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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 22 — Nov. 4, 2013
  • pp: 26671–26676
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High contrast three-dimensional photoacoustic imaging through scattering media by localized optical fluence enhancement

Antonio M. Caravaca-Aguirre, Donald B. Conkey, Jacob D. Dove, Hengyi Ju, Todd W. Murray, and Rafael Piestun  »View Author Affiliations


Optics Express, Vol. 21, Issue 22, pp. 26671-26676 (2013)
http://dx.doi.org/10.1364/OE.21.026671


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Abstract

We demonstrate enhanced three-dimensional photoacoustic imaging behind a scattering material by increasing the fluence in the ultrasound transducer focus. We enhance the optical intensity using wavefront shaping before the scatterer. The photoacoustic signal induced by an object placed behind the scattering medium serves as feedback to optimize the wavefront, enabling one order of magnitude enhancement of the photoacoustic amplitude. Using the enhanced optical intensity, we scan the object in two-dimensions before post-processing of the data to reconstruct the image. The temporal profile of the photoacoustic signal provides the information used to reconstruct the third dimension.

© 2013 Optical Society of America

1. Introduction

Scattering from turbid materials limits the depth through which images can be obtained. However, due to its deterministic nature, the scattering can be compensated with the help of a feedback mechanism using wavefront shaping. As a result, a focused spot can be created behind the scattering medium [1

1. I. M. Vellekoop and A. P. Mosk, “Focusing coherent light through opaque strongly scattering media,” Opt. Lett. 32(16), 2309–2311 (2007). [CrossRef] [PubMed]

]. The initial techniques have been limited by the inability to provide a feedback without access to the back side of the scatterer [1

1. I. M. Vellekoop and A. P. Mosk, “Focusing coherent light through opaque strongly scattering media,” Opt. Lett. 32(16), 2309–2311 (2007). [CrossRef] [PubMed]

3

3. Z. Yaqoob, D. Psaltis, M. S. Feld, and C. Yang, “Optical phase conjugation for turbidity suppression in biological samples,” Nat. Photonics 2(2), 110–115 (2008). [CrossRef] [PubMed]

]. Lately, new feedback mechanisms have been proposed such as iterative optimization feedback from fluorescence [4

4. I. M. Vellekoop, E. G. van Putten, A. Lagendijk, and A. P. Mosk, “Demixing light paths inside disordered metamaterials,” Opt. Express 16(1), 67–80 (2008). [CrossRef] [PubMed]

,5

5. J. Bertolotti, E. G. van Putten, C. Blum, A. Lagendijk, W. L. Vos, and A. P. Mosk, “Non-invasive imaging through opaque scattering layers,” Nature 491(7423), 232–234 (2012). [CrossRef] [PubMed]

] or digital OPC with second harmonic generation nanoparticles [6

6. C.-L. Hsieh, Y. Pu, R. Grange, and D. Psaltis, “Digital phase conjugation of second harmonic radiation emitted by nanoparticles in turbid media,” Opt. Express 18(12), 12283–12290 (2010). [CrossRef] [PubMed]

] used as guide stars. These techniques are limited by a scarcity constraint that requires that the feedback signal only comes from a single particle to ensure single focus creation. However, this constraint can be overcome with iterative focusing methods by using a nonlinear feedback, such as two photon fluorescence [7

7. K. Si, R. Fiolka, and M. Cui, “Fluorescence imaging beyond the ballistic regime by ultrasound pulse guided digital phase conjugation,” Nat. Photonics 6(10), 657–661 (2012). [CrossRef] [PubMed]

,8

8. O. Katz, E. Small, Y. Bromberg, and Y. Silberberg, “Focusing and compression of ultrashort pulses through scattering media,” Nat. Photonics 5(6), 372–377 (2011). [CrossRef]

], which itself is limited by optical power. Another promising method for imaging into scattering materials, especially biological tissue, uses a guide star created with an ultrasound focus [9

9. X. Xu, H. Liu, and L. V. Wang, “Time-reversed ultrasonically encoded optical focusing into scattering media,” Nat. Photonics 5(3), 154–157 (2011). [CrossRef] [PubMed]

]. Ultrasonic waves propagate through soft tissue with three orders of magnitude less scattering than optical waves, allowing them to penetrate much deeper with minimal scattering. The ultrasound focus guide star localy modulates the frequency of light crossing it. These tagged photons are then used to record the scattered optical field, which when phase conjugated, delivers photons back to the ultrasound focus [9

