High-resolution images of human brain are critical for monitoring the neurological conditions in a portable and safe manner. Sound speed mapping of brain tissues provides unique information for such a purpose. In addition, it is particularly important for building digital human acoustic models, which form a reference for future ultrasound research. Conventional ultrasound modalities can hardly image the human brain at high spatial resolution inside the skull due to the strong impedance contrast between hard tissue and soft tissue. We carry out numerical experiments to demonstrate that the time-domain waveform inversion technique, originating from the geophysics community, is promising to deliver quantitative images of human brains within the skull at a sub-millimeter level by using ultra-sound signals. The successful implementation of such an approach to brain imaging requires the following items: signals of sub-megahertz frequencies transmitting across the inside of skull, an accurate numerical wave equation solver simulating the wave propagation, and well-designed inversion schemes to reconstruct the physical parameters of targeted model based on the optimization theory. Here we propose an innovative modality of multiscale deconvolutional waveform inversion that improves ultrasound imaging resolution, by evaluating the similarity between synthetic data and observed data through using limited length Wiener filter. We implement the proposed approach to iteratively update the parametric models of the human brain. The quantitative imaging method paves the way for building the accurate acoustic brain model to diagnose associated diseases, in a potentially more portable, more dynamic and safer way than magnetic resonance imaging and x-ray computed tomography.

There is widespread interest and concern about the evidence and hypothesis that the auditory system is involved in ultrasound neuromodulation. We have addressed this problem by performing acoustic shear wave simulations in mouse skull and behavioral experiments in deaf mice. The simulation results showed that shear waves propagating along the skull did not reach sufficient acoustic pressure in the auditory cortex to modulate neurons. Behavioral experiments were subsequently performed to awaken anesthetized mice with ultrasound targeting the motor cortex or ventral tegmental area. The experimental results showed that ultrasound stimulation of the target areas significantly increased arousal scores even in deaf mice, whereas loss of ultrasound gel abolished the effect. Immunofluorescence staining also showed that ultrasound can modulate neurons in the target area, whereas neurons in the auditory cortex required the involvement of the normal auditory system for activation. In summary, the shear waves propagating along the skull cannot reach the auditory cortex and induce neuronal activation. Ultrasound neuromodulation-induced arousal behavior needs direct action on functionally relevant stimulation targets in the absence of auditory system participation.

声辐射力和声辐射力矩的计算是实现粒子精准操控的重要基础。基于经典声散射理论的偏波级数展开法较难直接应用于复杂模型的研究，而纯数值的方法则不利于进行系统的参数化分析。基于Born近似的基本原理，推导了低频情况下零阶Bessel驻波场中心任意粒子的声辐射力和力矩表达式。在此基础上，以球形粒子、椭球形粒子和柱形粒子为例进行了详细的计算，并考虑了声参数的非均匀性对声辐射力和力矩的影响。仿真结果表明，在低频范围内Born近似具有很高的精度，随着频率的增加和粒子与流体的阻抗匹配变差，Born近似的精度逐渐下降。对于倾斜放置于零阶Bessel驻波场中的椭球形粒子和柱形粒子，非对称性会导致其受到声辐射力矩的作用。在粒子尺寸远小于波长的情况下，声辐射力特性与粒子的具体形状几乎无关，但声辐射力矩不然。最后，引入周围流体的粘滞效应并对声辐射力的表达式进行了修正。该研究预期可以为生物医学、材料科学等领域利用驻波场声镊子实现微小粒子的精准操控提供一定的理论指导。