Ultrasound Physics, Instrumentation, and Bioeffects/Safety
Image quality in fetal echocardiography depends on the trade-off between resolution and penetration. Higher-frequency transducers improve spatial resolution but attenuate more, so late gestation or maternal habitus may require a lower-frequency probe. Harmonic imaging and compound imaging improve contrast and border detection. Frame rate is determined by sector width, depth, line density, and number of focal zones; narrow the sector and reduce depth to improve temporal resolution for moving structures. Color Doppler displays direction and velocity but is limited by the Nyquist limit; aliasing occurs when velocity exceeds the scale, so adjust PRF/scale rather than assuming pathology. Pulsed-wave Doppler gives spectral velocities but higher energy; keep dwell time short and use only when needed. Safety follows ALARA: use the lowest output and shortest exposure that yield diagnostic information. Understand thermal index (TI) for heating and mechanical index (MI) for cavitation; Doppler modes raise both, especially with prolonged insonation. During early pregnancy, avoid unnecessary Doppler and limit exposure. Traps include assuming a higher-frequency probe always improves the image, interpreting color aliasing as stenosis, and ignoring output display indices. Worked example: in a late-third-trimester fetus with poor resolution, reduce depth, narrow the sector, and consider a lower-frequency transducer rather than increasing overall output. Rule of thumb: optimize first with frequency, depth, sector, and focus; use Doppler only after two-dimensional imaging answers the clinical question.