TecSan - TecSan : Technologies pour la santé et l’autonomie

Low frequency cMUTs technology: application to the Brain Motion Measurement and Bone Quality Assessment. – COSTUM

Submission summary

Of all the medical imaging techniques, ultrasonography methods are very interesting since they are non-radiating, non-ionizing and inexpensive. Longitudinal acoustic waves propagate well in human tissue and have a long travelling distance, meaning that the majority of organs can currently be explored with modern echographic imaging systems. However there are two exceptions in the field of public health linked to the ageing population: the skeleton and the brain. Bones poorly propagate ultrasound waves on account of the very strong attenuation and very low frequencies that must be used (under 2 MHz). Moreover the acoustic field through the skull is strongly defocused and the brain cannot be imaged with echography.
Recent results have shown that the natural movement of the brain produced by blood pulsation is measurable with elastography techniques, so that brain pulsatility and brain viscoelasticity can be analyzed jointly. Moreover, in-vitro measurements of cortical bones have shown that for this organ ultrasound is a waveguide in which a great number of acoustic modes can propagate up to 2 MHz. This therefore offers a new opportunity to quantify factors of bone quality and thus to predict risk of fracture. The propagation velocities of these modes depend on the bone mineral density, cortical bone thickness and the elastic properties of the tissue.
A break-through has been expected in the field of ultrasonic transduction: the development of new technology known as cMUT (Capacitive Micromachined Ultrasonic Transducers). cMUT probes are made of several hundred micromachined membranes, layered on a wafer and mechanically excited with electrostatic forces. These devices have shown a remarkable property for low frequency applications, they are able to produce very significant displacement (100 nm) for tension voltage amplitudes lower than 100 Vpp. This project is an experimental development which aims to design low frequency cMUT probes in order overcome one of the main limitations of medical imaging echography: clearing the bone barrier. Two applications are being targeted with this development, with major importance in the surveillance of the ageing population, i.e. the measurement of natural brain movement and the evaluation of bone quality.
A dedicated phased-array cMUT probe with 1.5 MHz central frequency will be developed to measure brain movement. The device will be integrated into a commercially available “open” fast echographic imaging system. Movement detection will be performed through post-processing of RF acoustic lines given by the machine and involving the development and implementation of relevant algorithms dedicated to measurement of delay. Cortical bone assessment will be performed with the “multi-mode” apparatus developed by the Laboratoire d'Imagerie Paramétrique (LIP – CNRS) as part of the EVA project (ANR-Tecsan 2006). The probe developed for this application will have the same topology as the currently available probe. For both applications, the integration of cMUT probes to each platform is the first stage planned and then in-vitro tests will be conducted to assess the performance of cMUT and to establish benchmarks with their counterparts in ceramic technology. Finally, clinical tests are planned using mainly healthy subjects, to evaluate the two imaging platforms.

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The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

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