Deep Biological Tissues Research Articles

Focusing Through Scattering Media Via 1D Speckle Signal Feedback

AbstractLight propagation in complex media results in strong scattering. While wavefront shaping (WFS) enables the focusing of light at depth, its speed is mainly limited by the frame rate of the area‐array detectors. The photodetector has been used to achieve fast focusing, but it cannot record sufficient speckle information, limiting its applications in multi‐point focusing and non‐invasive focusing. Here, a method for invasively or non‐invasively focusing through scattering media is proposed by using 1D speckle signals as feedback for wavefront shaping. Experimentally, not only invasive multi‐point focusing can be realized, but also by maximizing the contrast of linear fluorescent 1D speckle signals, non‐invasive focusing can be achieved, suggesting the effectiveness of the proposed method. This approach can be generalized to the utilization of linear array detectors in WFS and may hold interesting prospects for rapid focusing light within deep biological tissues.

Supernormal Temperature Sensing Performance Realized through the Blue Emitting Level of Er3+ along with Detection Ability in Deep Tissues.

Fluorescence intensity ratio (FIR)-type optical thermometers based on thermally coupled energy levels (TCLs) of rare earth ions are suitable candidates for noncontact temperature detection in living organisms, microelectronics apparatus, and so forth. Therefore, the improvement of the thermometric sensitivity of TCL-based thermometers has become a research hotspot in recent years. Herein, ultrahigh sensitivity and outstanding resolution for temperature sensing have been realized in YNbO4: Yb3+/Er3+. Unusually, the thermally coupled three-level system of Er3+: 4F7/2/2H11/2/4S3/2 is first employed for optical thermometry based on FIR technology. A supernormal thermometric sensitivity of 2.67% K-1 is obtained from the thermally coupled 4F7/2 and 4S3/2 states due to the large energy gap between them, significantly surpassing that of most temperature sensors in the same category. Furthermore, the existence of the intermediate level 2H11/2 can effectively prevent the decoupling effect between 4F7/2 and 4S3/2. Additionally, the temperature sensing behavior realized by the Stark sublevels of the Er3+: 4I13/2 → 4I15/2 transition, with a penetration depth of 8 mm, shows the potential of temperature measurement in deep biological tissues, benefiting from its excitation and emission wavelengths located in the biological window. All of the data reveal that YNbO4: Yb3+/Er3+ is an ultrasensitive optical thermometer and exhibits the capacity of temperature detection in deep tissues.

Fast finite difference solver for optical microscopy in deep biological tissue

Optical scattering poses a significant challenge to high-resolution microscopy within deep tissue. To accurately predict the performance of various microscopy techniques in thick samples, we present a computational model that efficiently solves Maxwell’s equation in highly scattering media. This toolkit simulates the deterioration of the laser beam point spread function (PSF) without making a paraxial approximation, enabling accurate modeling of high-numerical-aperture (NA) objective lenses commonly employed in experiments. Moreover, this framework is applicable to a broad range of scanning microscopy techniques including confocal microscopy, stimulated emission depletion (STED) microscopy, and ground-state depletion microscopy. Notably, the proposed method requires only readily obtainable macroscopic tissue parameters. As a practical demonstration, we investigate the performance of Laguerre–Gaussian (LG) versus Hermite–Gaussian (HG) depletion beams in STED microscopy.

Magneto-oscillatory localization for small-scale robots

Magnetism is widely used for the wireless localization and actuation of robots and devices for medical procedures. However, current static magnetic localization methods suffer from large required magnets and are limited to only five degrees of freedom due to a fundamental constraint of the rotational symmetry around the magnetic axis. We present the small-scale magneto-oscillatory localization (SMOL) method, which is capable of wirelessly localizing a millimeter-scale tracker with full six degrees of freedom in deep biological tissues. The SMOL device uses the temporal oscillation of a mechanically resonant cantilever with a magnetic dipole to break the rotational symmetry, and exploits the frequency-response to achieve a high signal-to-noise ratio with sub-millimeter accuracy over a large distance of up to 12 centimeters and quasi-continuous refresh rates up to 200 Hz. Integration into real-time closed-loop controlled robots and minimally-invasive surgical tools are demonstrated to reveal the vast potential of the SMOL method.

