Ultrasonic Characterization of Rat Vagus Nerves using 25 MHz Pulse Reflectometry

In this paper, a rat vagus nerve's (VN) speed of sound was extracted from the pulse-echo voltage responses which were transmitted and received using a spherically focused 25MHz center frequency immersion ultrasonic transducer (UT). Four VNs were extracted following standard lab protocols and a desktop scanning acoustic microscope (SAM), with a 3-axis motorized stage (having 40 µm precision and 12.5 mm travel), was programmed to determine the nerve's orientation, height, and profile after measuring the water's acoustic velocity (AV) using a PHT (pressure humidity temperature) sensor. Before every measurement, the transducer face was automatically placed 12.7 mm away from the specimen's substrate. A spherically focused transducer with high operating frequency was required to achieve sufficient resolution through the nerve's small diameter of ~0.5 mm. Using the specialized SAM, the time of flights corresponding to the first four echoes were captured, and the nerve's acoustic velocity was calculated to be between 1575–1582 m/s. Understanding the VN's acoustic characteristics will be necessary to achieve fascicle specificity using focused ultrasonic neuromodulators. Stimulating only desired VN fascicles with precision and repeatability will allow targeted treatment of illnesses with lesser side effects than current options.