9. X. Xu, H. Liu, and L. V. Wang, “Time-reversed ultrasonically encoded optical focusing into scattering media,” Nat. Photonics 5(3), 154–157 (2011). [CrossRef] [PubMed]

]. Later, similar techniques were improved to allow for a reduction of the optical focus spot size [10

10. B. Judkewitz, Y. M. Wang, R. Horstmeyer, A. Mathy, and C. Yang, “Speckle-scale focusing in the diffusive regime with time-reversal of variance-encoded light (TROVE),” Nat. Photonics 7(4), 300–305 (2013). [CrossRef] [PubMed]

,11

11. K. Si, R. Fiolka, and M. Cui, “Breaking the spatial resolution barrier via iterative sound-light interaction in deep tissue microscopy,” Sci. Rep. 2, 748 (2012). [CrossRef] [PubMed]

].

Therefore, in this work we use a photoacoustic feedback optimized optical focus for creating an image of an absorbing medium behind a scatterer. Analyzing the temporal profile of the photoacoustic wave, we form a three-dimensional (3D) image after a two dimensional scan. Remarkably, we achieve a significant improvement in signal-to-noise ratio of about one order of magnitude.

2. High contrast photoacoustic imaging system

The high contrast photoacoustic imaging system combines wavefront shaping, using a liquid crystal spatial light modulator (SLM), and an ultrasound transducer for measuring the photoacoustic signal. The SLM phase encodes the wavefront to maximize the intensity of light, while the transducer provides feedback for the iterative optimization algorithm [Fig. 1
Fig. 1 Experimental setup for localized fluence-enhanced 3D photoacoustic imaging. The sample is placed in a glass water tank. A 532 nm pulsed laser illuminates an SLM that controls the input wavefront focused through the glass diffuser. A 90 MHz ultrasound transducer detects the photoacoustic signal from a sample placed behind the scattering material. The signal is amplified and digitized before being analyzed in the PC. The coordinate axis defines the axis in the following Figs. SLM: spatial light modulator; f1, f2: lenses; MO: microscope objective; GD: glass diffuser; S: sample.
.]. More specifically, an attenuated, expanded, and collimated 5 ns laser pulse (Continuum Surelight I20, 20 Hz repetition rate, Nd:YAG frequency doubled to produce 532 nm wavelength) illuminates the entire screen of a phase-only liquid crystal spatial light modulator (SLM) (Boulder Non-linear Systems, 512x512 pixels). After the SLM the energy per pulse in the beam is ~21 μJ. In practice the pulse energy is limited by the damage threshold of the SLM and (more importantly) the sample. A 4f system (f1 = 150mm, f2 = 250mm) images the SLM onto the back aperture of a long working distance microscope objective (Mitutoyo, 34 mm working distance, 5x magnification, 0.14 NA). The beam focuses into a water tank and onto the surface of the scattering material (glass diffuser, Edmund optics, 120 grit). An absorbing sample used for wavefront optimization and imaging is placed ~8 mm behind the scattering material and mounted to a 2D translation stage to allow for scanning in the x and y dimensions. The distance between the sample and the glass diffuser is chosen to approximately match the size of one speckle grain with the size of the photoacoustic focal region. The photoacoustic signal produced by the sample propagates through the water and is detected by a 90 MHz transducer (Olympus, model V3512, 50Mhz bandwidth). The location of the transducer is chosen based on the geometry of the system. However, it could be placed on the same side as the illumination beam without loss of generality. After being pre-amplified (Femto HSA-X-2-40, low-noise 40dB) an oscilloscope records and digitizes the signal and sends it to a computer for analysis. In order to limit the size of the acoustic volume during the optimization process the signal is digitally high-pass filtered using a 2nd order Butterworth filter with a cut-off frequency of 80 MHz .

The system maximizes the photoacoustic signal by modulating the wavefront with the SLM and using an iterative algorithm for optimization. We use a genetic algorithm (GA) for the optimization because it has been demonstrated to work well with low SNR signals [17

17. D. B. Conkey, A. N. Brown, A. M. Caravaca-Aguirre, and R. Piestun, “Genetic algorithm optimization for focusing through turbid media in noisy environments,” Opt. Express 20(5), 4840–4849 (2012). [CrossRef] [PubMed]

], which is sometimes the case with photoacoustics. The GA cost function for the algorithm is the peak-to-peak voltage (proportional to pressure) of the acoustic signal. While in general this signal depends on the absorber size, in our case the absorber is much larger than the acoustic beam diameter so it does not affect the quality of the optimization process. Furthermore, during the optimization process, the absorber does not change, making the cost function depend only on the amount of light absorbed by the sample.