Explorative study using ultrasound time-harmonic elastography for stiffness-based quantification of skeletal muscle function

Time-harmonic elastography (THE) is an emerging ultrasound imaging technique that allows full-field mapping of the stiffness of deep biological tissues. THE's unique ability to rapidly capture stiffness in multiple tissues has never been applied for imaging skeletal muscle. Therefore, we addressed the lack of data on temporal changes in skeletal muscle stiffness while simultaneously covering stiffness of different muscles.Acquiring repeated THE scans every five seconds we quantified shear-wave speed (SWS) as a marker of stiffness of the long head (LHB) and short head (SHB) of biceps brachii and of the brachialis muscle (B) in ten healthy volunteers. SWS was continuously acquired during a 3-min isometric preloading phase, a 3-min loading phase with different weights (4, 8, and 12 kg), and a 9-min postloading phase. In addition, we analyzed temporal SWS standard deviation (SD) as a marker of muscle contraction regulation.Our results (median [min, max]) showed both SWS at preloading (LHB: 1.04 [0.94, 1.12] m/s, SHB: 0.86 [0.78, 0.94] m/s, B: 0.96 [0.87, 1.09] m/s, p < 0.001) and the increase in SWS with loading weight to be muscle-specific (LHB: 0.010 [0.002, 0.019] m/s/kg, SHB: 0.022 [0.017, 0.042] m/s/kg, B: 0.039 [0.019, 0.062] m/s/kg, p < 0.001). Additionally, SWS during loading increased continuously over time by 0.022 [0.004, 0.051] m/s/min (p < 0.01). Using an exponential decay model, we found an average relaxation time of 27 seconds during postloading. Analogously, SWS SD at preloading was also muscle-specific (LHB: 0.018 [0.011, 0.029] m/s, SHB: 0.021 [0.015, 0.027] m/s, B: 0.024 [0.018, 0.037] m/s, p < 0.05) and increased by 0.005 [0.003, 0.008] m/s/kg (p < 0.01) with loading. SWS SD did not change over loading time and decreased immediately in the postloading phase.Taken together, THE of skeletal muscle is a promising imaging technique for in vivo quantification of stiffness and stiffness changes in multiple muscle groups within seconds. Both the magnitude of stiffness changes and their temporal variation during isometric exercise may reflect the functional status of skeletal muscle and provide additional information to the morphological measures obtained by conventional imaging modalities.

Parallel compensation of anisoplanatic aberrations in patterned photostimulation for two-photon optogenetics

Two-photon optogenetics becomes an indispensable technique in deciphering neural circuits recently, in which patterned photostimulation is generally adopted due to its low time delay and jitter, as well as its finely sculpting ability in space. However, optical aberrations in light propagation often deteriorates patterned photostimulation, leading to decreased intensity of patterns and thus reduced excitation efficiency. Considering anisoplanatic aberrations at different positions, only correcting aberrations at one position may aggravate aberrations at other positions. Here we propose a parallel aberration compensation based Gerchberg–Saxton (PAC-GS) algorithm for generating multiple holographic extended patterns with anisoplanatic aberrations compensated simultaneously. As an example, we demonstrate that PAC-GS is able to parallelly compensate anisoplanatic aberrations of multiple holographic patterns under gradient index (GRIN) lens, thus effectively improving the intensity of each pattern, promising for two-photon optogenetics in deep biological tissues with GRIN lens.

Quantification of blood flow index in diffuse correlation spectroscopy using a robust deep learning method.

