Photoacoustic (PA) imaging has had limited clinical applicability for many reasons but one primary barrier to clinical translation is the bulky, expensive, and low repetition-rate laser typically used, resulting in low frame-rate images and a system with a large physical footprint. We have previously demonstrated a fast-scan approach delivering the frame rates required for real-time integrated PA/ultrasound (PAUS) imaging. In this paper, we present a new real-time PAUS system based on a swept-scanning source approach using a compact, recently-developed laser, providing pulse-to-pulse wavelength tuning at kHz rates and a scanning fiber-optic delivery system integrated with a high-frequency (15 MHz) US linear array. An array of fibers spanning the array are arranged on two lateral sides of the transducer and scanned sequentially based on optimized pulse sequences. By coherent compounding of multiple sub-images associated with each fiber light source, PA imaging with sufficient SNR at a frame rate of 50 Hz is achieved. Real-time in vivo multi-spectral imaging of nano-drug delivery to mice is demonstrated. With the same scanner footprint, our compact PAUS system can provide not only conventional high-quality scanned US imaging with all associated modes, but interleaved, multispectral PA imaging at video rates appropriate for real-time clinical applications.
In vision correction surgeries, the corneal stroma is subject to limbal-relaxing incisions which change the focusing power of the cornea, but can damage tissue and put the patient at risk of complication. A non-invasive method to launch a mechanical wave in tissue, referred to as Acoustic Micro-Tapping (AuT), is demonstrated with phase-sensitive spectral domain OCT (SD-OCT) to probe for biomechanical changes in porcine and human cornea samples following arcuate keratotomy (AK). This method uses an air-coupled ultrasound transducer to deliver sufficient displacement on the corneal surface to launch a mechanical wave propagating as a guided mode. Rayleigh-Lamb wave propagation is captured at 100 spatial locations 6 mm across the corneal surface, resulting in a high resolution elastogram. The SD-OCT system operates in the MB mode at a functional frame rate of 47 kHz to detect local wave behavior for analysis of the group velocity, group displacement amplitude, displacement attenuation, phase velocity over the bandwidth of the excitation, mean frequency, and bandwidth. An analysis of mechanical wave behavior shows reduced wave speed up to 20% following an incision through 3/4th of the cornea in porcine tissue samples, indicating a potential reduction in elastic modulus. This technique was performed on porcine and human corneas following PRK incision to demonstrate progress toward clinical translation.
Optical coherence elastography (OCE) holds great promise for quantitative characterization of corneal elasticity including robust measurements of the intraocular pressure (IOP) independent of corneal mechanical properties. To translate this method into a viable clinical tool, however, requires wideband, highly accurate mechanical wave measurements using mechanical stimulation requiring no physical contact with the cornea. We have developed a method of non-contact mechanical stimulation of soft media with precise spatial and temporal shaping. We call it acoustic micro-tapping (AuT) because it employs focused, air-coupled ultrasound (US) to induce significant mechanical displacement at the boundary of a soft material using reflection-based radiation force. Combining it with high-speed, four-dimensional (three space dimensions plus time) phase-sensitive optical coherence tomography (PhS-OCT) creates a non-contact tool for high-resolution and quantitative dynamic elastography of soft tissue at near real-time imaging rates. To demonstrate this approach, we present OCE results on a porcine cornea using a homemade, focused 1 MHz air-coupled piezoelectric transducer with a matching layer to launch an US wave through air onto the sample surface. To provide an acoustic line source approximating a 1-D excitation, the transducer was made from a cylindrical segment of a piezoelectric tube. A high-speed (1.6 MHz A-Scan rate) PhS-OCT system was utilized to measure acoustic wave propagation in the cornea at different intraocular pressures (IOPs). Results from this OCE study demonstrate that an air-coupled US wave reflected from an air/tissue interface provides significant radiation force to generate displacement for elasticity imaging for full mechanical characterization of the cornea.
