Frequency-swept time-reversed ultrasonically encoded optical focusing

Abstract

A technique to rapidly scan an optical focus inside a turbid medium is attractive for various biomedical applications. Time-reversed ultrasonically encoded (TRUE) optical focusing has previously demonstrated light focusing into a turbid medium, using both analog and digital devices. Although the digital implementation can generate a focus with high energy, it has been time consuming to scan the TRUE focus inside a sample. Here, by sweeping the frequencies of both ultrasound and light, we demonstrate a multiplex recording of ultrasonically encoded wavefronts, accelerating the generation of multiple TRUE foci. Using this technique, we obtained a 2-D image of a fluorescent target centered inside a turbid sample having a thickness of 2.4 transport mean free paths. Fluorescence imaging is widely used to obtain biological images by scanning an optical focus. 1 However, due to scattering, optical focusing using an ordinary lens is limited to shallow depths of one transport mean free path, or 1 l t 0 ($1 mm in human skin), 2 beyond which scattering both reduces the amount of light arriving at the target and blurs the resulting image.One way to overcome this limitation is to use timereversed ultrasonically encoded (TRUE) focusing. 3,4 In TRUE focusing, a focused ultrasonic (US) pulse, applied inside a turbid sample, frequency modulates (or encodes) light within the acoustic volume. A phase-conjugated version of the encoded wavefront is then generated, using either analog or digital phase conjugate mirrors (PCMs). 3-6 Digital PCMs, consisting of a camera and a spatial light modulator (SLM), are attractive for higher energy focusing. [4][5][6] To record the encoded wavefront, a reference beam interferes with the encoded light on the camera, causing the intensity of the interferogram to beat at their difference frequency. 6 The encoded wavefront is extracted from the beat, and then its phase-conjugated version is reproduced using the SLM. Upon back-propagation to the sample, the phase-conjugated beam forms an optical focus at the original location of the ultrasound volume.It has been previously shown that TRUE focusing improves the resolution, thereby allowing deep fluorescence imaging beyond 1 l t 0 inside a scattering medium. 4-6 However, one of the challenges of applying digital TRUE focusing for imaging was the long time (several seconds) taken to generate a single optical focus. 5,6 The low signal-to-noise ratio (SNR) of the encoded-light detection typically mandates that multiple frames of interferograms be recorded and averaged to obtain a single encoded wavefront.Here, we propose a method called frequency-swept TRUE focusing, which takes advantage of the multiple recorded frames used for encoded-light detection to accelerate TRUE focal scanning. By sweeping the frequency of both the ultrasound and the light at the same time, we achieve simultaneous recording of multiple wavefronts, corresponding to different positions along the acoustic axis, without sacrificing SNR and using the same number of camera frames. A simi...

Focusing light inside dynamic scattering media with millisecond digital optical phase conjugation

“…The only difference is that the PBR of the focus would be much lower when focusing light inside tissue, compared with when focusing light in between two pieces of tissue, because the speckle size inside tissue is much smaller than that between two pieces of tissue. When focusing light inside tissue, this small-speckle-size-induced low PBR is a major challenge to all acoustic-wave-guided wavefront shaping techniques [4,15,19,24–31,41,47,58–60]. This low PBR is a separate problem to solve that is beyond the scope of this work, which concentrates on improving the speed, rather than improving the PBR of TRUE focusing.…”

“…To take full advantage of the SLM and further improve the speed of ultrasound-guided DOPC, we develop a double-exposure binary wavefront measurement method. The speed of our system is one to two orders of magnit ude higher than those of previous ultrasound-guided DOPC systems [16,24–31], and our method achieves the fastest light focusing inside a scattering medium among all the digital wavefront shaping methods developed to date [3–6]. …”

Fast optical wavefront engineering for controlling light propagation in dynamic turbid media

“…16bits) needed to be utilized and there was heavy load for data transfer. 49,56,[58][59][60] Besides, the shot noise inside raw signal with high background portion sharply reduced the SNR of collected light¯eld. To addressing these challenges, Liu et al in 2016 proposed a novel lock-in camera-based digital TRUE system.…”

Bit-efficient, sub-millisecond wavefront measurement using a lock-in camera for time-reversal based optical focusing inside scattering media

“…Even worse, phase-shifting holography needs to record and transfer at least four frames of images to calculate the phase map on a computer, so the speed of wavefront determination is severely limited by the low frame rates of conventional cameras and the heavy load during data transfer. When averaging is needed, even more frames need to be recorded and transferred, so it takes seconds to acquire a phase map (with 1920 × 1080 pixels) before time-reversed focusing can be performed in previous works [5,6,11,12]. To apply this technique in vivo , systems with higher speeds are strongly desired to accommodate the fast speckle decorrelation (on a time scale from 0.1 ms to 1 ms) primarily due to blood flow [9].…”