Diffuse correlation spectroscopy (DCS) is a powerful, noninvasive optical technique for measuring blood flow. Traditionally the blood flow index (BFi) is derived through nonlinear least-square fitting the measured intensity autocorrelation function (ACF). However, the fitting process is computationally intensive, susceptible to measurement noise, and easily influenced by optical properties (absorption coefficient and reduced scattering coefficient ) and scalp and skull thicknesses. We aim to develop a data-driven method that enables rapid and robust analysis of multiple-scattered light's temporal ACFs. Moreover, the proposed method can be applied to a range of source-detector distances instead of being limited to a specific source-detector distance. We present a deep learning architecture with one-dimensional convolution neural networks, called DCS neural network (DCS-NET), for BFi and coherent factor () estimation. This DCS-NET was performed using simulated DCS data based on a three-layer brain model. We quantified the impact from physiologically relevant optical property variations, layer thicknesses, realistic noise levels, and multiple source-detector distances (5, 10, 15, 20, 25, and 30mm) on BFi and estimations among DCS-NET, semi-infinite, and three-layer fitting models. DCS-NET shows a much faster analysis speed, around 17,000-fold and 32-fold faster than the traditional three-layer and semi-infinite models, respectively. It offers higher intrinsic sensitivity to deep tissues compared with fitting methods. DCS-NET shows excellent anti-noise features and is less sensitive to variations of and at a source-detector separation of 30mm. Also, we have demonstrated that relative BFi (rBFi) can be extracted by DCS-NET with a much lower error of 8.35%. By contrast, the semi-infinite and three-layer fitting models result in significant errors in rBFi of 43.76% and 19.66%, respectively. DCS-NET can robustly quantify blood flow measurements at considerable source-detector distances, corresponding to much deeper biological tissues. It has excellent potential for hardware implementation, promising continuous real-time blood flow measurements.

Performance enhancement in wavefront shaping of multiply scattered light: a review.

In nonballistic regime, optical scattering impedes high-resolution imaging through/inside complex media, such as milky liquid, fog, multimode fiber, and biological tissues, where confocal and multiphoton modalities fail. The significant tissue inhomogeneity-induced distortions need to be overcome and a technique referred as optical wavefront shaping (WFS), first proposed in 2007, has been becoming a promising solution, allowing for flexible and powerful light control. Understanding the principle and development of WFS may inspire exciting innovations for effective optical manipulation, imaging, stimulation, and therapy at depths in tissue or tissue-like complex media. We aim to provide insights about what limits the WFS towards biomedical applications, and how recent efforts advance the performance of WFS among different trade-offs. By differentiating the two implementation directions in the field, i.e., precompensation WFS and optical phase conjugation (OPC), improvement strategies are summarized and discussed. For biomedical applications, improving the speed of WFS is most essential in both directions, and a system-compatible wavefront modulator driven by fast apparatus is desired. In addition to that, algorithm efficiency and adaptability to perturbations/noise is of concern in precompensation WFS, while for OPC significant improvements rely heavily on integrating physical mechanisms and delicate system design for faster response and higher energy gain. Substantial improvements in WFS implementations, from the aspects of physics, engineering, and computing, have inspired many novel and exciting optical applications that used to be optically inaccessible. It is envisioned that continuous efforts in the field can further advance WFS towards biomedical applications and guide our vision into deep biological tissues.

Multipath optical thermometry realized by electronic levels and Stark sublevels of Er3+

High-performance optical thermometer that can operate in the long-wavelength range of the second near infrared (1500 nm–1700 nm, NIR-IIb) biological window is in desperate need for bio-medical treatment. In this paper, a multipath temperature sensor Y3NbO7: Yb3+/Er3+ with pure cubic phase is synthesized by the high temperature solid state method. Under the excitation of 980 nm wavelength, efficient green upconversion emission attributed to thermally coupled Er3+: 2H11/2/4S3/2 levels are found, through which an excellent temperature sensing properties are realized with the maximal absolute and relative sensitivity of approximately 0.54 % K−1 and 1.37 % K−1 as well as the minimal temperature resolution of about 0.024 K. More importantly, the sample exhibits remarkable temperature detection capacity in deep tissues, which is implemented by the thermally coupled Stark sublevels of Er3+: 4I13/2 locating just in the NIR-IIb sub-window. Thanks to this appropriate response wavelength range, the maximal penetration depth of the NIR-IIb optical thermometer in the biological tissues can reach up to 8 mm. All the findings indicate that the present sample is a supernormal multipath thermometric probe with detectability in the deep biological tissues.

Photoacoustic Imaging and Characterization of Bone in Medicine: Overview, Applications, and Outlook.

