High-frequency ultrasound detection of the temporal evolution of protein cross linking in myocardial tissue

Making a science out of preanalytics: An analytical method to determine optimal tissue fixation in real-time

Modern histopathology is built on the cornerstone principle of tissue fixation, however there are currently no analytical methods of detecting fixation and as a result, in clinical practice fixation is highly variable and a persistent source of error. We have previously shown that immersion in cold formalin followed by heated formalin is beneficial for preservation of histomorphology and have combined two-temperature fixation with ultra-sensitive acoustic monitoring technology that can actively detect formalin diffusing into a tissue. Here we expand on our previous work by developing a predictive statistical model to determine when a tissue is properly diffused based on the real-time acoustic signal. We trained the model based on the morphology and characteristic diffusion curves of 30 tonsil cores. To test our model, a set of 87 different tonsil samples were fixed with four different protocols: dynamic fixation according to our predictive algorithm (C/H:Dynamic, N = 18), gold-standard 24 hour room temperature (RT:24hr, N = 24), 6 hours in cold formalin followed by 1 hour in heated formalin (C/H:6+1, N = 21), and 2 hours in cold formalin followed by 1 hour in heated formalin (C/H:2+1, N = 24). Digital pathology analysis revealed that the C/H:Dynamic samples had FOXP3 staining that was spatially uniform and statistically equivalent to RT:24hr and C/H:6+1 fixation protocols. For comparison, the intentionally underfixed C/H:2+1 samples had significantly suppressed FOXP3 staining (p<0.002). Furthermore, our dynamic fixation protocol produced bcl-2 staining concordant with standard fixation techniques. The dynamically fixed samples were on average only submerged in cold formalin for 4.2 hours, representing a significant workflow improvement. We have successfully demonstrated a first-of-its-kind analytical method to assess the quality of fixation in real-time and have confirmed its performance with quantitative analysis of downstream staining. This innovative technology could be used to ensure high-quality and standardized staining as part of an expedited and fully documented preanalytical workflow.

Effect of ultrasound on the flocculation-sedimentation and thickening of unclassified tailings

In back-fill mining, how to rapidly increase the concentration of tailings is an important problem facing mining engineers. In this paper, the effects of ultrasound frequency (17-25 kHz), power (50-100 W), duration (5-20 min) and start time (3-12 min) on the final underflow concentration (FUC) of unclassified tailings (UTs) were investigated. The flocculation-sedimentation and thickening of tailings were compared with and without ultrasound application. The response surface method was applied to analyze the primary and secondary relationships and interactive relationships between the various ultrasound operating parameters and the FUC, and the optimal conditions were determined. In addition, Environmental Scanning Electron Microscope (E-SEM) was used to analyze the structural changes of underflow aggregates and clearly demonstrated a denser underflow after ultrasound treatment. The results indicated that ultrasound can significantly improve the underflow concentration of the UTs slurry. The frequency and power are the most important influencing factors. The best conditions for ultrasound application are a frequency of 20.4 kHz, power of 90 W, duration of 6.2 min and start time at 15.0 min. The FUC reached 71.75% after several minutes of sonication, which is 4.31% higher than the FUC of free flocculation. The results of E-SEM analysis showed significant differences among the microstructures of settled tailings (STs) after free flocculation and 17 and 20 kHz ultrasound treatment. Ultrasound at a frequency of 20 kHz has a more effective mechanical vibration and cavitation action and can therefore effectively break the polymer flocculant chains into shorter chains and promote the compaction of tailings and the release of water. The size distributions of the flocs before and after sonication also support this conclusion.