Using the feedback from the photoacoustic signal the GA finds a phase mask which maximizes the cost function and consequently the light intensity within the acoustic focus volume. After the optimization process the best phase mask is projected on the SLM to prepare for image scanning.

3. Photoacoustic enhancement and 3D imaging

As a first demonstration of the performance of the system, we test its ability to increase the photoacoustic signal produced inside a polypropylene tube (90 µm inner and 120 µm outer diameter) filled with India ink and placed behind a glass diffuser. The GA optimization runs with a population size of 20 and 804 input modes through 1200 phase mask measurements, or 60 generations, to find the optimal phase mask. As in [17

17. D. B. Conkey, A. N. Brown, A. M. Caravaca-Aguirre, and R. Piestun, “Genetic algorithm optimization for focusing through turbid media in noisy environments,” Opt. Express 20(5), 4840–4849 (2012). [CrossRef] [PubMed]

] the mutation rate decreased as the optimization progressed. Figure 2(a)
Fig. 2 (a) Experimental photoacoustic enhancement evolution of the optimization process. (b) Photoacoustic amplitude signal for a flat input phase mask (40 averaged samples). (c) Photoacoustic amplitude signal for the optimal input wavefront (5 averaged samples).
illustrates how the enhancement evolved as the optimization proceeded. Here we define the enhancement as the value of the cost function of the projected mask divided by the mean of the cost function from each member of the initial population. An amplitude enhancement of 10 of the photoacoustic signal is observed, indicative of a 10-fold increase in absorbed light in the focal region. In Figs. 2(b) and 2(c) the photoacoustic response for a flat phase and the optimized phase mask are compared. To minimize the noise of the photoacoustic signal we initially average 40 samples of the signal. As the signal strength increases the number of averaged samples decreases gradually to 5 in order to decrease the optimization time. As a result the signals in Figs. 2(b) and 2(c) are taken with 40 and 5 averaged samples, respectively. The optimization process takes about 15 minutes, limited by the repetition rate of the laser. These figures show clearly the improvement in the photoacoustic signal produced using the GA optimization approach.

After verification of photoacoustic signal enhancement, we demonstrate 3D imaging with enhanced localized optical fluence. As a first demonstration we scan the sample around the optimized focus with the automated translation stage while keeping the scattering material at a fixed location. The photoacoustic signal amplitude, recorded from each position [x-y plane, Fig. 1], is processed to reconstruct the 3D maximum intensity projection of the two tubes [Fig. 3(a)
Fig. 3 3D imaging of two capillaries. First row corresponds to the optimized phase mask. Second row corresponds to a flat phase projected on the SLM. (a),(d) 3D scan with a 2% intensity threshold; (b), (e) 2D scan; and (c), (f) 1D scan of the capillary tubes.
]. The temporal profile of the photoacoustic signal [Fig. 2(c)] encodes the z (axial) information. By sliding a window through the signal and selecting the maximum value for each window position many z values can be fixed to each x,y position to create the third dimension. The size of the window is determined by the axial resolution, δz, of the transducer, which comes from its bandwidth, B, and the speed of sound, cs: δz=cs/B30μm. The transducer also determines the transverse resolution of the acoustic beam: BD(6dB)=1.02Fcs/Df36μm,where BD is the acoustic beam diameter at the focus plane, F is the focal distance, D is the diameter of the transducer, and f is the central frequency. In these experiments we approximately match the size of the speckle with the acoustic BD during optimization. Hence, the resolution of our system is given by the speckle size at the transducer focal region.

