Waveguide Magnetic Resonance Elastography in a Pressure-Varying Porcine Model (original) (raw)
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In vivo quantification of myocardial stiffness in hypertensive porcine hearts using MR elastography
Journal of magnetic resonance imaging : JMRI, 2016
To determine alteration in left ventricular (LV) myocardial stiffness (MS) with hypertension (HTN). Cardiac MR elastography (MRE) was used to estimate MS in HTN induced pigs and MRE-derived MS measurements were compared against LV pressure, thickness and circumferential strain. Renal-wrapping surgery was performed to induce HTN in eight pigs. LV catheterization (to measure pressure) and cardiac MRI (1.5 Tesla; gradient echo-MRE and tagging) was performed pre-surgery at baseline (Bx), and post-surgery at month 1 (M1) and month 2 (M2). Images were analyzed to estimate LV-MS, thickness, and circumferential strain across the cardiac cycle. The associations between end-diastolic (ED) and end-systolic (ES) MS and (i) mean LV pressure; (ii) ED and ES thickness, respectively; and (iii) circumferential strain were evaluated using Spearman's correlation method. From Bx to M2, mean pressure, MRE-derived stiffness, and thickness increased while circumferential strain decreased significantly...
Journal of magnetic resonance imaging : JMRI, 2016
To estimate change in left ventricular (LV) end-systolic and end-diastolic myocardial stiffness (MS) in pigs induced with myocardial infarction (MI) with disease progression using cardiac magnetic resonance elastography (MRE) and to compare it against ex vivo mechanical testing, LV circumferential strain, and magnetic resonance imaging (MRI) relaxometry parameters (T1 , T2 , and extracellular volume fraction [ECV]). MRI (1.5T) was performed on seven pigs, before surgery (Bx), and 10 (D10), and 21 (D21) days after creating MI. Cardiac MRE-derived MS was measured in infarcted region (MIR) and remote region (RR), and validated against mechanical testing-derived MS obtained postsacrifice on D21. Circumferential strain and MRI relaxometry parameters (T2 , T1 , and ECV) were also obtained. Multiparametric analysis was performed to determine correlation between cardiac MRE-derived MS and 1) strain, 2) relaxometry parameters, and 3) mechanical testing. Mean diastolic (D10: 5.09 ± 0.6 kPa; D...
Magnetic Resonance in Medicine, 2005
Approximately half of patients experiencing congestive heart failure present with a normal left ventricular ejection fraction. Perturbations in material properties affecting ventricular pressure/ volume relationships likely play an important role in the "stiff heart syndrome" yet noninvasive tools permitting the accurate assessment of myocardial elasticity are extremely limited. We developed an MRI-based technique to examine regional left ventricular stress/strain relationships by incorporating displacement-encoding with stimulated-echoes (DENSE) and phase-contrast (PC) velocity mapping and compared regional elastic moduli (EM) and viscous delay time constants (VDTCs) (N = 10) with immediate postmortem direct strain gauge measurements (N = 8) and global chamber compliance (literature) in normal dogs. EMs by MRI were significantly greater in papillary muscle columns when compared with lateral wall and septal locations by MRI (7.59 ± 1.65 versus 3.40 ± 0.87 versus 2.55 ± 0.93 kPa, P < 0.0001) and were in agreement with direct strain gauge measurements (3.78 ± 0.93 and 2.96 ± 0.88 kPa for the lateral wall and the septum, P = ns for both versus MRI). MRI-determined VDTCs were similar in the three regions (VDTC = −1.15 ± 12.37 versus 3.04 ± 7.25 versus 4.17 ± 5.76 ms, P = ns) and did not differ from lateral and septal wall strain gauge assessment (VDTC = 3.09 ± 0.40 and 4.57 ± 1.86 ms, P = ns for both versus MRI). Viscoelastic measurements obtained in six normal volunteers demonstrated the feasibility of this technique in humans. Noninvasive, regional assessment of myocardial stiffness using DENSE and PC velocity mapping techniques is accurate in a canine model and feasible in humans.
