Ultrasonic tissue characterization of infarcted myocardium by scanning acoustic microscopy

Noninvasive Quantification of Contractile Dynamics in Cardiac Cells, Spheroids, and Organs-on-a-Chip Using High-Frequency Ultrasound

Cell-based models that mimic in vivo heart physiology are poised to make significant advances in cardiac disease modeling and drug discovery. In these systems, cardiomyocyte (CM) contractility is an important functional metric, but current measurement methods are inaccurate and low-throughput or require complex setups. To address this need, we developed a standalone noninvasive, label-free ultrasound technique operating at 40-200 MHz to measure the contractile kinetics of cardiac models, ranging from single adult CMs to 3D microtissue constructs in standard cell culture formats. The high temporal resolution of 1000 fps resolved the beat profile of single mouse CMs paced at up to 9 Hz, revealing limitations of lower speed optical based measurements to resolve beat kinetics or characterize aberrant beats. Coupling of ultrasound with traction force microscopy enabled the measurement of the CM longitudinal modulus and facile estimation of adult mouse CM contractile forces of 2.34 ± 1.40 μN, comparable to more complex measurement techniques. Similarly, the beat rate, rhythm, and drug responses of CM spheroid and microtissue models were measured, including in configurations without optical access. In conclusion, ultrasound can be used for the rapid characterization of CM contractile function in a wide range of commonly studied configurations ranging from single cells to 3D tissue constructs using standard well plates and custom microdevices, with applications in cardiac drug discovery and cardiotoxicity evaluation.

Scanning acoustic microscopy for material evaluation

Scanning acoustic microscopy (SAM) or Acoustic Micro Imaging (AMI) is a powerful, non-destructive technique that can detect hidden defects in elastic and biological samples as well as non-transparent hard materials. By monitoring the internal features of a sample in three-dimensional integration, this technique can efficiently find physical defects such as cracks, voids, and delamination with high sensitivity. In recent years, advanced techniques such as ultrasound impedance microscopy, ultrasound speed microscopy, and scanning acoustic gigahertz microscopy have been developed for applications in industries and in the medical field to provide additional information on the internal stress, viscoelastic, and anisotropic, or nonlinear properties. X-ray, magnetic resonance, and infrared techniques are the other competitive and widely used methods. However, they have their own advantages and limitations owing to their inherent properties such as different light sources and sensors.This paper provides an overview of the principle of SAM and presents a few results to demonstrate the applications of modern acoustic imaging technology. A variety of inspection modes, such as vertical, horizontal, and diagonal cross-sections have been presented by employing the focus pathway and image reconstruction algorithm. Images have been reconstructed from the reflected echoes resulting from the change in the acoustic impedance at the interface of the material layers or defects. The results described in this paper indicate that the novel acoustic technology can expand the scope of SAM as a versatile diagnostic tool requiring less time and having a high efficiency.

Development of a Shock-Wave Catheter Ablation System for Ventricular Tachyarrhythmias: Validation Study in Pigs In Vivo

Background

Although radiofrequency catheter ablation is the current state‐of‐the‐art treatment for ventricular tachyarrhythmias, it has limited success for several reasons, including insufficient lesion depth, prolonged inflammation with subsequent recurrence, and thromboembolisms due to myoendocardial thermal injury. Because shock waves can be applied to deep lesions without heat, we have been developing a shock‐wave catheter ablation (SWCA) system to overcome these fundamental limitations of radiofrequency catheter ablation. In this study, we evaluated the efficacy and safety of our SWCA system for clinical application to treat ventricular tachyarrhythmia.

Methods and Results

In 33 pigs, we examined SWCA in vivo for the following 4 protocols. First, in an epicardial substrate model (n=8), endocardial SWCA significantly decreased the sensing threshold (pre‐ versus postablation: 11.4±3.8 versus 6.8±3.6 mV; P<0.05) and increased the pacing threshold (pre‐ versus postablation: 1.6±0.8 versus 2.0±1.1 V; P<0.05), whereas endocardial radiofrequency catheter ablation failed to do so. Second, in a myocardial infarction model (n=3), epicardial SWCA of the border zone of the infarcted lesion was as effective as ablation of the normal myocardium. Third, in a coronary artery application model (n=10), direct application of shock waves to the epicardial coronary arteries caused no adverse effects in either the acute or chronic phase. Fourth, with an epicardial approach (n=8), we found that 90 shots per site provided an ideal therapeutic condition to create deep lesions with less superficial damage.

Conclusions

These results indicate that our new SWCA system is effective and safe for treatment of ventricular tachyarrhythmias with deep arrhythmogenic substrates.

Enhancement of Acoustic Microscopy Lateral Resolution: A Comparison Between Deep Learning and Two Deconvolution Methods

Scanning acoustic microscopy (SAM) provides high-resolution images of biological tissues. Since higher transducer frequencies limit penetration depth, image resolution enhancement techniques could help in maintaining sufficient lateral resolution without sacrificing penetration depth. Compared with existing SAM research, this work introduces two novelties. First, deep learning (DL) is used to improve lateral resolution of 180-MHz SAM images, comparing it with two deconvolution-based approaches. Second, 316-MHz images are used as ground truth in order to quantitatively evaluate image resolution enhancement. The samples used were mouse and rat brain sections. The results demonstrate that DL can closely approximate ground truth (NRMSE = 0.056 and PSNR = 28.4 dB) even with a relatively limited training set (four images, each smaller than 1 mm ×1 mm). This study suggests the high potential of using DL as a single image superresolution method in SAM.

Acoustic Histology with Specific Dyes and Antibodies

The present study aims to identify specific staining methods for acoustic histology. We compared attenuation-of-sound (AOS) images from scanning acoustic microscopy (SAM) with light microscopy (LM) images. Ethanol-fixed tissue or cytology samples and formalin-fixed surgical or autopsy specimens were examined. Nuclei, collagen, elastic fibers and polysaccharides and various antigens, including cell surface, cytoplasmic, nuclear and stromal substances, were observed. Samples with various fixation methods were used. Hematoxylin staining had significantly higher AOS values in accordance with staining duration. Specific staining for collagen, elastic fibers and polysaccharides increased the AOS values of the specific substance. Using diaminobenzidine tetrahydrochloride in NiCl₂ solution as a substrate for horseradish peroxidase increased the AOS values to those suitable for acoustic immunostaining. Collagenase digestion after collagen staining decreased AOS values, reflecting collagen density and distribution. Staining with specific dyes or acoustic immunostaining enabled the histologic localization of specific substances by SAM, similar to LM.

