Ultrasound Backscatter Microscopy Research Articles

Non-destructive, continuous monitoring of biochemical, mechanical, and structural maturation in engineered tissue

Regulatory guidelines for tissue engineered products require stringent characterization during production and necessitate the development of novel, non-destructive methods to quantify key functional parameters for clinical translation. Traditional assessments of engineered tissues are destructive, expensive, and time consuming. Here, we introduce a non-destructive, inexpensive, and rapid sampling and analysis system that can continuously monitor the mechanical, biochemical, and structural properties of a single sample over extended periods of time. The label-free system combines the imaging modalities of fluorescent lifetime imaging and ultrasound backscatter microscopy through a fiber-based interface for sterile monitoring of tissue quality. We tested the multimodal system using tissue engineered articular cartilage as an experimental model. We identified strong correlations between optical and destructive testing. Combining FLIm and UBM results, we created a novel statistical model of tissue homogeneity that can be applied to tissue engineered constructs prior to implantation. Continuous monitoring of engineered tissues with this non-destructive system has the potential for in-process monitoring of tissue engineered products, reducing costs and improving quality controls in research, manufacturing, and clinical applications.

Multimodal Label-Free Imaging for Detecting Maturation of Engineered Osteogenic Grafts.

There is a critical need to develop noninvasive, nondestructive methods for assessing the quality of engineered constructs prior to implantation. Currently, the composition and maturity of engineered tissues are assessed using destructive, costly, and time-consuming biochemical and mechanical analyses. The goal of this study was to use noninvasive, multimodal imaging to monitor osteogenic differentiation and matrix deposition by human mesenchymal stem/stromal cells (MSCs) during in vitro culture. MSCs were encapsulated in alginate hydrogels and cultured in osteogenic conditions for 4 weeks. Samples were evaluated using fluorescence lifetime imaging (FLIm) and ultrasound backscatter microscopy (UBM) prior to traditional biochemical and mechanical testing. Using linear regression analysis, we identified strong correlations between imaging parameters (e.g., fluorescence lifetime and acoustic attenuation coefficient) and destructive mechanical and biochemical tests to assess the maturation of osteogenically induced constructs. These data demonstrate the promise of nondestructive label-free imaging techniques to noninvasively ascertain the progression and maturity of tissue engineered bone grafts.

Time-resolved fluorescence spectroscopy and ultrasound backscatter microscopy for nondestructive evaluation of vascular grafts.

Quantitative and qualitative evaluations of structure and composition are important in monitoring development of engineered vascular tissue both in vitro and in vivo. Destructive techniques are an obstacle for performing time-lapse analyses from a single sample or animal. This study demonstrates the ability of time-resolved fluorescence spectroscopy (TRFS) and ultrasound backscatter microscopy (UBM), as nondestructive and synergistic techniques, for compositional and morphological analyses of tissue grafts, respectively. UBM images and integrated backscatter coefficients demonstrate the ability to visualize and quantify postimplantation changes in vascular graft biomaterials such as loss of the external elastic lamina and intimal/medial thickening over the grafted region as well as graft integration with the surrounding tissue. TRFS results show significant changes in spectra, average lifetime, and fluorescence decay parameters owing to changes in collagen, elastin, and cellular content between normal and grafted tissue regions. These results lay the foundation for the application of a catheter-based technique for in vivo evaluation of vascular grafts using TRFS and UBM.

