammation and infective endocarditis. ◦ Table 1: Radiopharmaceuticals for Clinical Nuclear Cardiology ◦ Technetium-99m sestamibi / tetrofosmin: SPECT; 6h half-life; Myocardial perfusion. ◦ Thallium-201: SPECT; 72h half-life; Myocardial perfusion. ◦ Iodine-123 MIBG: SPECT; 13h half-life; Cardiac sympathetic innervation. ◦ Rubidium-82: PET; 76s half-life; Myocardial perfusion. ◦ 13N-ammonia: PET; 10 min half-life; Myocardial perfusion. ◦ 18F-fluorodeoxyglucose (FDG): PET; 110 min half-life; Viability, infection, inflammation. ◦ Technetium-99m PYP/HMDP/DPD: SPECT; 6h half-life; Cardiac amyloidosis. • Cardiac Magnetic Resonance (CMR): ◦ Mechanism: Based on proton imaging in hydrogen; uses T1 and T2 weighted sequences. ◦ T1 Mapping: Used to scale myocardial inflammation or fibrosis. ◦ T2 Mapping: Used for myocardial edema. ◦ Table 2: Clinical Cardiac Magnetic Resonance Pulse Sequences ◦ Still frame (black/bright blood): Cardiac structures. ◦ Cine imaging: LV volume and function. ◦ Cine myocardial tagging: LV deformation (strain). ◦ Velocity-encoded phase contrast: Cardiac and great vessel flow. ◦ Myocardial perfusion imaging: Regional myocardial blood flow. ◦ Late gadolinium enhancement (LGE): Myocardial infarction and infiltrative disease. ◦ T2-weighted: Myocardial edema. ◦ Iron content imaging: Myocardial iron infiltration. ◦ MRI Angiography: Aorta, peripheral, and coronary arteries.

CLINICAL FEATURES

• Left Ventricular Systolic Function: ◦ Assessment: Evaluated via echocardiography (primary), CMR, and cardiac CT. ◦ Myocardial Strain: More robust measure of contractile function than volume-based EF. • Left Ventricular Diastolic Function: ◦ E/E' Ratio: Correlates with LV filling pressures; E' is mitral annular relaxation velocity. ◦ Flowchart 1: Assessment of Left Ventricular Diastolic Function 1. Normal Filling: E/A > 1 AND DT < 220 m/s → Normal 2. Impaired Relaxation: E/A < 1 AND DT > 220 m/s → Impaired Relaxation 3. Pseudonormal Pattern: E/A > 1 AND DT 150-200 m/s → Pseudonormal 4. Restrictive Pattern: E/A > 1.5 AND DT < 150 m/s → Restrictive Pattern • Right Ventricular Function: ◦ McConnell Sign: Preservation of basal/apical RV motion with mid-free wall dyskinesis → highly specific for acute pulmonary embolism.

DIAGNOSTIC APPROACH

  1. Stress Myocardial Perfusion Imaging (MPI): • Techniques: SPECT or PET. • Purpose: Identify myocardial ischemia, assess viability, and evaluate inflammation/infection. • Comparison: PET offers superior spatial resolution and absolute measures of perfusion compared to SPECT.
  2. Coronary CT Assessment: • Anatomical Evaluation: Identification of coronary artery calcification (CAC) and lumen narrowing. • Functional Evaluation (FFR_{CT}): ◦ Calculation: Determines if a stenosis is hemodynamically significant. ◦ Threshold: FFR_{CT} leq 0.80 → indicates flow-limiting lesion requiring intervention. • Flowchart 2: Assessment of Flow-Limiting Coronary Artery Disease
  3. CCTA Analysis: Identify anatomical stenosis (e.g., LAD occlusion).
  4. Functional Calculation: Calculate FFR_{CT}.
  5. Decision Point: If FFR_{CT} leq 0.80 → confirm hemodynamically significant stenosis.
  6. Echocardiographic Assessment of Diastolic Function: • Parameters: Use E/A ratio and Deceleration Time (DT). • Decision Logic: ◦ E/A > 1.5 AND DT < 150 m/s → Restrictive Pattern. ◦ E/A < 1 AND DT > 220 m/s → Impaired Relaxation.

MANAGEMENT & TREATMENT

  1. Clinical Decision Making via Imaging: • Ischemia vs. Scar: Use LGE (CMR) and perfusion-metabolism mismatch (PET) to determine if tissue is viable for revascularization. • Valvular Surgery Timing: Accurate assessment of ventricular size (via Echo or CMR) is essential for timing surgical correction of mitral/aortic regurgitation.
  2. Infection Monitoring: • FDG-PET/CT: Used to monitor response to antibiotic therapy in endocarditis; reduction in FDG signal indicates treatment success.

KEY PEARLS & HIGH-YIELD POINTS

• High-Yield Imaging Rules: ◦ T1 Mapping: Inflammation/Fibrosis. ◦ T2 Mapping: Edema. ◦ LGE (CMR): Pathognomonic for infarction in coronary distributions; used to differentiate myocarditis from ischemia. ◦ POCUS: Excellent screening tool with full diagnostic capabilities. • Clinical Correlation: ◦ Calcification vs. Flow: High CAC scores do not always correlate with flow-limiting disease (e.g., Figure 10). ◦ Symmetry of Findings: Discrepancy between anatomical stenosis and functional ischemia is a key clinical distinction (Figure 9).

Reference Tables

TABLE 248-1 Radiopharmaceuticals for Clinical Nuclear Cardiology RADIOPHARMACEUTICAL Technetium-99m sestamibi…

Harrison's 22e, p.1878

RADIOPHARMACEUTICAL IMAGING TECHNIQUE PHYSICAL HALF-LIFE APPLICATION
Technetium-99m sestamibi SPECT 6 h Myocardial perfusion imaging
SPECT 6 h
Thalium-201 SPECT 72 h Myocardial perfusion imaging
SPECT 13 h
Rubidium-82 PET 76 s Myocardial perfusion imaging
PET 10 min
18F-fluorodeoxyglucose PET 110 min Myocardial viability, infection and inflammation imaging
SPECT 6 h

TABLE 248-2 Clinical Cardiac Magnetic Resonance Pulse Sequences and Their Application

Harrison's 22e, p.1881

PULSE SEQUENCE KEY IMAGING INTERESTS
Cardiac Morphology
Still frame imaging (black or
bright blood)		 Cardiac structures
Cardiac Function
Blood Flow Imaging
Velocity-encoded phase contrast Cardiac and great vessel flow
Stress Testing

TABLE 248-3 Comparative Diagnostic Accuracy of Cardiac Imaging Approaches to Coronary Artery Disease

Harrison's 22e, p.1884

IMAGING MODALITY PUBLISHED DATA SENSITIVITY SPECIFICITY
Exercise echocardiography 15 studies (n = 1849 patients) 84% 82%
28 studies (n = 2246 patients) 80%
SPECT MPI 113 studies (n = 11,212 patients) 88% 76%
9 studies (n = 650 patients) 93%
CMR perfusion 37 studies (n = 2841 patients) 91% 81%
14 studies (n = 754 patients) 83%
Coronary CTA 18 studies (n = 1286 patients) 99% 89%