Optical coherence elastography (OCE), as the use of OCT to perform elastography has come to be known, began in 1998, around ten years after the rest of the field of elastography – the use of imaging to deduce mechanical properties of tissues. After a slow start, the maturation of OCT technology in the early to mid 2000s has underpinned a recent acceleration in the field. With more than 20 papers published in 2015, and more than 25 in 2016, OCE is growing fast, but still small compared to the companion fields of cell mechanics research methods, and medical elastography. In this review, we describe the early developments in OCE, and the factors that led to the current acceleration. Much of our attention is on the key recent advances, with a strong emphasis on future prospects, which are exceptionally bright.

Tissue optical clearing technique provides a prospective solution for the application of advanced optical methods in life sciences. This paper gives a review of recent developments in tissue optical clearing techniques. The physical, molecular and physiological mechanisms of tissue optical clearing are overviewed and discussed. Various methods for enhancing penetration of optical‐clearing agents into tissue, such as physical methods, chemical‐penetration enhancers and combination of physical and chemical methods are introduced. Combining the tissue optical clearing technique with advanced microscopy image or labeling technique, applications for 3D microstructure of whole tissues such as brain and central nervous system with unprecedented resolution are demonstrated. Moreover, the difference in diffusion and/or clearing ability of selected agents in healthy versus pathological tissues can provide a highly sensitive indicator of the tissue health/pathology condition. Finally, recent advances in optical clearing of soft or hard tissue for in vivo imaging and phototherapy are introduced.

We report on a noncontact low-coherence optical phase-based imaging method, termed shear wave imaging optical coherence tomography (SWI-OCT), which enables 2D depth-resolved visualization of the low-amplitude elastic wave propagation in tissue with ultrahigh frame rate. SWI-OCT is based on 1D transverse scanning of the M-mode OCT imaging that is precisely synchronized with a low-pressure short-duration air-puff loading system. This approach of scanning and data recording allows visualization of the induced tissue deformation at high frame rate. The applied phase-resolved interferometric technique, with sensitivity on the nanometer scale, makes the low-amplitude tissue displacement detectable. For the demonstration of this method, and to study its application for tissue biomechanics, we performed pilot experiments on agar phantoms and ex vivo rabbit corneas. Samples with different elastic properties can be differentiated based on the velocity of the elastic wave propagation that is directly visualized with a 25 kHz frame rate. Our results indicate that SWI-OCT has the potential to be further developed as a major technique for depth-resolved high-resolution tissue elastography in vivo.

To improve the safety and efficacy of thermal therapy, it is necessary to map tissue temperature in real time with submillimetre spatial resolution. Accurate temperature maps may provide the necessary control of the boundaries of the heated regions and minimize thermal damage to surrounding normal tissues. Current imaging modalities fail to monitor tissue temperature in real time with high resolution and accuracy. We investigated a non-invasive optoacoustic method for accurate, real-time monitoring of tissue temperature during thermotherapy. In this study, we induced temperature gradients in tissue and tissue-like samples and monitored the temperature distribution using the optoacoustic technique. The fundamental harmonic of a Q-switched Nd : YAG laser (λ = 1064 nm) was used for optoacoustic wave generation and probing of tissue temperature. The tissue temperature was also monitored with a multi-sensor temperature probe inserted in the samples. Good agreement between optoacoustically measured and actual tissue temperatures was obtained. The accuracy of temperature monitoring was better than 1ûC, while the spatial resolution was about 1 mm. These data suggest that the optoacoustic technique has the potential to be used for non-invasive, real-time temperature monitoring during thermotherapy.

Accurate non-invasive assessment of tissue elasticity in vivo is required for early diagnostics of many tissue abnormalities. We have developed a focused air-pulse system that produces a low-pressure and short-duration air stream, which can be used to excite transient surface waves (SWs) in soft tissues. System characteristics were studied using a high-resolution analog pressure transducer to describe the excitation pressure. Results indicate that the excitation pressure provided by the air-pulse system can be easily controlled by the air source pressure, the angle of delivery, and the distance between the tissue surface and the port of the air-pulse system. Furthermore, we integrated this focused air-pulse system with phase-sensitive optical coherence tomography (PhS-OCT) to make non-contact measurements of tissue elasticity. The PhS-OCT system is used to assess the group velocity of SW propagation, which can be used to determine Young’s modulus. Pilot experiments were performed on gelatin phantoms with different concentrations (10%, 12% and 14% w/w). The results demonstrate the feasibility of using this focused air-pulse system combined with PhS-OCT to estimate tissue elasticity. This easily controlled non-contact technique is potentially useful to study the biomechanical properties of ocular and other tissues in vivo.