Improving the imaging ability of ultrasound-modulated optical tomography with spectral-hole burning

Ultrasound-modulated optical tomography is a hybrid imaging technique based on detection of the diffused light modulated by a focused ultrasonic wave inside a scattering medium. With the combined advantages of ultrasonic resolution and optical contrast, UOT is ideal for deep tissue imaging. Its growth in popularity and application, however, is hindered by the low efficiency in detecting the modulated diffused photons. Research activities on UOT have therefore been centered on improving its signal detection efficiency by exploring various technical solutions. A prime example is the use of spectral-hole burning (SHB) in UOT. By applying SHB crystal as a spectral filter, one modulation sideband of the diffused light can be efficiently selected while all the other frequency components are strongly suppressed. Immune to both the spatial and temporal incoherence of the signal with a high enough on/off ratio, SHB can boost the UOT imaging ability dramatically and push it towards practical applications. We compare SHB with the other technologies that have been applied to UOT, and identify the unique features that make SHB a preferable tool for UOT. We also discuss the desired improvements from the SHB side, which will help UOT pave the way from research to everyday life

Focusing light into turbid media: time-reversed ultrasonically encoded (TRUE) focusing

In turbid media such as biological tissues, light undergoes multiple scattering. Consequently, it is not possible to focus light at depths beyond one transport mean free path in such media. To break through this limit, we proposed and experimentally demonstrated a novel technique, based on ultrasonic encoding of diffused laser light and optical time reversal, which effectively focuses light into a turbid medium. In the experimental implementation of the Time-Reversed Ultrasonically Encoded (TRUE) optical focusing, a turbid medium was illuminated by a laser beam with a long coherence length. The incident light was multiply scattered inside the medium and ultrasonically encoded within the ultrasonic focal zone. The wavefront of the ultrasonically encoded light was then time reversed by a Phase Conjugate Mirror (PCM) outside the medium. The time-reversed (or phase conjugated) optical wavefront traced back the trajectories of the ultrasonically encoded diffused light, and converged to the ultrasonic focal zone. With a commercially available photorefractive crystal as the PCM, the main approaches for increasing focusing depth are to improve the efficiencies of ultrasonic encoding and time reversal. Our recent experiments showed that light can be focused into a 5-mm thick tissue-mimicking phantom (optical thickness = 50, i.e., geometric thickness = 50 mean free paths) with a dynamically adjustable focus. The TRUE optical focusing opens a door to focusing light into turbid media or manipulating light-matter interactions

Time-reversed ultrasonically encoded optical focusing into tissue-mimicking media with thickness up to 70 mean free paths

In turbid media such as biological tissue, multiple scattering hinders direct light focusing at depths beyond one transport mean free path. As a solution to this problem, time-reversed ultrasonically encoded (TRUE) optical focusing is proposed based on ultrasonic encoding of diffused laser light and optical time reversal. In TRUE focusing, a laser beam of long coherence length illuminates a turbid medium, where the incident light undergoes multiple scattering and part of it gets ultrasonically encoded within the ultrasonic focal zone. A conjugated wavefront of the ultrasonically encoded light is then generated by a phase conjugate mirror outside the medium, which traces back the trajectories of the ultrasonically encoded diffused light and converges light to the ultrasonic focal zone. Here, we report the latest experimental improvement in TRUE optical focusing that increases its penetration in tissue-mimicking media from a thickness of 3.75 to 7.00 mm. We also demonstrate that the TRUE focus depends on the focal diameter of the ultrasonic transducer

Time-reversed ultrasonically encoded optical focusing in biological tissue

We report an experimental investigation of time-reversed ultrasonically encoded optical focusing in biological tissue. This technology combines the concepts of optical phase conjugation and ultrasound modulation of diffused coherent light. The ultrasonically encoded (or tagged) diffused light from a tissue sample is collected in reflection mode and interferes with a reference light in a photorefractive crystal (used as a phase conjugation mirror) to form a hologram. Then a time-reversed copy of the tagged light is generated and traces back the original trajectories to the ultrasonic focus inside the tissue sample. With our current setup, we can achieve a maximum penetration depth of 5 mm in a chicken breast sample and image optical contrasts within a tissue sample with a spatial resolution approximately equaling 1/√2 of the ultrasound focal diameter

Ultrasonic encoding of diffused light: from optical imaging to light focusing in turbid media

