Adaptive anisotropic gaussian filtering to reduce acquisition time in cardiac diffusion tensor imaging

Diffusion tensor imaging (DTI) is used to quantify myocardial fiber orientation based on helical angles (HA). Accurate HA measurements require multiple excitations (NEX) and/or several diffusion encoding directions (DED). However, increasing NEX and/or DED increases acquisition time (TA). Therefore, in this study, we propose to reduce TA by implementing a 3D adaptive anisotropic Gaussian filter (AAGF) on the DTI data acquired from ex-vivo healthy and infarcted porcine hearts. DTI was performed on ex-vivo hearts [9-healthy, 3-myocardial infarction (MI)] with several combinations of DED and NEX. AAGF, mean (AVF) and median filters (MF) were applied on the primary eigenvectors of the diffusion tensor prior to HA estimation. The performance of AAGF was compared against AVF and MF. Root mean square error (RMSE), concordance correlation-coefficients and Bland–Altman’s technique was used to determine optimal combination of DED and NEX that generated the best HA maps in the least possible TA. Lastly, the effect of implementing AAGF on the infarcted porcine hearts was also investigated. RMSE in HA estimation for AAGF was lower compared to AVF or MF. Post-filtering (AAGF) fewer DED and NEX were required to achieve HA maps with similar integrity as those obtained from higher NEX and/or DED. Pathological alterations caused in HA orientation in the MI model were preserved post-filtering (AAGF). Our results demonstrate that AAGF reduces TA without affecting the integrity of the myocardial microstructure.     

Magnetic resonance elastography of the lung parenchyma in an in situ porcine model with a noninvasive mechanical driver: Correlation of shear stiffness with trans‐respiratory system pressures

Quantification of the mechanical properties of lung parenchyma is an active field of research due to the association of this metric with normal function, disease initiation and progression. A phase contrast MRI‐based elasticity imaging technique known as magnetic resonance elastography is being investigated as a method for measuring the shear stiffness of lung parenchyma. Previous experiments performed with small animals using invasive drivers in direct contact with the lungs have indicated that the quantification of lung shear modulus with ¹H based magnetic resonance elastography is feasible. This technique has been extended to an in situ porcine model with a noninvasive mechanical driver placed on the chest wall. This approach was tested to measure the change in parenchymal stiffness as a function of airway opening pressure (P_ao) in 10 adult pigs. In all animals, shear stiffness was successfully quantified at four different P_ao values. Mean (±STD error of mean) pulmonary parenchyma density corrected stiffness values were calculated to be 1.48 (±0.09), 1.68 (±0.10), 2.05 (±0.13), and 2.23 (±0.17) kPa for P_ao values of 5, 10, 15, and 20 cm H2O, respectively. Shear stiffness increased with increasing P_ao, in agreement with the literature. It is concluded that in an in situ porcine lung shear stiffness can be quantitated with ¹H magnetic resonance elastography using a noninvasive mechanical driver and that it is feasible to measure the change in shear stiffness due to change in P_ao. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Index of biventricular interdependence calculated using cardiac MRI: a proof of concept study in patients with and without constrictive pericarditis

We sought to propose a magnetic resonance (MR) imaging-derived index of biventricular interdependence as a diagnostic parameter to distinguish patients with surgically-confirmed pericardial constriction from those without. Free-breathing real time MR pulse sequences of seventeen subjects with surgically proven constrictive pericarditis and thirty-five patients referred for clinically-indicated cardiac MR examinations but without documented constriction were analyzed using a novel index of biventricular interdependence. Cross-sectional biventricular areas at end diastole using the epicardial surface were traced at the mid left ventricular level at end-inspiration and end-expiration and an index of biventricular interdependence, defined as the ratio of (biventricular end-diastolic area at end-inspiration)/(biventricular end-diastolic area at end-expiration) was calculated for each subject. The mean index for both groups was calculated and results were statistically compared. The index of biventricular interdependence approximated unity (mean index 1.03 ± 0.03 SD) in patients with surgically confirmed pericardial constriction, indicating similar biventricular area at end-inspiration and end-expiration, and was significantly lower than in individuals without constrictive pericarditis (mean index 1.28 ± 0.10 SD; p < 0.0001). The MR-derived index of biventricular interdependence was significantly different between subjects with surgically-confirmed pericardial constriction and subjects where pericardial constraint was not suspected and may serve as a useful metric in the hemodynamic assessment of patients with a potential diagnosis of constrictive pericarditis.     

In vivo assessment of MR elastography-derived effective end-diastolic myocardial stiffness under different loading conditions

Purpose:

To compare magnetic resonance elastography (MRE) effective stiffness to end‐diastolic pressure at different loading conditions to demonstrate a relationship between myocardial MRE effective stiffness and end‐diastolic left ventricular (LV) pressure.

Materials and Methods:

MRE was performed on four pigs to measure the end‐diastolic effective stiffness under different loading conditions. End‐diastolic pressure was increased by infusing Dextran‐40 (20% of blood volume). For each infusion of Dextran‐40, end‐diastolic pressure was recorded and end‐diastolic effective stiffness was measured using MRE. In each pig, least‐square linear regression was performed to determine the correlation between end‐diastolic effective stiffness and end‐diastolic LV pressure.

Results:

A linear correlation was found between end‐diastolic LV pressure and end‐diastolic effective stiffness with R² ranging from 0.73–0.9. A linear correlation with R² = 0.26 was found between end‐diastolic LV pressure and end‐diastolic effective stiffness when pooling data points from all pigs.

Conclusion:

End‐diastolic effective myocardial stiffness increases linearly with end‐diastolic LV pressure. J. Magn. Reson. Imaging 2011;33:1224–1228. © 2011 Wiley‐Liss, Inc.     

MR elastography as a method for the assessment of myocardial stiffness: Comparison with an established pressure–volume model in a left ventricular model of the heart

Magnetic resonance elastography (MRE) measurements of shear stiffness (μ) in a spherical phantom experiencing both static and cyclic pressure variations were compared to those derived from an established pressure–volume (P‐V)‐based model. A spherical phantom was constructed using a silicone rubber composite of 10 cm inner diameter and 1.3 cm thickness. A gradient echo MRE sequence was used to determine μ within the phantom at static and cyclic pressures ranging from 55 to 90 mmHg. Average values of μ using MRE were obtained within a region of interest and were compared to the P‐V‐derived estimates. Under both static and cyclic pressure conditions, the P‐V‐ and MRE‐based estimates of μ ranged from 98.2 to 155.1 kPa and 96.2 to 150.8 kPa, respectively. Correlation coefficients (R²) of 0.98 and 0.97 between the P‐V and MRE‐based estimates of shear stiffness measurements were obtained. For both static and cyclic pressures, MRE‐based measures of μ agree with those derived from a P‐V model, suggesting that MRE can be used as a new, noninvasive method of assessing μ in sphere‐like fluid‐filled organs such as the heart. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.