A review on the applications of ultrasonic technology in membrane bioreactors

Membrane bioreactors (MBRs) have received increasing attention in the field of wastewater treatment in recent years. However, membrane fouling is the main problem of MBRs, limiting their widespread and large applications. Membrane cleaning methods can be mainly classified into four types including chemical, physical, physico-chemical and biological clean the fouled membrane. In recent years, ultrasonication has been reported as a promising cleaning technique for the membranes fouled in MBRs. Ultrasonic irradiation can clean the fouled membrane by creating important physical phenomena including microjets, microstreams and shock waves. Moreover, the ultrasonic method can be combined with other cleaning methods e.g. chemical cleaning and backwashing in order to improve the cleaning efficiency. It should be noted that the application of ultrasonic in the MBR system is not limited to the cleaning of membrane. The pretreatment of the wastewater by ultrasonic irradiation or ultrasound coupled with other methods, e.g. ozonation, prior to MBR system, can decrease the organic loading of the wastewater and subsequently postpone the fouling of the membrane. This paper critically reviews the recent advances in the applications of ultrasound in MBR systems. Emerging issues associated with application of on-line ultrasound and also hybrid on-line ultrasound for controlling the membrane fouling in MBR systems are critically reviewed. Moreover, application of the ultrasound in ex-situ form for cleaning the fouled membranes and pretreatment of wastewater prior to the MBR system is discussed.

Toward 3-D Echocardiographic Determination of Regional Myofiber Structure

As a step toward the goal of relating changes in underlying myocardial structure to observed altered cardiac function in the hearts of individual patients, this study addresses the feasibility of creating echocardiography-derived maps of regional myocardial fiber structure for entire, intact, excised sheep hearts. Backscatter data were obtained from apical echocardiographic images acquired with a clinical ultrasonic imaging system and used to determine local fiber orientations in each of seven hearts. Systematic acquisition across the entire heart volume provided information sufficient to give a complete map for each heart. Results from the echocardiography-derived fiber maps compare favorably with corresponding results derived from diffusion tensor magnetic resonance imaging. The results of this study provide evidence of the feasibility of using echocardiographic methods to generate individualized whole heart fiber maps for patients.

Beyond Cervical Length: A Pilot Study of Ultrasonic Attenuation for Early Detection of Preterm Birth Risk

The purpose of this study was to determine whether cervical ultrasonic attenuation could identify women at risk of spontaneous preterm birth. During pregnancy, women (n = 67) underwent from one to five transvaginal ultrasonic examinations to estimate cervical ultrasonic attenuation and cervical length. Ultrasonic data were obtained with a Zonare ultrasound system with a 5- to 9-MHz endovaginal transducer and processed offline. Cervical ultrasonic attenuation was lower at 17-21 wk of gestation in the SPTB group (1.02 dB/cm-MHz) than in the full-term birth groups (1.34 dB/cm-MHz) (p = 0.04). Cervical length was shorter (3.16 cm) at 22-26 wk in the SPTB group than in the women delivering full term (3.68 cm) (p = 0.004); cervical attenuation was not significantly different at this time point. These findings suggest that low attenuation may be an additional early cervical marker to identify women at risk for SPTB.

Cervical attenuation as a measure of preterm delivery: impact of different region of interest sizes

One to five transvaginal ultrasound scans were taken of 63 women to estimate the microstructural changes in cervix using ultrasonic attenuation. Spectral log difference algorithm showed a clear decrease in attenuation as the time to delivery comes closer. The decrease in attenuation occurs earlier in preterm birth compared to full term birth which can be used as a predictor for preterm birth. Attenuation estimate did not improve as the ROI size increased.

Cervical strain determined by ultrasound elastography and its association with spontaneous preterm delivery

OBJECTIVE

To determine if there is an association between cervical strain, evaluated using ultrasound elastography, and spontaneous preterm delivery (sPTD) <37 weeks of gestation.

METHODS

One hundred and eighty nine (189) women at 16-24 weeks of gestation were evaluated. Ultrasound elastography was used to estimate cervical strain in three anatomical planes: one mid-sagittal in the same plane used for cervical length measurement, and two cross sectional images: one at the level of the internal cervical os, and the other at the level of the external cervical os. In each plane, two regions of interest (endocervix and entire cervix) were examined; a total of six regions of interest were evaluated.

