Heechul Yoon

The impact of intraocular pressure on elastic wave velocity estimates in the crystalline lens

Intraocular pressure (IOP) is believed to influence the mechanical properties of ocular tissues including cornea and sclera. The elastic properties of the crystalline lens have been mainly investigated with regard to presbyopia, the age-related loss of accommodation power of the eye. However, the relationship between the elastic properties of the lens and IOP remains to be established. The objective of this study is to measure the elastic wave velocity, which represents the mechanical properties of tissue, in the crystalline lens ex vivo in response to changes in IOP. The elastic wave velocities in the cornea and lens from seven enucleated bovine globe samples were estimated using ultrasound shear wave elasticity imaging. To generate and then image the elastic wave propagation, an ultrasound imaging system was used to transmit a 600 µs pushing pulse at 4.5 MHz center frequency and to acquire ultrasound tracking frames at 6 kHz frame rate. The pushing beams were separately applied to the cornea and lens. IOP in the eyeballs was varied from 5 to 50 mmHg. The results indicate that while the elastic wave velocity in the cornea increased from 0.96  ±  0.30 m s-1 to 6.27  ±  0.75 m s-1 as IOP was elevated from 5 to 50 mmHg, there were insignificant changes in the elastic wave velocity in the crystalline lens with the minimum and the maximum speeds of 1.44  ±  0.27 m s-1 and 2.03  ±  0.46 m s-1, respectively. This study shows that ultrasound shear wave elasticity imaging can be used to assess the biomechanical properties of the crystalline lens noninvasively. Also, it was observed that the dependency of the crystalline lens stiffness on the IOP was significantly lower in comparison with that of cornea.

Contrast‐enhanced ultrasound imaging in vivo with laser‐activated nanodroplets

Purpose This study introduces a real‐time contrast‐enhanced ultrasound imaging method with recently developed laser‐activated nanodroplets (LANDs), a new class of phase‐change nanometer‐scale contrast agents that provides perceptible, sustained high‐contrast with ultrasound. Methods In response to pulsed laser irradiation, the LANDs—, which contain liquid perfluorohexane and optical fuses—blink (vaporize and recondense). That is, they change their state from liquid nanodroplets to gas microbubbles, and then back to liquid nanodroplets. In their gaseous microbubble state, the LANDs provide high‐contrast ultrasound, but the microbubbles formed in situ typically recondense in tens of milliseconds. As a result, LAND visualization by standard, real‐time ultrasound is limited. However, the periodic optical triggering of LANDs allows us to observe corresponding transient, periodic changes in ultrasound contrast. This study formulates a probability function that measures how ultrasound temporal signals vary in periodicity. Then, the estimated probability is mapped onto a B‐scan image to construct a LAND‐localized, contrast‐enhanced image. We verified our method through phantom and in vivo experiments using an ultrasound system (Vevo 2100, FUJIFILM VisualSonics, Inc., Toronto, ON, Canada) operating with a 40‐MHz linear array and interfaced with a 10 Hz Nd:YAG laser (Phocus, Opotek Inc., Carlsbad, CA, USA) operating at the fundamental 1064 nm wavelength. Results From the phantom study, the results showed improvements in the contrast‐to‐noise ratio of our approach over conventional ultrasound ranging from 129% to 267%, with corresponding execution times of 0.10 to 0.29 s, meaning that the developed method is computationally efficient while yielding high‐contrast ultrasound. Furthermore, in vivo sentinel lymph node (SLN) imaging results demonstrated that our technique could accurately identify the SLN. Conclusions The results indicate that our approach enables efficient and robust LAND localization in real time with substantially improved contrast, which is essential for the successful translation of this contrast agent platform to clinical settings.

