Karla P. Mercado

Scholte wave generation during single tracking location shear wave elasticity imaging of engineered tissues.

The physical environment of engineered tissues can influence cellular functions that are important for tissue regeneration. Thus, there is a critical need for noninvasive technologies capable of monitoring mechanical properties of engineered tissues during fabrication and development. This work investigates the feasibility of using single tracking location shear wave elasticity imaging (STL-SWEI) for quantifying the shear moduli of tissue-mimicking phantoms and engineered tissues in tissue engineering environments. Scholte surface waves were observed when STL-SWEI was performed through a fluid standoff, and confounded shear moduli estimates leading to an underestimation of moduli in regions near the fluid-tissue interface.

Quantitative Ultrasound for Nondestructive Characterization of Engineered Tissues and Biomaterials

Non-invasive, non-destructive technologies for imaging and quantitatively monitoring the development of artificial tissues are critical for the advancement of tissue engineering. Current standard techniques for evaluating engineered tissues, including histology, biochemical assays and mechanical testing, are destructive approaches. Ultrasound is emerging as a valuable tool for imaging and quantitatively monitoring the properties of engineered tissues and biomaterials longitudinally during fabrication and post-implantation. Ultrasound techniques are rapid, non-invasive, non-destructive and can be easily integrated into sterile environments necessary for tissue engineering. Furthermore, high-frequency quantitative ultrasound techniques can enable volumetric characterization of the structural, biological, and mechanical properties of engineered tissues during fabrication and post-implantation. This review provides an overview of ultrasound imaging, quantitative ultrasound techniques, and elastography, with representative examples of applications of these ultrasound-based techniques to the field of tissue engineering.

Size-isolation of ultrasound-mediated phase change perfluorocarbon droplets using differential centrifugation

Perfluorocarbon droplets that are capable of an ultrasound-mediated phase transition have applications in diagnostic and therapeutic ultrasound. Techniques to modify the droplet size distribution are of interest because of the size-dependent acoustic response of the droplets. Differential centrifugation has been used to isolate specific sizes of microbubbles. In this work, differential centrifugation was employed to isolate droplets with diameters between 1 and 3 μm and 2 and 5 μm from an initially polydisperse distribution. Further, an empirical model was developed for predicting the droplet size distribution following differential centrifugation and to facilitate the selection of centrifugation parameters for obtaining desired size distributions.

Compensating for Scholte waves in single track location shearwave elasticity imaging

The estimation of shearwave velocity in biological tissues using Single Track Location Shearwave Elasticity Imaging (STL-SWEI) depends on the assumption that the ultrasonically observed particle displacements are due to the propagation of shearwaves in an approximately infinite space. When this assumption is violated, erroneous estimates of the shearwave speed may occur leading to image artifacts. One particularly troubling error occurs when slowly propagating Scholte waves are generated at solid-fluid interfaces. These interface waves travel at a slower speed than the shearwaves produced in STL-SWEI. However, the signals produced appear similar to that of shearwaves and cannot be readily distinguished in the typical STL-SWEI imaging sequence. Instead, alternative sequences are needed to identify and correct for these anomalous wave types. In this work, the surface wave phenomena is examined in the context of STL-SWEI imaging. The appearance of these waves is demonstrated in simulation, tissue mimicking p...

Characterizing collagen microstructure using high frequency ultrasound

Collagen is the most abundant extracellular matrix protein in mammals and is widely investigated as a scaffold material for tissue engineering. Collagen provides structural properties for scaffolds and, importantly, the microstructure of collagen can affect key cell behaviors such as cell migration and proliferation. This study investigated the feasibility of using high-frequency quantitative ultrasound to characterize collagen microstructure, namely, collagen fiber density and size, nondestructively. The integrated backscatter coefficient (IBC) was employed as a quantitative ultrasound parameter to characterize collagen microstructure in 3-D engineered hydrogels. To determine the relationship between the IBC and collagen fiber density, hydrogels were fabricated with different collagen concentrations (1–4 mg/mL). Further, collagen hydrogels polymerized at different temperatures (22–37°C) were investigated to determine the relationship between the IBC and collagen microfiber size. The IBC was computed from...

