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

Multi-parametric MRI as an indirect evaluation tool of the mechanical properties of in-vitro cardiac tissues

Background

Early detection of heart failure is essential to effectively reduce related mortality. The quantification of the mechanical properties of the myocardium, a primordial indicator of the viability of the cardiac tissue, is a key element in patient’s care. Despite an incremental utilization of multi-parametric magnetic resonance imaging (MRI) for cardiac tissue characteristics and function, the link between multi-parametric MRI and the mechanical properties of the heart has not been established. We sought to determine the parametric relationship between the myocardial mechanical properties and the MR parameters. The specific aim was to develop a reproducible evaluative quantitative tool of the mechanical properties of cardiac tissue using multi-parametric MRI associated to principal component analysis.

Methods

Samples from porcine hearts were submitted to a multi-parametric MRI acquisition followed by a uniaxial tensile test. Multi linear regressions were performed between dependent (Young’s modulus E) and independent (relaxation times T1, T2 and T2*, magnetization transfer ratio MTR, apparent diffusion coefficient ADC and fractional anisotropy FA) variables. A principal component analysis was used to convert the set of possibly correlated variables into a set of linearly uncorrelated variables.

Results

Values of 46.1±12.7 MPa for E, 729±21 ms for T1, 61±6 ms for T2, 26±7 for T2*, 35±5% for MTRx100, 33.8±4.7 for FAx10^-2, and 5.85±0.21 mm²/s for ADCx10^-4 were measured. Multi linear regressions showed that only 45% of E can be explained by the MRI parameters. The principal component analysis reduced our seven variables to two principal components with a cumulative variability of 63%, which increased to 80% when considering the third principal component.

Conclusions

The proposed multi-parametric MRI protocol associated to principal component analysis is a promising tool for the evaluation of mechanical properties within the left ventricle in the in vitro porcine model. Our in vitro experiments will now allow us focused in vivo testing on healthy and infracted hearts in order to determine useful quantitative MR-based biomarkers.

Simulation of elastic wave scattering in cells and tissues at the microscopic level

The scattering of longitudinal and shear waves from spherical, nucleated cells and three-dimensional tissues with simple and hierarchical microstructures was numerically modeled at the microscopic level using an iterative multipole approach. The cells were modeled with a concentric core-shell (nucleus-cytoplasm) structure embedded in an extracellular matrix. Using vector multipole expansions and boundary conditions, scattering solutions were derived for single cells with either solid or fluid properties for each of the cell components. Tissues were modeled as structured packings of cells. Multiple scattering between cells was simulated using addition theorems to translate the multipole fields from cell to cell in an iterative process. Backscattering simulations of single cells indicated that changes in the shear properties and nuclear diameter had the greatest effect on the frequency spectra. Simulated wave field images and high-frequency spectra (15-75 MHz) from tissues containing 1211-2137 cells exhibited up to 20% enhancement of the field amplitudes at the plasma membrane, significant changes in spectral features due to neoplastic and other microstructural alterations, and a detection threshold of approximately 8.5% infiltration of tumor cells into normal tissue. These findings suggest that histology-based simulations may provide insight into fundamental ultrasound-tissue interactions and help in the development of new medical technologies.

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.

Histology-based simulations for the ultrasonic detection of microscopic cancer in vivo

Ultrasonic spectroscopy may offer an alternative to imaging methods for the in vivo detection of microscopic cancer. To investigate this potential, a numerical model that incorporates multiple scattering, wave-mode conversion, and hierarchical microstructures was developed to simulate ultrasonic interactions in biological tissue at the microscopic level. Simulated high-frequency (20-75 MHz) spectra of up to 2137 cells displayed significant correlations to nucleus diameter and malignant cell infiltration, and indicated as few as 300 malignant cells may be detectable in normal tissue. The results suggest that ultrasonic spectroscopy combined with simulation-based interpretive models may provide real-time histopathology during surgeries, biopsies, and endoscopies.

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.

Anisotropy of the backscatter coefficient of formalin-fixed ovine myocardium

The objective of this study was to measure the backscatter coefficient of formalin-fixed myocardial tissue as a function of angle of insonification relative to the myocardial fiber direction. Backscatter measurements were performed on eight cylindrical formalin-fixed lamb myocardial specimens and compensated for attenuation and diffraction effects to determine the backscatter coefficient. The backscatter coefficient at 5 MHz was found to be maximum for insonification perpendicular to the predominant myofiber orientation and minimum for parallel insonification, with values of (17+/-14) and (1.2+/-0.7) x 10(-4) cm(-1) sr(-1) (mean+/-standard deviation), respectively.

Characterization of anisotropic myocardial backscatter using spectral slope, intercept and midband fit parameters

The specific myocardial structural components that contribute to the observed level of backscatter from the heart and its dependence on the angle of insonification have not been completely identified: The objectives of this study were to measure the anisotropy of backscatter from myocardium using the approach first introduced by Lizzi et al. [J Acoust Soc Am 73, 1366-1373 (1983)] and to use the extracted spectral parameters (spectral slope, intercept and midband fit) to characterize changes in the apparent scatterer size, spatial concentration and acoustic impedance properties as functions of the angle of insonification. Backscatter measurements were performed in vitro on eight cylindrical formalin-fixed lamb myocardial specimens using a 5 MHz focused transducer. The backscattered spectral data as a function of angle of insonification relative to the myocardial fiber direction were analyzed over the frequency range of 4 to 6 MHz. The spectral parameters describing features of backscatter were determined by applying a linear fit to attenuation-compensated normalized spectra. Results show that values of the spectral slope do not exhibit a significant dependence on the angle of insonification within uncertainties; however, the zero-frequency intercept showed clear anisotropy and was found to be a maximum for insonification perpendicular to the predominant myofiber orientation and a minimum for parallel insonification. A comparison of midband fit values at 5 MHz with attenuation-compensated integrated backscatter values showed excellent agreement for all angles of insonification. These data suggest that measurements of spectral slope, intercept, and midband fit can provide insights regarding aspects of tissue microstructure underlying the observed anisotropy of myocardial scattering properties. Measurements of the slope parameter suggest a very modest change in effective scatterer size with angle of insonification. However, the observed anisotropy in intercept and midband fit and apparent absence of anisotropy in the spectral slope suggests an angle of insonification dependence of acoustic concentration, the combination of effective spatial scatterer concentration and acoustic impedance properties, without a significant contribution from changes in effective scatterer size.