Theoretical Quality Assessment of Myocardial Elastography with In Vivo Validation
IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control, 2007
Myocardial elastography (ME), a radio frequency (RF)-based speckle tracking technique with one-dimensional (1-D) cross correlation and novel recorrelation methods in a 2-D search was proposed to estimate and fully image 2-1) transmural deformation field and to detect abnormal cardiac function. A theoretical framework was first developed in order to evaluate the performance of 2-D myocardial elastography based on a previously developed 3-D finite-element model of the canine left ventricle. A normal (control) and an ischemic (left-circumflex, LCx) model, which more completely represented myocardial deformation than a kinematic model, were considered. A 2-D convolu-tional image formation model was first used to generate RF signals for quality assessment of ME in the normal and ischemic cases. A 3-D image formation model was further developed to investigate the effect of the out-of-plane motion on the 2-D, in-plane motion estimation. Both orthogonal, in-plane displacement components (i.e., lateral and axial) between consecutive RF frames were iteratively estimated. All the estimated incremental 2-D displacements from end-diastole (ED) to end-systole (ES) were then accumulated to acquire the cumulative 2-D displacements, which were further used to calculate the cumulative 2-D systolic finite strains. Furthermore, the cumulative systolic radial and circumferential strains, which were angle-and frame-rate independent, were obtained from the 2-D finite-strain components and imaged in full view to detect the ischemic region. We also explored the theoretical understanding of the limitations of our technique for the accurate depiction of disease and validated it in vivo against tagged magnetic resonance imaging (tMRI) in the case of a normal human myocardium in a 2-D short-axis (SA) echocardiographic view. The theoretical framework succeeded in demonstrating that the 2-D myocardial elastography technique was a reliable tool for the complete estimation and depiction of the in-p- - lane myocardial deformation field as well as for accurate identification of pathological mechanical function using established finite-element, left-ventricular canine models. In a preliminary study, the 2-D myocardial elastography was shown capable of imaging myocardial deformation comparable to equivalent tMRI estimates in a clinical setting.
Diagnostics
Subclinical alterations in myocardial structure and function occur early during the natural disease course. In contrast, clinically overt signs and symptoms occur during late phases, being associated with worse outcomes. Identification of such subclinical changes is critical for timely diagnosis and accurate management. Hence, implementing cost-effective imaging techniques with accuracy and reproducibility may improve long-term prognosis. A growing body of evidence supports using cardiac magnetic resonance (CMR) to quantify deformation parameters. Tissue-tagging (TT-CMR) and feature-tracking CMR (FT-CMR) can measure longitudinal, circumferential, and radial strains and recent research emphasize their diagnostic and prognostic roles in ischemic heart disease and primary myocardial illnesses. Additionally, these methods can accurately determine LV wringing and functional dynamic geometry parameters, such as LV torsion, twist/untwist, LV sphericity index, and long-axis strain, and seve...
Cardiac MR Elastography: Comparison with left ventricular pressure measurement
Journal of Cardiovascular Magnetic Resonance, 2009
Purpose of study: To compare magnetic resonance elastography (MRE) with ventricular pressure changes in an animal model. Methods: Three pigs of different cardiac physiology (weight, 25 to 53 kg; heart rate, 61 to 93 bpm; left ventricular [LV] end-diastolic volume, 35 to 70 ml) were subjected to invasive LV pressure measurement by catheter and noninvasive cardiac MRE. Cardiac MRE was performed in a shortaxis view of the heart and applying a 48.3-Hz shear-wave stimulus. Relative changes in LV-shear wave amplitudes during the cardiac cycle were analyzed. Correlation coefficients between wave amplitudes and LV pressure as well as between wave amplitudes and LV diameter were determined. Results: A relationship between MRE and LV pressure was observed in all three animals (R 2 ≥ 0.76). No correlation was observed between MRE and LV diameter (R 2 ≤ 0.15). Instead, shear wave amplitudes decreased 102 ± 58 ms earlier than LV diameters at systole and amplitudes increased 175 ± 40 ms before LV dilatation at diastole. Amplitude ratios between diastole and systole ranged from 2.0 to 2.8, corresponding to LV pressure differences of 60 to 73 mmHg. Conclusion: Externally induced shear waves provide information reflecting intraventricular pressure changes which, if substantiated in further experiments, has potential to make cardiac MRE a unique noninvasive imaging modality for measuring pressure-volume function of the heart.
