Numerical analysis of ultrasound propagation and reflection intensity for biological acoustic impedance microscope

Noninvasive imaging of rat-derived microglia and its reactivity to inflammatory molecules via acoustic impedance microscopy

PURPOSE

Microglia, the brain's immune cells, play important roles in neuronal differentiation, survival, and death. The function of microglia is deeply related to the morphologies; however, it is too complex to observe conventionally and identify the condition of living microglia using optical microscopes. Herein, we proposed a new method to observe living cultured microglia and their reactivity to inflammation via the acoustic impedance mode of a scanning acoustic microscope.

METHODS

Primary cultured microglia collected from rat pups exposed to acetamiprid, an insecticide, in utero were observed with both acoustic interface impedance mode (C-mode) and transparent three-dimensional impedance mode (B-mode).

RESULTS

We characterized microglia into four types based on the results obtained from acoustic impedance, cytoskeletal information, and laser confocal imaging. Biphasic acoustic observation using B-mode and C-mode gave us information regarding the dynamic morphologies of living microglia treated with adenosine triphosphate (ATP) (600 μmol/L), which reflects distress signals from inflamed neurons. Acetamiprid exposure induced microglia response even in the neonatal period. ATP stimulus altered the shape and thickness of microglia with a change in the bulk modulus of the cell. Three-dimensional alteration with ATP stimulus could be observed only after biphasic acoustic observation using B-mode and C-mode. This acoustic observation was consistent with confocal observation using anti-Iba-1 and P2Y12 immunocytochemistry.

CONCLUSION

This study demonstrated the adequacy of using a scanning acoustic microscope in analyzing microglia's shape, motility, and response to inflammation.

Time and frequency domain deconvolution for cross-sectional cultured cell observation using an acoustic impedance microscope

Herein, we propose a method to estimate the reflection coefficient of the ultrasonic wave transmitted onto an object and to display this with acoustic impedance distribution. The observation targets were glial cells, which have a rigid cytoskeleton and spread out well on a culture substrate. A reflection coefficient derived only from the cells was then obtained using a deconvolution process. In the conventional method, the deconvolution process that was performed only in the frequency domain would cause an error in the reconstructed signal, and it formed an artifact when the result was converted into the acoustic impedance image. To solve this problem, two types of deconvolution techniques were applied in either the full frequency or time-frequency domain. The results of both methods were then compared. Since the characteristic acoustic impedance is a physical property substantially equivalent to the bulk modulus, it can be considered that the internal elastic parameter is thus estimated. An analysis of the nucleus based on its position in the acoustic impedance image was then performed. The results indicated that the proposed time-frequency domain deconvolution method is able to maintain the structure of the cell, while the cell itself is free from unwanted artifacts. The nucleus was also estimated to be located toward the center of the cell, with lower acoustic impedance value than the cytoskeleton. The results of this study could contribute to establishing a method for monitoring the internal condition of cultured cells in regenerative medicine and drug discovery.

Three-dimensional acoustic impedance mapping of cultured biological cells

The acoustic microscope is a powerful tool for the observation of biological matters. Non-invasive in-situ observation can be performed without any staining process. Acoustic microscopy is contrasted by elastic parameters like sound speed and acoustic impedance. We have proposed an acoustic microscope that can acquire three-dimensional acoustic impedance profile. The technique was applied to cell-size observation. Glial cells were cultured on a 70 μm-thick polypropylene film substrate. A highly focused ultrasound beam was transmitted from the rear side of the substrate, and the reflection was received by the same transducer. An acoustic pulse, its spectrum spreading briefly 100 through 450 MHz, was transmitted. By analyzing the internal reflections in the cell, the distribution of characteristic acoustic impedance along the beam direction was determined. Three-dimensional acoustic impedance mapping was realized by scanning the transducer, exhibiting the intra-cellular structure including nucleus and cytoskeleton.

Noninvasive ultrasound imaging for assessment of intact microstructure of extracellular matrix in tissue engineering

Development of artificial tissues or organs is one of the actual tasks in regenerative medicine that requires observation and evaluation of intact volume microstructure of tissue engineering products at all stages of their formation, from native donor tissues and decellularized scaffolds to recipient cell migration in the matrix. Unfortunately in practice, methods of vital noninvasive imaging of volume microstructure in matrixes are absent. In this work, we propose a new approach based on high-frequency acoustic microscopy for noninvasive evaluation and visualization of volume microstructure in tissue engineering products. The results present the ultrasound characterization of native rat diaphragms and lungs and their decellularized scaffolds. Verification of the method for visualization of tissue formation in the matrix volume was described in the model samples of diaphragm scaffolds with stepwise collagenization. Results demonstrate acoustic microscopic sensitivity to cell content concentration, variation in local density, and orientation of protein fibers in the volume, micron air inclusions, and other inhomogeneities of matrixes.

