Ultrasonic sculpting of virtual optical waveguides in tissue

Abstract

Optical imaging and stimulation are widely used to study biological events. However, scattering processes limit the depth to which externally focused light can penetrate tissue. Optical fibers and waveguides are commonly inserted into tissue when delivering light deeper than a few millimeters. This approach, however, introduces complications arising from tissue damage. In addition, it makes it difficult to steer light. Here, we demonstrate that ultrasound can be used to define and steer the trajectory of light within scattering media by exploiting local pressure differences created by acoustic waves that result in refractive index contrasts. We show that virtual light pipes can be created deep into the tissue (>18 scattering mean free paths). We demonstrate the application of this technology in confining light through mouse brain tissue. This technology is likely extendable to form arbitrary light patterns within tissue, extending both the reach and the flexibility of light-based methods.

Transversally travelling ultrasound for light guiding deep into scattering media

“…Our proposed TULW/transient US-IRIS concept is related to similar approaches by Chamanzar and co-workers [25][26][27] and Ishijima et al 28 , where light waveguiding by transversal US application has been achieved by using either a cylinder-shaped UST array (radius 19 mm, height 30 mm) 25 or a cylindrical absorber for generation of nonlinear PA waves (maximum sample sizẽ 3 × 10 mm) 28 . In both cases, the sample size is restricted in both directions.…”

“…Then, assuming the scattering anisotropy factor of IL g = 0.9 (refs. 25,26 ), the optical thickness τ of the phantoms could be calculated as τ ¼ 10 μ 0 s z, where the acquisition depth z (sample thickness) is measured from the boundary of the scattering phantom (see z = 0 in Fig. 2a, b).…”

“…It is important to note that the mechanisms used in our approach are not relying on light coherence 51 . While previously considered too weak for experiment [51][52][53][54][55] , this particular type of light-US interaction is regaining interest [25][26][27][28]38,[56][57][58] and our approach presents a different viewpoint to enrich the understanding of the complex processes involved. Complete uncovering the mechanism of the waveguiding at low pressures requires further investigation and the development of an appropriate theoretical model.…”

“…As US is locally changing the refractive index of the medium upon propagation 23,24 , several concepts of US-induced light waveguiding have been proposed recently to aid light delivery in depth. So far, they are based on US waves in a cylindrical cavity, either formed by a cylinder-shaped ultrasound transducer (UST) [25][26][27] , or by an absorber, acting as a cylindrical source of nonlinear PA waves 28 . These studies demonstrate that the US-induced variations in a scattering medium could provide sufficient AO-interaction to aid light delivery in-depth.…”

“…However, the difficulties of upscaling the cylindrical sources and application of the acoustic load to the entire volume of the sample might limit the applicability of the proposed concepts [25][26][27][28] in vivo. These constraints could be eased by the use of transient US waves for light guiding into scattering media 29 .…”