RESULTS

The prevalence of sPTD was 11% (21/189). Strain values from each of the six cervical regions correlated weakly with cervical length (from r=-0.24, P<0.001 to r=-0.03, P=0.69). Strain measurements obtained in a cross sectional view of the internal cervical os were significantly associated with sPTD. Women with strain values ≤25th centile in the endocervical canal (0.19) and in the entire cervix (0.14) were 80% less likely to have a sPTD than women with strain values >25th centile [endocervical: odds ratio (OR) 0.2; 95% confidence interval (CI), 0.03-0.96; entire cervix: OR 0.17; 95% CI, 0.03-0.9]. Additional adjustment for gestational age, race, smoking status, parity, maternal age, pre-pregnancy body mass index, and previous preterm delivery did not appreciably alter the magnitude or statistical significance of these associations. Strain values obtained from the external cervical os and from the sagittal view were not associated with sPTD.

CONCLUSION

Low strain values in the internal cervical os were associated with a significantly lower risk of spontaneous preterm delivery <37 weeks of gestation.

Novel methods for assessment of right heart structure and function in pulmonary hypertension

Long-term increases in pulmonary vascular resistance and pulmonary arterial pressure resulting from structural alterations and abnormal vasoreactivity of the pulmonary vasculature may lead to right ventricular (RV) remodeling. Conventional methods of assessment of RV structure and function do not provide sensitive markers of RV remodeling for prognostic information. Advances in cardiac imaging have provided the capability to obtain quantitative information on the RV structure and function. This article reviews the clinical conditions that result in PH and discusses the novel and emerging methods for the assessment of right heart structure and function in PH in infants and children.

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.

Layer-dependent variation in the anisotropy of apparent integrated backscatter from human coronary arteries

Clinical imaging of the coronary arteries in the cardiac catheterization laboratory using intravascular ultrasound (IVUS) is known to display a three-layered appearance, corresponding to the intima/plaque, media and adventitia. It is not known whether ultrasonic anisotropy arising from these tissues may alter this pattern in future IVUS systems that insonify in the forward direction or obliquely. In anticipation of such devices, the current study was carried out by imaging fresh human coronary arteries in two orthogonal directions in vitro. Twenty-six sites from 12 arteries were imaged with a side-looking IVUS system, and with an acoustic microscope both radially and axially. Side-looking IVUS and radial acoustic microscopy scans demonstrated the typical "bright-dark-bright" pattern of the backscatter, with the media being significantly darker than the other two layers. Images obtained in the axial orientation exhibited a markedly different pattern, with the relative brightness of the media significantly larger than that of the intima/plaque.

Ultrasonic pretreatment of sludge: a review

Ultrasonication is an emerging and very effective mechanical pretreatment method to enhance the biodegradability of the sludge, and it would be very useful to all wastewater treatment plants in treating and disposing sewage sludge. Ultrasonication enhances the sludge digestibility by disrupting the physical, chemical and biological properties of the sludge. The degree of disintegration depends on the sonication parameters and also on sludge characteristics, therefore the evaluation of the optimum parameters varies with the type of sonicater and sludge to be treated. The full-scale installations of ultrasonication have demonstrated that there is 50% increase in the biogas generation, and in addition evaluation of energy balance showed that the average ratio of the net energy gain to electric consumed by the ultrasound device is 2.5. This review article summarizes the benefits of ultrasonication of sludge, the effect of sonication parameters, impact of sludge characteristics on sludge disintegration, and thereby the increase in biogas production in anaerobic digester. Due to uncertainty in the unit representation by many researchers and nonavailability of the data, comparison of these results is complicated. Comparison of ultrasonication with other pretreatment options is necessary to evaluate the best economical and environmental pretreatment technology for sludge treatment and disposal. The optimum parameters for the ultrasonication vary with sludge characteristics.

