Echocardiographic Image Acquisition & Doppler

Basic Ultrasound Physics

• Sound waves = mechanical, travel in straight line, longitudinal (particles move same direction as wave).

• Frequency (Hz): cycles per second (↑frequency = ↓period).

• Period: time for one cycle.

• Wavelength (mm): distance for one cycle.

• Propagation speed: ~1540 m/s in soft tissue.

• Amplitude/Power/Intensity: describe wave "strength."

Key Formula:

• Wavelength = Propagation speed ÷ Frequency

Attenuation (weakening of sound)

• Causes:

  • Reflection (echo back to probe → image formed)

  • Scattering (spread in many directions)

  • Absorption (converted to heat)

• Attenuation ↑ with: longer path & higher frequency.

• Attenuation coefficient = f ÷ 2 (dB/cm/MHz).

• Impedance = density × stiffness (difference in impedance → reflection).

Transducer

• Function: Converts electricity ⇄ sound (piezoelectric effect).

• Basic parts: piezoelectric crystal + housing.

• During transmit: electricity → sound.

• During receive: sound → electricity.

Sound Beam Anatomy

• Near Zone (Fresnel): beam narrows → focus.

• Focus: narrowest point = best image.

• Far Zone (Fraunhofer): beam widens again.

• Focal Zone: region around focus, best resolution.

Resolution

• Axial Resolution (front-to-back):

  • Better than lateral.

  • Improved with shorter pulses.

  • Mnemonic: LAARD = Longitudinal, Axial, Range, Radial, Depth.

• Lateral Resolution (side-to-side):

  • Best at focal zone.

  • Equation: Lateral resolution = Beam diameter.

Doppler & Scattering

• Doppler uses blood cell scattering.

• Affected by: particle size, hematocrit, transducer frequency, blood compressibility.