Regional changes in the elastic properties of myopic Guinea pig sclera

Biomechanical changes in the sclera likely underlie the excessive eye elongation of axial myopia. We studied the biomechanical characteristics of myopic sclera at the microscopic level using scanning acoustic microscopy (SAM) with 7-μm in-plane resolution. Guinea pigs underwent form-deprivation (FD) in one eye from 4 to 12 days of age to induce myopia, and 12-μm-thick scleral cryosections were scanned using a custom-made SAM. Two-dimensional maps of the bulk modulus (K) and mass density (ρ) were derived from the SAM data using a frequency-domain approach. We assessed the effect on K and ρ exerted by: 1) level of induced myopia, 2) region (superior, inferior, nasal or temporal) and 3) eccentricity from the nerve using univariate and multivariate regression analyses. Induced myopia ranged between -3D and -9.3D (Mean intraocular difference of -6.2 ± 1.7D, N = 11). K decreased by 0.036 GPa for every 1.0 D increase in induced myopia across vertical sections (p < 0.001). Among induced myopia right eyes, K values in the inherently more myopic superior region were 0.088 GPa less than the inferior region (p = 0.002) and K in the proximal nasal region containing the central axis were 0.10 GPa less than temporal K (p = 0.036). K also increased 0.12 GPa for every 1 mm increase in superior vertical distance (p < 0.001), an effect that was blunted after 1 week of FD. Overall, trends for ρ were less apparent than for K. ρ values increased by 20.7 mg/cm3 for every 1.00 D increase in induced myopia across horizontal sections (p < 0.001), and were greatest in the region containing the central posterior pole. ρ values in the inherently more myopic superior region were 13.1 mg/cm3 greater than that found in inferior regions among control eyes (p = 0.002), and increased by 11.2 mg/cm3 for every 1 mm increase in vertical distance (p = 0.001). This peripheral increase in ρ was blunted after 1 week of FD. Scleral material properties vary depending on the location in the sclera and the level of induced myopia. Bulk modulus was most reduced in the most myopic regions (both induced myopia and inherent regional myopia), and suggests that FD causes microscopic local decreases in sclera stiffness, while scleral mass density was most increased in the most myopic regions.

Autoregressive Signal Processing Applied to High-Frequency Acoustic Microscopy of Soft Tissues

Quantitative acoustic microscopy (QAM) at frequencies exceeding 100 MHz has become an established imaging tool to depict acoustical and mechanical properties of soft biological tissues at microscopic resolutions. In this study, we investigate a novel autoregressive (AR) model to improve signal processing and parameter estimation and to test its applicability to QAM. The performance of the AR model for estimating acoustical parameters of soft tissues (i.e., acoustic impedance, speed of sound, and attenuation) was compared to the performance of the Hozumi model using simulated ultrasonic QAM signals and using experimentally measured signals from thin (i.e., 12 and ) sections of human lymph-node and pig-cornea tissue specimens. Results showed that the AR and Hozumi methods performed equally well (i.e., produced an estimation error of 0) in signals with low, linear attenuation in the tissue and high impedance contrast between the tissue and the coupling medium. However, the AR model outperformed the Hozumi model in estimation accuracy and stability (i.e., parameter error variation and number of outliers) in cases of 1) thin tissue-sample thickness and high tissue-sample speed of sound, 2) small impedance contrast between the tissue sample and the coupling medium, 3) high attenuation in the tissue sample, and 4) nonlinear attenuation in the tissue sample. Furthermore, the AR model allows estimating the exponent of nonlinear attenuation. The results of this study suggest that the AR model approach can improve current QAM by providing more reliable, quantitative, tissue-property estimates and also provides additional values of parameters related to nonlinear attenuation.

Triplex micron-resolution acoustic, photoacoustic, and optical transmission microscopy via photoacoustic radiometry

We present a new sensing technique, termed photoacoustic radiometry (PAR), for mapping the optical attenuation properties of a sample. In PAR, laser pulses attenuated via transmission through the sample impinge on the ultrasound transducer and generate a photoacoustic (PA) signal within it. Spatial variation of the optical attenuation properties of the sample influences the amplitude of the PAR signal, providing image contrast. Performed simultaneously with pulse-echo ultrasound and PA imaging, this triplex imaging technique enables rapid characterization of samples with micrometer-resolution in a single scan. In this work, we demonstrate that the PAR technique can be easily integrated into existing PA microscopy systems, with applications in imaging biological samples and non-destructive evaluation of optically opaque materials such as silicon wafers.

The Structure of Human Sebaceous Glands and Its Relation to Skin Viscoelasticity

High-frequency ultrasound has realized high-resolution observation of deep part of the dermis in vivo. The size of sebaceous glands was evaluated by three-dimensional ultrasound microscopy with the ultrasonic frequency of 120 MHz. The viscoelasticity of the same regions was measured by well-established biomechanical equipment. There was no significant difference between the size of sebaceous glands in cheek and forearm. The skin's ability to recover to its initial position after deformation was significantly higher in forearm than in cheek. Both sizes of sebaceous glands in cheek and forearm were positively correlated with the parameter of viscoelasticity. The size of the sebaceous glands in the deep part of the dermis can be a parameter of skin viscoelasticity. High-frequency ultrasound imaging contributes to the evaluation of human skin morphology as well as functions.

Improved High-Frequency Ultrasound Corneal Biometric Accuracy by Micrometer-Resolution Acoustic-Property Maps of the Cornea

Purpose

Mapping of epithelial thickness (ET) is useful for detection of keratoconus, a disease characterized by corneal thinning and bulging in which epithelial thinning occurs over the apex. In prior clinical studies, optical coherence tomography (OCT) measurements of ET were systematically thinner than those obtained by 40-MHz high-frequency ultrasound (HFU) where a constant speed of sound (c) of 1636 m/s was used for all corneal layers. The purpose of this work was to study the acoustic properties, that is, c, acoustic impedance (Z), and attenuation (α) of the corneal epithelium and stroma independently using a scanning acoustic microscope (SAM) to investigate the discrepancy between OCT and HFU estimates of ET.

Methods

Twelve unfixed pig corneas were snap-frozen and 6-μm sections were scanned using a custom-built SAM with an F-1.08, 500-MHz transducer and a 264-MHz bandwidth. Two-dimensional maps of c, Z, and α with a spatial resolution of 4 μm were derived.

Results

SAM showed that the value of c in the epithelium (i.e., 1548 ± 18 m/s) is substantially lower than the value of c in the stroma (i.e., 1686 ± 33 m/s).

Conclusion

SAM results demonstrated that the assumption of a constant value of c for all corneal layers is incorrect and explains the prior discrepancy between OCT and HFU ET determinations.

Translational Relevance

The findings of this study have important implications for HFU-based ET measurements and will improve future keratoconus diagnosis by providing more-accurate ET estimates.

A method for the design of ultrasonic devices for scanning acoustic microscopy using impulsive signals

Scanning acoustic microscopy (SAM) using impulsive signals is useful for characterization of biological tissues and cells. The operating center frequency of an ultrasonic device strongly depends on the performance characteristics of the device if the measurement is conducted by using impulsive signals. In this paper, a method for the design of ultrasonic devices for SAM using impulsive signals was developed. A new plane-wave model was introduced to calculate frequency characteristics of loss of ultrasonic devices by taking into account the conversion loss at the ultrasonic transducer, the transmission loss at the acoustic anti-reflection coating, and the propagation loss in the couplant. Ultrasonic devices were fabricated with a ZnO ultrasonic transducer using two acoustic lenses with aperture radii of 1.0 mm and 0.5 mm, respectively. The frequencies at which measured losses became minima corresponded to the calculation results by the plane-wave model. This numerical calculation method is useful for designing ultrasonic devices for acoustic microscopy using impulsive signals.

