Anisotropy of Young's modulus of human tibial cortical bone.

The anisotropy of Young's modulus in human cortical bone was determined for all spatial directions by performing coordinate rotations of a 6 by 6 elastic stiffness matrix. The elastic stiffness coefficients were determined experimentally from ultrasonic velocity measurements on 96 samples of normal cortical bone removed from the right tibia of eight human cadavers. The following measured values were used for our analysis: c11 = 19.5 GPa, c22 = 20.1 GPa, c33 = 30.9 GPa, c44 = 5.72 GPa, c55 = 5.17 GPa, c66 = 4.05 GPa, c23 = 12.5 GPa. The remaining coefficients were determined by assuming that the specimens possessed at least an orthorhombic elastic symmetry, and further assuming that c13 = c23 c12 = c11 - 2c66. Our analysis revealed a substantial anisotropy in Young's modulus in the plane containing the long axis of the tibia, with maxima of 20.9 GPa parallel to the long axis, and minima of 11.8 GPa perpendicular to this axis. A less pronounced anisotropy was observed in the plane perpendicular to the long axis of the tibia. To display our results for the full three-dimensional anisotropy of cortical bone, a closed surface was used to represent Young's modulus in all spatial directions.

Effects of tissue anisotropy and contrast acoustic properties on myocardial scattering in contrast echocardiography.

In this study we explored the potential effects that tissue anisotropy, in conjunction with the acoustic properties of contrast, may have on quantitative measurements of myocardial perfusion with the use of ultrasonic contrast agents. We used a computer simulation of the parasternal short-axis view, based on previously measured values for the anisotropy of backscatter and attenuation of myocardium, to predict the backscattered energy from 18 specific regions within the heart before and after myocardial contrast perfusion. Results demonstrated a regional variation of contrast enhancement in the short-axis view and variations caused by incremental increases in contrast level for specific myocardial regions. Thus quantitative assessment of myocardial perfusion with contrast echocardiography is influenced by the anisotropic properties of the myocardium, and the resulting postcontrast image will depend on the interaction between tissue properties and contrast acoustic properties. The degree of myocardial enhancement caused by the presence of contrast may depend on the spatial position of the specific region investigated with respect to the transducer and the amount of contrast in the myocardium.

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.

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 degrees 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.

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.

Substance abuse in nursing. Forming policies.

A written policy addressing impaired practice increases the likelihood that all nurses will be treated similarly. It also protects the organization legally. One large Midwestern medical center's approach to policy development, completed after 1 year of task force work, is described.

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 Young's modulus of these tissues. For formalin fixed tendon a value of 1.37 GPa is obtained for Young's 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 Young's 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.

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.

The impaired nurse. Organizational and professional models of response.

1. Two models of response, organizational and professional, affect the impaired nurse, who is both an employee and a professional. 2. The organizational model of response uses an Employee Assistance Program (EAP) to detect and intervene with impaired employees. 3. The professional model of response involves mechanisms established through the board of nursing, including posttreatment monitoring. 4. When both models are available, linking mechanisms such as coordinators, policies, and reentry practices can combine the strengths of each model.