Christakis Constantinides

A high-resolution cardiovascular magnetic resonance diffusion tensor map from ex-vivo C57BL/6 murine hearts

BackgroundThe complex cardiac fiber structural organization and spatial arrangement of cardiomyocytes in laminar sheetlets contributes greatly to cardiac functional and contractile ejection patterns. This study presents the first comprehensive, ultra-high resolution, fully quantitative statistical tensor map of the fixed murine heart at isotropic resolution of 43 μm using diffusion tensor (DT) cardiovascular magnetic resonance (CMR).MethodsImaging was completed in approximately 12 hours using a six-directional encoding scheme, in five ex vivo healthy C57BL/6 mouse hearts. The tensor map constructed from this data provides an average description of the murine fiber architecture visualized with fiber tractography, and its population variability, using the latest advances in image tensor analysis and statistics.ResultsResults show that non-normalized cardiac tensor maps are associated with mean fractional anisotropy of 0.25 ± 0.07 and mean diffusivity of 8.9 ± 1.6 × 10−4 mm2/s. Moreover, average mid-ventricular helical angle distributions ranged between –41 ± 3° and +52 ± 5° and were highly correlated with transmural depth, in agreement with prior published results in humans and canines. Calculated variabilities of local myocyte orientations were 2.0° and 1.4°. Laminar sheet orientation variability was found to be less stable at 2.6°. Despite such variations, the murine heart seems to be highly structured, particularly when compared to canines and humans.ConclusionsThis tensor map has the potential to yield an accurate mean representation and identification of common or unique features of the cardiac myocyte architecture, to establish a baseline standard reference of DTI indices, and to improve detection of biomarkers, especially in pathological states or post-transgenetic modifications.

MRI-based morphological modeling, synthesis and characterization of cardiac tissue-mimicking materials

This study uses standard synthetic methodologies to produce tissue-mimicking materials that match the morphology and emulate the in vivo murine and human cardiac mechanical and imaging characteristics, with dynamic mechanical analysis, atomic force microscopy (AFM), scanning electron microscopy (SEM) and magnetic resonance imaging. In accordance with such aims, poly(glycerol sebacate) (PGS) elastomeric materials were synthesized (at two different glycerol (G)-sebacic (S) acid molar ratios; the first was synthesized using a G:S molar ratio of 2:2, while the second from a 2:5 G:S molar ratio, resulting in PGS2:2 and PGS2:5 elastomers, respectively). Unlike the synthesized PGS2:2 elastomers, the PGS2:5 materials were characterized by an overall mechanical instability in their loading behavior under the three successive loading conditions tested. An oscillatory response in the mechanical properties of the synthesized elastomers was observed throughout the loading cycles, with measured increased storage modulus values at the first loading cycle, stabilizing to lower values at subsequent cycles. These elastomers were characterized at 4 °C and were found to have storage modulus values of 850 and 1430 kPa at the third loading cycle, respectively, in agreement with previously reported values of the rat and human myocardium. SEM of surface topology indicated minor degradation of synthesized materials at 10 and 20 d post-immersion in the PBS buffer solution, with a noted cluster formation on the PGS2:5 elastomers. AFM nanoindentation experiments were also conducted for the measurement of the Young modulus of the sample surface (no bulk contribution). Correspondingly, the PGS2:2 elastomer indicated significantly decreased surface Youngs modulus values 20 d post-PBS immersion, compared to dry conditions (Youngs modulus = 1160 ± 290 kPa (dry) and 200 ± 120 kPa (20 d)). In addition to the two-dimensional (2D) elastomers, an integrative platform for accurate construction of three-dimensional tissue-mimicking models of cardiac anatomy from 2D MR images using rapid prototyping manufacturing processes was developed. For synthesized elastomers, doping strategies with two different concentrations of the MRI contrast agent Dotarem allowed independent and concurrent control of the imaging characteristics (contrast and relaxivity) during the synthetic process for increased contrast agent absorption, with tremendous potential for non-destructive in vivo use and applications to cardiovascular and cerebrovascular diseases.