Recently we used ultrasound from an air-coupled transducer for non-contact excitation of broadband mechanical waves in soft tissue such as cornea. The transient displacement, generated by “Acoustic Micro-Tapping” (AuT), was then measured using phase-sensitive spectral domain OCT (SD-OCT). In addition traditional surface wave speed measurement, we investigated complementary methods to characterize the mechanical properties of the target material. We note that the maximum frequency, as well as the group velocity, of the surface wave is related to both the phase velocity of the material and the spatial width of the acoustic pulse. If the spatial and temporal profile of the excitation is well defined, it may be possible to infer elastic modulus from the frequency profile of a propagating mechanical wave. To assess the effect of the spatial profiles of the AuT excitation on frequency profiles of resulting mechanical waves, acoustic pulses with different spatial width (from 0.1 to 1 mm) were applied to agar phantoms with different shear modulus (from 1 to 100 kPa) to generate mechanical waves, and a SD-OCT system with a functional frame rate of 47 kHz was used to track wave propagation. For validation, simulations with the same acoustic and mechanical properties were performed using a finite element method (FEM) to analyze induced wave propagation. The phantom experiment and simulation exhibited similar increase in the maximum frequency with decreasing excitation width. Both estimates also agreed well with previous theoretical results.
The formation of amyloid – aggregate of misfolded proteins – is associated with more than 50 human pathologies, including Alzheimer’s disease, Parkinson’s disease, and Type 2 diabetes mellitus. Investigating protein aggregation is a critical step in drug discovery and development of therapeutics targeted to these pathologies. However, screens to identify protein aggregates are challenging due to the stochastic character of aggregate nucleation. Here we employ photoacoustics (PA) to screen thermodynamic conditions and solution components leading to formation of protein aggregates. Particularly, we study the temperature dependence of the Gruneisen parameter in optically-contrasted, undersaturated and supersaturated solutions of glycoside hydrolase (lysozyme). As nucleation of protein aggregates proceeds in two steps, where the first is liquid-liquid separation (rearrangement of solute’s density), the PA response from complex solutions and its temperature-dependence monitor nucleation and differentiate undersaturated and supersaturated protein solutions. We demonstrate that in the temperature range from 22 to 0° C the PA response of contrasted undersaturated protein solution behaves similar to water and exhibits zero thermal expansion at 4°C or below, while the response of contrasted supersaturated protein solution is nearly temperature independent, similar to the behavior of oils. These results can be used to develop a PA assay for high-throughput screening of multi-parametric conditions (pH, ionic strength, chaperone, etc.) for protein aggregation that can become a key tool in drug discovery, targeting aggregate formation for a variety of amyloids.
Optical coherence elastography (OCE) can provide clinically valuable information based on local measurements of tissue stiffness. Improved light sources and scanning methods in optical coherence tomography (OCT) have led to rapid growth in systems for high-resolution, quantitative elastography using imaged displacements and strains within soft tissue to infer local mechanical properties. We describe in some detail the physical processes underlying tissue mechanical response based on static and dynamic displacement methods. Namely, the assumptions commonly used to interpret displacement and strain measurements in terms of tissue elasticity for static OCE and propagating wave modes in dynamic OCE are discussed with the ultimate focus on OCT system design for ophthalmic applications. Practical OCT motion-tracking methods used to map tissue elasticity are also presented to fully describe technical developments in OCE, particularly noting those focused on the anterior segment of the eye. Clinical issues and future directions are discussed in the hope that OCE techniques will rapidly move forward to translational studies and clinical applications.
Acoustic radiation force (ARF) is commonly used in ultrasound (US)-based elastography to generate shear waves deep within soft tissue. These waves can be detected with different methods, e.g. contact conventional ultrasound imaging probes or contact free magnetic resonance or optical coherence tomography (OCT). For many clinical applications, however, for instance the eye, a totally non-contact system for generation/detection of mechanical waves is needed.
Here, we present a method for efficient non-contact excitation of broadband transverse mechanical waves in soft media. The approach is based on pushing the medium under study with a 1 MHz chirped US wave focused to its surface from air. The US beam reflected from the air/medium interface provides the ARF force to the medium surface launching a transient mechanical wave in the transverse (lateral) direction.
The design and performance of the air-coupled transducer is discussed. The focal zone, peak pressure and acoustic intensity are measured for transducers with different numerical apertures. Time and frequency characteristics of the propagating mechanical waves, generated in soft tissue, are tracked with a phase-sensitive ultra-fast frame rate OCT imaging system. Application of the proposed method for non-contact, non-invasive, sub-mm resolution elasticity measurement in soft tissue is proposed.
