Overview
This lecture covers the fundamental principles of echocardiography, including its history, physics, imaging modes, image formation, common tomographic views, and the progression from 2D to real-time 3D imaging of the heart.
Historical Background of Echocardiography
- Echocardiography uses ultrasound to non-invasively examine the heart.
- The technique originated in the 1950s using industrial ultrasonic flaw detectors.
- Clinical applications began with M-mode for mitral valve assessment.
- Major advances: contrast echo (1960s), 2D imaging, transesophageal echo, and pulsed Doppler.
- Real-time 2D and later 3D imaging revolutionized the field.
Ultrasound Physics & Production
- Ultrasound is sound with frequencies >20 kHz; echocardiography uses 1.5–7.5 MHz.
- Sound velocity depends on medium (heart tissue: 1540 m/s).
- The piezoelectric effect allows certain crystals to convert electric signals to sound waves and vice versa.
- Transducers generate and receive ultrasound pulses, forming the basis of imaging.
Characteristics of the Sound Wave
- Key properties: frequency (Hz), period (s), amplitude (W), intensity (W/m²), wavelength (m), propagation speed (m/s).
- Frequency affects resolution and penetration: higher frequency = better resolution, less penetration.
- Period and frequency are inversely related; amplitude/intensity describe beam strength.
- Wavelength is inversely related to frequency.
- Propagation speed formula: V = f × λ.
Modes of Image Display
- A-mode: amplitude spikes (obsolete).
- B-mode: brightness mode for 2D/real-time images.
- M-mode: motion mode for moving structures, useful for cardiac valves and chamber walls.
- Doppler modes (pulsed, continuous, color): assess blood flow velocities and direction.
Image Formation & Resolution
- Pulsed ultrasound allows for image creation by firing and listening cycles.
- Reflected echoes from tissue interfaces are used to form images.
- Image quality depends on frame rate (temporal resolution), sector size, and line density.
- Lateral resolution: ability to distinguish side-by-side structures, best at the focal zone.
- Longitudinal (axial) resolution: distinguishes front-to-back structures, improved by shorter pulses.
Echocardiographic Tomographic Views
- Parasternal long axis: shows aortic/mitral valves, ascending aorta, and chambers.
- Parasternal short axis: visualizes cardiac structures in cross-section at various levels.
- Apical four-chamber: shows all four heart chambers.
- Apical five-chamber: includes the aorta and outflow tracts.
- Apical two-chamber & long axis: focus on left heart structures.
- Subcostal four-chamber: best for interatrial septum and vena cava.
- Suprasternal view: visualizes aortic arch and great vessels.
Advancements: 2D to Real-Time 3D Imaging
- 3D echo uses phased-array transducers for volumetric data acquisition.
- Imaging modes: narrow angle, zoom, full-volume (multi-cycle), and 3D color Doppler.
- Technological improvements enable higher resolution, faster scanning, and comprehensive spatial imaging.
Key Terms & Definitions
- Echocardiography (ECHO) — Diagnostic use of ultrasound to visualize the heart.
- Ultrasound — Sound waves with frequencies above 20 kHz.
- Piezoelectric effect — Conversion of electrical signals to mechanical (sound) waves by certain crystals.
- Transducer — Device that emits and receives ultrasound in echocardiography.
- Frame rate — The number of images produced per second, affecting temporal resolution.
- Lateral resolution — Ability to differentiate two parallel structures side by side.
- Axial resolution — Ability to distinguish structures positioned front to back.
Action Items / Next Steps
- Review and memorize key echocardiographic views and their anatomical landmarks.
- Study the physical principles, especially frequency, wavelength, and resolution relationships.
- Prepare for practical identification of imaging modes and view orientation.