A Polymer-Based CMUT Probe for Imaging the Spinal Cord in Rats

The ability of ultrasound imaging to deliver real-time visualization of tissue structures and surgical instruments can provide essential benefits in guiding medical interventions. In spinal cord injury research, small animal models are commonly used, but their size restricts the applicability of many standard ultrasound systems. Capacitive micromachined ultrasonic transducers (CMUTs) offer advantages over traditional piezoelectric transducers, including a smaller form factor, high design flexibility, and improved acoustic performance. CMUT structures made of polymers (polyCMUTs) can be produced cost-effectively and quickly, while potentially offering flexible, biocompatible transducers for next-generation ultrasound systems. This study introduces the first polyCMUT probe designed for imaging spinal cords in rats. A compact 11 MHz, 64 channel probe with a $12.9\times 6.5$ mm small tip was developed through a three-stage fabrication process, combining in-house manufactured polyCMUT arrays with electronics, integrated in a research imaging system. Performance evaluation included electrical impedance measurements, acoustic characterization, and in vitro and ex-vivo imaging. Quality analysis validated the stability of the fabrication process, demonstrating high yield and minimal variability, with a standard deviation in resonance frequency of less than 1%. The probe successfully visualized key anatomical structures like the central canal as well as real-time imaging of needle insertion into tissue. However, distinguishing between gray and white matter remained challenging due to limitations in frequency and bandwidth. This study demonstrates the potential of the polyCMUT technology for developing tailored ultrasound solutions. Future work will focus on optimizing high-frequency performance and advancing toward in vivo applications to provide meaningful tools in spinal cord injury research and therapeutic interventions.