IQ demodulation and bandpass filtering are standard components of ultrasound beamforming. However, their influence on spatial sampling and aliasing has received limited attention. We investigate how demodulation frequency and filtering bandwidth affect image formation in ultrafast intracardiac echocardiography (ICE), where miniaturized arrays impose trade-offs between element pitch, aperture size, and field of view, particularly near steering limits. Due to the coupling of temporal and spatial domains, a selected temporal frequency band directly maps to a spatial frequency band. If this exceeds the array’s spatial sampling limit, aliasing occurs, leading to grating-lobe-like artifacts. This can be interpreted as a receive-side extension of the spatial Nyquist constraint d ≤ λ/2. We demonstrate these effects in vivo: higher-frequency demodulation within the system bandwidth leads to artifacts and phase wrapping, while lower-frequency settings improve localization and preserve phase coherence. These results show that demodulation and filtering act as spatial frequency selectors, enabling a trade-off between resolution and aliasing in ultrafast imaging.