Material Properties of Human Ocular Tissue at 7-µm Resolution

Quantitative assessment of the material properties of ocular tissues can provide valuable information for investigating several ophthalmic diseases. Quantitative acoustic microscopy (QAM) offers a means of obtaining such information, but few QAM investigations have been conducted on human ocular tissue. We imaged the optic nerve (ON) and iridocorneal angle in 12-µm deparaffinized sections of the human eye using a custom-built acoustic microscope with a 250-MHz transducer (7-µm lateral resolution). The two-dimensional QAM maps of ultrasound attenuation (α), speed of sound ( c), acoustic impedance ( Z), bulk modulus ( K), and mass density (ρ) were generated. Scanned samples were then stained and imaged by light microscopy for comparison with QAM maps. The spatial resolution and contrast of scanning acoustic microscopy (SAM) maps were sufficient to resolve anatomic layers of the retina (Re); anatomic features in SAM maps corresponded to those seen by light microscopy. Significant variations of the acoustic parameters were found. For example, the sclera was 220 MPa stiffer than Re, choroid, and ON tissue. To the authors' knowledge, this is the first systematic study to assess c, Z, K, ρ, and α of human ocular tissue at the high ultrasound frequencies used in this study.

A Novel Quantitative 500-MHz Acoustic Microscopy System for Ophthalmologic Tissues

OBJECTIVE

This paper describes development of a novel 500-MHz scanning acoustic microscope (SAM) for assessing the mechanical properties of ocular tissues at fine resolution. The mechanical properties of some ocular tissues, such as lamina cribrosa (LC) in the optic nerve head, are believed to play a pivotal role in eye pathogenesis.

METHODS

A novel etching technology was used to fabricate silicon-based lens for a 500-MHz transducer. The transducer was tested in a custom-designed scanning system on human eyes. Two-dimensional (2-D) maps of bulk modulus (K) and mass density (ρ) were derived using improved versions of current state-of-the-art signal processing approaches.

RESULTS

The transducer employed a lens radius of 125 μm and had a center frequency of 479 MHz with a -6-dB bandwidth of 264 MHz and a lateral resolution of 4 μm. The LC, Bruch's membrane (BM) at the interface of the retina and choroid, and Bowman's layer (BL) at the interface of the corneal epithelium and stroma, were successfully imaged and resolved. Analysis of the 2-D parameter maps revealed average values of LC, BM, and BL with KLC = 2.81 ±0.17; GPa, KBM = 2.89 ±0.18; GPa, KBL = 2.6 ±0.09 ; GPa, ρ LC = 0.96 ±0.03 g/cm³; ρ BM = 0.97 ±0.04 g/cm³; ρ BL = 0.98 ±0.04 g/cm³.

SIGNIFICANCE

This novel SAM was shown to be capable of measuring mechanical properties of soft biological tissues at microscopic resolution; it is currently the only system that allows simultaneous measurement of K, ρ, and attenuation in large lateral scales (field area >9 mm²) and at fine resolutions.

Fine-resolution maps of acoustic properties at 250 MHz of unstained fixed murine retinal layers

Ex vivo assessment of microscale tissue biomechanical properties of the mammalian retina could offer insights into diseases such as keratoconus, and macular degeneration. A 250-MHz scanning acoustic microscope (7-μm resolution) has been constructed to derive two-dimensional quantitative maps of attenuation (α), speed of sound (c), acoustic impedance (Z), bulk modulus (B), and mass density ( ρ). The two-dimensional maps were compared to coregistered hematoxylin-and-eosin stained sections. This study is the first to quantitatively assess  α, c, Z, B, and ρ of individual retinal layers of mammalian animals at high ultrasound frequencies. Significant differences in these parameters between the layers were demonstrated.

Acoustic impedance microscopy for biological tissue characterization

A new method for two-dimensional acoustic impedance imaging for biological tissue characterization with micro-scale resolution was proposed. A biological tissue was placed on a plastic substrate with a thickness of 0.5mm. A focused acoustic pulse with a wide frequency band was irradiated from the "rear side" of the substrate. In order to generate the acoustic wave, an electric pulse with two nanoseconds in width was applied to a PVDF-TrFE type transducer. The component of echo intensity at an appropriate frequency was extracted from the signal received at the same transducer, by performing a time-frequency domain analysis. The spectrum intensity was interpreted into local acoustic impedance of the target tissue. The acoustic impedance of the substrate was carefully assessed prior to the measurement, since it strongly affects the echo intensity. In addition, a calibration was performed using a reference material of which acoustic impedance was known. The reference material was attached on the same substrate at different position in the field of view. An acoustic impedance microscopy with 200×200 pixels, its typical field of view being 2×2 mm, was obtained by scanning the transducer. The development of parallel fiber in cerebella cultures was clearly observed as the contrast in acoustic impedance, without staining the specimen. The technique is believed to be a powerful tool for biological tissue characterization, as no staining nor slicing is required.

Detection and quantification of bacterial biofilms combining high-frequency acoustic microscopy and targeted lipid microparticles

BACKGROUND

Immuno-compromised patients such as those undergoing cancer chemotherapy are susceptible to bacterial infections leading to biofilm matrix formation. This surrounding biofilm matrix acts as a diffusion barrier that binds up antibiotics and antibodies, promoting resistance to treatment. Developing non-invasive imaging methods that detect biofilm matrix in the clinic are needed. The use of ultrasound in conjunction with targeted ultrasound contrast agents (UCAs) may provide detection of early stage biofilm matrix formation and facilitate optimal treatment.

RESULTS

Ligand-targeted UCAs were investigated as a novel method for pre-clinical non-invasive molecular imaging of early and late stage biofilms. These agents were used to target, image and detect Staphylococcus aureus biofilm matrix in vitro. Binding efficacy was assessed on biofilm matrices with respect to their increasing biomass ranging from 3.126 × 103 ± 427 UCAs per mm(2) of biofilm surface area within 12 h to 21.985 × 103 ± 855 per mm(2) of biofilm matrix surface area at 96 h. High-frequency acoustic microscopy was used to ultrasonically detect targeted UCAs bound to a biofilm matrix and to assess biofilm matrix mechanoelastic physical properties. Acoustic impedance data demonstrated that biofilm matrices exhibit impedance values (1.9 MRayl) close to human tissue (1.35 - 1.85 MRayl for soft tissues). Moreover, the acoustic signature of mature biofilm matrices were evaluated in terms of integrated backscatter (0.0278 - 0.0848 mm(-1) × sr(-1)) and acoustic attenuation (3.9 Np/mm for bound UCAs; 6.58 Np/mm for biofilm alone).

CONCLUSIONS

Early diagnosis of biofilm matrix formation is a challenge in treating cancer patients with infection-associated biofilms. We report for the first time a combined optical and acoustic evaluation of infectious biofilm matrices. We demonstrate that acoustic impedance of biofilms is similar to the impedance of human tissues, making in vivo imaging and detection of biofilm matrices difficult. The combination of ultrasound and targeted UCAs can be used to enhance biofilm imaging and early detection. Our findings suggest that the combination of targeted UCAs and ultrasound is a novel molecular imaging technique for the detection of biofilms. We show that high-frequency acoustic microscopy provides sufficient spatial resolution for quantification of biofilm mechanoelastic properties.