Ultrasonic attenuation estimation of the pregnant cervix: a preliminary report

OBJECTIVE

Estimates of ultrasonic attenuation (the loss of energy as an ultrasonic wave propagates through tissue) have been used to evaluate the structure and function of tissues in health and disease. The purpose of this research was to develop a method to estimate ultrasonic cervical attenuation during human pregnancy using a clinical ultrasound system.

METHODS

Forty women underwent a cervical scan once during pregnancy with the Zonare z.one clinical ultrasound system using a 4-9-MHz endovaginal transducer. This ultrasound system provides access to radiofrequency (RF) image data for processing and analysis. In addition, a scan of a tissue-mimicking phantom with a known attenuation coefficient was acquired and used as a reference. The same settings and transducer used in the clinical scan were used in the reference scan. Digital data of the beam-formed image were saved in Digital Imaging and Communications in Medicine (DICOM) format on a flash drive and converted to RF data on a personal computer using a Matlab program supplied by Zonare. Attenuation estimates were obtained using an algorithm that was independently validated using tissue-mimicking ultrasonic phantoms.

RESULTS

RF data were acquired and analyzed to estimate attenuation of the human pregnant cervix. Regression analysis revealed that attenuation was: a predictor of the interval from ultrasound examination to delivery (beta = 0.43, P = 0.01); not a predictor of gestational age at time of examination (beta = - 0.23, P = 0.15); and not a predictor of cervical length (beta = 0.077, P = 0.65).

CONCLUSIONS

Ultrasonic attenuation estimates have the potential to be an early and objective non-invasive method to detect interval between examination and delivery. We hypothesize that a larger sample size and a longitudinal study design will be needed to detect gestational age-associated changes in cervical attenuation.

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.

Regional variation in the measured apparent ultrasonic backscatter of mid-gestational fetal pig hearts

The goal of this study was to characterize and compare regional backscatter properties of fetal hearts through measurements of the apparent integrated backscatter. Sixteen excised, formalin-fixed fetal pig hearts, representing an estimated 53 to 63 days of gestation, were investigated. Spatially localized measurements of integrated backscatter from these specimens were acquired using a 50 MHz single-element transducer. The apparent integrated backscatter measurements demonstrate different patterns of backscatter from the myocardium of the right ventricle compared with that of the left ventricle. These backscatter measurements appear to be consistent with the anisotropy of the fiber orientation observed in histologic assessment of the same specimens. For each of the 16 hearts, the apparent integrated backscatter from the right ventricular myocardium was larger than that from the left ventricular myocardium, exhibiting mean apparent backscatter values of -35.9 +/- 2.0 dB and -40.1 +/- 1.9 dB (mean +/- standard deviation; n = 16; p < 0.001), respectively. This study suggests that the intrinsic ultrasonic properties of the left and right ventricular myocardium are distinct in fetal pig hearts at mid-gestation.

Ultrasonic detection of the anisotropy of protein cross linking in myocardium at diagnostic frequencies

Increased myocardial stiffness in aging and diabetes that may result in pathologies such as diastolic dysfunction has been attributed, in part, to an increase in cross linking of extracellular matrix proteins such as collagen. With the development of new approaches to cardiovascular therapy, it becomes increasingly important to develop noninvasive approaches for monitoring changes in myocardial cross linking. The objective of this study was to use ultrasound at frequencies used in clinical echocardiography to measure changes in myocardial attenuation resulting from increased cross linking as a function of angle of insonification over a complete rotation. Through-transmission radiofrequency-based measurements were performed on 36 specimens from 12 freshly excised ovine hearts at room temperature, which were then fixed in formalin to induce protein cross linking prior to repeated measurements. For angles near perpendicular to the myofiber direction, the measured slope of attenuation increased from 0.52 +/- 0.07 dB/(cm MHz) (mean +/- one standard deviation) for freshly excised to 0.85 +/- 0.08 dB/(cm MHz) for formalin-fixed myocardium. In contrast, results for parallel insonification exhibit considerable overlap (1.88 +/- 0.17 for freshly excised and 1.75 +/- 0.19 dB/(cm MHz) for formalin-fixed myocardium). Results of this study suggest that the response of the extracellular collagenous matrix to changes in cross linking is directionally dependent. The anisotropy of ultrasonic attenuation thus may provide an approach for noninvasive monitoring of the extent and progression of myocardial disease associated with changes in protein cross linking. Accounting for effects due to anisotropy may be essential for the future detection of such changes using ultrasonic attenuation in vivo.