Murine Cardiac Catheterizations and Hemodynamics: On the issue of Parallel Conductance

Catheter-based measurements are extensively used nowadays in animal models to quantify global left ventricular (LV) cardiac function and hemodynamics. Conductance catheter measurements yield estimates of LV volumes. Such estimates, however, are confounded by the catheters nonhomogeneous emission field and the contribution to the total conductance of surrounding tissue or blood conductance values (other than LV blood), a term often known as parallel conductance. In practice, in most studies, volume estimates are based on the assumptions that the catheters electric field is homogeneous and that parallel conductance is constant, despite prior results showing that these assumptions are incorrect. This study challenges the assumption for spatial homogeneity of electric field excitation of miniature catheters and investigated the electric field distribution of miniature catheters in the murine heart, based on cardiac model-driven (geometric, lump component) simulations and noninvasive imaging, at both systolic and diastolic cardiac phases. Results confirm the nonuniform catheter emission field, confined spatially within the LV cavity and myocardium, falling to 10% of its peak value at the ring electrode surface, within 1.1-2.0 mm, given a relative tissue permittivity of 33 615. Additionally, <;1% of power leaks were observed into surrounding cavities or organs at end-diastole. Temporally varying parallel conductance effects are also confirmed, becoming more prominent at end-systole.

Elimination of mutual inductance in NMR phased arrays: the paddle design revisited.

This study proposes a method to empirically minimize mutual inductance, using passive end-ring circular paddles, with neighboring coil loops placed in a non-overlapped configuration. The proposed concepts are validated through B(1)-field simulations for resonant coils at f(o)=300.5 MHz, having various sizes (3-10 cm), and for paddles with sizes ranging from 16 to 30 mm, and bench tests on constructed 4×4cm(2) two- (1×2) and four-coil loop (2×2) planar arrays. Simulation results yield total mean percentage B(1)-field differences of only 7.03% between the two non-overlapping coil array configurations (paddles vs. no-paddles). Pair-wise comparisons of elicited mean B(1)-field differences from the use of different circular and rectangular paddle sizes, yield values <5.3%. Theoretical calculation of the normalized mutual coupling coefficient in the non-overlapped coil configuration reduces to almost zero with optimally sized-paddles having a radius of approximately 28% the coils largest dimension. In the absence of paddles, differences in the split of resonance peaks of 9.9 MHz were observed for the two coils in the 1×2 array, which vanished with paddle placement. Single coil responses (unloaded/loaded) without paddles, and responses from array coils with use of optimally-sized paddles yielded quality factor ratios that ranged between 1.1-1.86 and 1.0-1.5, respectively. Phantom and mouse loaded reflection coefficients S(11)/S(22) were -16.7/-16.2dB and -28.2/-16.1 dB, for the two array loops, respectively. Under unloaded conditions and in the absence of paddles, split resonances were observed for the 1×2 array, yielding transmission coefficients of -5.5 to -8.1 dB, reversing to single resonance responses upon paddle placements, with transmission coefficients of -14.4 to -15.6 dB.

Morphological studies of the murine heart based on probabilistic and statistical atlases

This study directly compares morphological features of the mouse heart in its end-relaxed state based on constructed morphometric maps and atlases using principal component analysis in C57BL/6J (n=8) and DBA (n=5) mice. In probabilistic atlases, a gradient probability exists for both strains in longitudinal locations from base to apex. Based on the statistical atlases, differences in size (49.8%), apical direction (15.6%), basal ventricular blood pool size (13.2%), and papillary muscle shape and position (17.2%) account for the most significant modes of shape variability for the left ventricle of the C57BL/6J mice. For DBA mice, differences in left ventricular size and direction (67.4%), basal size (15.7%), and position of papillary muscles (16.8%) account for significant variability.