Fluid flow measurement for diagnosis of ventricular shunt malfunction using nonlinear responses of microbubbles in the contrast-enhanced ultrasound imaging

Contrast-enhanced ultrasound imaging utilizing the nonlinear responses of microbubbles is proposed for identifying ventricular shunt malfunction. The developed method suppresses the signal from walls of a shunt catheter and background tissues, and allows accurate measurements of the cerebrospinal fluid flow within the shunt catheter using a relatively small concentration of microbubbles. The flow rates estimated in the linear mode were significantly underestimated (65% at 0.1 ml/min and 5.0% microbubble concentration) while estimates using the nonlinear mode were not. Overall, the nonlinear responses of microbubbles improve the estimation of flow rates in a shunt catheter at low concentrations of microbubbles.

Dual-Phase Transmit Focusing for Multiangle Compound Shear-Wave Elasticity Imaging

Shear-wave elasticity imaging (SWEI) enables the quantitative assessment of the mechanical properties of tissue. In SWEI, the effective generation of acoustic radiation force is of paramount importance. Consequently, several research groups have investigated various transmit beamforming and pulse-sequencing methods. To further improve the efficiency of the shear-wave generation, and therefore, to increase the quality of SWEI, we introduce a technique referred to as “multiangle compound SWEI” (MAC-SWEI), which uses simultaneous multiangular push beams created by dual-phase transmit focusing. By applying a constant phase offset on every other element of an array transducer, dual-phase transmit focusing creates both main and grating lobes (i.e., multiangular push beams for pushing) to simultaneously generate shear waves with several wavefront angles. The shear waves propagating at different angles are separated by multidirectional filtering in the frequency domain, leading to the reconstruction of multiple spatially co-registered shear-wave velocity maps. To form a single-elasticity image, these maps are combined, while regions associated with known artifacts created by the push beams are omitted. Overall, we developed and tested the MAC-SWEI method using Field II quantitative simulations and the experiments performed using a programmable ultrasound imaging system. Our results suggest that MAC-SWEI with dual-phase transmit focusing may improve the quality of elasticity maps.

Impact of depth-dependent optical attenuation on wavelength selection for spectroscopic photoacoustic imaging

An optical wavelength selection method based on the stability of the absorption cross-section matrix to improve spectroscopic photoacoustic (sPA) imaging was recently introduced. However, spatially-varying chromophore concentrations cause the wavelength- and depth-dependent variations of the optical fluence, which degrades the accuracy of quantitative sPA imaging. This study introduces a depth-optimized method that determines an optimal wavelength set minimizing an inverse of the multiplication of absorption cross-section matrix and fluence matrix to minimize the errors in concentration estimation. This method assumes that the optical fluence distribution is known or can be attained otherwise. We used a Monte Carlo simulation of light propagation in tissue with various depths and concentrations of deoxy-/oxy-hemoglobin. We quantitatively compared the developed and current approaches, indicating that the choice of wavelength is critical and our approach is effective especially when quantifying deeper imaging targets.

Time-shifted multi-tracking of shear waves for the characterization of scleral biomechanics

The biomechanics of the sclera can play a critical role in glaucoma diagnosis and monitoring new treatment methods. Previous studies have used strain elastography and investigated the relationship between scleral stiffness and intraocular pressure. However, applying shear wave elastography (SWE) to scleral tissue is challenging because sclera (100–1000 kPa) is significantly stiffer than normal soft tissue (10–100 kPa). We introduce a time-shifted multi-tracking approach capable of measuring the propagation of fast shear waves in stiff tissues, such as the sclera.

Multispectral ultrafast ultrasound imaging: A versatile tool probing dynamic phase-change contrast agents

Optically triggered perfluorohexane nanodroplets (PFHnDs) can repeatedly vaporize and recondense, providing photoacoustic (PA) and blinking ultrasound (US) signals in response to pulsed laser irradiation. This property of PFHnDs has led to the development of various US imaging modes, including super-resolution and background-free contrast-enhanced imaging. However, to utilize the full potential of PFHnDs, multi-wavelength PA imaging, followed by ultrafast US imaging, referred to as multispectral ultrafast US/PA imaging, is required. Thus, we built a combined system that can trigger and image the dynamic behavior of PFHnDs with various optical wavelengths.