Photoacoustic techniques have shown promise in identifying molecular changes in bone tissue and visualizing tissue microstructure. This capability represents significant advantages over gold standards (i.e., dual-energy X-ray absorptiometry) for bone evaluation without requiring ionizing radiation. Instead, photoacoustic imaging uses light to penetrate through bone, followed by acoustic pressure generation, resulting in highly sensitive optical absorption contrast in deep biological tissues. This review covers multiple bone-related photoacoustic imaging contributions to clinical applications, spanning bone cancer, joint pathologies, spinal disorders, osteoporosis, bone-related surgical guidance, consolidation monitoring, and transsphenoidal and transcranial imaging. We also present a summary of photoacoustic-based techniques for characterizing biomechanical properties of bone, including temperature, guided waves, spectral parameters, and spectroscopy. We conclude with a future outlook based on the current state of technological developments, recent achievements, and possible new directions.

Development of a Low-Frequency Piezoelectric Ultrasonic Transducer for Biological Tissue Sonication.

The safety of ultrasound exposure is very important for a patient's well-being. High-frequency (1-10 MHz) ultrasound waves are highly absorbed by biological tissue and have limited therapeutic effects on internal organs. This article presents the results of the development and application of a low-frequency (20-100 kHz) ultrasonic transducer for sonication of biological tissues. Using the methodology of digital twins, consisting of virtual and physical twins, an ultrasonic transducer has been developed that emits a focused ultrasound signal that penetrates into deeper biological tissues. For this purpose, the ring-shaped end surface of this transducer is excited not only by the main longitudinal vibrational mode, which is typical of the flat end surface transducers used to date, but also by higher mode radial vibrations. The virtual twin simulation shows that the acoustic signal emitted by the ring-shaped transducer, which is excited by a higher vibrational mode, is concentrated into a narrower and more precise acoustic wave that penetrates deeper into the biological tissue and affects only the part of the body to be treated, but not the whole body.

Real-time passive cavitation mapping with high spatial-temporal resolution

Shock wave lithotripsy (SWL) and laser lithotripsy (LL) have been widely used for clinical treatment of kidney stones. Cavitation plays an important role in stone fragmentation in both SWL and LL, yet it may also contribute to renal tissue injury. It is therefore crucial to determine the spatiotemporal distributions of cavitation activities to maximize stone fragmentation while minimizing tissue injury. Passive cavitation mapping (PCM) has most practical applications in deep biological tissues and is most promising for clinical translation. We have developed a set of technologies for 2D/3D PCM that can be seamlessly integrated with ultrasound imaging and photoacoustic imaging. Our 2D/3D PCM has achieved a spatial resolution of hundreds of micrometers and a temporal resolution of several microseconds. We also developed a transient angular spectrum approach for PCM reconstruction, which is ten times faster than the traditional delay-and-sum method. Using the 2D/3D PCM system, we imaged shockwave- and laser-induced single cavitation bubbles in both free field and constricted space, as well as on large animal models. Collectively, our results have demonstrated the high reliability and spatial-temporal accuracy of the 2D/3D PCM approach, which paves the way for future in vivo applications and human studies during SWL and LL.

Water-soluble polymer brush-substituted squaraine NIR-II dye for efficient photothermal therapy.

Near-infrared II (NIR-II, 1000-1700 nm) fluorescence imaging has the advantages of low light scattering and weak biological autofluorescence compared with conventional NIR (600-900 nm) fluorescence imaging and can obtain a high signal-to-noise ratio in deeper biological tissues, as well as micron-level high resolution. A great deal of effort has been directed toward the construction of conjugated polymers for effective NIR-triggered fluorescence imaging (FI) and photothermal therapy (PTT) combined therapy. However, NIR-II fluorescent materials are mainly nanoparticles prepared by coprecipitation methods, and water-soluble NIR-II materials need to be further developed. In this paper, we synthesized novel water-soluble squaric acid nanoparticles (SQ-POEGMA) with low toxicity and excellent photostability by attaching a water-soluble oligomer (POEGMA) to the small molecule squaric acid through a click chemistry reaction. The photothermal conversion efficiency of SQ-POEGMA is 33% in vitro, which can effectively inhibit the growth of cancer cells with 94% tumor inhibition rate in vivo under 808 nm laser irradiation, while no appreciable side effects were observed.