Heart rate and blood pressure variability effects as a result of oxygen and nitrous oxide administration in the anesthetized mouse

This study examines the effects of changing oxygen fractional inspiration ratio (FiO2), and nitrous oxide (N2O) for the improvement of cardiovascular control of mean arterial blood pressure (MAP) and heart rate (HR) in C57BL/6 mice under isoflurane anesthesia (1.5%) for up to 90 minutes post-induction. Heart rate variability (HRV) indices are also quantified under these conditions. The results indicate that changing the FiO2 does result in lower MAP and HR values compared to the case of N2O (50%) administration to the isoflurane gas mixture. HRV indices declined over the course of all anesthetic regimens, suggesting a decrease in parasympathetic tone. We conclude that the most optimal anesthetic condition is achieved when N2O (50%) is added to the gas mixture.

In vivo epicardial force and strain characterisation in normal and MLP-knockout murine hearts

The studys objective is to quantify in vivo epicardial force and strain in the normal and transgenic myocardium using microsensors.Male mice (n = 39), including C57BL/6 (n = 26), 129/Sv (n = 5), wild-type (WT) C57  ×  129Sv (n = 5), and muscle LIM protein (MLP) knock-out (n = 3), were studied under 1.5% isoflurane anaesthesia. Microsurgery allowed the placement of two piezoelectric crystals at longitudinal epicardial loci at the basal, middle, and apical LV regions, and the independent (and/or concurrent) placement of a cantilever force sensor. The findings demonstrate longitudinal contractile and relaxation strains that ranged between 4.8-9.3% in the basal, middle, and apical regions of C57BL/6 mice, and in the mid-ventricular regions of 129/Sv, WT, and MLP mice. Measured forces ranged between 3.1-8.9 mN. The techniques feasibility is also demonstrated in normal mice following afterload, occlusion-reperfusion challenges.Furthermore, the total mid-ventricular forces developed in MLP mice were significantly reduced compared to the WT controls (5.9  ±  0.4 versus 8.9  ±  0.2 mN, p < 0.0001), possibly owing to the fibrotic and stiffer myocardium. No significant strain differences were noted between WT and MLP mice.The possibility of quantifying in vivo force and strain from the normal murine heart is demonstrated with a potential usefulness in the characterisation of transgenic and diseased mice, where regional myocardial function may be significantly altered.

A novel spiral radiofrequency coil for high field mouse cardiac imaging

The idea of a novel MR surface coil based on multi-turn spiral geometry is presented for use in mouse cardiac MRI. The benefits from flat, cylindrical arrangement of the coil are compared using computer simulations and MRI experiments in various cases of free space, phantom and animal loading conditions. Results show that the cylindrical case compares well with a commercially available birdcage coil offering a 50% signal intensity improvement for depths of penetration up to 6.1 mm from coil surface. There is also adequate B1 field penetration that allows visualization of the lateral and inferior walls of the murine heart.

A Prototype Device for 3D Regional, Passive Elasticity Measurements of the Murine Myocardium Using AFM

This study presents the design and implementation of a prototype device that interfaces with commercial atomic force microscopy (AFM) instruments to accurately measure ex vivo, regional, passive elasticity measurements of the murine myocardium in three-dimensions (3D). The constructed prototype has a translation stage base allows fine adjustment of planar location at the micrometer level of accuracy. Four degrees of motional freedom are allowed in total, permitting fine, independent rotations of the murine heart (oriented at right-angles with respect to its normal mode of operation) at 2–5° increments, along its two axes of symmetry. Successful mounting of the constructed prototype was realized on the AFM’s base platform, with Zpositioning maintained through the instrument’s electronics. Measurements of normal epicardial tissue elasticity in one excised male C57BL/6 heart led to a mean value (over 15 epicardial locations) of 51.8 ± 5.4 kPa. A flexible, versatile, and low-cost prototype assembly was designed, implemented, and tested to conduct epicardial elasticity measurements in a semi-automatedmanner, using AFM. The epicardial elasticity values are within expected, published ranges for the normal myocardium of 10–55 kPa. Efforts are ongoing to allow 3D regional elasticity measurements at finer resolution with potential usefulness to studies of genetically altered mice.