Although the induced refractive index contrast is small, and the induced waveguide would accept only a small fraction of scattered photons, it is sufficient to provide an advantage over the reference case e in the point of observation o. f A fraction of light, scattered within a small angle close to the beam axis, that would be scattered away otherwise, is coupled into the ultrasound-induced waveguide and its propagation into depth is promoted. e In the absence of the ultrasound-induced waveguide, the light from a coupling lens is scattered close to the surface of the sample. d At a rather shallow depth or in low scattering samples light could be focused in the point of observation o by an external lens, however, as scattering is increased such focusing is destroyed. c As the ultrasound pulse propagates along the scattering medium (bottom-up), the associated refractive index pattern creates a waveguide, which could support the propagation of a synchronous short light pulse into the depth of the medium. b Light can be guided deeper into the tissue by a carefully crafted ultrasound field, emitted by an ultrasound transducer (UST), and detrimental effects of scattering are reduced. 1a).Ī Collimated laser light beam impinging on the highly scattering tissue forms a typical forward-scattering pattern: the straight line propagation is broken, and light penetration depth is limited. Therefore, their application towards deeper tissue regions is also dependent on achievements in light delivery (Fig. However, even PA- and AO-aided LWS (PA/AO-LWS) require that threshold illumination levels are achieved first. Subjected to lower propagation losses of ultrasound (US) as compared to light, both modalities are able to increase the sensing range and thus are able to extend the depths where the LWS methods may be used 13, 14, 15. Hence, it is possible to employ photoacoustic (PA) and acoustooptic (AO) interactions to recover light intensity information from the deep tissue. This technique requires the knowledge of the light intensity at some point in-depth as a feedback signal to optimize the incident light field and to improve light delivery to the given position. For instance, the amount of scattering events can be diminished using invasive solutions 3, 7 and optical clearing 8, 9, or affecting the distribution of scatterers 10.Īmong the most successful recent active approaches used to counteract light scattering and to improve light delivery is light wavefront shaping (LWS) 11, 12. Passive methods include the use of near-infrared instead of visible light to access the biomedical transparency window(s), or the use of sophisticated filtering concepts for imaging, as is done in ballistic imaging, optical coherence tomography, and diffuse optical tomography 2, 6.Īdditionally, several active strategies to handle light scattering and its effects by influencing light propagation were introduced. Over time, numerous ways to mitigate or circumvent these difficulties were developed. Therefore, the applicability of light-based methods to image, activate, and cure deep tissue remains limited. Their practical applicability, however, is severely impeded by light scattering, hindering the ability to deliver light into regions deeper than a few millimeters 2, 5. By virtue of these advantages, various optical concepts are currently used for light-based tissue imaging 2, activation 3, and treatment 4. Optical methods are safe for biomedical applications and provide high speed, high resolution, and selectivity 1.
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