A 2D slice is extracted from the 3D image to measure the distance between the two tubes. In Fig. 3(b) the normalized 2D image corresponding to an intermediate plane shows the maximum photoacoustic signal from both tubes are separated by 170 µm. From the size of the outer diameter we infer the tubes are 50 µm apart. A normalized 1D scan from the x = 0.02 mm profile is shown in Fig. 3(c). We observe the inner diameter is 50µm, significantly smaller than 90µm diameter given by the manufacture. This can be understood by considering that the photoacoustic feedback came from ink inside of the polypropylene tube. During the optimization, the wavefront correction accounted for the refraction of light produced by the polypropylene tube. In other words, the tube acted as a lens because of the higher index of refraction (1.49) of the polypropylene tubing material. As the optical focus moved away from the center of the tube during the image scan procedure, the wavefront no longer matched the curvature of the tube and the focus inside the tube was destroyed, thus producing negligible signal at the tube edges. Despite this, the tubes are clearly defined with high absorption contrast. For comparison, the 3D, 2D and 1D reconstruction with an unoptimized wavefront projected on the SLM are shown in Figs. 3(d)-3(f) respectively. In this case, the width of the two tubes and their separation agree with the actual tube size, although the SNR is more than 10 times lower than the optimized case. Figure 3(c) also compares the photoacoustic intensities (proportional to acoustic pressure squared) of the optimized and unoptimized scans.

4. Discussion and conclusion

We have demonstrated a one order of magnitude enhancement of the photoacoustic signal amplitude by GA optimization of the phase of the input wavefront to compensate for scattering and increase the optical intensity of light in the acoustic focus. This enhancement allowed for the imaging of two 90 µm inner diameter tubes with excellent signal-to-noise ratio as compared to a flat phase wavefront. Furthermore, by using the time of arrival information from the photoacoustic signal, the depth information was recovered and a 3D image reconstructed after scanning the sample in two dimensions.

In this experimental system we scan the sample around the focus because of the speed limitations imposed by the low repetition rate of the laser. With a higher repetition rate laser source and a faster wavefront modulation device [18

18. D. B. Conkey, A. M. Caravaca-Aguirre, and R. Piestun, “High-speed scattering medium characterization with application to focusing light through turbid media,” Opt. Express 20(2), 1733–1740 (2012). [CrossRef] [PubMed]

], sub-second optimization of the input phase mask could potentially be enabled. This would open up the possibility of scanning the transducer instead of the sample and re-optimizing for each position, providing opportunities for in-vivo testing of biologically relevant samples and to increase the penetration depth of current photoacoustic microscopy techniques.

In conclusion, we have shown for the first time photoacoustic images created by locally enhancing fluence using wavefront shaping and hence the image signal-to-noise ratio. Furthermore, this is the first demonstration of imaging in three dimensions through a highly scattering medium (without using the memory effect [5

5. J. Bertolotti, E. G. van Putten, C. Blum, A. Lagendijk, W. L. Vos, and A. P. Mosk, “Non-invasive imaging through opaque scattering layers,” Nature 491(7423), 232–234 (2012). [CrossRef] [PubMed]

,19

19. O. Katz, E. Small, and Y. Silberberg, “Looking around corners and through thin turbid layers in real time with scattered incoherent light,” Nat. Photonics 6(8), 549–553 (2012). [CrossRef]

]) and without need to access the back side of the scatterer where the object is located.

Acknowledgments

We acknowledge support from the National Science Foundation award DGE-0801680. We also thank Youzhi Li for exciting discussions.

References and Links

1.

I. M. Vellekoop and A. P. Mosk, “Focusing coherent light through opaque strongly scattering media,” Opt. Lett. 32(16), 2309–2311 (2007). [CrossRef] [PubMed]

2.

S. M. Popoff, G. Lerosey, R. Carminati, M. Fink, A. C. Boccara, and S. Gigan, “Measuring the transmission matrix in optics: an approach to the study and control of light Propagation in disordered media,” Phys. Rev. Lett. 104(10), 100601 (2010). [CrossRef] [PubMed]

3.

Z. Yaqoob, D. Psaltis, M. S. Feld, and C. Yang, “Optical phase conjugation for turbidity suppression in biological samples,” Nat. Photonics 2(2), 110–115 (2008). [CrossRef] [PubMed]

4.

I. M. Vellekoop, E. G. van Putten, A. Lagendijk, and A. P. Mosk, “Demixing light paths inside disordered metamaterials,” Opt. Express 16(1), 67–80 (2008). [CrossRef] [PubMed]

5.

J. Bertolotti, E. G. van Putten, C. Blum, A. Lagendijk, W. L. Vos, and A. P. Mosk, “Non-invasive imaging through opaque scattering layers,” Nature 491(7423), 232–234 (2012). [CrossRef] [PubMed]

6.

C.-L. Hsieh, Y. Pu, R. Grange, and D. Psaltis, “Digital phase conjugation of second harmonic radiation emitted by nanoparticles in turbid media,” Opt. Express 18(12), 12283–12290 (2010). [CrossRef] [PubMed]

7.

