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Idiopathic pulmonary fibrosis (IPF) affects 200 000 patients in the United States of America. IPF is responsible for changes in the micro-architecture of the lung parenchyma, such as thickening of the alveolar walls, which reduces compliance and elasticity. In this study, we verify the hypothesis that changes in the microarchitecture of the lung parenchyma can be characterized by exploiting multiple scattering of ultrasound waves by the alveolar structure. Ultrasound propagation in a highly scattering regime follows a diffusion process, which can be characterized using the diffusion constant. We hypothesize that in a fibrotic lung, the thickening of the alveolar wall reduces the amount of air (compared with a healthy lung), thereby minimizing the scattering events. Pulmonary fibrosis is created in Sprague-Dawley rats by instilling bleomycin into the airway. The rats are studied within 3 weeks after bleomycin administration. Using a 128-element linear array transducer operating at 7.8 MHz, in vivo experimental data are obtained from Sprague-Dawley rats and the transport mean free path (L*) and backscatter frequency shift (BFS) are evaluated. Significant differences ( ) in the L* values between control and fibrotic rats and in the BFS values between fibrotic and edematous rats showcase the potential of these parameters for diagnosis and monitoring of IPF.

We present an ultrasound algorithm [lesion imaging and target detection in multiple scattering (LITMUS)] suited to image lesions (hypoechoic) or targets (hyperechoic) in highly complex structures. In such media, standard ultrasound imaging techniques fail to detect lesions or targets due to aberrations and the loss of linearity between propagation distance and propagation time, caused by multiple scattering of ultrasound waves. The present algorithm (LITMUS) has the capability to predict the location as well as the size of such lesions/targets by using the multiple scattered ultrasound signals to its advantage. In this experimental and computational study, we use an ultrasound linear array. Lesions/targets are embedded at varying depths inside multiple scattering media with varying density of scatterers. In the simulations, plastic scatterers are used as the source of multiple scattering in a propagation medium (water). In the experiments, melamine sponges are used, with air alveoli as the scattering source. For multiple locations along the transducer, the incoherent backscattered intensity of the backscattered signals is extracted and the linear growth of the diffusive halo over time is tracked. Sudden changes in this growth indicate the presence of a region with reduced heterogeneity, indicative of the presence of a lesion/target. This methodology is combined with a depression detection algorithm to predict the size and location of the lesion/targets with high fidelity, despite the presence of strong heterogeneity and multiple scattering.

Self-bending (or self-accelerating) and nondiffracting acoustic beams, such as Airy beams, have the potential to focus around obstacles that are directly in the beam path. Here, we demonstrate the self-bending and focusing properties of Airy beams in the ultrasound domain using finite difference time-domain simulations at 5.2 MHz. The phase profiles of self-bending Airy beams are determined from the Airy function. This beam is then transmitted experimentally using a linear array transducer connected to a 128 channel Vantage Verasonics operating at 5.2 MHz. The performance of self-bending beams is compared to conventional focused ultrasound beams in the presence of a strong scattering obstacle (steel rod). The ability of self-bending Airy beams to bypass obstacles is characterized in terms of their relative energy retention at peak intensity, that was found experimentally to be 50.5% for traditional focused beams whereas 71.5% for Airy beams, proving that self-bending beams performed better than conventional beams in terms of relative energy retention with no significant change in the focal profiles. However, it is observed that, in absolute terms, self-bending beams focus less energy than traditional focused beams.