Acousto optic imaging beyond the acoustic diffraction limit using speckle decorrelation

Abstract

Acousto-optic tomography (AOT) enables optical-contrast imaging deep inside scattering samples via localized ultrasound modulation of scattered light. However, the resolution of AOT is inherently limited by the ultrasound focus size, prohibiting microscopic investigations. In the last few years, advances in the field of digital wavefront-shaping have allowed the development of novel approaches for overcoming the acoustic resolution limit of AOT. However, these novel approaches require the execution of thousands of wavefront measurements within the sample speckle decorrelation time, limiting their application to static samples. Here, we show that it is possible to surpass the acoustic resolution limit with a conventional AOT system by exploiting the natural dynamics of speckle decorrelations rather than trying to overcome them. We achieve this by adapting the principles of super-resolution optical fluctuations imaging (SOFI), originally developed for imaging blinking fluorophores, to AOT. We show that naturally fluctuating optical speckle grains can serve as the analogues of blinking fluorophores, enabling super-resolution by statistical analysis of fluctuating acousto-optic signals.

Acousto-optic Ptychography

“…The diffused light is ultrasonically tagged at a chosen position inside the sample by a focused ultrasound pulse at a central frequency f US . The acousto-optic modulated (ultrasound-tagged) light field at frequency f AO = f opt + f US is measured by a camera placed outside the sample using a pulsed reference beam that is synchronized with the ultrasound pulses, via off-axis phase-shifting interferometry [20,30] (Supplementary section 1).…”

“…Since the conventional AOI image (Fig. 1(g)) is a convolution between the target object and the ultrasound-focus pressure distribution [20], its resolution is limited by the acoustic diffraction limit.…”

“…In addition, in practice, these techniques do not allow a resolution increase of more than a factor of ×3 − ×6 improvement from the acoustical diffraction-limit, when sub-micron optical speckle grains are considered [18]. Recently, approaches that do not rely on wavefront shaping, and exploit the dynamic fluctuations to enable improved resolution [20] or fluorescent imaging [21] have been demonstrated, but these do not practically allow a resolution increase of more than a factor of 2-3. Closing the two orders of magnitude gap between the ultrasound resolution and the optical diffraction-limit is thus still an open challenge.…”

Embracing the uncertainty: the evolution of SOFI into a diverse family of fluctuation-based super-resolution microscopy methods

“…While we limit the scope of the current review to microscopy methods that apply fluorescence fluctuations, it is interesting to note that the principles of SOFI have inspired multiple original imaging techniques that use fluctuations in the illumination pattern for super-resolution in fluorescence [27][28][29][30] and photo-acoustic [31][32][33] imaging. This choice of scope is not only a semantic distinction, rather it has an important implication on the resolution.…”

Fast non-line-of-sight imaging with high-resolution and wide field of view using synthetic wavelength holography

“…T here are numerous instances of imaging within the physical sciences wherein an opaque barrier (such as a wall) or a scattering medium (such as fog or tissue) impedes direct view of the object. Over the years, many attempts [1][2][3][4][5][6][7][8][9][10][11][12][13] have been made to non-invasively recover images of objects obscured from direct view. These techniques are collectively referred to as Non Line-of-Sight Imaging (NLoS) in our work.…”