MANDUCA et Al. per year is increasing rapidly (Figure 1). However, numerous types of acquisitions and processing techniques are in use, and MRE results have been expressed in many different quantities: shear modulus (possibly complex), storage modulus, magnitude of the complex shear modulus, shear stiffness (defined in different ways), wave speed, propagation, loss modulus, attenuation, loss tangent or loss factor, phase angle, damping ratio, attenuation, and penetration rate. This list is not exhaustive and does not include additional quantities related to fitting specific assumed material models or anisotropic materials. This diversity of terminology and absence of standardization can lead to confusion (particularly among clinicians) as to the meaning of certain terms, or how to interpret or compare certain types of MRE results. This paper is written by the MRE Guidelines Committee, a group formalized at the first meeting of the ISMRM MRE Study Group, to attempt to clarify and (to some extent) standardize MRE terminology and practice. Specifically, the purpose of this paper is to (1) explain MRE terminology to those not familiar with it, (2) define "good practices" for practitioners of MRE, and (3) discuss some practices and terms that we believe should be standardized and some that should be discouraged. F I G U R E 2 Schematic stress-strain relationship for soft tissue unloaded and at three different tissue-loading states. Magnetic resonance elastography measures the slope of this curve at a given point, as indicated by the tangent line at ε 3
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). Myocardial stiffness relates myocardial deformation (strain) to loading (stress) and is thought to affect the heart's function. To date, the primary method of evaluating myocardial stiffness in vivo has been by inferring it from pressure-volume (P-V) relationships (1,2). For example, it has been shown that patients with diastolic heart failure exhibit increased chamber stiffness (dP/dV) (3), as do patients with myocardial ischemia and patients with myocardial infarction (4). However, P-V methods are invasive, require technical precision, assess the left ventricular (LV) chamber rather than the true intrinsic properties of the myocardium, and provide only a global measure of stiffness. Therefore, there is a need for a technique capable of noninvasively assessing true intrinsic mechanical properties of the myocardium such as shear modulus (i.e., shear stiffness or stiffness) (m).MR elastography (MRE) is a novel imaging technique that can be used to measure shear stiffness (5-9). In MRE, cyclic motion is applied to a tissue and a phasecontrast MR image is acquired in which motion-encoding gradients are synchronized with the external motion. This produces MRI images of the waves propagating in the tissue. The wave displacements obtained from these images can be mathematically converted to stiffness maps.To date, MRE has been shown to resolve the shear stiffness of static tissues (10,11). However, there are challenges to applying this technique to dynamic organs such as the heart. These include performing faster data acquisition to capture the different phases of cardiac cycle and introducing external shear waves into the heart while avoiding bulk motion artifacts. Previous studies (12,13) have shown the feasibility of using a cine MRE acquisition strategy in a simulated, dynamic LV spherical phantom when the acquisition is appropriately synchronized with the motion of the phantom. One of the studies (12) demonstrated a linear correlation between effectiv...
Purpose To determine the correlation in abdominal aortic stiffness obtained using magnetic resonance elastography (MRE) (μMRE) and MRI-based pulse wave velocity (PWV) shear stiffness (μPWV) estimates in normal volunteers of varying age; and also to determine the correlation between μMRE and μPWV. Methods In-vivo aortic MRE and MRI were performed on 21 healthy volunteers with ages ranging from 18 to 65 years to obtain wave and velocity data along the long-axis of the abdominal aorta. The MRE wave images were analyzed to obtain mean stiffness, and the phase contrast images were analyzed to obtain PWV measurements and indirectly estimate stiffness values from Moens-Korteweg equation. Results Both μMRE and μPWV measurements increased with age, demonstrating linear correlations with R2 values of 0.81 and 0.67, respectively. Significant difference (p≤0.001) in mean μMRE and μPWV between young and old healthy volunteers was also observed. Furthermore, a poor linear correlation of R2 value of 0.43 was determined between μMRE and μPWV in initial pool of volunteers. Conclusion The results of this study indicate linear correlations between μMRE and μPWV with normal aging of the abdominal aorta. Significant differences in mean μMRE and μPWV between young and old healthy volunteers were observed.
Purpose To assess reproducibility in measuring left ventricular (LV) myocardial stiffness in volunteers throughout the cardiac cycle using magnetic resonance elastography (MRE) and to determine its correlation with age. Methods Cardiac MRE (CMRE) was performed on 29 normal volunteers, with ages ranging from 21 to 73 years. For assessing reproducibility of CMRE-derived stiffness measurements, scans were repeated per volunteer. Wave images were acquired throughout the LV myocardium, and were analyzed to obtain mean stiffness during the cardiac cycle. CMRE-derived stiffness values were correlated to age. Results Concordance correlation coefficient revealed good inter-scan agreement with rc of 0.77, with p-value<0.0001. Significantly higher myocardial stiffness was observed during end-systole (ES) compared to end-diastole (ED) across all subjects. Additionally, increased deviation between ES and ED stiffness was observed with increased age. Conclusion CMRE-derived stiffness is reproducible, with myocardial stiffness changing cyclically across the cardiac cycle. Stiffness is significantly higher during ES compared to ED. With age, ES myocardial stiffness increases more than ED, giving rise to an increased deviation between the two.
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