Editorial for “Reproducibility of Cardiac Multifrequency MR Elastography in Assessing Left Ventricular Stiffness and Viscosity”

Diastolic dysfunction, a key contributor to heart failure with preserved ejection fraction, presents significant diagnostic challenges in clinical practice.1 Despite its prevalence, accurately characterizing diastolic dysfunction remains challenging due to its complex nature. Diastole involves several intricate physiological processes, including left ventricular relaxation, filling pressures, and myocardial stiffness, all influenced by systemic conditions such as hypertension and aging. Echocardiography is the most widely used tool for assessing diastolic function due to its accessibility and noninvasive nature.2 However, it has several limitations, particularly when assessing myocardial stiffness and early-stage diastolic dysfunction. Usual echocardiographic parameters such as the E/A ratio, E/e' ratio, and left atrial volume index can suggest diastolic dysfunction but lack sensitivity in certain patient populations. These measurements often fail to assess myocardial mechanical properties directly and are influenced by loading conditions. A recent study published by Pak et al demonstrated the low accuracy of American Society of Echocardiography/European Association of Cardiovascular Imaging guidelines in assessing diastolic function compared to invasive left atrial pressure measurements.3 Additionally, their diagnostic utility diminishes in patients with more complex heart diseases such as hypertrophic cardiomyopathy.4 This challenge highlights the need for advanced imaging modalities to enhance the assessment of diastolic function. Magnetic resonance elastography (MRE) is a noninvasive imaging technique that applies low-frequency mechanical vibrations to generate shear waves in tissues, which are then captured and analyzed to determine tissue stiffness.5 The principle behind MRE is based on the propagation of shear waves through biological tissues: stiffer tissues transmit waves more rapidly, while softer tissues attenuate them. By applying mechanical vibrations and imaging the resulting wave propagation, MRE provides a direct measurement of tissue stiffness. This technique is well-established in assessing liver fibrosis and has been progressively adapted for cardiac applications, allowing researchers to evaluate myocardial mechanical properties in vivo. However, cardiac MRI elastography faces several limitations, including poor temporal and spatial resolution, the false assumption of isotropic heart tissue, challenges from cardiac motion, breath-hold requirements, complex shear wave propagation, and dependence on actuator placement. The transition from traditional MRE to multifrequency MR elastography (MMRE) involves applying multiple vibration frequencies, thereby enhancing the spatial resolution and robustness of the stiffness maps. Tomoelastography, an advanced form of MMRE, further refines this technique by integrating multifrequency data with advanced inversion algorithms to generate high-resolution maps of tissue stiffness and viscosity.6 This could be interesting in the myocardium, where stiffness varies not only between healthy and pathological states but also regionally within the heart. In this issue of JMRI,7 Castelein et al aimed to evaluate the reproducibility of MMRE-derived markers of left ventricular stiffness and viscosity across healthy subjects and patients with aortic stenosis, a condition known to alter myocardial mechanics. Given that reproducibility is essential for the clinical application of any imaging technique, this study addresses a fundamental aspect of MMRE's utility in routine practice. The study design involved repeating the MMRE examination twice on each participant, the same day, ensuring that the reliability of the measurements could be thoroughly assessed. One of the key findings is the excellent intra-participant reproducibility of myocardial stiffness and viscosity measurements, with interscan reliability confirmed through Bland–Altman analysis and intraclass correlation coefficients. The study also demonstrated that variations in actuator positioning during the MMRE procedure had minimal impact on measurement reliability, indicating the robustness of this technique in capturing left ventricular mechanical properties. However, the study's focus on reproducibility, while crucial, leaves some questions unanswered. As the authors themselves acknowledge, no direct comparison between healthy subjects and patients with aortic stenosis was made regarding myocardial stiffness values, nor were these findings correlated with other standard noninvasive metrics of diastolic function, such as echocardiographic parameters or conventional cardiac MRI markers. Incorporating these comparisons would not only validate MMRE's potential diagnostic value but also establish its role alongside other diastolic function measures. Despite these limitations, the findings offer a compelling case for further development and clinical integration of MMRE as a tool for assessing myocardial stiffness and viscosity. While MMRE represents a novel approach to quantifying myocardial stiffness, it should not be viewed in isolation. Cardiac MRI offers a suite of advanced imaging techniques such as four-dimensional flow or feature tracking which can complement MMRE in the comprehensive assessment of diastolic function.8, 9 By integrating MMRE with these other advanced imaging techniques, future studies could create a more holistic picture of diastolic dysfunction, combining direct mechanical assessments with flow and deformation analyses to improve diagnostic accuracy and patient care. Meanwhile, as with these other novel biomarkers, a methodological consensus should be established for the assessment and postprocessing of MMRE datasets. That will help achieve better correlation with established noninvasive and invasive diastolic function assessment, and to better differentiate between healthy and pathological states. Cardiac MMRE could represent a promising frontier in the noninvasive assessment of myocardial stiffness, offering direct insights into the mechanical properties that underpin diastolic dysfunction. This study on reproducibility is a crucial step in validating MMRE as a reliable tool, but further research is needed to explore its diagnostic accuracy and clinical relevance. Hichem Sakhi: writing the paper. Virgile Chevance: review and editing. Arshid Azarine: review and editing. None (all authors).