High contrast three-dimensional photoacoustic imaging through scattering media by localized optical fluence enhancement

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.

Wavefront Shaping and Its Application to Enhance Photoacoustic Imaging

“…Moreover, ultrasound has orders of magnitude weaker scattering than light does, and it allows deep penetration and precise pinpointing to the targeted position. Therefore, photoacoustically-guided wavefront shaping has rapidly gained attention and been followed by several other research teams [26,28,[45][46][47]. However, since the diffused optical energy is maximized within the ultrasonic focus, the dimension of the resultant optical focal spot is typically ultrasonic-diffraction limited.…”

“…On the other hand, if only shallow tissue depths (e.g., within one transport mean free path) are sought, where ballistic or quasi-ballistic photons dominate, the optical spot size can be much smaller than that of the ultrasonic focus, the resolution of PA imaging is determined by the optical-but not the acoustic-focal size. In this scenario, the technique is often referred to as optical-resolution photoacoustic imaging or microscopy [28,29].…”

“…However, recently, researchers began to notice that the seemingly random scattering events and the resultant speckles are actually deterministic within a certain temporal window [37,38], and it is possible to reverse [39][40][41] or compensate for [42] the scattering-induced phase scrambling. To do so, researchers have developed several wavefront shaping (sometimes also referred to wavefront engineering) techniques, such as iterative wavefront optimization [23][24][25][26]28,[42][43][44][45][46][47][48][49][50][51], measuring the transmission matrix of the scattering medium [21,22,[52][53][54][55][56], and optical time reversal via phase conjugation [39,40,[57][58][59][60][61][62][63][64][65]. Nevertheless, the goals of these implementations are identical, i.e., to make light wavelets traveling along different optical paths interfere coherently at a region of interest (ROI) and form a bright optical spot (focus) out of the much darker background.…”

“…For that purpose, researchers have proposed injected or embedded probes, such as small optical sensors [66,67], fluorescence particles [68,69], and nonlinear beads [57], ultrasound mediation [40,58,61,70,71], as well as absorption perturbation [22,25,46,48,[72][73][74][75][76]. Among them, PA signal has been proved very attractive [22,25,28,47,48,53,55,73,74,76], as it is noninvasive, relatively deeply-penetrating, and it can pinpoint the focal spot accurately (the acoustic and optical foci overlap). Furthermore, with linear [22,25,28,53,73,74] or nonlinear [48] PA signals as the guide star, acousticand optical-diffraction limited optical focal spot can be achieved, respectively, benefiting from the nature of PA signal generation.…”

“…Among them, PA signal has been proved very attractive [22,25,28,47,48,53,55,73,74,76], as it is noninvasive, relatively deeply-penetrating, and it can pinpoint the focal spot accurately (the acoustic and optical foci overlap). Furthermore, with linear [22,25,28,53,73,74] or nonlinear [48] PA signals as the guide star, acousticand optical-diffraction limited optical focal spot can be achieved, respectively, benefiting from the nature of PA signal generation. In turn, during the process of such PA-guided wavefront optimization, the in situ optical fluence is gradually increased, and so is the strength of PA signals.…”