On-demand gas-generating nanoparticles as an ultrasound imaging contrast agent

Microbubbles (MBs) are often used as ultrasound imaging contrast agents. Given their micrometer size, MBs are confined in vascular compartments because they cannot escape through endothelial barriers and, as such, MBs are primarily useful in microcirculation or vascular-targeting molecule imaging. To penetrate outside of vasculature, the size of the ultrasound contrast agent should be in the nanometer scale range. When gas bubbles are reduced to nanometer size, they have low echogenicity and limited response to acoustic field. A desired ultrasound contrast agent should consist of nanoparticles that are capable of escaping from vasculature, penetrating into tissue, and generating sufficient contrast once they reach the target site. Here we report the development of a novel contrast agent (panel A) consisting of gold nanoparticles that are covered by azide compounds and capable of on-demand laser-induced gas generation via photolysis.

Super-resolution imaging with ultrafast ultrasound and laser-activated nanodroplets (Conference Presentation)

Super-resolution ultrasound imaging techniques have shown promising potential in non-invasive imaging of deep-lying tissue. However, these methods utilize microbubbles, limiting its utility to visualization of vasculature with moving bubbles. To resolve extravascular targets, our group previously introduced a method for super-resolution ultrasound imaging based on laser-activated nanodroplets (LANDs) that repeatedly vaporize and recondense in response to optical irradiation. The method resolves the location of LANDs from the difference between two imaging frames capturing vaporization and recondensation of individual LANDs. However, since only two neighboring frames are used to produce a difference frame, this method is sensitive to noise-related errors limiting the improvement in spatial resolution. In this study, we introduce a new approach to super-resolution imaging. In our approach, ultrafast imaging, which typically captures images at over several thousand frames per second, was used for spatio-temporal compounding. Specifically, multiple successive ultrasound frames were used to obtain the difference frame with improved reliability and repeatability thus enhanced spatial resolution. To evaluate our approach, we imaged a phantom containing uniformly-distributed LANDs using an ultrasound system equipped with a linear array transducer and interfaced with pulsed laser. An ultrafast plane-wave compounding approach was used to capture ultrasound images at 6 kHz frame rate. We achieved a four-fold improvement in spatial resolution over the previous approach. In addition, three-dimensional super-resolution imaging of a phantom with microcapillaries containing LANDs was performed illustrating the robustness of our method. These results suggest that our approach has the potential for high-resolution molecular imaging of intravascular and extravascular targets.

Ultrasound and photoacoustic imaging to monitor ocular stem cell delivery and tissue regeneration (Conference Presentation)

Glaucoma is associated with dysfunction of the trabecular meshwork (TM), a fluid drainage tissue in the anterior eye. A promising treatment involves delivery of stem cells to the TM to restore tissue function. Currently histology is the gold standard for tracking stem cell delivery and differentiation. To expedite clinical translation, non-invasive longitudinal monitoring in vivo is desired. Our current research explores a technique combining ultrasound (US) and photoacoustic (PA) imaging to track mesenchymal stem cells (MSCs) after intraocular injection. Adipose-derived MSCs were incubated with gold nanospheres to label cells (AuNS-MSCs) for PA imaging. Successful labeling was first verified with in vitro phantom studies. Next, MSC delivery was imaged ex vivo in porcine eyes, while intraocular pressure was hydrostatically clamped to maintain a physiological flow rate through the TM. US/PA imaging was performed before, during, and after AuNS-MSC delivery. Additionally, spectroscopic PA imaging was implemented to isolate PA signals from AuNS-MSCs. In vitro cell imaging showed AuNS-MSCs produce strong PA signals, suggesting that MSCs can be tracked using PA imaging. While the cornea, sclera, iris, and TM region can be visualized with US imaging, pigmented tissues also produce PA signals. Both modalities provide valuable anatomical landmarks for MSC localization. During delivery, PA imaging can visualize AuNS-MSC motion and location, creating a unique opportunity to guide ocular cell delivery. Lastly, distinct spectral signatures of AuNS-MSCs allow unmixing, with potential for quantitative PA imaging. In conclusion, results show proof-of-concept for monitoring MSC ocular delivery, raising opportunities for in vivo image-guided cell delivery.