Scott M. Handley

ULTRASONIC DETERMINATION OF THE ANISOTROPY OF YOUNG'S MODULUS OF FIXED TENDON AND FIXED MYOCARDIUM

The linear elastic properties of a soft tissue exhibiting a unidirectional arrangement of reinforcing fibers may be described in terms of the five independent elastic stiffness coefficients C11, C13, C33, C44, and C66. In previous studies, ultrasonic measurements of these coefficients for formalin fixed specimens of bovine Achilles tendon and normal human myocardium were reported. In the present study these results are used to analyze the anisotropy of Youngs modulus of these tissues. For formalin fixed tendon a value of 1.37 GPa is obtained for Youngs modulus along the fiber axis of the tissue, and a value of 0.0706 GPa is obtained perpendicular to the fibers. For formalin fixed myocardium, values of 0.101 and 0.0311 GPa parallel and perpendicular to the fibers, respectively, are obtained. Based on the results for the angular dependence of Youngs modulus from unidirectional specimens of myocardium, a model is introduced to estimate these features for the more complicated fiber architecture of the left ventricular wall.

Effects of myocardial fiber orientation in echocardiography: Quantitative measurements and computer simulation of the regional dependence of backscattered ultrasound in the parasternal short-axis view

We measured the regional disparity in backscattered ultrasound by means of obtaining integrated backscatter images of 10 healthy subjects and placing a region of interest in 18 distinct positions. A computer model simulating the short-axis view was implemented on the basis of previously measured values for the anisotropic ultrasonic properties of myocardium. Measurements showed that the integrated backscatter value was greatest for the anterior septum and decreased by 15.9 +/- 3.5 dB for the lateral wall and 17.7 +/- 3.5 dB for the inferior septum. The value in the posterior wall was 8.1 +/- 3.8 dB below the value for the anterior septum. The regional variation of backscatter predicted with the simulation correlated well with the clinical measurements. These results suggested that analyses based on measurements of backscatter may require compensation for the inherent anisotropic properties of myocardium.

Estimation of the elastic stiffness coefficient c13 of fixed tendon and fixed myocardium

Recent studies from our laboratory have detailed the anisotropy of velocity of quasilongitudinal-mode ultrasonic waves through formalin fixed samples of normal human myocardium and bovine Achilles tendon. Results of these studies were used to determine the elastic stiffness coefficients c33, corresponding to the propagation of longitudinal-mode waves parallel to the fiber axis of the tissue, and c11, corresponding to the propagation of longitudinal-mode waves perpendicular to the fiber axis. For a tissue possessing a unidirectional arrangement of fibers with a random transverse distribution, three additional coefficients, c13, c44, and c12, are needed to describe its linear mechanical properties completely. Direct ultrasonic measurements of these coefficients in solids typically require the propagation of transverse-mode waves through the sample. Such measurements are difficult to perform in soft tissues because transverse-mode ultrasonic waves are very highly attenuated by the tissue. This study therefore employs a technique to estimate c13 based on measurements of the velocity of quasilongitudinal-mode ultrasonic waves for numerous angles of propagation relative to the fiber axis of the tissue. Analysis of data obtained from formalin fixed bovine Achilles tendon and human myocardium yield estimated values for c13 of 3.17 and 2.46 GPa, respectively.

Backscatter imaging and myocardial tissue characterization

The goal of myocardial ultrasonic tissue characterization is to complement two-dimensional and Doppler echocardiography by providing information (such as assessment of regional viability based on localized values of backscatter) beyond that derived from an assessment of myocardial dimensions and motion. Quantitative backscatter imaging can be subdivided into three broad areas: (1) direct applications, in which specific pathologies are identified and monitored, (2) indirect applications, in which quantitative techniques designed for use in tissue characterization serve to expand the role of echocardiography, and (3) contributions to the understanding of cardiac structure and function.

Comparison of integrated backscatter values obtained with acoustic densitometry with values derived from spectral analysis of digitized signals from a clinical imaging system

Time-domain-based integrated backscatter values obtained with the use of acoustic densitometry (AD) were compared with values determined from a spectral-based analysis of the radio-frequency (RF) signals with a modified Hewlett-Packard Sonos 1500 imaging system. Integrated backscatter images of five specimens of bovine tendon were acquired in the AD acquisition mode, and the corresponding signals related to the backscattered RF were digitized for each angle of insonification as the specimens were rotated in 10-degree increments. The integrated backscatter images were analyzed with the AD analysis package, and the corresponding values determined from the RF power spectra were obtained from the digitized ultrasonic signals. Good agreement was found between the two methods over the entire range of measured values. The mean anisotropy in the measured integrated backscatter (mean +/- standard error) was found to be 27 +/- 2 dB for time-domain-based analysis and 25 +/- 2 dB for RF spectral-based analysis.