Scanning acoustic microscopy for characterization of neoplastic and inflammatory lesions of lymph nodes

A scanning acoustic microscope (SAM) imaging system calculates and color codes speed of sound (SOS). We evaluated the SAM results for lymph node imaging and compared these results with those of light microscopy (LM). SAM showed normal structures and localized/diffuse lesions of the lymph node. Our results revealed that as a rule, soft areas such as cystic necrosis presented less SOS while harder areas such as coagulative necrosis, granulomas, and fibrosis exhibited greater SOS. SOS increased according to stromal desmoplastic reactions and cellular concentration. In neoplastic lesions, statistically significant differences in SOS were observed among scirrhous carcinomas, lymphomas, and medullary carcinomas. SAM provided the following benefits over LM: (1) images reflected the tissue elasticity of each lesion, (2) digitized SOS data could be statistically comparable, (3) images were acquired in a few minutes without special staining, (4) SAM images and echographic images were comparable for clinical ultrasound imaging study.

Echogenicity in transrectal ultrasound is determined by sound speed of prostate tissue components

Typically, conventional transrectal ultrasound (TRUS) imaging of the cancer tissue is hypoechoic in echo texture. However, TRUS does not reliably distinguish between cancerous and non-cancerous tissue in the prostate. In the present study, sound speed of prostate needle biopsy specimens were measured by ultrasound speed microscope (USM) to construct a database for interpreting clinical TRUS images. Biopsy specimens were formalin-fixed and sectioned approximately 5 µm in thickness. They were mounted on glass slides without cover slips. The ultrasonic transducer with the central frequency of 120 MHz was mechanically scanned over the specimen to measure sound speed distribution. Echo intensity of TRUS images were qualitatively classified into three categories; hyperechoic, iso-echoic and hypoechoic areas. Sound speed was 1596.9 ± 28.2 m/s in hyperechoic, 1571.2 ± 35.8 m/s in iso-echoic and 1562.6 ± 35.1 m/s in hypoechoic area, respectively. However, echo intensity showed no significant relationship to malignancy of prostatic tissue. Echo intensity of TRUS is significantly affected with tissue components and USM findings would provide important information for interpretation of TRUS images.

Relation between morphology of sebaceous glands inside human skin and viscoelastic properties

Three-dimensional ultrasound microscopy with the central frequency of 120 MHz made it possible to observe in vivo sebaceous glands at the deep part of the dermis at microscopic level. The deformation displacements were measured by an established testing device, and the viscoelasticity was estimated from the measured displacements and Voigt model. The occupancy, density or average size of sebaceous glands was compared with the viscoleasticity. There were three major findings in the comparisons. First, the occupancy of sebaceous gland showed negative correlation with the elasticity. Second, the density of sebaceous glands showed positive correlation with the viscosity. Third, the average size of sebaceous glands showed negative correlation with the viscosity. In conclusion, viscoelastic property of human skin is strongly influenced by the morphology of the sebaceous glands.

Pulmonary imaging with a scanning acoustic microscope discriminates speed-of-sound and shows structural characteristics of disease

Tissue elasticity can be detected using a scanning acoustic microscope (SAM), whereby acoustic images are created from the speed of sound through tissues. This system discriminated pulmonary tissue components and demonstrated distinct acoustic images of the lung; these results corresponded well to those obtained using the conventional microscope. SAM provides the following benefits: (1) images are acquired in only few minutes without requirement for staining, (2) basic data is obtained for low-frequency ultrasonic examination, and (3) speed of sound from each lesion is digital and comparable among diseases. Comparative analysis of cancer invasion, post-inflammatory fibrosis, and deposition disease was possible using the data obtained with the system, and the results showed good correlation with those using the conventional microscope and by clinical diagnosis. The SAM system is applicable not only to pulmonary diseases but also to various diseases in other organs.

High-resolution ultrasound imaging of human skin in vivo by using three-dimensional ultrasound microscopy

Observing the morphology of human skin is important in the diagnosis of skin cancer and inflammation and in the assessment of skin aging. High-frequency ultrasound imaging provides high spatial resolution of the deep layers of the skin, which cannot be visualized by optical methods. The objectives of the present study were to develop a three-dimensional (3-D) ultrasound microscope and to observe the morphology of normal human skin in vivo. A concave polyvinylidene fluoride transducer with a central frequency of 120 MHz was excited using an electric pulse generated by semiconductor switching. The transducer was scanned two-dimensionally by using two linear motors on the region-of-interest and the ultrasonic reflection was digitized with 2-GHz sampling. Consecutive B-mode images perpendicular to the skin surface were reconstructed to generate multiplanar reconstructed images and 3-D volume-rendering images clearly showing microstructures such as sebaceous glands and hair follicles. The 3-D ultrasound microscope could be used to successfully image the morphology of human skin noninvasively and may provide important information on skin structure.

Ultrasonic tissue characterization of prostate biopsy tissues by ultrasound speed microscope

Ultrasound speed microscope was developed for quantitative measurement of ultrasonic parameters of soft tissues. The system can measure the ultrasonic attenuation and sound speed in the tissue using fast Fourier transform of a single pulsed wave instead of burst waves used in conventional acoustic microscopy. Prostate biopsy tissues were formalin-fixed and sectioned approximately 5-6 μm in thickness. They were mounted on glass slides without cover slips. The ultrasonic transducer was mechanically scanned over the specimen. Attenuation was 1.42 ± 0.08 dB/mm and the sound speed was 1584 ± 12 m/s in prostatic cancer while both values were 1.86 ± 0.14 dB/mm and 1614 ± 30 m/s in normal prostate. The basic measurements of ultrasonic properties would help understanding the interpretation of clinical echography in diagnosis of prostate cancer.

Imaging of sebaceous glands of human skin by three-dimensional ultrasound microscopy and its relation to elasticity

High frequency ultrasound imaging has realized high resolution in vivo imaging of the biological tissues at a microscopic level. Human skin structure, especially sebaceous glands at the deep part of the dermis, was observed by three-dimensional ultrasound microscopy with the central frequency of 120 MHz. The visco-elasticity and surface sebum level of the observed region were measured by established testing devices. Both sebaceous glands density and surface sebum level were higher in cheek than those in forearm. The viscosity of forearm was lower than that of cheek. These results suggest that sebaceous glands may act as cushions of the skin besides their classical role of secreting sebum and some hormones. High frequency ultrasound imaging contributes to the evaluation of human skin aging.

Localised micro-mechanical stiffening in the ageing aorta

Age-related loss of tissue elasticity is a common cause of human morbidity and arteriosclerosis (vascular stiffening) is associated with the development of both fatal strokes and heart failure. However, in the absence of appropriate micro-mechanical testing methodologies, multiple structural remodelling events have been proposed as the cause of arteriosclerosis. Therefore, using a model of ageing in female sheep aorta (young: <18 months, old: >8 years) we: (i) quantified age-related macro-mechanical stiffness, (ii) localised in situ micro-metre scale changes in acoustic wave speed (a measure of tissue stiffness) and (iii) characterised collagen and elastic fibre remodelling. With age, there was an increase in both macro-mechanical stiffness and mean microscopic wave speed (and hence stiffness; young wave speed: 1701 ± 1 m s⁻¹, old wave speed: 1710 ± 1 m s⁻¹, p < 0.001) which was localized to collagen fibril-rich regions located between large elastic lamellae. These micro-mechanical changes were associated with increases in both collagen and elastic fibre content (collagen tissue area, young: 31 ± 2%, old: 40 ± 4%, p < 0.05; elastic fibre tissue area, young: 55 ± 3%, old: 69 ± 4%, p < 0.001). Localised collagen fibrosis may therefore play a key role in mediating age-related arteriosclerosis. Furthermore, high frequency scanning acoustic microscopy is capable of co-localising micro-mechanical and micro-structural changes in ageing tissues.

