Echocardiography Instrumentation and Principles
Physics essentials
Know how piezoelectric transducers convert electrical energy to sound and back. Frequency determines resolution and penetration: higher frequency gives better resolution but less penetration. Axial resolution depends on pulse length (spatial pulse length = number of cycles × wavelength); lateral resolution depends on beam width and is worst in the far field. Understand the near field (Fresnel) and far field (Fraunhofer), and that the transition depends on transducer diameter and wavelength.
Doppler and artifacts
Distinguish pulsed-wave (range-specific, limited by Nyquist) from continuous-wave (unlimited velocities, no range resolution). Know the Doppler equation, the meaning of the angle of insonation (cosine dependence; keep ≤20° for accuracy), and aliasing. Understand color flow, tissue Doppler, and the artifacts: shadowing, enhancement, reverberation, comet tail, side lobes, and mirror image.
Traps
Questions often test that increasing frequency improves axial resolution but worsens penetration, or that CW Doppler cannot localize a signal. Another trap: confusing frame rate with depth-increasing depth lowers frame rate. Remember harmonic imaging improves endocardial definition by using twice the transmitted frequency.
Rule of thumb
Axial resolution ≈ spatial pulse length ÷ 2; shorten the pulse (fewer cycles, higher frequency) to improve it.
Worked example
If a 5 MHz probe has a wavelength of 0.31 mm and a 2-cycle pulse, spatial pulse length ≈ 0.62 mm and axial resolution ≈ 0.31 mm.