Multifrequency transcranial ultrasound holography with acoustic lenses
Résumé
Focusing ultrasound through the skull requires accurate correction of skull induced aberrations. Acoustic lenses provide an inexpensive and effective measure for correcting these distortions. This study explores a phase unwrapping technique for designing multi-frequency acoustic lenses coupled with geometrically focused transducers. Here, we demonstrate numerically and theoretically that in a homogeneous medium and for a single target steered from the natural focus of the transducer, a wrapped lens can only work at its design frequency, while a phase unwrapped lens is broadband. This concept is further explored in experiments where three human skulls are inserted between a transducer and its natural focus. Unwrapped lenses are used to correct for the skull aberrating effect at multiple frequencies and are compared to their wrapped counterparts. Such experiments demonstrate that the unwrapped lenses are the only ones capable of correcting for skull aberrations in a broad range of frequencies. At the design frequency of the lenses, the unwrapped lenses perform quantitatively better than their wrapped counterparts since the average intensity behind three skulls was found to be 49% lower with a phase wrapped lens compared to a phase unwrapped lens, even if both allow to qualitatively refocus the wave at the desired targets. In addition, such results confirm the necessity to correct for skull aberrations, since the field may be highly distorted in the absence of aberration correction.
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