Ultrasound Backscatter Microscopy Image-Guided Intraventricular Gene Delivery at Murine Embryonic Age 9.5 and 10.5 Produces Distinct Transgene Expression Patterns at the Adult Stage

In utero injection of a retroviral vector into the embryonic telencephalon aided by ultrasound backscatter microscopy permits introduction of a gene of interest at an early stage of development. In this study, we compared the tissue distribution of gene expression in adult mice injected with retroviral vectors at different embryonic ages in utero. Following ultrasound image-guided gene delivery (UIGD) into the embryonic telencephalon, adult mice were subjected to whole-body luciferase imaging and immunohistochemical analysis at 6 weeks and 1 year postinjection. Luciferase activity was observed in a wide range of tissues in animals injected at embryonic age 9.5 (E9.5), whereas animals injected at E10.5 showed brain-localized reporter gene expression. These results suggest that mouse embryonic brain creates a closed and impermeable structure around E10. Therefore, by injecting a transgene before or after E10, transgene expression can be manipulated to be local or systemic. Our results also provide information that widens the applicability of UIGD beyond neuroscience studies.

Ultrasound Backscatter Microscopy for Imaging of Oral Carcinoma

Ultrasound backscatter microscopy (UBM), or ultrasound biomicroscopy, is a noninvasive, label-free, and ionizing radiation-free technique allowing high-resolution 3-dimensional structural imaging. The goal of this study was to evaluate UBM for resolving anatomic features associated with squamous cell carcinoma of the oral cavity. The study was conducted in a hamster buccal pouch model. A carcinogen was topically applied to cheeks of 14 golden Syrian hamsters. Six additional hamsters served as healthy controls. A high-frequency (41 MHz, 6-mm focal depth, lateral and axial resolutions of 65 and 37 μm, respectively) UBM system was used for scanning the oral cavity after 14 weeks of carcinogen application. Histologic analyses were conducted on scanned regions. The histologic structure of buccal tissue and microvasculature networks could be visualized from the UBM images. Epithelial and mucosal hypertrophy and neoplastic changes were identified in animals subjected to the carcinogen. In animals with invasive squamous cell carcinoma, lesion development and destruction of the structural integrity of tissue layers were noted. In this pilot study, UBM generated sufficient contrast for morphologic features associated with oral carcinoma compared to healthy tissue. This modality may present a practical technique for detection of oral neoplasms that is potentially translatable to humans.

Multimodal in vivo imaging of oral cancer using fluorescence lifetime, photoacoustic and ultrasound techniques

This work reports a multimodal system for label-free tissue diagnosis combining fluorescence lifetime imaging (FLIm), ultrasound backscatter microscopy (UBM), and photoacoustic imaging (PAI). This system provides complementary biochemical, structural and functional features allowing for enhanced in vivo detection of oral carcinoma. Results from a hamster oral carcinoma model (normal, precancer and carcinoma) are presented demonstrating the ability of FLIm to delineate biochemical composition at the tissue surface, UBM and related radiofrequency parameters to identify disruptions in the tissue microarchitecture and PAI to map optical absorption associated with specific tissue morphology and physiology.

Preliminary animal studies on observation of injured spinal cord with intraoperative ultrasound backscatter microscopy

Many studies have shown that strategies of nerve regeneration and cell-based transplantation are valid based on animal models of spinal cord injury (SCI). To apply these strategies and bridge spinal cord defects, the identification and precise localization of lesions during spinal cord surgery is necessary. The aim of the present experiment was to evaluate the capabilities of ultrasound backscatter microscopy (UBM) in identifying morphologic changes after SCI. After laminectomy, high-resolution ultrasound images of the spinal cord were obtained in one normal and seven spinal cord-injured adult Wistar rats using a UBM system with a 55-MHz center frequency scanner. Comparison between histoanatomic and UBM images was also performed. The results showed that UBM can identify cysts after the experimental SCI is removed in adult rats. In addition, the glial scar formed in secondary injury showed obvious hyperechoic speckle in the UBM image and correlated with the histoanatomic image. UBM has obvious clinical value in nerve regeneration and cell-based transplantation strategies in injured spinal cords.

