Focal volume, steering, and aberration correction in transcranial focused ultrasound: reconsidering the tradeoffs between single-element transducers, phased arrays, and acoustic holograms.

Transducer architecture is an increasingly important design consideration in transcranial focused ultrasound, where focal precision, electronic steering, skull-aberration correction, workflow complexity, and cost must be balanced. Under matched aperture, transmit frequency, focal depth, effective source geometry, and equivalent aberration correction, subdividing an aperture into multiple independently driven elements does not by itself reduce the diffraction-limited focal volume. Phased arrays provide important capabilities that single-element transducers generally lack, including electronic steering, dynamic phase correction, and flexible control of energy deposition. These capabilities can improve the realized intracranial field, particularly when skull-induced aberrations would otherwise broaden, shift, or split the focus. Single-element systems may also be extended through patient-specific acoustic holograms or lenses, which impose spatially varying phase delays to compensate for aberrations, steer the focus, or shape the pressure field. These approaches may reduce system complexity but are typically static and less adaptable than phased arrays. We therefore frame transducer selection as an application-driven design-matching problem: the relevant question is not which architecture is inherently "more focused" under matched conditions, but which best meets a study's requirements for steering, skull-aberration correction, temporal reconfigurability, spatial degrees of freedom, workflow burden, cost, positioning robustness, and safety.