A new light and sound sensitive nanoemulsion contrast agent is presented. The agents feature a low boiling point liquid perfluorocarbon core and a broad light spectrum absorbing polypyrrole (PPy) polymer shell. The PPy coated nanoemulsions can reversibly convert from liquid to gas phase upon cavitation of the liquid perfluorocarbon core. Cavitation can be initiated using a sufficiently high intensity acoustic pulse or from heat generation due to light absorption from a laser pulse. The emulsions can be made between 150 and 350 nm in diameter and PPy has a broad optical absorption covering both the visible spectrum and extending into the near-infrared spectrum (peak absorption ~1053 nm).
The size, structure, and optical absorption properties of the PPy coated nanoemulsions were characterized and compared to PPy nanoparticles (no liquid core) using dynamic light scattering, ultraviolet-visible spectrophotometry, transmission electron microscopy, and small angle X-ray scattering. The cavitation threshold and signal intensity were measured as a function of both acoustic pressure and laser fluence. Overlapping simultaneous transmission of an acoustic and laser pulse can significantly reduce the activation energy of the contrast agents to levels lower than optical or acoustic activation alone. We also demonstrate that simultaneous light and sound cavitation of the agents can be used in a new sono-photoacoustic imaging method, which enables greater sensitivity than traditional photoacoustic imaging.
Shear wave OCE (SW-OCE) uses an OCT system to track propagating mechanical waves, providing the information
needed to map the elasticity of the target sample. In this study we demonstrate high speed, 4D imaging to capture
transient mechanical wave propagation. Using a high-speed Fourier domain mode-locked (FDML) swept-source OCT
(SS-OCT) system operating at ~1.62 MHz A-line rate, the equivalent volume rate of mechanical wave imaging is 16
kvps (kilo-volumes per second), and total imaging time for a 6 x 6 x 3 mm volume is only 0.32 s. With a displacement
sensitivity of ~10 nanometers, the proposed 4D imaging technique provides sufficient temporal and spatial resolution for
real-time optical coherence elastography (OCE). Combined with a new air-coupled, high-frequency focused ultrasound
stimulator requiring no contact or coupling media, this near real-time system can provide quantitative information on
localized viscoelastic properties. SW-OCE measurements are demonstrated on tissue-mimicking phantoms and porcine
cornea under various intra-ocular pressures. In addition, elasticity anisotropy in the cornea is observed. Images of the
mechanical wave group velocity, which correlates with tissue elasticity, show velocities ranging from 4-20 m/s
depending on pressure and propagation direction. These initial results strong suggest that 4D imaging for real-time OCE
may enable high-resolution quantitative mapping of tissue biomechanical properties in clinical applications.
Optical coherence tomography (OCT)-based elasticity imaging can map soft tissue elasticity based on speckle-tracking of elastic wave propagation using highly sensitive phase measurements of OCT signals. Using a fixed elastic wave source and moving detection, current imaging sequences have difficulty in reconstructing tissue elasticity within speckle-free regions, for example, within the crystalline lens of the eye. We present a moving acoustic radiation force imaging sequence to reconstruct elastic properties within a speckle-free region by tracking elastic wave propagation from multiple laterally moving sources across the field of view. We demonstrate the proposed strategy using heterogeneous and partial speckle-free tissue-mimicking phantoms. Harder inclusions within the speckle-free region can be detected, and the contrast-to-noise ratio slightly enhanced compared to current OCE imaging sequences. The results suggest that a moving source approach may be appropriate for OCE studies within the large speckle-free regions of the crystalline lens.
Phase-sensitive optical coherence tomography (PhS-OCT) can be utilized for quantitative shear-wave elastography using speckle tracking. However, current approaches cannot directly reconstruct elastic properties in speckle-less or speckle-free regions, for example within the crystalline lens in ophthalmology. Investigating the elasticity of the crystalline lens could improve understanding and help manage presbyopia-related pathologies that change biomechanical properties. We propose to reconstruct the elastic properties in speckle-less regions by sequentially launching shear waves with moving acoustic radiation force (mARF), and then detecting the displacement at a specific speckle-generating position, or limited set of positions, with PhS-OCT. A linear ultrasound array (with a center frequency of 5 MHz) interfaced with a programmable imaging system was designed to launch shear waves by mARF. Acoustic sources were electronically translated to launch shear waves at laterally shifted positions, where displacements were detected by speckle tracking images produced by PhS-OCT operating in M-B mode with a 125-kHz A-line rate. Local displacements were calculated and stitched together sequentially based on the distance between the acoustic source and the detection beam. Shear wave speed, and the associated elasticity map, were then reconstructed based on a time-of-flight algorithm. In this study, moving-source shear wave elasticity imaging (SWEI) can highlight a stiff inclusion within an otherwise homogeneous phantom but with a CNR increased by 3.15 dB compared to a similar image reconstructed with moving-detector SWEI. Partial speckle-free phantoms were also investigated to demonstrate that the moving-source sequence could reconstruct the elastic properties of speckle-free regions. Results show that harder inclusions within the speckle-free region can be detected, suggesting that this imaging method may be able to detect the elastic properties of the crystalline lens.