Density imaging using a multiple-frequency DBIM approach

Current inverse scattering methods for quantitative density imaging have limitations that keep them from practical experimental implementations. In this work, an improved approach, termed the multiple-frequency distorted Born iterative method (MF-DBIM) algorithm, was developed for imaging density variations. The MF-DBIM approach consists of inverting the wave equation by solving for a single function that depends on both sound speed and density variations at multiple frequencies. Density information was isolated by using a linear combination of the reconstructed single-frequency profiles. Reconstructions of targets using MF-DBIM from simulated data were compared with reconstructions using methods currently available in the literature, i.e., the dual-frequency DBIM (DF-DBIM) and T-matrix approaches. Useful density reconstructions, i.e., root mean square errors (RMSEs) less than 30%, were obtained with MF-DBIM even with 2% Gaussian noise in the simulated data and using frequency ranges spanning less than an order of magnitude. Therefore, the MFDBIM approach outperformed both the DF-DBIM method (which has problems converging with noise even an order of magnitude smaller) and the T-matrix method (which requires a ka factor close to unity to achieve convergence). However, the convergence of all the density imaging algorithms was compromised when imaging targets with object functions exhibiting high spatial frequency content.

Quantitative measurements of apoptotic cell properties using acoustic microscopy

Time-resolved acoustic microscopy was used to measure properties of cells such as the thickness, sound velocity, acoustic impedance, density, bulk modulus, and attenuation, before and after apoptosis. A total of 12 cells were measured, 5 apoptotic and 7 non-apoptotic. Measurements made at 375 MHz showed a statistically significant increase in the cell thickness from 13.6 ± 3.1 μm to 17.3 ± 1.6 μm, and in the attenuation from 1.08 ± 0.21 dB/cm/MHz to 1.74 ± 0.36 dB/cm/MHz. The other parameters, such as the sound velocity, density, acoustic impedance, and bulk modulus remained similar within experimental error. Acoustic images obtained at 1.0 GHz showed increased RF-signal backscatter and a clear delineation of the nucleus and cytoplasm from apoptotic cells compared with non-apoptotic cells. Extensive activity was observed optically and acoustically within apoptotic cells. Acoustic measurements made one minute apart showed variations in the ultrasonic backscatter but not attenuation in the cells, which indicated rapid structural changes were occurring but not changes in bulk composition. The normalized crosscorrelation coefficient was used to quantify the variations in the backscatter RF-signal during apoptosis by comparing the first RF signal measured to each successive RF signal every 10 s. A coefficient of 1 indicates strong correlation, whereas a coefficient of 0 indicates no correlation. An average correlation coefficient of 0.93 ± 0.05 was measured for non-apoptotic cells, compared with 0.68 ± 0.17 for apoptotic cells, indicating that the RF signal as a function of time varied rapidly during apoptosis.

Effect of estrogen on tissue elasticity of the ligament proper in rabbit anterior cruciate ligament: measurements using scanning acoustic microscopy

BACKGROUND

Previous epidemiological studies revealed that anterior cruciate ligament (ACL) injuries were more frequently seen in female athletes than in male athletes. To elucidate the pathogenetic roles of estrogen in ACL ruptures, the elasticity of ACL tissue was measured using a scanning acoustic microscope (SAM) in an estrogen-controlled animal model.

METHODS

A total of 40 ovariectomized Japanese white rabbits were randomly divided into four groups according to the administered dose of 17beta-estradiol (groups L, M, H, and C). Injection of 17beta-estradiol was performed 1, 2, 3, and 4 weeks after surgery, and doses in groups L, M, and H were 50, 100, and 500 microg/kg, respectively. Group C received no estradiol. Only groups L, M, and C were used for current analyses because their mean serum estrogen levels were within the physiological range (groups C, L, M, and H: 37, 50, 60, and 231 pg/ml, respectively). Five weeks after ovariectomy, the lateral portion of the ligament was harvested. Specimens were fixed with 10% neutralized formalin and embedded in paraffin. Then, 10 mum thick sections were cut perpendicular to the ligament fibers for routine histological staining and measurement with SAM.

RESULTS

The mean tissue sound speeds of groups C, L, and M were 1727 +/- 32, 1683 +/- 53, and 1665 +/- 63 m/s, respectively. Group M presented significantly lower tissue sound speed than group C (P = 0.021). Furthermore, a negative correlation was found between the mean serum estrogen level and mean tissue sound speed of the ACL among all animals in groups C, L, and M (r = 0.47, P = 0.016).

CONCLUSION

The results of the present study indicated that estrogen altered the tissue elasticity of rabbit ACL. Estrogen may constitute one of the pathogenetic factors in ACL rupture in female athletes.

Propagation of vibration caused by electrical excitation in the normal human heart

The ability to noninvasively detect regional dynamic myocardial damage related to action potentials and mechanical properties affected by heart disease is of great clinical importance. Though there are invaluable clinical tools for diagnosis of a broad range of cardiac conditions, such myocardial properties cannot be evaluated. We have previously shown that pulsive vibration occurs on the myocardium after electrical stimulation of an isolated heart. In this study, using a novel technique for ultrasonic measurement of the myocardial motion, we detected pulsive vibrations spontaneously caused by electrical excitation and by valve closure. Using a sparse sector scan, the vibrations were measured almost simultaneously at about 10,000 points set in the heart wall at a high temporal resolution. The consecutive spatial distributions of the phase of the vibrations revealed wave propagation along the wall in healthy subjects for the first time in vivo. At around the time of the Q-wave of the electrocardiogram, the propagation started from the interventricular septum and extended to both the base and apical sides of the heart with a speed of 1 m/s, which corresponds to the propagation of electrical excitation from the Purkinje fiber-myocyte junction in the interventricular septum. Other vibrations then propagated from the base at several m/s, although some of them had dispersion properties. These are shear waves caused by the mitral-valve closure, corresponding to the first heart sound. These phenomena have potential for detection of regional myocardial tissue damage related to propagation of the action potentials and regional myocardial viscoelasticity.

Comparison of left ventricular diastolic filling with myocyte bulk modulus using Doppler echocardiography and acoustic microscopy in pressure-overload left ventricular hypertrophy and cardiac amyloidosis

BACKGROUND

The myocardial bulk modulus has been described as the constitutive properties of the left ventricular (LV) wall and is measured as rho V2 (rho = density, V = sound speed) using acoustic microscopy.

HYPOTHESIS

The study was undertaken to assess the relationship between the myocyte bulk modulus and transmitral inflow patterns in patients with pressure-overload LV hypertrophy (LVH) and cardiac amyloidosis (AMD).

METHODS

In 8 patients with LVH, 8 with AMD, and 10 controls without heart disease, the transmitral inflow pattern was recorded by Doppler echocardiography before death, and myocardial tissue specimens were obtained at autopsy. The tissue density and sound speed in the myocytes were measured by microgravimetry and acoustic microscopy, respectively. The diameters of the myocytes were measured on histopathologic specimens stained by the elastica Van Gieson method.

