Minimally Invasive InVivo Functional Ultrasound Imaging Using a 40 MHz Phased Array Endoscope: Mapping the Auditory Response in Rats

Abstract

Recently, functional ultrasound imaging has emerged as a new tool for monitoring changes in cerebral blood volume associated with neural activity. The combination of high spatiotemporal resolution as well as low cost and high portability have positioned ultrasound to potentially become a groundbreaking imaging modality for functional imaging of the brain. To date, studies have been limited to large craniotomies, neonatal, and thin skull applications. In this study we extend the application of functional ultrasound to small burr hole surgeries by using a 64-element 40 MHz phased array packaged in a 2.5 × 3.1 mm endoscopic form factor. In anesthetized rats, the probe was inserted into a small 3.0 × 6.5 mm hole for functional imaging of the inferior colliculus (IC) which is an important node in the auditory neural pathway. 4, 8, and 15 kHz tones were presented at 97 dB for fixed intervals. Imaging was undertaken during these intervals by coherently compounding 16 diverging waves at a pulse repetition frequency of 40 kHz on a custom 64-channel beamforming platform. Image acquisition and stimulus were synchronized and automated using a custom Python 2.7 software interface. Beamforming, singular-value decomposition (SVD) filtering, and post-processing were performed in MATLAB. Increases in cerebral blood volume as high as 85% were measured in response to auditory stimuli in the IC. Mapping was performed with very high spatial resolution, 40 and 100 µm of axial and lateral resolution respectively. The position of the measured functional activation is in excellent agreement with the anatomical position of the IC. This study shows that functional mapping through small burr hole surgeries is possible, vastly increasing the number of potential use-cases for functional ultrasound imaging.