Using frequency-sum beamforming in passive cavitation imaging

Passive Cavitation Imaging (PCI) is a method for locating cavitation emissions to study biological effects of ultrasound on tissues. In this method, an image is formed by beamforming passively recorded acoustic emissions with an array. The image resolution depends on the ultrasound frequency and array geometry. Acoustic emissions can be scattered due to tissue inhomogeneity, which may degrade the image resolution. Emissions at higher frequencies are more susceptible to such degradation. Frequency-sum beamforming is a nonlinear technique that alters this sensitivity to scattering by manufacturing higher frequency information from lower frequency components via a quadratic product of complex signal amplitudes. This presentation evaluates the performance of frequency-sum beamforming in a scattering environment using simulations and experiments, conducted in the kHz and MHz frequency regimes. First, 50 and 100 kHz signals were broadcasted from a single source to an array of 16 hydrophones in a water tank with...

Effect of diluent fluid viscosity on acoustic droplet vaporization-mediated dissolved oxygen scavenging

Acoustic droplet vaporization (ADV) can be used to scavenge dissolved oxygen and reduce the partial pressure of oxygen (pO2) in a fluid containing perfluoropentane droplets. The impact of the diluent fluid’s viscosity on ADV-mediated pO2 reduction was investigated. Polyvinylpyrrolidone (PVP) was dissolved in saline to modify the solution’s viscosity. The diluent fluid viscosity (η) and surface tension (γ) were measured. Droplets were manufactured using amalgamation and differential centrifugation to yield diameters between 1-6 μm. Droplets were diluted to 6.5x106 droplets/mL in saline (γ = 68 mN/m, η = 0.7 cP), 3 mg/mL PVP solution (γ = 65 mN/m, η = 1.2 cP), or 15 mg/mL PVP solution (γ = 65 mN/m, η = 4 cP). The viscosities of the 3 mg/mL and 15 mg/mL PVP solutions mimicked those of plasma and whole blood, respectively. Droplet solutions were exposed to ultrasound (5 MHz, 4.25 MPa peak negative pressure in situ, 10 cycles) in a 37°C in vitro flow system. The initial pO2 in the fluids was 113±2 mmHg, simila...

Frequency-sum beamforming in a random scattering environment

In a uniform environment, sound propagation direction(s) or the location of a sound source may be determined from array-recorded signals by beamforming. However, the beamforming results may be degraded when there is random scattering between the source and the receivers. Such sensitivity to mild scattering may be altered through use of an unconventional beamforming technique that manufactures higher frequency information from lower-frequency signal components via a quadratic product of complex signal amplitudes. This presentation will describe frequency-sum beamforming, and then illustrate it with simulation results and near-field acoustic experiments. The simulations suggest that frequency-sum beamforming may be beneficial when there is one loud source and the environment provides one primary propagation path. The experiments were conducted in either a 1.0-m-deep 1.07-m-diameter cylindrical water tank using 50 kHz and 100 kHz signals broadcast from a single source to an array of 16 hydrophones when discr...

An empirical model of size-isolated ultrasound-triggered phase shift emulsions

High-speed mechanical agitation is commonly used to produce microbubbles and droplets for ultrasound imaging and therapy. This technique results in a high concentration (~1010 particles/mL) of polydisperse particles (less than 400 nm to greater than 15 μm in diameter). Differential centrifugation has been used to isolate microbubbles and droplets of specific sizes. In our prior work, we have isolated droplets between 2 μm and 5 μm. In the current work, we have isolated different sizes of droplets by adjusting centrifugation speeds. Our size-isolation protocol increased the fraction of droplets between 1 μm and 3 μm from 8% for non-centrifuged droplets to 87% for differentially centrifuged droplets. An empirical model for the size distribution after differential centrifugation was developed. The measured fraction of droplets in the supernatant and pellet for all sizes after a single centrifugation was used in the empirical model. There was a 3% difference in the volume-weighted mean diameter of the experim...

Parametric imaging of three-dimensional engineered tissue constructs using high-frequency ultrasound

The goal of this study was to use high-frequency ultrasound to nondestructively characterize three-dimensional engineered tissues. We hypothesized that backscatter spectral parameters, such as the integrated backscatter coefficient (IBC), can be used to quantify differences in cell concentration in engineered tissues. We chose the IBC parameter since it estimates the backscattering efficiency of scatterers per unit volume. In this study, acoustic fields were generated using single-element, focused transducers (center frequencies of 30 and 40 MHz) operating over a frequency range of 13 to 47 MHz. Three-dimensional engineered tissue constructs were fabricated with mouse embryonic fibroblasts homogenously embedded within agarose. Constructs with cell concentrations ranging from 1x104 to 1x106 cells/mL were investigated. The IBC was computed from the backscatter spectra, and parametric images of spatial variations in the IBC were generated. Results showed that the IBC increased linearly with cell concentratio...