RESULTS

In the subendocardium, the myocyte bulk modulus was larger in LVH (2.98 x 10(9) N/m2, p < 0.001) and smaller in AMD (2.61 x 10(9) N/m2, p < 0.001) than in the controls (2.87 x 10(9) N/m2). The myocyte diameter in LVH (26 +/- 1 microns) was larger than that in the control (21 +/- 1 microns, p < 0.001) and AMD (20 +/- 1 microns, p < 0.001). The bulk modulus in the subendocardial myocyte significantly correlated with the deceleration time (DT) of the early transmitral inflow (r = 0.689, p = 0.028 in control, r = 0.774, p = 0.024 in LVH, and r = 0.786, p = 0.021 in AMD).

CONCLUSION

The changes in the myocyte elasticity as represented by the bulk modulus were limited to the subendocardial layers and may be related to relaxation abnormalities in LVH and a reduction in LV compliance in AMD.

Measurements of ultrasonic attenuation properties of midgestational fetal pig hearts

The objectives of this study were to measure the relative attenuation properties of the left and right ventricles in fetal pig hearts and to compare the spatial variation in attenuation measurements with those observed in previously published backscatter measurements. Approximately 1.0-mm-thick, short-axis slices of excised, formalin-fixed heart were examined from 15 midgestational fetal pigs using a 50-MHz single-element transducer. Measurements of the attenuation properties demonstrate regional differences in the left and right ventricular myocardium that appear consistent with the previously reported regional differences in apparent integrated backscatter measurements of the same fetal pig hearts. For regions of perpendicular insonification relative to the myofiber orientation, the right ventricular free wall showed larger values for the slope of the attenuation coefficient from 30-60 MHz (1.48 +/- 0.22 dB/(cm x MHz) (mean +/- SD) and attenuation coefficient at 45 MHz (46.3 +/- 7.3 dB/cm [mean +/- SD]) than the left ventricular free wall (1.18 +/- 0.24 dB/(cm x MHz) and 37.0 +/- 7.9 dB/cm (mean +/- SD) for slope of attenuation coefficient and attenuation coefficient at 45 MHz, respectively). This attenuation study supports the hypothesis that intrinsic differences in the myocardium of the left and right ventricles exist in fetal pig hearts at midgestation.

Degenerated coracoacromial ligament in shoulders with rotator cuff tears shows higher elastic modulus: measurement with scanning acoustic microscopy

PURPOSE

The purpose of this study was to determine the elasticity of the coracoacromial ligament in shoulders with and without rotator cuff tears.

METHODS

The coracoacromial ligaments from 20 cadaveric shoulders (average patient age 79.5 years; 8 men, 12 women) were divided into six portions--three portions (acromial, central, and coracoid) in two layers (superficial and deep). A total of 120 samples were studied. First, the samples were classified by the collagen fiber orientation into three degeneration patterns: wavy, straight, irregular. For each pattern, the tissue sound speed, which shows a positive correlation with elasticity, was measured with scanning acoustic microscopy. Next, the samples were divided into three groups: 60 samples from shoulders with rotator cuff tears (RCT group), 30 samples from shoulders with an intact rotator cuff and a subacromial spur (spur group), and 30 samples from shoulders with an intact rotator cuff without a subacromial spur (control group). All shoulders with rotator cuff tears had subacromial spurs. The tissue sound speed and the histological findings were compared among the groups.

RESULTS

The sound speeds in the wavy, straight, and irregular patterns were 1592 +/- 17.2 m/s (mean +/- SD), 1626 +/- 28.0 m/s, and 1607 +/- 29.8 m/s, respectively (P < 0.0001). The sound speed in the straight pattern was higher than that in the wavy pattern (P < 0.0001), and that in the irregular pattern was lower than that in the straight pattern (P = 0.0023). The RCT group and the spur group had more straight patterns (P = 0.0002) and fewer wavy patterns (P < 0.0001) than did the control group. Significant differences in the sound speed were observed between the groups (P < 0.0001): 1596 +/- 19.1 m/s in the control group, 1630 +/- 31.5 m/s in the spur group, 1612 +/- 28.6 m/s in the RCT group.

CONCLUSIONS

The coracoacromial ligament in shoulders with rotator cuff tears shows higher elastic modulus than in age-matched normal shoulders due to degeneration of the ligament.

Ultrasonic radio-frequency spectrum analysis differentiates normal and edematous brain tissue from meningioma intraoperatively

Intraoperative ultrasound imaging of the brain is used for tumor localization and resection control. The aim of the present study was to prove whether spectral analysis of radio-frequency (rf) signals is able to improve its diagnostic capabilities by adding quantitative acoustical parameters to pure visual analysis. Meningioma was chosen as a first model because of its distinct borders during surgery as well as in ultrasound imaging. Rf signals were captured intraoperatively. Spectral analysis of rf signals was performed off-line in areas of normal brain, edematous tissue, and meningioma within the bandwidth of the transducer. At 5.0 MHz, attenuation allowed significant differentiation for normal brain versus edema (P= .00002), normal brain versus meningioma (P= .000004), and edema versus meningioma (P= .002). The slope of attenuation reached significant levels among the three groups, too. Backscatter analysis consisted of determination of the power spectral density with a significant difference for edema versus meningioma at 5 MHz (P= .02). The same was true for a relative integrated backscatter coefficient (P= .01). Frequency-dependent backscatter coefficients were estimated using a standard phantom with edema showing the highest values followed by parenchyma and meningioma. Spectral analysis of rf signals has the potential of differentiating intracranial tissues as could be shown exemplarily with meningioma in this study. If this is also true for infiltrating tumors, the method might serve as a tool to better define tumor borders, thus improving the extent of resection.

High frequency ultrasound imaging of surface and subsurface structures of fingerprints

High frequency ultrasound is suitable for non-invasive evaluation of skin because it can obtain both morphological and biomechanical information. A specially developed acoustic microscope system with the central frequency of 100 MHz was developed. The system was capable of (1) conventional C-mode acoustic microscope imaging of thinly sliced tissue, (2) ultrasound impedance imaging of the surface of in vivo thick tissue and (3) 3D ultrasound imaging of inside of the in vivo tissue. In the present study, ultrasound impedance imaging and 3D ultrasound imaging of in vivo fingerprints were obtained. The impedance image showed pores of the sweat glands in the surface of fingerprint and 3D ultrasound imaging showed glands of the rear surface of fingerprint. Both findings were not visualized by normal optical imaging, thus the system can be applied to pathological diagnosis of skin lesions and assessment of aging of the skin in cosmetic point of view.

Ultrasound speed and impedance microscopy for in vivo imaging

Ultrasound speed and impedance microscopy was developed in order to develop in vivo imaging system. The sound speed mode realized non-contact high resolution imaging of cultured cells. This mode can be applied for assessment of biomechanics of the cells and thinly sliced tissues. The impedance mode visualized fine structures of the surface of the rat's brain. This mode can be applied for intra-operative pathological examination because it does not require slicing or staining.