Nondestructive Evaluation of Tissue Engineered Articular Cartilage Using Time-Resolved Fluorescence Spectroscopy and Ultrasound Backscatter Microscopy

The goal of this study is to evaluate the ability of a bimodal technique integrating time-resolved fluorescence spectroscopy (TRFS) and ultrasound backscatter microscopy (UBM) for nondestructive detection of changes in the biochemical, structural, and mechanical properties of self-assembled engineered articular cartilage constructs. The cartilage constructs were treated with three chemical agents (collagenase, chondroitinase-ABC, and ribose) to induce changes in biochemical content (collagen and glycosaminoglycan [GAG]) of matured constructs (4 weeks); and to subsequently alter the mechanical properties of the construct. The biochemical changes were evaluated using TRFS. The microstructure and the thickness of the engineered cartilage samples were characterized by UBM. The optical and ultrasound results were validated against those acquired via conventional techniques including collagen and GAG quantification and measurement of construct stiffness. Current results demonstrated that a set of optical parameters (e.g., average fluorescence lifetime and decay constants) showed significant correlation (p<0.05) with biochemical and mechanical data. The high-resolution ultrasound images provided complementary cross-section information of the cartilage samples morphology. Therefore, the technique was capable of nondestructively evaluating the composition of extracellular matrix and the microstructure of engineered tissue, demonstrating great potential as an alternative to traditional destructive assays.

Noninvasive Multimodal Evaluation of Bioengineered Cartilage Constructs Combining Time-Resolved Fluorescence and Ultrasound Imaging

A multimodal diagnostic system that integrates time-resolved fluorescence spectroscopy, fluorescence lifetime imaging microscopy, and ultrasound backscatter microscopy is evaluated here as a potential tool for assessing changes in engineered tissue composition and microstructure nondestructively and noninvasively. The development of techniques capable of monitoring the quality of engineered tissue, determined by extracellular matrix (ECM) content, before implantation would alleviate the need for destructive assays over multiple time points and advance the widespread development and clinical application of engineered tissues. Using a prototype system combining time-resolved fluorescence spectroscopy, FLIM, and UBM, we measured changes in ECM content occurring during chondrogenic differentiation of equine adipose stem cells on 3D biodegradable matrices. The optical and ultrasound results were validated against those acquired via conventional techniques, including collagen II immunohistochemistry, picrosirius red staining, and measurement of construct stiffness. Current results confirm the ability of this multimodal approach to follow the progression of tissue maturation along the chondrogenic lineage by monitoring ECM production (namely, collagen type II) and by detecting resulting changes in mechanical properties of tissue constructs. Although this study was directed toward monitoring chondrogenic tissue maturation, these data demonstrate the feasibility of this approach for multiple applications toward engineering other tissues, including bone and vascular grafts.

In Vivo Ultrasound Imaging Of Pancreatic Islets

Introduction: Imaging and quantifying pancreatic islets in vivo could revolutionize the treatment of diabetes mellitus. Currently, insulin levels and hemoglobin A1C are our main methods for determining beta cell mass in diabetic patients. These insensitive measures are grossly inadequate for proper guidance of therapy. An office-based, non-invasive method for determining islet mass serially in diabetics has long been sought-after, but with no success. Here we show that a sub-harmonic ultrasound probe, in conjunction with microbubble intravenous contrast, allows islets visualization in the mouse pancreas based on the increased blood flow compared to surrounding pancreatic tissue. Methods: We injected lipid microbubbles intravenously into the circulation of mice, and then used ultrasound backscatter microscopy with a sub-harmonic probe to specifically scan only the reflected ultrasound signal from microbubbles and not the surrounding pancreatic tissue. We used Maximum-Intensity-Persistence (MIP) sensing by the probe, together with a motorized stage to scan through a block of pancreas at 30μM intervals, and thus generate a three-dimensional rendering of the MIP-generated signal. Results: The subharmonic ultrasound visualization rendered clearly delineated large blood vessels of the scanned region in the pancreas. We were also able to identify discrete, three-dimensional hyper-perfused areas that were of the size, number, and distribution of islets. To validate that these hyperperfused areas were indeed islets, we scanned the pancreas of transgenic mice that express GFP under the mouse insulin promoter. Conclusions: Using a mouse model, we now have strong evidence to show the potential feasibility of using ultrasound combined with intravenous administration of microbubbles to visualize and quantify islet mass.

