MR elastography of the in vivo abdominal aorta: a feasibility study for comparing aortic stiffness between hypertensives and normotensives

Purpose:

To demonstrate feasibility of using MR elastography (MRE) to identify hypertensive changes in the abdominal aorta when compared with normotensives based on the stiffness measurements.

Materials and Methods:

MRE was performed on eight volunteers (four normotensives and four hypertensives) to measure the effective stiffness of the abdominal aorta. MRE wave images are directionally filtered and phase gradient analysis was performed to determine the stiffness of the aorta. Student's t‐test was performed to determine significant difference in stiffness measurements between normotensives and hypertensives.

Results:

The normotensive group demonstrated a mean abdominal aortic stiffness of 3.7 ± 0.8 kPa, while the controlled‐hypertensive demonstrated a mean abdominal aortic stiffness of 9.3 ± 1.9 kPa. MRE effective stiffness of abdominal aorta in hypertensives was significantly greater than that of normotensives with p = 0.02.

Conclusion:

Feasibility of in vivo aortic MRE is demonstrated. Hypertensives have significantly higher aortic stiffness assessed through MRE than normotensives. J. Magn. Reson. Imaging 2012;35:582‐586. © 2011 Wiley Periodicals, Inc.     

Magnetic resonance elastography as a method to estimate myocardial contractility

Purpose:

To determine whether increasing epinephrine infusion in an in vivo pig model is associated with an increase in end‐systolic magnetic resonance elastography (MRE)‐derived effective stiffness.

Materials and Methods:

Finite element modeling (FEM) was performed to determine the range of myocardial wall thicknesses that could be used for analysis. Then MRE was performed on five pigs to measure the end‐systolic effective stiffness with epinephrine infusion. Epinephrine was continuously infused intravenously in each pig to increase the heart rate in increments of 20%. For each such increase end‐systolic effective stiffness was measured using MRE. In each pig, Student's t‐test was used to compare effective end‐systolic stiffness at baseline and at initial infusion of epinephrine. Least‐square linear regression was performed to determine the correlation between normalized end‐systolic effective stiffness and increase in heart rate with epinephrine infusion.

Results:

FEM showed that phase gradient inversion could be performed on wall thickness ≈≥1.5 cm. In pigs, effective end‐systolic stiffness significantly increased from baseline to the first infusion in all pigs (P = 0.047). A linear correlation was found between normalized effective end‐systolic stiffness and percent increase in heart rate by epinephrine infusion with R² ranging from 0.86–0.99 in four pigs. In one of the pigs the R² value was 0.1. A linear correlation with R² = 0.58 was found between normalized effective end‐systolic stiffness and percent increase in heart rate when pooling data points from all pigs.

Conclusion:

Noninvasive MRE‐derived end‐systolic effective myocardial stiffness may be a surrogate for myocardial contractility. J. Magn. Reson. Imaging 2012;36:120–127. © 2012 Wiley Periodicals, Inc.     

Magnetic resonance elastography as a method for the assessment of effective myocardial stiffness throughout the cardiac cycle

MR elastography (MRE) is a noninvasive technique in which images of externally generated waves propagating in tissue are used to measure stiffness. The first aim is to determine, from a range of driver configurations, the optimal driver for the purpose of generating waves within the heart in vivo. The second aim is to quantify the shear stiffness of normal myocardium throughout the cardiac cycle using MRE and to compare MRE stiffness to left ventricular chamber pressure in an in vivo pig model. MRE was performed in six pigs with six different driver setups, including no motion, three noninvasive drivers, and two invasive drivers. MRE wave displacement amplitudes were calculated for each driver. During the same MRI examination, left ventricular pressure and MRI‐measured left ventricular volume were obtained, and MRE myocardial stiffness was calculated for 20 phases of the cardiac cycle. No discernible waves were imaged when no external motion was applied, and a single pneumatic drum driver produced higher amplitude waves than the other noninvasive drivers (P < 0.05). Pressure–volume loops overlaid onto stiffness–volume loops showed good visual agreement. Pressure and MRE‐measured effective stiffness showed good correlation (R² = 0.84). MRE shows potential as a noninvasive method for estimating effective myocardial stiffness throughout the cardiac cycle. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

MR elastography of human lung parenchyma: technical development, theoretical modeling and in vivo validation

Purpose:

To develop a novel MR‐based method for visualizing the elastic properties of human lung parenchyma in vivo and to evaluate the ability of this method to resolve differences in parenchymal stiffness at different respiration states in healthy volunteers.

Materials and Methods:

A spin‐echo MR Elastography (MRE) pulse sequence was developed to provide both high shear wave motion sensitivity and short TE for improved visualization of lung parenchyma. The improved motion sensitivity of this approach was modeled and tested with phantom experiments. In vivo testing was then performed on 10 healthy volunteers at the respiratory states of residual volume (RV) and total lung capacity (TLC).

Results:

Shear wave propagation was visualized within the lungs of all volunteers and was processed to provide parenchymal shear stiffness maps for all 10 subjects. Density corrected stiffness values at TLC (1.83 ± 0.22 kPa) were higher than those at the RV (1.14 ± 0.14 kPa) with the difference being statistically significant (P < 0.0001).

Conclusion:

¹H‐based MR elastography can noninvasively measure the shear stiffness of human lung parenchyma in vivo and can quantitate the change in shear stiffness due to respiration. The values obtained were consistent with previously reported in vitro assessments of cadaver lungs. Further work is required to increase the flexibility of the current acquisition and to investigate the clinical potential of lung MRE. J. Magn. Reson. Imaging 2011;33:1351–1361. © 2011 Wiley‐Liss, Inc.