Elastic stiffness coefficients (c11, C33, and C13) for freshly excised and formalin-fixed myocardium from ultrasonic velocity measurements

The goal of this study was to measure elastic stiffness coefficients of freshly excised and subsequently formalin-fixed myocardial tissue. Our approach was to measure the angle-dependent phase velocities associated with the propagation of a longitudinal ultrasonic wave (3-8 MHz) in ovine myocardium using phase spectroscopy techniques and to interpret the results in the context of orthotropic and transversely isotropic models describing the elastic properties of myocardium. The phase velocity results together with density measurements were used to obtain the elastic stiffness coefficients c11, c33, and c13 for both symmetries. The results for the elastic stiffness coefficients c11, c33, and c13 are the same for both symmetries. Measurements for freshly excised myocardium and the same tissue after a period of formalin fixation were compared to examine the impact of fixation on the elastic stiffness coefficients.

Anisotropy of apparent backscatter in the short-axis view of mouse hearts

The goals of this investigation were to measure the anisotropy of backscattered ultrasound observed in the short-axis view of mouse hearts in systole and diastole and to compare these measurements with predictions from a computer simulation. Measurements of midmyocardial apparent backscatter were obtained from analyses of the hearts of seven wild-type mice using a clinical imaging system utilizing a linear array with a nominal center frequency of 13 MHz. A computer model simulating the short-axis view was implemented based on previous measurements of the angle of insonification dependence of myocardial backscatter and attenuation. Results demonstrate that the measured backscatter was largest for those myocardial regions corresponding to approximately perpendicular insonification relative to the myofibers and the smallest for regions of approximately parallel insonification, with the minimum to maximum values of apparent backscatter differing by approximately 10 dB. The measured anisotropy of backscatter was similar for end-systole and end-diastole and was in good agreement with the predicted anisotropy obtained from the computer simulations.

Effects of region-of-interest length on estimates of myocardial ultrasonic attenuation and backscatter

Measurements of tissue properties using an image-based technique that makes use of an external reference may have the potential for practical clinical implementation in echocardiography. The objective of this study was to quantify the ability of this technique to distinguish myocardial attenuation and backscatter properties for specific lengths of the region-of-interest (ROI). We chose to exploit the anisotropic properties of the myocardium as a model for distinguishing tissue with different acoustic properties. Excised lateral wall segments from seven healthy adult sheep hearts were imaged using a commercially available (Philips/ATL) clinical scanner operating in the fundamental imaging mode with a linear array (L 7-4). Statistical and receiver operating characteristic (ROC) analyses were used to evaluate the ability of the video signal analysis method to differentiate midmyocardial from subendocardial regions based on measurements of the acoustic properties for specific lengths of the ROI. Results demonstrate that the ability to distinguish tissue properties increases with ROI length for both slope of attenuation and backscatter coefficient measurements. Statistically significant differences were observed for measurements utilizing the ROI lengths as short as 0.4 cm with corresponding progressively increasing areas under the ROC curves for increasing ROI lengths. [NIH R37 HL40302]

Ultrasonic delineation of aortic microstructure: the relative contribution of elastin and collagen to aortic elasticity

