A capacitive micromachined ultrasonic transducer probe for assessment of cortical bone

Abstract : A wide range of ultrasound methods has been proposed to assess the mechanical strength of bone. Axial transmission technique, which consists of measuring guided elastic modes through the cortical shell of long bones such as the radius and the 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 as 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 – 2 MHz frequency range. However, there are certain limitations in the design of low frequency arrays using traditional PZT technology, and these limitations have triggered active research to find alternative solutions. Capacitive Micromachined Ultrasonic Transducers (cMUTs) present the potential to overcome these limitations and to improve axial transmission measurement significantly. The aim of the study presented here 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 to probe packaging. The fabricated device was carefully characterized using both electrical and optical measurements in order 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 to results obtained with a counterpart PZT-based array.
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Audren Boulmé, Sophie Ngo, Jean-Gabriel Minonzio, Mathieu Legros, Maryline Talmant, et al.. A capacitive micromachined ultrasonic transducer probe for assessment of cortical bone. IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Institute of Electrical and Electronics Engineers, 2014, 61 (4), pp.710-723. ⟨10.1109/TUFFC.2014.2959⟩. ⟨hal-01302015⟩

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