Measurement of soft tissue elasticity in the congenital clubfoot using scanning acoustic microscope

To compare the soft-tissue elasticity between the medial, lateral, and posterior aspects, the deltoid and calcaneofibular ligaments, and the medial, lateral, and posterior capsular tissues were collected from 27 feet of 16 congenital-clubfoot patients. The tissue sound speed, which closely correlates to the Young's modulus, was measured using a scanning acoustic microscope. Contrary to our expectations, lateral ligament showed a significantly higher sound speed than medial ligament (P=0.0023). Lateral capsule also showed a higher sound speed than the medial one (P=0.0338). The results of the study indicated that the lateral soft tissues including the ligaments and capsule underwent severe contracture in congenital clubfoot.

Ultrasonic tissue characterization of atherosclerosis by a speed-of-sound microscanning system

We have been developing a scanning acoustic microscope (SAM) system for medicine and biology featuring quantitative measurement of ultrasonic parameters of soft tissues. In the present study, we propose a new concept sound speed microscopy that can measure the thickness and speed of sound in the tissue using fast Fourier transform of a single pulsed wave instead of burst waves used in conventional SAM systems. Two coronary arteries were frozen and sectioned approximately 10 microm in thickness. They were mounted on glass slides without cover slips. The scanning time of a frame with 300 x 300 pixels was 90 s and two-dimensional distribution of speed of sound was obtained. The speed of sound was 1680 +/- 30 m/s in the thickened intima with collagen fiber, 1520 +/- 8 m/s in the lipid deposition underlying the fibrous cap, and 1810 +/- 25 m/s in a calcified lesion in the intima. These basic measurements will help in the understanding of echo intensity and pattern in intravascular ultrasound images.

Ultrasonic characterization of whole cells and isolated nuclei

High frequency ultrasound imaging (20 to 60 MHz) is increasingly being used in small animal imaging, molecular imaging and for the detection of structural changes during cell and tissue death. Ultrasonic tissue characterization techniques were used to measure the speed of sound, attenuation coefficient and integrated backscatter coefficient for (a) acute myeloid leukemia cells and corresponding isolated nuclei, (b) human epithelial kidney cells and corresponding isolated nuclei, (c) multinucleated human epithelial kidney cells and d) human breast cancer cells. The speed of sound for cells varied from 1522 to 1535 m/s, while values for nuclei were lower, ranging from 1493 to 1514 m/s. The attenuation coefficient slopes ranged from 0.0798 to 0.1073 dB mm(-1) MHz(-1) for cells and 0.0408 to 0.0530 dB mm(-1) MHz(-1) for nuclei. Integrated backscatter coefficient values for cells and isolated nuclei showed much greater variation and increased from 1.71 x 10(-4) Sr(-1) mm(-1) for the smallest nuclei to 26.47 x 10(-4) Sr(-1) mm(-1) for the cells with the largest nuclei. The findings suggest that integrated backscatter coefficient values, but not attenuation or speed of sound, are correlated with the size of the nuclei.

Non-mineralized fibrocartilage shows the lowest elastic modulus in the rabbit supraspinatus tendon insertion: measurement with scanning acoustic microscopy

The acoustic properties of rabbit supraspinatus tendon insertions were measured by scanning acoustic microscopy. After cutting parallel to the supraspinatus tendon fibers, specimens were fixed with 10% neutralized formalin, embedded in paraffin, and sectioned. Both the sound speed and the attenuation constant were measured at the insertion site. The 2-dimensional distribution of the sound speed and that of the attenuation constant were displayed with color-coded scales. The acoustic properties reflected both the histologic architecture and the collagen type. In the tendon proper and the non-mineralized fibrocartilage, the sound speed and attenuation constant gradually decreased as the predominant collagen type changed from I to II. In the mineralized fibrocartilage, they increased markedly with the mineralization of the fibrocartilaginous tissue. These results indicate that the non-mineralized fibrocartilage shows the lowest elastic modulus among 4 zones at the insertion site, which could be interpreted as an adaptation to various types of biomechanical stress.

Does decalcification alter the tissue sound speed of rabbit supraspinatus tendon insertion? In vitro measurement using scanning acoustic microscopy

Failure of the tendon or ligament insertions is one of the most common injuries in the Orthopaedic field. To elucidate the pathogenesis of those injuries, the authors had attempted to measure the tissue sound speed that could be correlated to its elasticity using scanning acoustic microscopy (SAM). For the application of SAM to tendon or ligament insertions, it was necessary to determine the role of decalcification in SAM measurements since mineralized tissues including bone or mineralized fibrocartilage were present at the insertion site. To assess whether decalcification alters the tissue sound speed or not, supraspinatus tendon insertion of six Japanese white rabbits were measured with SAM operating in the frequency range of 50-150 MHz. Right supraspinatus tendons attached to the humeral head were cut into two pieces at the center of the tendon. Then, they were fixed with 10% neutralized formalin for 12 h. In each specimen, medial half was not decalcified, while lateral half was decalcified with ethylene-diamine-tetra-acetic acid (EDTA). After embedding in paraffin, 5 microm thick specimens were prepared for the measurement using SAM. The mean sound speed in each histologic zone was evaluated, and subsequently compared to that measured in the undecalcified and the decalcified specimens. Mean sound speed of non-mineralized fibrocartilage was 1544 m/s in the undecalcified specimens, while the value of 1541 m/s was determined in the decalcified ones. On the other hand, it decreased 2-3% after decalcification in the mineralized tissue including mineralized fibrocartilage and bone (mineralized fibrocartilage: undecalcified = 1648 m/s, decalcified = 1604 m/s; bone: undecalcified = 1716 m/s, decalcified = 1677 m/s). However, no significant differences were found between the undecalcified and the decalcified specimens (non-mineralized fibrocartilage: p = 0.84, mineralized fibrocartilage: p = 0.35, bone: p = 0.28). These results indicate that SAM could be applied to determine the properties of the tendon or the ligament insertions after decalcification with EDTA. Although SAM is applicable only for in vitro experimental study, it is expected that these data will contribute to better understanding concerning the biomechanics of tendon or ligament insertions as well as the pathogenesis of their failure at a microscopic level.

The frequency dependence of ultrasonic velocity and the anisotropy of dispersion in both freshly excised and formalin-fixed myocardium

The objectives of this study were to measure the frequency dependence of the ultrasonic velocity in myocardium and to quantify the frequency dependence of phase velocity as a function of the insonification angle relative to the predominant direction of the myofibers. Broadband phase spectroscopy data were acquired, spanning a frequency range of 3 to 8 MHz. Measurements were made on 36 tissue specimens cored from 12 freshly excised lamb hearts and were repeated after fixation with formalin. Measured phase velocities were found to be well characterized by a logarithmic fit. For freshly excised myocardium, the dispersion over the 3 to 8 MHz bandwidth was dependent on the direction of insonification, ranging from 1.2 m/s change for perpendicular insonification (across the myofibers) to 3.7 m/s for parallel insonification (along the myofibers). The effects of formalin-fixation resulted in a significant increase in dispersion for perpendicular insonification, but did not appreciably alter the dispersion for parallel insonification.

Increased elasticity of capsule after immobilization in a rat knee experimental model assessed by scanning acoustic microscopy

OBJECTIVES

The mechanical property of immobilized joints is not well understood. The present study was designed to investigate the tissue elasticity of the anterior and posterior synovial membrane (SM) in a rat immobilized knee model using scanning acoustic microscopy (SAM). Moreover, the structural characteristics of the SM after immobilization were examined by transmission electron microscopy (TEM).

