A capacitive micromachined ultrasonic transducer probe for assessment of cortical bone

A wide range of ultrasound methods has been proposed to assess the mechanical strength of bone. The axial transmission technique, which consists of measuring guided elastic modes through the cortical shell of long bones such as the radius or tibia, has recently emerged as one of the most promising approaches of all bone exploration methods. Determination of dispersion curves of guided waves is therefore of prime interest because they provide a large set of input data required to perform inverse process, and hence to evaluate bone properties (elastic and geometric). The cortical thickness of long bones ranges from approximately 1 to 7 mm, resulting in wide inter-individual variability in the guided wave response. This variability can be overcome by using a single probe able to operate with a tunable central frequency, typically within the 100 kHz to 2 MHz frequency range. However, there are certain limitations in the design of low-frequency arrays using traditional PZT technology; these limitations have triggered active research to find alternative solutions. Capacitive micromachined ultrasonic transducers (cMUTs) have the potential to overcome these limitations and to improve axial transmission measurement significantly. The objective of this study was to design and construct a new cMUT-based axial transmission probe and to validate the approach. We report all the steps followed to construct such a prototype, from the description of the fabrication of the cMUT (based on a surface micromachining process) through probe packaging. The fabricated device was carefully characterized using both electrical and optical measurements to check the homogeneity of the device, first from cMUT to cMUT and then from element to element. Finally, axial transmission measurements carried out with the prototype cMUT probe are shown and compared with results obtained with a PZT-based array.