Metallic nanoparticles have been widely used in a variety of imaging and therapeutic applications due to their unique
optical properties in the visible and near-infrared (NIR) regions - for example, various plasmonic nanoparticles are used
for molecular photoacoustic imaging and photothermal therapy. However, there are concerns that these agents may not
be safe under physiological conditions, because these nanoparticles are not biodegradable, could accumulate and,
therefore, could be toxic long-term. We investigate the feasibility of using biodegradable gold nanoclusters as a contrast
agent for highly sensitive photoacoustic imaging. The size of these biodegradable nanoclusters, consisting of sub-5 nm
primary gold particles and a biodegradable polymer binder, is less than 100 nm. Due to plasmon coupling, these
nanoclusters are characterized by a broad extinction spectrum that extends to the near infrared (NIR) spectral range.
Photoacoustic imaging of tissue models containing inclusions with different concentrations of nanoparticles was
performed using a tunable pulsed laser system. The results indicate that the biodegradable nanoclusters, comprised of
small gold nanoparticles, can be used as contrast agents in photoacoustic imaging.
We investigated the effect of rough surface on the performance of extinction-based localized surface plasmon resonance
(LSPR) biosensors. The sensor measures resonance wavelength shifts in transmittance due to biomolecular interactions
amplified by periodic nanostructures. The numerical calculation was conducted using rigorous coupled-wave analysis
with Gaussian random profiles. The results suggest that, when a surface has a roughness smaller than 2 nm, the
sensitivity of an LSPR biosensor is not significantly affected regardless of correlation length (CL). However, we found
that extinction peak amplitude and curve width are affected substantially with a decrease in CL. At CL less than 100 nm,
surface roughness may induce interference between localized surface plasmons excited by the surface and nanowires,
which can lead to significant degradation of sensor performance.
Surface plasmon resonance (SPR) coupled fluorescence uses an evanescent electromagnetic field to excite fluorophores
in the vicinity of surface. We investigated the influence of enhanced evanescent fields at SPR on the induced
fluorescence intensity. The system of this study is based on angle scanning with a half-cylinder prism (SF10) and dual
motorized rotation stages to observe the correlation between the evanescent fields and fluorescent intensity of
microbeads. With this system, emission from fluorophores only exists in close proximity to the surface of the microbead.
The results show that evanescent fields produced at SPR provide more sensitive fluorescence images compared to those
measured at a total internal reflection angle.
In this study, we investigated the impact of surface roughness on the sensitivity of conventional and nanowire-based
surface plasmon resonance (SPR) biosensors. The theoretical research was conducted using rigorous coupled-wave
analysis with Gaussian surface profiles of gold films determined by atomic force microscopy. The results suggest that,
when surface roughness ranges 1 nm, the sensitivity of a conventional SPR system is not significantly affected regardless
of the correlation length. For a nanowire-based SPR biosensor, however, we found that the sensitivity degrades
substantially with a decreasing correlation length. Particularly, at a correlation length smaller than 100 nm, random
rough surface may induce destructive coupling between excited localized surface plasmons, which can lead to prominent
reduction of sensitivity enhancement.
In this study, we experimentally confirmed the sensitivity enhancement by the nanowire-based surface plasmon
resonance (SPR) sensor structure. Gold nanowire samples with a period of 500 nm were fabricated by interference
lithography on a gold-SF10 glass substrate. Sensitivity enhancement compared to a conventional SPR structure was
measured to be 31% when evaluated using a varied concentration of ethanol at a dielectric surrounding layer. This result
is consistent with numerical data of rigorous coupled-wave analysis. Rough surfaces of thin gold film and gold
nanowires are deemed to induce the sensitivity degradation by more than 10%. More significant sensitivity improvement
can be achieved by implementing finer nanowires.
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