K. Si, R. Fiolka, and M. Cui, “Fluorescence imaging beyond the ballistic regime by ultrasound pulse guided digital phase conjugation,” Nat. Photonics 6(10), 657–661 (2012). [CrossRef] [PubMed]

8.

O. Katz, E. Small, Y. Bromberg, and Y. Silberberg, “Focusing and compression of ultrashort pulses through scattering media,” Nat. Photonics 5(6), 372–377 (2011). [CrossRef]

9.

X. Xu, H. Liu, and L. V. Wang, “Time-reversed ultrasonically encoded optical focusing into scattering media,” Nat. Photonics 5(3), 154–157 (2011). [CrossRef] [PubMed]

10.

B. Judkewitz, Y. M. Wang, R. Horstmeyer, A. Mathy, and C. Yang, “Speckle-scale focusing in the diffusive regime with time-reversal of variance-encoded light (TROVE),” Nat. Photonics 7(4), 300–305 (2013). [CrossRef] [PubMed]

11.

K. Si, R. Fiolka, and M. Cui, “Breaking the spatial resolution barrier via iterative sound-light interaction in deep tissue microscopy,” Sci. Rep. 2, 748 (2012). [CrossRef] [PubMed]

12.

A. G. Bell, “Upon the production and reproduction of sound by light,” J. Soc. Telegr. Eng. 9, 404–426 (1880).

13.

H. F. Zhang, K. Maslov, G. Stoica, and L. V. Wang, “Functional photoacoustic microscopy for high-resolution and noninvasive in vivo imaging,” Nat. Biotechnol. 24(7), 848–851 (2006). [CrossRef] [PubMed]

14.

C. G. A. Hoelen, F. F. M. de Mul, R. Pongers, and A. Dekker, “Three-dimensional photoacoustic imaging of blood vessels in tissue,” Opt. Lett. 23(8), 648–650 (1998). [CrossRef] [PubMed]

15.

F. Kong, R. H. Silverman, L. Liu, P. V. Chitnis, K. K. Lee, and Y. C. Chen, “Photoacoustic-guided convergence of light through optically diffusive media,” Opt. Lett. 36(11), 2053–2055 (2011). [CrossRef] [PubMed]

16.

T. Chaigne, O. Katz, A. C. Boccara, M. Fink, E. Bossy, and S. Gigan, “Controlling light in scattering media noninvasively using the photo-acoustic transmission-matrix” arXiv:optics 1305.6246, (2013).

17.

D. B. Conkey, A. N. Brown, A. M. Caravaca-Aguirre, and R. Piestun, “Genetic algorithm optimization for focusing through turbid media in noisy environments,” Opt. Express 20(5), 4840–4849 (2012). [CrossRef] [PubMed]

18.

D. B. Conkey, A. M. Caravaca-Aguirre, and R. Piestun, “High-speed scattering medium characterization with application to focusing light through turbid media,” Opt. Express 20(2), 1733–1740 (2012). [CrossRef] [PubMed]

19.

O. Katz, E. Small, and Y. Silberberg, “Looking around corners and through thin turbid layers in real time with scattered incoherent light,” Nat. Photonics 6(8), 549–553 (2012). [CrossRef]

OCIS Codes
(110.5120) Imaging systems : Photoacoustic imaging
(110.7050) Imaging systems : Turbid media
(110.0113) Imaging systems : Imaging through turbid media
(110.1080) Imaging systems : Active or adaptive optics

ToC Category:
Imaging Systems

History
Original Manuscript: August 5, 2013
Revised Manuscript: September 26, 2013
Manuscript Accepted: October 23, 2013
Published: October 29, 2013

Virtual Issues
Vol. 9, Iss. 1 Virtual Journal for Biomedical Optics

Citation
Antonio M. Caravaca-Aguirre, Donald B. Conkey, Jacob D. Dove, Hengyi Ju, Todd W. Murray, and Rafael Piestun, "High contrast three-dimensional photoacoustic imaging through scattering media by localized optical fluence enhancement," Opt. Express 21, 26671-26676 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-22-26671