Hybrid Optical-Ultrasonic Technique for Biomedical Diagnostics.

We report the development of a diagnostic system combining time-resolved fluorescence spectroscopy and ultrasound backscatter microscopy and its application in diagnosis of tumors and atherosclerotic disease. This system allows for concurrent evaluation of distinct compositional, functional, and micro-anatomical features of normal and diseased tissues.

Development of a Multi-modal Tissue Diagnostic System Combining High Frequency Ultrasound and Photoacoustic Imaging with Lifetime Fluorescence Spectroscopy.

We report the development and validate a multi-modal tissue diagnostic technology, which combines three complementary techniques into one system including ultrasound backscatter microscopy (UBM), photoacoustic imaging (PAI), and time-resolved laser-induced fluorescence spectroscopy (TR-LIFS). UBM enables the reconstruction of the tissue microanatomy. PAI maps the optical absorption heterogeneity of the tissue associated with structure information and has the potential to provide functional imaging of the tissue. Examination of the UBM and PAI images allows for localization of regions of interest for TR-LIFS evaluation of the tissue composition. The hybrid probe consists of a single element ring transducer with concentric fiber optics for multi-modal data acquisition. Validation and characterization of the multi-modal system and ultrasonic, photoacoustic, and spectroscopic data coregistration were conducted in a physical phantom with properties of ultrasound scattering, optical absorption, and fluorescence. The UBM system with the 41 MHz ring transducer can reach the axial and lateral resolution of 30 and 65 μm, respectively. The PAI system with 532 nm excitation light from a Nd:YAG laser shows great contrast for the distribution of optical absorbers. The TR-LIFS system records the fluorescence decay with the time resolution of ~300 ps and a high sensitivity of nM concentration range. Biological phantom constructed with different types of tissues (tendon and fat) was used to demonstrate the complementary information provided by the three modalities. Fluorescence spectra and lifetimes were compared to differentiate chemical composition of tissues at the regions of interest determined by the coregistered high resolution UBM and PAI image. Current results demonstrate that the fusion of these techniques enables sequentially detection of functional, morphological, and compositional features of biological tissue, suggesting potential applications in diagnosis of tumors and atherosclerotic plaques.

Temperature dependence of acoustic properties of human tissues with a high‐resolution ultrasound backscatter microscope

The B‐mode in vitro images of human skin and ocular tissues are obtained using a high‐resolution ultrasound backscatter microscope (UBM) at the frequencies of 40, 80, and 100 MHz. It shows that a high‐resolution UBM can visualize more detailed superficial microstructures of skin and eye at higher frequencies. In order to improve high‐frequency ultrasound images, we have investigated the acoustic properties, such as sound velocity, sound attenuation, and backscatter coefficient, of human tissues with this UBM system over the frequency range from 25 to 110 MHz. Their acoustic properties are also investigated as a function of temperature in the range from 20<th>°C to 65<th>°C, which may provide bases for ultrasound image monitoring of thermal therapy. [Work supported by Strategic National R&D Program of Ministry of Commerce, Industry and Energy (MOCIE), Republic of Korea.]

High-resolution ultrasonography of subretinal structure and assessment of retina degeneration in rat