In optical scattering media such as biological tissue, light propagation is randomized by multiple scattering. Beyond one transport mean free path, where photon propagation is in the diffusive regime, direct light focusing becomes infeasible. The resulting loss of light localization poses serious challenge to optical imaging in thick scattering media. Ultrasound modulated optical tomography (UOT) combines high optical contrast and good ultrasonic resolution, and is therefore an ideal imaging modality for soft biological tissue. A variety of detection techniques have been developed in UOT in an effort to discriminate the ultrasonically encoded diffused light as the imaging signal. We developed a photorefractive crystal based detection system, which has the ability to image both the optical and acoustic properties of biological tissues. With the improved photorefractive crystal based detection, tissue-mimicking phantom samples as thick as 9.4 cm can be imaged. We further exploit the virtual source concept in UOT and combine it with optical time reversal to achieve diffusive light focusing into scattering media. Experimental implementation of this new technology is presented

Time-reversed ultrasonically encoded (TRUE) optical focusing in reflection mode: demonstrations in tissue mimicking phantoms and ex vivo tissue

The problem of how to effectively deliver light dynamically to a small volume inside turbid media has been intensively investigated for imaging and therapeutic purposes. Most recently, a new modality termed Time-Reversed Ultrasonically Encoded (TRUE) optical focusing was proposed by integrating the concepts of ultrasound modulation of diffused light with optical phase conjugation. In this work, the diffused photons that travel through the ultrasound focal region are "tagged" with a frequency shift due to the ultrasound modulation. Part of the tagged light is collected in reflection mode and transmitted to a photorefractive crystal, forming there a stationary hologram through interference with a coherent reference optical beam. The hologram is later read by a conjugated optical beam, generating a phase conjugated wavefront of the tagged light. It is conveyed back to the turbid medium in reflection mode, and eventually converges to the ultrasound focal zone. Optical focusing effects from this system are demonstrated experimentally in tissue-mimicking phantoms and ex vivo chicken breast tissue, achieving effective round-trip optical penetration pathlength (extinction coefficient multiplied by round-trip focusing depth) exceeding 160 and 100, respectively. Examples of imaging optical inclusions with this system are also reported

Reflection-mode time-reversed ultrasonically encoded optical focusing into turbid media

Time-reversed ultrasonically encoded (TRUE) optical focusing was recently proposed to deliver light dynamically to a tight region inside a scattering medium. In this letter, we report the first development of a reflection-mode TRUE optical focusing system. A high numerical aperture light guide is used to transmit the diffusely reflected light from a turbid medium to a phase-conjugate mirror (PCM), which is sensitive only to the ultrasound-tagged light. From the PCM, a phase conjugated wavefront of the tagged light is generated and conveyed by the same light guide back to the turbid medium, subsequently converging to the ultrasonic focal zone. We present experimental results from this system, which has the ability to focus light in a highly scattering medium with a round-trip optical penetration thickness (extinction coefficient multiplied by round-trip depth) as large as 160

Focusing light into turbid media: time-reversed ultrasonically encoded (TRUE) focusing

In turbid media such as biological tissues, light undergoes multiple scattering. Consequently, it is not possible to focus light at depths beyond one transport mean free path in such media. To break through this limit, we proposed and experimentally demonstrated a novel technique, based on ultrasonic encoding of diffused laser light and optical time reversal, which effectively focuses light into a turbid medium. In the experimental implementation of the Time-Reversed Ultrasonically Encoded (TRUE) optical focusing, a turbid medium was illuminated by a laser beam with a long coherence length. The incident light was multiply scattered inside the medium and ultrasonically encoded within the ultrasonic focal zone. The wavefront of the ultrasonically encoded light was then time reversed by a Phase Conjugate Mirror (PCM) outside the medium. The time-reversed (or phase conjugated) optical wavefront traced back the trajectories of the ultrasonically encoded diffused light, and converged to the ultrasonic focal zone. With a commercially available photorefractive crystal as the PCM, the main approaches for increasing focusing depth are to improve the efficiencies of ultrasonic encoding and time reversal. Our recent experiments showed that light can be focused into a 5-mm thick tissue-mimicking phantom (optical thickness = 50, i.e., geometric thickness = 50 mean free paths) with a dynamically adjustable focus. The TRUE optical focusing opens a door to focusing light into turbid media or manipulating light-matter interactions