Nano-lipospheres as acoustically active ultrasound contrast agents: evolving tumor imaging and therapy technique

Applying nanobubbles (NBs) for contrast-enhanced ultrasound imaging has received increased attention. NBs are biocompatible, multifunctional, theranostic agents. Their properties of high echogenicity and stability create an agent suitable for ultrasonography diagnosis. Their favorable properties of size, in vivo stability, and ease of modification are being exploited to implement a theranostic platform for cancer treatment. The considerable development offers the potential to overcome drug resistance and adverse side effects that are associated with traditional chemotherapy. This review outlines the principles of ultrasonography and angiogenesis. Microbubbles and micelles are also discussed to underline the superior capabilities of NBs for the application. NBs could passively accumulate to tumor tissue by enhanced permeability and retention effect. In addition, it can also achieve the active transportation by surface modification. Active targeting modalities and stimuli-responsive drug delivery modifications generate a therapeutic vehicle. The cytotoxicity of NBs formulations, multimodal imaging capability, active targeting mechanisms, and drug delivery methods are highlighted to confirm the NB as a vehicle for targeted treatment and enhanced ultrasound imaging.

Determining sodium diffusion through acoustic impedance measurements using 80 MHz Scanning Acoustic Microscopy: Agarose phantom verification

The purpose of this study is to explore the feasibility of time-dependent acoustic impedance measurement by Scanning Acoustic Microscopy (SAM) for analyzing the sodium diffusion. The purpose is motivated by the fact that sodium monitoring is challenging and still in the area of exploratory analysis despite its biological importance. To our knowledge, this is the first study in which sodium diffusion has been investigated by time-dependent acoustic impedance measurements provided by SAM. We first tested the idea in an agarose phantom as a proof-of-concept. Accordingly, we designed the agarose phantom which initially contains a well of sodium chloride (NaCl) solution moving radially into the phantom. By using NaCl diffusion in the phantom, we obtained two-dimensional (2D) acoustic impedance (Z) maps over time through SAM operating with 80 MHz ultrasonic transducer having a lateral resolution of 20 μm. A linear correlation between the changes in the concentration profile of the phantom and its acoustic impedance was introduced. Analysis of experimental data proved that spatially changing acoustic impedance could be ascribed to the diffusion process and produced a diffusion coefficient in the order of 10^-5 cm²/s which matches well with the literature. Our results showed that SAM could monitor the time-dependent alterations in acoustic impedance resulting from the diffusion of sodium inside the agarose phantom. With this study, SAM shows a promise as a monitoring tool not only to obtain static images but also to perform dynamic investigations of sodium ions with the advantages of providing images in micrometer resolution with a scanning time no longer than 2 min for an image area of 4.8 mm × 4.8 mm.

Fingerprint Imaging System Based on Capacitive Micromachined Ultrasonic Transducer by Using Impediography Method Including Direct Touch and Waveguide Methods

Fingerprint imaging is a powerful tool in biometric identification systems. This paper proposes a fingerprint imaging method that involves applying an impedance signal to a fingerprint via ultrasonic impediography using a capacitive micromachined ultrasonic transducer (CMUT). We fabricated CMUT sparse array which has a center frequency of 6.25 MHz and collapse voltage of 50 V by local oxidation of silicon process. We develop a prototype by using waveguide and ultrasonic impediography method based on the CMUT. By using a waveguide made from a hard material, the prototype can ensure device protection and image lateral resolution improvement. The proposed technique successfully images the acoustic input impedance differences between the ridges and valleys of a fingerprint. The images created using direct touch and waveguide models are compared and analyzed. In addition, we present a phenomenon in the waveguide system in terms of energy loss by using numerical simulation. Therefore, this study facilitates ultrasonic fingerprint impediography measurement based on the basic direct touch and quartz glass waveguide system.

Numerical analysis of acoustic impedance microscope utilizing acoustic lens transducer to examine cultured cells

A new technique is proposed for non-contact quantitative cell observation using focused ultrasonic waves. This technique interprets acoustic reflection intensity into the characteristic acoustic impedance of the biological cell. The cells are cultured on a plastic film substrate. A focused acoustic beam is transmitted through the substrate to its interface with the cell. A two-dimensional (2-D) reflection intensity profile is obtained by scanning the focal point along the interface. A reference substance is observed under the same conditions. These two reflections are compared and interpreted into the characteristic acoustic impedance of the cell based on a calibration curve that was created prior to the observation. To create the calibration curve, a numerical analysis of the sound field is performed using Fourier Transforms and is verified using several saline solutions. Because the cells are suspended by two plastic films, no contamination is introduced during the observation. In a practical observation, a sapphire lens transducer with a center frequency of 300 MHz was employed using ZnO thin film. The objects studied were co-cultured rat-derived glial (astrocyte) cells and glioma cells. The result was the clear observation of the internal structure of the cells. The acoustic impedance of the cells was spreading between 1.62 and 1.72 MNs/m(3). Cytoskeleton was indicated by high acoustic impedance. The introduction of cytochalasin-B led to a significant reduction in the acoustic impedance of the glioma cells; its effect on the glial cells was less significant. It is believed that this non-contact observation method will be useful for continuous cell inspections.