Effects of tissue anisotropy on the spectral characteristics of ultrasonic backscatter measured with a clinical imaging system

In this paper, we report the effects of inherent tissue anisotropy on the spectral properties of backscattered ultrasound when measured with a commercially-available imaging system. We insonified five specimens of bovine tendon immersed in a water tank and rotated in 10° increments while being imaged with a Hewlett-Packard Sonos 1500 system. The backscattered RF signals corresponding to each angle of insonification were digitized and the spectral characteristics of the backscattered ultrasound were determined. The mean anisotropy, defined as the average difference between values at perpendicular and parallel insonification, for band-limited estimates of backscattered power, centroid frequency, upper-band to lower-band power ratio, and upper-band to total-band power ratio were found to be 24.6 ± 1.1 dB, 142 ± 27 kHz, 32 ± 13%, and 22 ± 5%, respectively (mean ± SE). The magnitude of each of these backscatter spectral parameters was larger at perpendicular insonification compared with the corresponding values at parallel insonification, consistent with previous measurements of the inherent anisotropy of ultrasonic attenuation and backscatter in tissue.

Effects of bleeder cloth impressions on the use of polar backscatter to detect porosity

The potential of ultrasonic polar backscatter measurements for detecting and characterizing porosity in composite laminates has been investigated in a number of laboratories[l–l1]. The objective of this study was to evaluate the influence of the nature of the composite’s surface on such measurements. The deleterious effects of bleeder cloth impressions, previously noted by Bar-Cohen[12], led to the hypothesis that the periodic surface features due to bleeder cloth impressions remaining after the cure process contribute significantly to the received backscattered signal, possibly masking the anisotropy of backscatter which is used to estimate porosity.

A Relationship between Frequency Dependent Ultrasonic Attenuation and Porosity in Composite Laminates

The detrimental effects of porosity on material strength are well known. The work of Hsu, Rose, and Adler[l] provides a means of estimating the volume fraction of pores and the average pore radius in isotropic elastic media from the value of frequency at which the attenuation coefficient becomes frequency independent and the magnitude of the attenuation coefficient at that plateau. Quantitative results for the isotropic case depend on numerical factors obtained by Gubernatis et al. [2] which are functions of the ratio of the transverse to longitudinal sound velocities, i.e., on the Poisson ratio. Mobley et al. [3] have tested these theories by making measurments of attenuation covering a frequency range that extended well into the frequency independent plateau. The experimental results of these investigators suggest that the theoretical results obtained by Rose et al. are qualitatively correct even though some of the features of wave propagation in layered, anisotropic media are not explicitly incorporated into the scattering model.

Backscatter from Specific Regions of Human Hearts Obtained from Standard Echocardiography Views

Further development of quantitative diagnostic procedures will enhance the selection and implementation of specific therapies that can reduce the damage to the heart muscle when applied to the victims of a heart attack. Ultrasonic imaging is one modality that provides real-time images of the beating heart that may be useful in forming diagnoses and evaluating therapies applied as well as providing insight into the underlying physiology. In characterizing the state of the cardiac muscle after a heart attack a clinician would like the ability to differentiate a segment of myocardial tissue with an old infarct (scarred tissue) from a region with acute ischemic injury where the muscle tissue may still be viable and could potentially be salvaged with the application of an appropriate therapy. Furthermore, once a specific therapy has been applied, it may be useful to monitor the reperfusion of the affected myocardial region. Our goal in ultrasonic tissue characterization is to provide an assessment of the state of the tissue based on quantitative analyses of the ultrasonic signal returned from a specific region of tissue. Implementation of quantitative ultrasonic tissue characterization procedures will complement conventional ultrasonic imaging which provides information regarding the dimensions and motion of the heart.

Detection of Disbonded Regions in Bonded Aluminum Plates Using an Ultrasonic 7.5 MHz Linear Array Medical Imaging System

Current concern for ensuring the air-worthiness of the aging commercial air fleet has prompted the establishment of broad-agency programs to develop NDT technologies that address specific aging-aircraft issues.[1, 2] One of the crucial technological needs that has been identified is the development of rapid, quantitative systems for depot-level inspection of bonded aluminum lap joints on aircraft.[1–3] Research results for characterization of disbond and corrosion based on normal-incidence pulse-echo measurement geometries are showing promise, but are limited by the single-site nature of the measurement which requires manual or mechanical scanning to inspect an area. [4–7] One approach to developing efficient systems may be to transfer specific aspects of current medical imaging technology to the NDT arena. Ultrasonic medical imaging systems offer many desirable attributes for large scale inspection. They are portable, provide real-time imaging, and have integrated video tape recorder and printer capabilities available for documentation and post-inspection review. Furthermore, these systems are available at a relatively low cost (approximately