Deep optical imaging within complex scattering media

Optical imaging has had a central role in elucidating the underlying biological and physiological mechanisms in living specimens owing to its high spatial resolution, molecular specificity and minimal invasiveness. However, its working depth for in vivo imaging is extremely shallow, and thus reactions occurring deep inside living specimens remain out of reach. This problem originates primarily from multiple light scattering caused by the inhomogeneity of tissue obscuring the desired image information. Adaptive optical microscopy, which minimizes the effect of sample-induced aberrations, has to date been the most effective approach to addressing this problem, but its performance has plateaued because it can suppress only lower-order perturbations. To achieve an imaging depth beyond this conventional limit, there is increasing interest in exploiting the physics governing multiple light scattering. New approaches have emerged based on the deterministic measurement and/or control of multiple-scattered waves, rather than their stochastic and statistical treatment. In this Review, we provide an overview of recent developments in this area, with a focus on approaches that achieve a microscopic spatial resolution while remaining useful for in vivo imaging, and discuss their present limitations and future prospects. Optical microscopy is limited to shallow in vivo imaging depths owing to the exponential extinction of single-scattered waves by multiple light scattering. In this Review, we survey methodologies for deep optical imaging that maintain microscopic resolution by making deterministic use of multiple-scattered waves. Optical microscopy is an indispensable tool in biology and medicine owing to its high spatial resolution, molecular specificity and minimal invasiveness, but it is limited to the interrogation of superficial layers for in vivo imaging. The intensity of single-scattered waves used in conventional imaging decreases exponentially with depth; thus, the imaging depth limits are set by the detector dynamic range and the efficiency of the gating operations. Approaches that make deterministic use of the abundant multiple-scattered (MS) waves have been proposed to enable deep optical imaging while maintaining the microscopic spatial resolving power. Recording and controlling the wavefront of MS waves enables a complex scattering layer to be converted into a focusing lens, leading to the development of an ultrathin endoscope. Acousto-optic interactions and wavefront sensing and/or control are integrated to exploit the large penetration depth of ultrasound and high spatial resolution of optical imaging. Reflection-matrix approaches that record and process all MS waves enable the correction of sample-induced aberrations, exploitation of multiple scattering signals and suppression of multiple scattering noise, allowing for imaging at depths greater than those accessible with conventional confocal and adaptive optics microscopy.