METHODS

Thirty rats had their knee joints immobilized with a plate and metal screws. The rats were fixed at 1, 2, 4, 8 and 16 weeks after surgery and the knee joints were sectioned sagittally for SAM. Selected specimens were processed for TEM. A new concept SAM using a single pulsed wave instead of continuous waves was applied to measure the sound speed of the anterior and posterior SM, comparing it with the corresponding light microscopic images.

RESULTS

The sound speed of the posterior SM increased significantly in the 8- and 16-week experimental group compared with that in the control group. The sound speed of the anterior SM showed no statistical difference between the experimental and the control groups at any period of immobilization. The posterior SM of the experimental group was different from that of the control group in the ultrastructural characteristics of extracellular matrices.

CONCLUSIONS

Our data suggest that the increased elasticity and structural changes of the posterior SM are one of the main causes of limited extension after a long period of immobilization in flexion using SAM, which is a powerful tool for evaluating the elasticity of targeted tissues.

Measurements of the anisotropy of ultrasonic velocity in freshly excised and formalin-fixed myocardial tissue

The objective of this study was to quantify the anisotropy of ultrasonic velocity in freshly excised myocardial tissue and to examine the effects of formalin-fixation. Through-transmission radio-frequency-based measurements were performed on ovine and bovine myocardial specimens from 24 different hearts. A total of 81 specimens were obtained from specific locations within each heart to investigate the possibility of regional differences in anisotropy of velocity in the left ventricular wall and septum. No regional differences were observed for either lamb or cow myocardial specimens. In addition, no specific species-dependent differences were observed between ovine and bovine myocardium. Average values of velocity at room temperature for perpendicular and parallel insonification were 1556.9 +/- 0.6 and 1565.2 +/- 0.7 m/s (mean +/- standard error), respectively, for bovine myocardium (N=45) and 1556.3 +/- 0.6 and 1564.7 +/- 0.7 m/s for ovine myocardium (N=36). Immediately after measurements of freshly excised myocardium, ovine specimens were fixed in formalin for at least one month and then measurements were repeated. Formalin-fixation appears to increase the overall velocity at all angles of insonification and to increase the magnitude of anisotropy of velocity.

Paclitaxel prevents loss of pulmonary endothelial barrier integrity during cold preservation

BACKGROUND

Cold preservation is the most practical method to maintain the viability of isolated lungs. However, rapid cooling may affect pulmonary endothelial function. We examined the effects of microtubule stabilization with paclitaxel on pulmonary endothelial barrier integrity under cold temperature.

METHODS

Human pulmonary arterial endothelial cells were incubated at 4 degrees C for 2 hr in the presence or absence of paclitaxel (2.5 micromol/L). Microtubules was visualized using immunocytochemical techniques. Ultrasonic attenuation was measured with scanning acoustic microscopy. Endothelial barrier integrity was measured as transendothelial electric resistance. In addition, we examined graft function in a rat lung transplantation model, in which the donor lung had been preserved in the presence of paclitaxel (2.5 micromol/L) at 4 degrees C for 12 hr.

RESULTS

Low temperature caused a reversible microtubule disassembly, but the structure of microtubules was preserved by paclitaxel. Paclitaxel prevented the cooling-induced decrease in ultrasonic attenuation and transendothelial electric resistance. In a rat transplantation model, we found that preservation with paclitaxel successfully improved the oxygenation performance of the donor lung, which demonstrated only mild congestion and less significant interstitial edema without fluid accumulation in the alveolar spaces.

CONCLUSIONS

Our results indicate that microtubule stabilization with paclitaxel may be beneficial to prevent the loss of the endothelial barrier during cold preservation. We conclude that the use of paclitaxel in organ preservation solutions is useful in protecting pulmonary endothelial barrier integrity during cold preservation, thereby reducing the occurrence of early graft failure.

Acoustic properties of aortic aneurysm obtained with scanning acoustic microscopy

In aortic aneurysm tissues, macrophages and their secretion of matrix metalloproteinases (MMPs) are playing important role for tissue degeneration. Some studies have shown that weakening of the mechanical properties of the degenerated tissues may progress the expansion of the aneurysm. However, actual measurement of the mechanical properties has not been investigated at microscopic level. The objective of the present study is to assess the mechanical properties of aortic aneurysm tissues by measuring acoustic properties by scanning acoustic microscopy (SAM). Twenty-one cases of aortic aneurysm including renal and common iliac aneurysm tissues were surgically excised. Each tissue was fixed by 4% formaldehyde and the specimens were treated as (1) picrosirius red staining for normal and polarized light microscopy, (2) CD68 staining for macrophage detection, and (3) no staining for acoustic microscopy. A specially developed SAM system operating in the frequency range of 100-200 MHz, was employed in the measurement. Images of amplitude and phase are obtained in a field of 2x2 mm. The intima was mainly consisted of degenerated collagen without polarization of picrosirius red staining. Macrophages stained by CD68 were observed near the degenerated collagen fibers. The sound speed was 1567 m/s in the intima, 1576 m/s in the media, 1640 m/s in the adventitia, respectively. Infiltration of macrophages showed higher values of attenuation and sound speed than the surrounding tissues. The sound speed of the intima was significantly lower than our previous measurement of atherosclerotic aorta without aneurismal change. As the tissue elasticity is closely correlated with the sound speed, the elasticity of the intima was considered to be lower in aneurysm tissues. This mechanical weakness may contribute to the expansion of the diameter of the aneurysm. Acoustic microscopy provided important data for assessing tissue mechanical properties of abdominal aneurysm.

Time-frequency analysis for pulse driven ultrasonic microscopy for biological tissue characterization

The authors have proposed a new type of ultrasonic microscopy for biological tissue characterization. The system is driven by a nanosecond pulse voltage, the generated acoustic wave being reflected at the front and rear side of the sliced tissue. In this report, a time-frequency analysis was applied to determine the sound speed thorough the tissue. Frequency dependence of sound speed was obtained with a myocardium of a rat sliced into 10 microm. As the reflected waveform had a significant amount of oscillating component, the waveform was once subjected to the deconvolution process. As the result, two reflections were clearly separated in time domain. Then these two reflections were separately analyzed by time-frequency analysis. Each reflection was extracted by using a proper window function. Phase angles of these reflections at the same frequency were compared. A sound speed micrograph at an arbitrary frequency in between 50 and 150 MHz was successfully obtained. A tendency was found that the sound speed slightly increases with frequency.

Acoustic inhomogeneity of carotid arterial plaques determined by GHz frequency range acoustic microscopy

Characterization of the structure and composition of carotid arterial plaques is important to predict the risk of embolic and thrombotic events. In clinical echography, echolucent lesions seem to be more associated with stroke than echogenic lesions, and a low grey-scale median is associated with a fivefold increase in the incidence of silent infarcts. In the present study, five fixed human carotid atherosclerotic lesions were observed by GHz-range scanning acoustic microscopy (SAM). The atherosclerotic lesions were characterized by either thickened fibrosis with dense collagen fibers or lipid accumulation with sparse collagen network by optical microscopy. SAM revealed that the fibrosis was classified into type I and III collagen by attenuation of ultrasound (US) and that the sound field of lipid accumulation lesions became inhomogeneous. The results would provide the scientific basis for the images of vulnerable plaques being produced in diagnostic US.