Real-time passive cavitation mapping with high spatial-temporal resolution

Shock wave lithotripsy (SWL) and laser lithotripsy (LL) have been widely used for clinical treatment of kidney stones. Cavitation plays an important role in stone fragmentation in both SWL and LL, yet it may also contribute to renal tissue injury. It is therefore crucial to determine the spatiotemporal distributions of cavitation activities to maximize stone fragmentation while minimizing tissue injury. Passive cavitation mapping (PCM) has most practical applications in deep biological tissues and is most promising for clinical translation. We have developed a set of technologies for 2D/3D PCM that can be seamlessly integrated with ultrasound imaging and photoacoustic imaging. Our 2D/3D PCM has achieved a spatial resolution of hundreds of micrometers and a temporal resolution of several microseconds. We also developed a transient angular spectrum approach for PCM reconstruction, which is ten times faster than the traditional delay-and-sum method. Using the 2D/3D PCM system, we imaged shockwave- and laser-induced single cavitation bubbles in both free field and constricted space, as well as on large animal models. Collectively, our results have demonstrated the high reliability and spatial-temporal accuracy of the 2D/3D PCM approach, which paves the way for future in vivo applications and human studies during SWL and LL.

Numerical investigation on losses and heat-transfer characteristics of Magnetic-heat therapy in deep biological tissue based on magnetothermal gel with medium frequency vortex

This paper proposes a medium frequency magnetothermal treatment method, which can be used for a certain depth of thermal hyperthermia of tumor tissue in vivo. Firstly, a magnetothermal treatment program using a vortex coil is designed with a medium frequency alternating magnetic field, which can better realize the external implementation. Next, the electromagnetic field of the vortex coil is simulated by finite element method, and the electromagnetic field distribution characteristics of the system are obtained for treatment reference. Then, the preparation and composition design of the gel suitable for the magnetothermal effect are studied, and the temperature rise experiment is carried out to verify the reliability of the temperature rise of the magnetothermal gel. Finally, the human thoracic cavity tissue model is established to calculate the magnetothermal effect of the gel on the lung tumor tissue. And the thermal effect and temperature distribution of different physical parameters on the tissue are obtained. The results show that the magnetothermal therapy can penetrate non-magnetic barriers and effectively generate a temperature rise on the magnetothermal gel injected into the body, so that the tumor tissue in a certain space is inhibited by the temperature rise in a short time. Therefore, the research results in this article provide important theoretical and clinical value for non-invasive hyperthermia of tumors in vivo.

Polarization microwave-induced thermoacoustic imaging for quantitative characterization of deep biological tissue microstructures

Polarization optical imaging can be used to characterize anisotropy in biological tissue microstructures and has been demonstrated to be a powerful tool for clinical diagnosis. However, the approach is limited by an inability to image targets deeper than ∼ 1 mm due to strong optical scattering in biological tissues. As such, we propose a novel polarization microwave-induced thermoacoustic imaging (P-MTAI) technique to noninvasively detect variations in deep tissue by exploiting the thermoacoustic signals induced by four pulsed microwaves of varying polarization orientations. The proposed P-MTAI method overcomes the penetration limits of conventional polarization optical imaging and provides submillimeter resolution over depths of several centimeters. As part of the paper, the structural characteristics of tissues were quantified using a new parameter, the degree of microwave absorption anisotropy. P-MTAI was also applied to the noninvasive detection of morphological changes in cardiomyocytes as they transitioned from ordered to disordered states, providing a potential indication of myocardial infarction.

Polarized photoacoustic microscopy based on high-order harmonics for anisotropy detection.

BackgroundAnisotropy which encodes rich structure and function information is one of the key and unique characteristics of tissues. Polarized photoacoustic imaging shows tremendous potential for the detection and quantification of the anisotropy of tissues. The existing polarized photoacoustic imaging methods cannot quantify anisotropy and detect the orientation of the optical axis in 3D imaging.MethodsWe proposed a versatile polarized photoacoustic imaging method based on the detection of high-order harmonics of the photoacoustic signal, which can be used for both 2D and 3D polarized photoacoustic imaging, This method can detect and quantify the anisotropy and the orientation of the optical axis of the anisotropic objects by the amplitude and initial phase of the high-order harmonics. A double-focusing polarized photoacoustic microscopy was developed to validate the proposed method. Experiments were conducted on 2D and 3D anisotropic phantoms.ResultsThe results showed that the anisotropy and the orientation of the optical axis of the anisotropic object can be detected and quantified accurately by the amplitude and initial phase of the high-order harmonics, even at a depth of triple transport mean free path. The imaging depth of the polarized photoacoustic microscopy is mainly limited by laser energy attenuation rather than depolarization.ConclusionsPolarized photoacoustic microscopy based on high-order harmonics has tremendous potential for imaging the anisotropy of deep biological tissues in vivo. It also extends the capability of photoacoustic microscopy to image the anisotropy of tissues.