The purpose of this work was to evaluate the ability of 80 MHz ultrasonography to differentiate intra-retinal layers and quantitatively assess photoreceptor dystrophy in small animal models. Four groups of 10 RCS rats each (five dystrophic and five controls) were explored at 25, 35, 45 and 55 days post-natal (PN). A series of retina cross-sections were obtained ex vivo from outside intact eyes using an 80 MHz three-dimensional ultrasound backscatter microscope (20-μm-axial resolution). Ultrasound features of normal retina were correlated to those of corresponding histology and thickness measurements of photoreceptor segment and nuclear layers were performed on all groups. To show the ability of 80 MHz ultrasonography to distinguish the retinal degeneration in vivo, one RCS rat was explored at 25 and 55 days post-natal. Ultrasound image of normal retina displayed four distinct layers marked by reflections at neurites/nuclei interfaces and permitted to differentiate the photoreceptor segment and nuclear layers. The backscatter level from the retina was shown to be related to the size, density and organization of the intra-layer structure. Ultrasound thickness measurements highly correlated with histologic measurements. A thinning ( p<0.05) of outer nuclear layer (ONL) was detected over time for controls and was thought to be assigned to retina maturation. Retinal degeneration started at PN35 and resulted in a more pronounced ONL thinning ( p<0.05) over time. ONL degeneration was accompanied by segment layer thickening ( p<0.05) at PN35 and thinning thereafter. These changes may indicate accumulation of outer segment debris at PN35 then progressive destruction. In vivo images of rat intra-retinal structure showed the ability of the method to distinguish the photoreceptor layer changes. Our results indicate that 80 MHz ultrasonography reveals intra-retinal layers and is sensitive to age and degenerative changes of photoreceptors. This technique has great potential to follow-up retinal dystrophy and therapeutic effects in vivo.

High resolution ultrasound elastomicroscopy imaging of soft tissues: system development and feasibility

Research in elasticity imaging typically relies on 1–10 MHz ultrasound. Elasticity imaging at these frequencies can provide strain maps with a resolution in the order of millimetres, but this is not sufficient for applications to skin, articular cartilage or other fine structures. We developed a prototype high resolution elastomicroscopy system consisting of a 50 MHz ultrasound backscatter microscope system and a calibrated compression device using a load cell to measure the pressure applied to the specimen, which was installed between a rigidly fixed face-plate and a specimen platform. Radiofrequency data were acquired in a B-scan format (10 mm wide × 3 mm deep) in specimens of mouse skin and bovine patellar cartilage. The scanning resolution along the B-scan plane direction was 50 µm, and the ultrasound signals were digitized at 500 MHz to achieve a sensitivity better than 1 µm for the axial displacement measurement. Because of elevated attenuation of ultrasound at high frequencies, special consideration was necessary to design a face-plate permitting efficient ultrasound transmission into the specimen and relative uniformity of the compression. Best results were obtained using a thin plastic film to cover a specially shaped slit in the face-plate. Local tissue strain maps were constructed by applying a cross-correlation tracking method to signals obtained at the same site at different compression levels. The speed of sound in the tissue specimen (1589.8 ± 7.8 m s−1 for cartilage and 1532.4 ± 4.4 m s−1 for skin) was simultaneously measured during the compression test. Preliminary results demonstrated that this ultrasound elastomicroscopy technique was able to map deformations of the skin and articular cartilage specimens to high resolution, in the order of 50 µm. This system can also be potentially used for the assessment of other biological tissues, bioengineered tissues or biomaterials with fine structures.

Telencephalic Neural Progenitors Appear To Be Restricted to Regional and Glial Fates before the Onset of Neurogenesis

The contribution of early cell lineage to regional fate in the mammalian forebrain remains poorly understood. Previous lineage-tracing studies using retroviral methods were only begun at mid-neurogenesis and have suffered from region-specific retroviral silencing. We have been able to study cell lineage in the telencephalon from the onset of neurogenesis by using ultrasound backscatter microscopy to label the forebrain neuroepithelium and a modified retroviral lineage library to overcome regional silencing. Our studies suggest that by embryonic day 9.5, forebrain clones are primarily restricted to territories within anatomically demarcated regional boundaries, such as the cortex, striatum and hypothalamus. In addition, we observed a subset of clones that appeared to be composed entirely of glia. These observations suggest that both regional and cell-type restrictions exist within progenitor populations before the first forebrain cells become postmitotic.

