Low Frequency cMUT Technology: Application to Measurement of Brain Movement and Assessment of Bone Quality
Résumé
Following recent advances in medical ultrasound imaging methods almost all human tissues can currently be examined. There are, however, two exceptions: the human skeleton and the brain, because bone tissue is a strongly attenuating and defocusing medium, rendering classical pulse-echo imaging methods inappropriate. Specific imaging approaches within low frequency bands, i.e. 200 kHz–2 MHz, have therefore recently been developed and the results are very promising: (1) the technique for the bone is axial transmission measurement, which consists of using elastic guided modes to characterize all elastic constants of the medium; (2) for brain exploration, it has been demonstrated that brain movement can be measured (i.e. brain pulsatility) with elastography techniques. However, there are certain limitations in the fabrication of low frequency probes with classical technology, which involve finding an alternative to the traditional PZT. Capacitive Micromachined Ultrasonic Transducers (cMUTs) can overcome these limitations and greatly improve these new imaging modalities. The study presented here represents technological development with several goals: (1) the design and fabrication of two different low frequency linear arrays for bone and brain exploration, respectively the testing of axial transmission measurements with a cMUT probe and; (2) comparison with a PZT probe; (3) the development of an imaging method based on the elastography of brain pulsatility, its implementation in a commercial ultrasound scanner and clinical trials for the validation. The results obtained with cMUT and PZT probes are compared.