Arunark Kolipaka

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.

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.

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

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.

Magnetic resonance elastography as a method to estimate myocardial contractility

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.

MR elastography of the ex vivo bovine globe

To evaluate the feasibility of using MR elastography (MRE) to assess the mechanical properties of the eye.

Measuring age-dependent myocardial stiffness across the cardiac cycle using MR elastography: A reproducibility study

To assess reproducibility in measuring left ventricular (LV) myocardial stiffness in volunteers throughout the cardiac cycle using MR elastography (MRE) and to determine its correlation with age.

Hepatic and Splenic Stiffness Augmentation Assessed with MR Elastography in an in vivo Porcine Portal Hypertension Model

To investigate the influence of portal pressure on the shear stiffness of the liver and spleen in a well‐controlled in vivo porcine model with magnetic resonance elastography (MRE). A significant correlation between portal pressure and tissue stiffness could be used to noninvasively assess increased portal venous pressure (portal hypertension), which is a frequent clinical condition caused by cirrhosis of the liver and is responsible for the development of many lethal complications.

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

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.

Evaluation of a rapid, multiphase MRE sequence in a heart-simulating phantom.

The aims of this study were to validate stiffness estimates of a phantom undergoing cyclic deformation obtained using a multiphase magnetic resonance elastography (MRE) imaging sequence by comparison with those obtained using a single‐phase MRE sequence and to quantify the stability of the multiphase‐derived stiffness estimates as a function of deformation frequency and imaging parameters. A spherical rubber shell of 10 cm diameter and 1 cm thickness was connected to a computerized flow pump to produce cyclic pressure variations within the phantom. The phantom was imaged at cyclic pressures between 18–72 bpm using single‐phase and multiphase MRE acquisitions. The shear stiffness of the phantom was resolved using a spherical shell wave inversion algorithm. Shear stiffness was averaged over the slice of interest and plotted against pressure within the phantom. A linear correlation was observed between stiffness and pressure. Good correlation (R2 = 0.98) was observed between the stiffness estimates obtained using the standard single‐phase and the multiphase pulse sequences. Stiffness estimates obtained using multiphase MRE were stable when the fraction of the deformation period required for acquisition of a single image was not greater than 42%. The results demonstrate the potential of multiphase MRE technique for imaging dynamic organs, such as the heart. Magn Reson Med, 2009.

Magnetic resonance elastography: Inversions in bounded media

Magnetic resonance elastography is a noninvasive imaging technique capable of quantifying and spatially resolving the shear stiffness of soft tissues by visualization of synchronized mechanical wave displacement fields. However, magnetic resonance elastography inversions generally assume that the measured tissue motion consists primarily of shear waves propagating in a uniform, infinite medium. This assumption is not valid in organs such as the heart, eye, bladder, skin, fascia, bone and spinal cord, in which the shear wavelength approaches the geometric dimensions of the object. The aim of this study was to develop and test mathematical inversion algorithms capable of resolving shear stiffness from displacement maps of flexural waves propagating in bounded media such as beams, plates, and spherical shells, using geometry‐specific equations of motion. Magnetic resonance elastography and finite element modeling of beam, plate, and spherical shell phantoms of various geometries were performed. Mechanical testing of the phantoms agreed with the stiffness values obtained from finite element modeling and magnetic resonance elastography data, and a linear correlation of r2 ≥ 0.99 was observed between the stiffness values obtained using magnetic resonance elastography and finite element modeling data. In conclusion, we have demonstrated new inversion methods for calculating shear stiffness that may be more appropriate for waves propagating in bounded media. Magn Reson Med, 2009.