An Acoustic Measurement Library for Non-Invasive Trans-Rodent Skull Ultrasonic Focusing at High Frequency

Objective:
To investigate the feasibility of developing an acoustic measurement library for non-invasive trans-rodent skull ultrasonic focusing at high frequency.
Methods:
A fiber-optic hydrophone (FOH) was positioned at the geometric focus of a spherically-curved phased array (64 elements, 25 mm diameter, 20 mm radius of curvature). Elements were driven sequentially (3.3 MHz driving frequency) and FOH waveforms were recorded with and without intervening
ex-vivo
rodent skullcaps. Measurements were carried out on 15 skullcaps (Sprague-Dawley rats, 182-209 g) across 3 fixed transmission regions per specimen. An element-wise measurement library of skull-induced phase differences was constructed using mean values across all specimens for each transmission region. Library-based transcranial phase differences were compared with direct FOH-based measurements across 5 additional skullcaps not included in the library.
Results:
Library-based phase corrections deviated less from FOH-based trans-skull phase difference values than those calculated for the water-path case, and restored partial transcranial focal quality relative to that recovered using invasive hydrophone-based corrections. Retrospective analysis suggests comparable performance can be obtained using smaller library sizes.
Conclusion:
An acoustic measurement library can facilitate non-invasive transcranial aberration correction in rodents at high frequency.
Significance:
Library-based focusing represents a practical approach for delivering high-frequency ultrasound brain treatments in small animals.