Optical focusing deep inside dynamic scattering media with near-infrared time-reversed ultrasonically encoded (TRUE) light

Abstract

Focusing light deep inside living tissue has not been achieved despite its promise to play a central role in biomedical imaging, optical manipulation and therapy. To address this challenge, internal-guide-star-based wavefront engineering techniques--for example, time-reversed ultrasonically encoded (TRUE) optical focusing--were developed. The speeds of these techniques, however, were limited to no greater than 1 Hz, preventing them from in vivo applications. Here we improve the speed of optical focusing deep inside scattering media by two orders of magnitude, and focus diffuse light inside a dynamic scattering medium having a speckle correlation time as short as 5.6 ms, typical of living tissue. By imaging a target, we demonstrate the first focusing of diffuse light inside a dynamic scattering medium containing living tissue. Since the achieved focusing speed approaches the tissue decorrelation rate, this work is an important step towards in vivo deep tissue noninvasive optical imaging, optogenetics and photodynamic therapy.

Figure 1. The influence of phase conjugation speed on the quality of TRUE optical focusing

(a) Illustration of the TRUE focusing concept. Laser light S with a frequency of f₀ + f_a illuminates a scattering medium and a portion of the diffuse light traversing the acoustic focus is frequency-downshifted to f₀ (the frequency of the acoustic wave is f_a). A PCM records the wavefront of these ultrasonically modulated light S_– (f₀) in a hologram and then phase-conjugates the light back to the ultrasonic focus, thereby forming a focus. Dashed arrows indicate time-reversed light. Plane A denotes the x–z plane intersecting the acoustic axis. (b,c) Simulated recorded holograms in a slow (b) and fast (c) PCM. Hologram blurring is clearly visible in b as a reduction in speckle contrast. (d,e) Simulated light intensity distribution on plane A by reading the hologram recorded in the slow (d) and fast (e) PCM. No focusing can be observed in d. All the images in b–e were normalized by their own maximum values. S, sample light; S_– , frequency-downshifted sample light (signal light); ${\mathrm{S}}_{-}^{\star }$, time-reversed signal light; TRUE, time-reversed ultrasonically encoded.

Figure 2. TRUE optical focusing inside a dynamic scattering medium containing a tissue-mimicking phantom

(a) Schematic of the set-up for characterizing the relationship between the speckle correlation time (τ_c) and the phantom movement speed. The front surface of the crystal and the object plane of the objective were mirrored planes. (b) The speckle correlation coefficient as a function of time when the phantom was moved at 0.010 mm s⁻¹. τ_c = 60 ms was determined for this speed. (c) The relationship between the speckle correlation time and the phantom movement speed. Error bar shows the s.e. of τ_c measured when light illuminated three different locations on the intralipid–gelatin phantom. (d) Schematic of the set-up for evaluating the quality of the time-reversed light at various sample decorrelation rates. (e–g) The time-reversed light pattern when the intralipid–gelatin phantom was static (τ_c>300 s, e), moved at 0.100 mm s⁻¹ (τ_c = 5.6 ms, f) and at 0.200 mm s ¹ (τ_c = 2.8 ms, g). (h) No time-reversed light was observed when the frequency of S was shifted by 100 kHz (τ_c = 0.01 ms). All the images in e–h were normalized by their own maximum values. (i) Schematic of the set-up for imaging an absorptive target with TRUE light. The target was scanned along the x direction. (j) One-dimensional images of the target acquired under different conditions. The circles, squares and diamonds denote experimental data. The solid and dashed lines denote curve fitting of the experimental data. The dotted line denotes the four-point moving average of the experimental data. AT, absorptive target; GD, ground glass diffuser; GG, gelatin gel; IP, intralipid–gelatin phantom; L, lens; LT, lens tube; M, mirror; Obj, Objective; PBS, polarizing beamsplitter; PD, photodiode; R, reference beam; R*, reading beam, phase conjugate to R; S, sample light; S_– , frequency-downshifted sample light (signal light); ${\mathrm{S}}_{-}^{\star }$, time-reversed signal light; SPS, Sn₂P₂S₆:Te 1% photorefractive crystal; TRUE, time-reversed ultrasonically encoded; US, ultrasound; UT, ultrasonic transducer. Scale bar, 1 mm.

Figure 3. TRUE optical focusing inside a dynamic scattering medium containing a living-mouse ear

(a) The speckle correlation coefficient as a function of time for a living-mouse ear. Three speckle decorrelation characteristics were identified. (b) The speckle correlation curves measured at five locations on the mouse ear. The speckle correlation time (τ_c) determined from the curves ranged from less than 0.44–10 ms. When the blood flow was blocked, τ_c became much larger. (c) Schematic of the set-up for imaging an absorptive target placed between a living-mouse ear and a diffuser. The target was scanned along the x direction. Inset: a photo showing the right ear of a mouse used as a dynamic scattering medium. The left ear was bent downwards to avoid blocking the light. Aluminium foil tapes were used to block the light that did not pass through the right ear. (d) 1D images of the absorptive target. The circles and diamonds denote experimental data. The solid line denotes curve fitting of the experimental data. The dotted line denotes the four-point moving average of the experimental data. AT, absorptive target; GD, ground glass diffuser; GG, gelatin gel; L, lens; LME, living-mouse ear; PBS, polarizing beamsplitter; PD, photodiode; R, reference beam; R*, reading beam, phase conjugate to R; S, sample light; S, frequency-downshifted sample light (signal light); ${\mathrm{S}}_{-}^{\star }$, time-reversed signal light; SPS, Sn₂P₂S₆:Te 1% photorefractive crystal; TRUE, time-reversed ultrasonically encoded; US, ultrasound; UT, ultrasonic transducer.