Near-Infrared Activation of Sensory Rhodopsin II Mediated by NIR-to-Blue Upconversion Nanoparticles.

Direct optical activation of microbial rhodopsins in deep biological tissue suffers from ineffective light delivery because visible light is strongly scattered and absorbed. NIR light has deeper tissue penetration, but NIR-activation requires a transducer that converts NIR light into visible light in proximity to proteins of interest. Lanthanide-doped upconversion nanoparticles (UCNPs) are ideal transducer as they absorb near-infrared (NIR) light and emit visible light. Therefore, UCNP-assisted excitation of microbial rhodopsins with NIR light has been intensively studied by electrophysiology technique. While electrophysiology is a powerful method to test the functional performance of microbial rhodopsins, conformational changes associated with the NIR light illumination in the presence of UCNPs remain poorly understood. Since UCNPs have generally multiple emission peaks at different wavelengths, it is important to reveal if UCNP-generated visible light induces similar structural changes of microbial rhodopsins as conventional visible light illumination does. Here, we synthesize the lanthanide-doped UCNPs that convert NIR light to blue light. Using these NIR-to-blue UCNPs, we monitor the NIR-triggered conformational changes in sensory rhodopsin II from Natronomonas pharaonis (NpSRII), blue light-sensitive microbial rhodospsin, by FTIR spectroscopy. FTIR difference spectrum of NpSRII was recorded under two different excitation conditions: (ⅰ) with conventional blue light, (ⅱ) with UCNP-generated blue light upon NIR excitation. Both spectra display similar spectral features characteristic of the long-lived M photointermediate state during the photocycle of NpSRII. This study demonstrates that NIR-activation of NpSRII mediated by UCNPs takes place in a similar way to direct blue light activation of NpSRII.

Numerical Investigation on Losses and Heat-Transfer Characteristics of Magnetic-Heat Therapy In Deep Biological Tissue Based on Magnetothermal Gel with Medium Frequency Vortex

Numerical Investigation on Losses and Heat-Transfer Characteristics of Magnetic-Heat Therapy In Deep Biological Tissue Based on Magnetothermal Gel with Medium Frequency Vortex

Modeling combined ultrasound and photoacoustic imaging: Simulations aiding device development and artificial intelligence

Combined ultrasound and photoacoustic (USPA) imaging has attracted several pre-clinical and clinical applications due to its ability to simultaneously display structural, functional, and molecular information of deep biological tissue in real time. However, the depth and wavelength dependent optical attenuation and the unknown optical and acoustic heterogeneities limit the USPA imaging performance in deep tissue regions. Novel instrumentation, image reconstruction, and artificial intelligence (AI) methods are currently being investigated to overcome these limitations and improve the USPA image quality. Effective implementation of these approaches requires a reliable USPA simulation tool capable of generating US based anatomical and PA based molecular contrasts of deep biological tissue. Here, we developed a hybrid USPA simulation platform by integrating finite element models of light (NIRFast) and ultrasound (k-Wave) propagations for co-simulation of B-mode US and PA images. The platform allows optimization of different design parameters for USPA devices, such as the aperture size and frequency of both light and ultrasound detector arrays. For designing tissue-realistic digital phantoms, a dictionary-based function has been added to k-Wave to generate various levels of ultrasound speckle contrast. The feasibility of modeling US imaging combined with optical fluence dependent multispectral PA imaging is demonstrated using homogeneous as well as heterogeneous tissue phantoms mimicking human organs (e.g., prostate and finger). In addition, we also demonstrate the potential of the simulation platform to generate large scale application-specific training and test datasets for AI enhanced USPA imaging. The complete USPA simulation codes together with the supplementary user guides have been posted to an open-source repository (https://github.com/KothapalliLabPSU/US-PA_simulation_codes).