Aortic elasticity is an important factor in hemodynamic health, and compromised aortic compliance affects not only arterial dynamics but also myocardial function. A variety of pathologic processes (e.g., diabetes, Marfan's syndrome, hypertension) can affect aortic elasticity by altering the microstructure and composition of the elastin and collagen fiber networks within the tunica media. Ultrasound tissue characterization techniques can be used to obtain direct measurements of the stiffness coefficients of aorta by measurement of the speed of sound in specific directions. In this study we sought to define the contributions of elastin and collagen to the mechanical properties of aortic media by measuring the magnitude and directional dependence of the speed of sound before and after selective isolation of either the collagen or elastin fiber matrix. Formalin-fixed porcine aortas were sectioned for insonification in the circumferential, longitudinal, or radial direction and examined using high-frequency (50 MHz) ultrasound microscopy. Isolation of the collagen or elastin fiber matrices was accomplished through treatment with NaOH or formic acid, respectively. The results suggest that elastin is the primary contributor to aortic medial stiffness in the unloaded state, and that there is relatively little anisotropy in the speed of sound or stiffness in the aortic wall.

Parametric analysis of carotid plaque using a clinical ultrasound imaging system

We evaluated quantitative ultrasonic methods for assessment of carotid plaque content. In vitro measurements of fixed, carotid plaque specimens obtained by surgical endarterectomy were performed using a clinical Philips HDI 5000 imaging system connected to a radiofrequency (RF) signal-acquisition system. We acquired RF signals and grey-scale images from carotid specimens (n = 17) and a tissue-mimicking reference phantom. Imaged plaque sections were then classified according to histology. Parametric images were constructed from the integrated backscatter (IBS), and the midband, slope and intercept values of a straight-line fit to the apparent backscatter transfer function. Analysis was performed on 82 regions-of-interest (ROIs). The IBS values for collagen, lipid and hemorrhage plaques were 5.8 +/- 5.4, 3.9 +/- 3.7, 2.8 +/- 2.2 dB, respectively. Midband and IBS parameter images exhibited good agreement in morphology with histology, whereas the slope and intercept parameter images were noisy. Mean IBS, midband, and grey-scale values of complex plaques were found to be statistically different (p < 0.05) from lipid, hemorrhage and fibrolipid plaques. The bias and limits of agreement (1.3 +/- 4.9 dB) between the grey-scale and IBS methods, however, indicated that the two methods were not interchangeable. Results indicate necessary improvements, such as reduction of large measurement variances and identification of robust parameters, that will permit multiparametric characterization of carotid plaque under in vivo conditions.

Characterization of aortic microstructure with ultrasound: implications for mechanisms of aortic function and dissection

Specific ultrasonic tissue characterization parameters were correlated with the three-dimensional architecture and material properties (density, compressibility, size, and orientation) of aortic elastic elements at the microscopic level. The medial layer of 10 samples of normal canine aorta were insonified in vitro utilizing acoustic microscopy from 30 to 44 MHz. The following quantitative indexes exhibited substantial anisotropic elastic behavior in radial (R), circumferential (C), and longitudinal (L) directions: backscatter coefficient (R:0.9 +/- 0.2; C:0.008 +/- 0.0008; LL:0.0077 +/- 0.0008 sr(-1) cm(-1)); frequency dependence of backscatter (R:3.3; C:1.4; L:1.5); attenuation coefficients 1(R:105 +/- 22; L:135 +/- 13; C:131 +/- 14 dB/cm). Thus, the ultrasonic indexes are anisotropic: equivalent in the C and L directions, but markedly different in the R direction. These data are indicative of an aortic microstructure that interacts with ultrasonic waves as thin sheet-like elastic layers instead of independent elastin fibers. This specific sheet-like organization of elastin microfibers may function to limit shear injury to concentric aortic lamellae and prevent aortic dissection. The marked anisotropic behavior of normal aortas suggests that ultrasound may be useful for nondestructive characterization of vascular integrity.