Shear elasticity-pressure relationship in normal and infarcted myocardium
European Heart Journal, 2013
Shear-wave methods (SW) by ultrasound or magnetic resonance have been introduced for noninvasive measurements of myocardial shear elasticity (μ). Recent studies have shown a correlation between μ and left ventricular pressure (LVP) or contractility. How these factor interact, particularly in vivo, remains unclear. Stiffness increases due to several factors, from active contraction to passive stretch, changes with disease, and so on. We studied whether there is a unique μ-LVP relationship in vivo in active vs. passive state. Methods: μ of anterior LV myocardium was measured in 10 pigs by 2 methods, the SW method (SDUV) and from classical stress-strain relationships (sonomicrometry and pressure data). In 5 animals, a reperfused MI was induced. The active (systole) and passive (volume loading) μ-LVP relations in the same animal were compared. Results: In all animals, there was a linear relationship between μ and LVP during the heart cycle (left Figure), regardless of whether the myocardium was normal or infarcted (R 2 : 0.84±0.10 in 10 normal animals and 0.87±0.14 in 5 animals with MI; P<0.05). The slopes of these relations tended to be lower post-MI. In MI, μ was higher at end-diastole (P<0.05) but pseudo-normal during systole. The reconstructed μ-LVP relations indicated that, at the same LVP, μ was higher in contracting myocardium vs. the passive state (right Figure). Conversely, relationships were similar in MI (right Figure). Animals with small subendocardial infarcts had more complex pattern. Conclusions: The results suggested that, in diseased myocardium, the absolute values of shear elasticity during systole as measured by SW do not purely reflect local contractile function. The contribution of passive tissue properties (pressuredependent) needs to be accounted for.
BMC Cardiovascular Disorders, 2013
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 mm 2 /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.
Ultrasound Strain Imaging of Altered Myocardial Stiffness
Circulation, 2004
Background— In this study we evaluate the diastolic deformation of ischemic/reperfused myocardium and relate this deformation to tissue elastic properties. Methods and Results— Farm pigs were subjected to left anterior descending coronary artery occlusion followed by reperfusion to create either stunning (n=12) or transmural myocardial infarction (n=12). Ultrasound-derived radial strain rates (SR) and strain were measured in the ischemic and remote walls. Myocardial stiffness was estimated from diastolic pressure–wall thickness relationship obtained from preload alterations. At reperfusion, end-systolic strain (ε sys ) was significantly reduced in both stunned and infarcted walls compared with their remote walls (3±3% versus 26±2% and 1±0% versus 33±5%, respectively; P <0.0001) or baseline values. Diastolic passive deformation (ε A ) and rates of deformation during early (E SR ) and late (A SR ) diastole were comparable between stunned and remote walls (ε A : 7.3±1.6% versus 7.9±...
Validation of myocardial elastography using MR tagging in normal and abnormal human hearts in vivo
2007 4th IEEE International Symposium on Biomedical Imaging: From Nano to Macro, 2007
In this paper, Myocardial Elastography (ME), a radiofrequency (RF) based speckle tracking technique, was employed in order to assess the contractility of a myocardium, and validated against tagged Magnetic Resonance Imaging (tMRI) in vivo in normal as well as abnormal cases. Both RF ultrasound and tMRI frames were acquired in 2D short-axis (SA) views from two healthy subjects and one with a history of infarction. In-plane (lateral and axial) incremental displacements were iteratively estimated using 1D cross-correlation and recorrelation techniques in a 2D search with a 1D matching kernel. The incremental displacements from end-diastole (ED) to end-systole (ES) were then accumulated to obtain cumulative systolic displacements. In tMRI, cardiac motion was obtained by a template-matching algorithm on a 2D grid-shaped mesh. The entire displacement distribution within the myocardium was obtained by a cubic B-splinebased method. In both ME and tMRI, 2D Lagrangian finite systolic strains were calculated from cumulative 2D displacements. Radial and circumferential strains were then computed from the 2D finite strains. Both qualitatively and quantitatively, ME is shown capable of measuring myocardial deformation in excellent agreement with tMRI estimates in normal and abnormal subjects.