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References

  1. I. M. Vellekoop and A. P. Mosk, “Focusing coherent light through opaque strongly scattering media,” Opt. Lett.32(16), 2309–2311 (2007). [CrossRef] [PubMed]
  2. S. M. Popoff, G. Lerosey, R. Carminati, M. Fink, A. C. Boccara, and S. Gigan, “Measuring the transmission matrix in optics: an approach to the study and control of light Propagation in disordered media,” Phys. Rev. Lett.104(10), 100601 (2010). [CrossRef] [PubMed]
  3. Z. Yaqoob, D. Psaltis, M. S. Feld, and C. Yang, “Optical phase conjugation for turbidity suppression in biological samples,” Nat. Photonics2(2), 110–115 (2008). [CrossRef] [PubMed]
  4. I. M. Vellekoop, E. G. van Putten, A. Lagendijk, and A. P. Mosk, “Demixing light paths inside disordered metamaterials,” Opt. Express16(1), 67–80 (2008). [CrossRef] [PubMed]
  5. J. Bertolotti, E. G. van Putten, C. Blum, A. Lagendijk, W. L. Vos, and A. P. Mosk, “Non-invasive imaging through opaque scattering layers,” Nature491(7423), 232–234 (2012). [CrossRef] [PubMed]
  6. C.-L. Hsieh, Y. Pu, R. Grange, and D. Psaltis, “Digital phase conjugation of second harmonic radiation emitted by nanoparticles in turbid media,” Opt. Express18(12), 12283–12290 (2010). [CrossRef] [PubMed]
  7. K. Si, R. Fiolka, and M. Cui, “Fluorescence imaging beyond the ballistic regime by ultrasound pulse guided digital phase conjugation,” Nat. Photonics6(10), 657–661 (2012). [CrossRef] [PubMed]
  8. O. Katz, E. Small, Y. Bromberg, and Y. Silberberg, “Focusing and compression of ultrashort pulses through scattering media,” Nat. Photonics5(6), 372–377 (2011). [CrossRef]
  9. X. Xu, H. Liu, and L. V. Wang, “Time-reversed ultrasonically encoded optical focusing into scattering media,” Nat. Photonics5(3), 154–157 (2011). [CrossRef] [PubMed]
  10. B. Judkewitz, Y. M. Wang, R. Horstmeyer, A. Mathy, and C. Yang, “Speckle-scale focusing in the diffusive regime with time-reversal of variance-encoded light (TROVE),” Nat. Photonics7(4), 300–305 (2013). [CrossRef] [PubMed]
  11. K. Si, R. Fiolka, and M. Cui, “Breaking the spatial resolution barrier via iterative sound-light interaction in deep tissue microscopy,” Sci. Rep.2, 748 (2012). [CrossRef] [PubMed]
  12. A. G. Bell, “Upon the production and reproduction of sound by light,” J. Soc. Telegr. Eng.9, 404–426 (1880).
  13. H. F. Zhang, K. Maslov, G. Stoica, and L. V. Wang, “Functional photoacoustic microscopy for high-resolution and noninvasive in vivo imaging,” Nat. Biotechnol.24(7), 848–851 (2006). [CrossRef] [PubMed]
  14. C. G. A. Hoelen, F. F. M. de Mul, R. Pongers, and A. Dekker, “Three-dimensional photoacoustic imaging of blood vessels in tissue,” Opt. Lett.23(8), 648–650 (1998). [CrossRef] [PubMed]
  15. F. Kong, R. H. Silverman, L. Liu, P. V. Chitnis, K. K. Lee, and Y. C. Chen, “Photoacoustic-guided convergence of light through optically diffusive media,” Opt. Lett.36(11), 2053–2055 (2011). [CrossRef] [PubMed]
  16. T. Chaigne, O. Katz, A. C. Boccara, M. Fink, E. Bossy, and S. Gigan, “Controlling light in scattering media noninvasively using the photo-acoustic transmission-matrix” arXiv:optics 1305.6246, (2013).
  17. D. B. Conkey, A. N. Brown, A. M. Caravaca-Aguirre, and R. Piestun, “Genetic algorithm optimization for focusing through turbid media in noisy environments,” Opt. Express20(5), 4840–4849 (2012). [CrossRef] [PubMed]
  18. D. B. Conkey, A. M. Caravaca-Aguirre, and R. Piestun, “High-speed scattering medium characterization with application to focusing light through turbid media,” Opt. Express20(2), 1733–1740 (2012). [CrossRef] [PubMed]
  19. O. Katz, E. Small, and Y. Silberberg, “Looking around corners and through thin turbid layers in real time with scattered incoherent light,” Nat. Photonics6(8), 549–553 (2012). [CrossRef]

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