Transmural variation of myocardial attenuation measured with a clinical imager

The objective of this study was to quantify the transmural variation in attenuation for the septal and lateral walls of the heart. Our approach was to utilize a commercially available ultrasonic imaging system to acquire images of excised sections of eight sheep hearts with an orientation similar to that encountered in the apical four-chamber view. The measured values (mean +/-SE) of the slope of attenuation for the transmural regions of the septum are: 1.40 +/-0.11, 0.99 +/-0.09, and 1.85 +/-0.16 (dB/cm/MHz) for the left subendocardial, midmyocardial, and right subendocardial zones, respectively. The analogous data from the lateral wall are: 1.42 +/-0.11, 0.83 +/-0.07, and 1.20 +/- 0.16 (dB/cm/MHz) for the subendocardial, midmyocardial, and subepicardial zones, respectively. These data demonstrate that ultrasonic attenuation associated with the septum and the lateral wall, when imaged in a manner similar to that of the apical four-chamber view, is anisotropic.

Transmural variation of myocardial attenuation and its potential effect on contrast-mediated estimates of regional myocardial perfusion

Promising technical developments suggest that it may be feasible to use contrast echocardiography to estimate regional myocardial perfusion. Although the optimal approach has not yet been determined, the use of a nonlinear (harmonic) response of the contrast agent is common to several recent advances. The purpose of this article is to delineate the relation between the anisotropic (angle-dependent) ultrasonic attenuation of the myocardium through which the sound wave has propagated and the regional, nonlinear response of the contrast agent. Apparent perfusion will be modulated by this regionally varying, path-dependent attenuation, which is determined by the local angle between the propagating sound wave and the myofiber orientation. We illustrate the potential magnitude of the effect of myocardial anisotropy for the apical 4-chamber view by examining propagation along the septum and the lateral wall. We present experimentally measured values of the attenuation of excised sheep myocardium, showing statistically significant differences in the attenuation in the mid wall compared with that in symmetrical zones to the left and right of the mid wall, reflecting the well-known myofiber orientations in these 3 regions. The nonlinear (harmonic) response of a contrast agent depends on the local pressure amplitude, which for a given mechanical index is determined by the attenuation accumulated along the path to the point where the regional perfusion is estimated.

Age-related alterations of cardiac tissue microstructure and material properties in Fischer 344 rats

The cardiac aging process is accompanied by global mechanical dysfunction that reflects increased myocardial stiffness. Accordingly, age-related changes in microscopic material properties of myocardium were delineated with high-frequency ultrasound (US) (30 to 44 MHz) tissue characterization methods for aging Fischer 344 rats at 6 (adult), 18 (aged), and 24 (senescent) months of age. The excised lateral wall of the left ventricle of rats (n = 10 per group) was insonified with a 50-MHz acoustic microscope for determination of integrated backscatter, backscatter coefficient and attenuation coefficient. Histological and biochemical analyses for collagen content and cardiac myocyte diameter were performed. Collagen concentration increased progressively with age, with the greatest increments occurring from 6 to 18 months (38.0 +/- 6.3 to 53.0 +/- 7.1 mg/g dry wt), and leveling off at 24 months (60.0 +/- 7.4 mg/g dry wt). Tissue microscopic material properties also changed progressively from 6 to 24 months of age, as determined by US methods: integrated backscatter increased (-44.7 +/- 1.8 vs. -40.8 +/- 1.9 dB, p < 0.05), attenuation increased (47.1 +/- 5.9 to 65.3 +/- 7.8 dB/cm, p < 0.05), and the backscatter coefficient increased (0.73 +/- 0.16 x 10(-5) to 3.76 +/- 1.6 x 10(-5) cm(-1), p < 0.05), from 6 to 24 months of age in each case. Age-related alterations in indices of cardiac microscopic material properties were closely correlated with the changes in cardiac microstructure. Ultrasonic tissue characterization may prove to be a sensitive tool to monitor changes in the cardiac microstructure, such as increased collagen deposition, that occur within age-related diastolic dysfunction.