Most recently, Xu et al.8 introduced a focused ultrasonic beam as an internal “guide star” for OPC, so that the resultant optical focal point can be positioned anywhere inside a turbid medium. In this technique, called time-reversed ultrasonically encoded (TRUE) optical focusing, a turbid medium is exposed simultaneously to coherent incident light and a focused ultrasound field. The incident photons experience a series of random elastic scattering events within the sample. Part of the scattered photons traverse the ultrasound region, i.e., the acousto-optic (AO) interaction volume, and are “tagged” with an ultrasonic frequency.9 The resultant diffused light, collected outside the sample, is sent to a photorefractive crystal (PRC) functioning as a PCM. Unlike the general OPC method, in TRUE optical focusing, only the ultrasonically tagged photons contribute to a stable hologram, which later generates a time-reversed wavefront. Since all tagged photons originate from the AO interaction volume, the conjugated copy will eventually travel back, although tortuously, to the AO interaction volume inside the turbid medium. Given the much broader spatial distribution of the diffused light, the ultrasound focus determines the AO interaction volume, hence the location and extent of the “time-reversed” focus. Therefore by moving the relative position of the ultrasound focus inside the turbid medium, one can dynamically focus light to anywhere predefined by the ultrasound focus.