Ultrasound backscatter microscopy of mouse embryos.

Extensive genetic information and transgenic techniques available in the mouse have led to its wide use in studies of mammalian development and models of human disease. A basic limitation of analyzing dynamic developmental processes in mouse embryos is their inaccessibility, because they are encased in the maternal uterus. In particular, cardiovascular development has been difficult to study and an understanding of the underlying mechanisms regulating heart development is incomplete, in part because of the lack of methods to measure cardio- and hemodynamics in live mouse embryos. Also, in contrast to lower vertebrate species such as frog, chick, and zebrafish, direct in utero manipulation of mouse embryos, through injection of dyes, cells, or retroviruses are difficult or impossible at most embryonic stages. High-frequency (40–50 MHz) ultrasound imaging and Doppler scanners, also called ultrasound backscatter microscopes (UBM), have recently been developed that allow noninvasive measurements of cardiovascular structure and function in live mouse embryos and that make possible direct in utero manipulations through high-resolution ultrasound-guided injections. This protocol describes methods that have been employed to image and measure blood flow in live mouse embryos and to inject cells, retroviruses and other agents into specific embryonic target sites using UBM imaging as a guidance system.

In utero ultrasound backscatter microscopy of early stage mouse embryos

A high resolution ultrasound imaging technique, ultrasound backscatter microscopy (UBM), has previously been shown to be useful for in utero imaging of mouse embryos, and for direct manipulation of mouse embryos through UBM-guided injections. UBM images from mouse embryos staged between 8.5 and 10.5 days of gestation are presented to demonstrate the range of anatomical structures which can be studied with this approach. Ultrasound contrast agents have been injected into the forebrain ventricle of 10.5 day embryos to characterize the resulting three-dimensional distribution of the injected agents. These studies provide important background data relevant to future use of this technique for in utero analysis of early brain and heart development, and for in utero manipulation of mouse embryos through UBM-guided injections.

Alteration of limb and brain patterning in early mouse embryos by ultrasound-guided injection of Shh-expressing cells

A basic limitation of the study of development in the mouse is the inaccessibility of the embryos, which are encased in the maternal uterus. We demonstrate the first use of ultrasound backscatter microscopy for guiding injections of cells and other agents into early stage mouse embryos. Cells were injected into the mouse neural tube cavity as early as 9.5 days post coitus (E9.5), and into the developing limb buds as early as E10.5. Furthermore, a cell line engineered to express the secreted factor Sonic Hedgehog ( Shh) was injected into early developing mouse brains or limbs. The Shh-expressing cells were found to induce ectopic expression of the Shh target genes Patched and Hnf3 β in the dorsal brain, and to alter digit patterning in the anterior limb bud. These results show that gene misexpression studies can be performed in mouse embryos using ultrasound-guided injection of transfected cells or retroviruses. In combination with the many available mouse mutants, this method offers a new approach for analyzing genetic interactions through gain-of-function studies performed in mutant mouse backgrounds.

Specification of Mouse Telencephalic and Mid-Hindbrain Progenitors Following Heterotopic Ultrasound-Guided Embryonic Transplantation

We have demonstrated the utility of ultrasound backscatter microscopy for targeted intraparenchymal injections into embryonic day (E) 13.5 mouse embryos. This system has been used to test the degree of commitment present in neural progenitors from the embryonic ventral telencephalon and mid-hindbrain region. Many E13.5 ventral telencephalic progenitors were observed to integrate and adopt local phenotypes following heterotopic transplantation into telencephalic or mid-hindbrain targets, whereas mid-hindbrain cells of the same stage were unable to integrate and change fate in the telencephalon. In contrast, many mid-hindbrain cells from an earlier developmental stage (E10.5) were capable of integrating and adopting a forebrain phenotype after grafting into the telencephalon, suggesting that mouse mid-hindbrain progenitors become restricted in their developmental potential between E10.5 and E13.5.