No abstract

During medical investigations of the head, ultrasound measurements can offer information with simple, non-invasive, and real-time procedure. However, for human adult applications, the clinical treatment of transcranial acoustic imaging remains a challenge by the presence of the skull, results in acoustic aberrations caused by two main phenomena, i.e., attenuation and distortion. These aberrations may affect the signal understanding because of the induced artifacts and the inaccuracy of the imaging target structural information. Variations of the physical properties of the skull, its thickness and porosity, will strongly affect the mechanical properties of the medium and thus the acoustic response. We propose a method to understand the influence of these characteristics on the signal degradation. In order to mimic the human adult skull, a large quantity of epoxy resin-based phantoms is created to explore all the possible physical characteristic variation in the bone. Additional components, titanium dioxide and seeds, will be added to the samples to recreate the acoustic scattering effects of a skull bone. Signal features from pulse-echo mode ultrasound, such as signal attenuation or broadening, will be extracted and studied in the time and frequency domain. In this paper, we are looking for relationship between these physical parameters and the signal features, with the objective to determine bone characteristics without any direct access in later experiments; and going a step further into aberration correction during transcranial imaging procedure.

Structures made of a metallic film deposited on a substrate are conventionally used as opto-acoustic transducers for picosecond ultrasonic experiments where detection in the time domain of the Brillouin scattering in a transparent sample is sought. In this paper, we substitute the metallic film for a periodic stack of nanometric layers made of gold and lithium fluoride to increase the amplitude, at the Brillouin frequency shift, of the strain generated by the photo-thermal effect. A model is used to analyze the generated strain amplification with the volume fraction and with the total thickness of this structure and to evaluate the gain in terms of sample dynamic reflectivity changes. Amplification by a factor of 20 is measured when using the composite structure with respect to signals detected with a transducer made of a single gold layer.