Figure 1: Schematic of the imaging principle.
From: Deep-tissue focal fluorescence imaging with digitally time-reversed ultrasound-encoded light
(a) In the recording step, a 0.8-mm wide sample beam (f0) scatters as it propagates through the tissue sample. A confined region of the scattered light in the tissue sample is frequency-shifted (f0±fUS) by a focused ultrasound pulse. The ultrasound focus thus becomes a virtual source within the tissue. Both the frequency-shifted light and the non-shifted light further scatter through the tissue and are collected. This output wavefront interferes with a reference beam (f0+fUS) and the resulting interference pattern is imaged onto a scientific CMOS (sCMOS) camera in the digital phase conjugate mirror module. The digital phase conjugate mirror selectively measures the phase map (φ(x, y)) of the frequency-shifted light through digital phase-shifting holography. The ultrasound is turned off after recording. (b) In the playback step, the conjugate of the recorded phase map (−Φ(x, y)) is displayed on a spatial light modulator (SLM) placed at the image plane of the sCMOS camera. The reference beam reflects off the SLM and is transformed into the phase conjugate beam that is propagated back into the tissue, reconstructing an optical focus at the ultrasound modulation location. Any excited fluorescence is collected and measured outside the tissue using a photodetector.
