Cardiovascular Magnetic Resonance Elastography: Current Evidence, Challenges, and Future Perspectives

Figures

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  Figure 1. Example of an MRE transducer setup. This schematic illustration demonstrates the mechanical coupling between the vibration source and the imaging subject. A transducer is placed on the target area and fixed in place to ensure stable wave transmission and to prevent motion-related artifacts. In acoustic MRE systems, vibrations are generated by a remote actuator (“wave source”) located in close proximity. These vibrations are then transmitted via a flexible waveguide to the transducer, which conveys them to the tissue. In electromechanical MRE systems, the physical arrangement is very similar, even though vibrations are produced in the transducer. The transducer is again positioned on the target area and is connected to a motor by way of a flexible drive shaft, which transmits rotational movement to the transducer. Abbreviations: magnetic resonance elastography, “MRE”.

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  Figure 2. GT transducer setup. (A) The main board of the GT transducer, which includes a 4 nm stepper motor and a control unit, is located next to the main console of the MRI. This placement makes it convenient to program the system outside the MRI exam room and adjust the settings of the MRI sequences. (B,C) The system includes a flexible PEEK rotation axis. (D) The flexible waveguide is connected to the back-end motor and leads into the (G) MRI examination room to the transducer (E), which is positioned on a phantom in this example. (F) The transducer housing consists of two PEEK rods, one of which is bonded with (a) a polytetrafluoroethylene eccentric compound, and both rods are fitted with (b) two PEEK bearings. The two PEEK rods are linked together via (c) two PEEK timing pulleys and a timing belt. Abbreviations: gravitational, “GT”; magnetic resonance imaging, “MRI”; polyether ether ketone, “PEEK”.

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  Figure 3. Example of in vivo positioning of the pencil beam. (A) At the moment of aortic valve closure, mechanical shear waves are generated and propagate from the base toward the apex of the heart (direction indicated by the blue arrow). Therefore, the 2D volume of the pencil beam (the bar overlapping the interventricular septum) was first projected within the interventricular septum positioned towards the apex base direction. (B) To mitigate partial volume effects and motion during image analysis, two pencil-beam volumes were positioned perpendicular to the interventricular septum, one at the base (blue bar) and one at the apex (orange bar). An in vivo positioning example is shown in a subject, depicting the short-axis view (C) and the horizontal long-axis view (D). Figure adapted from Ref. [4]. Abbreviations: two-dimensional, “2D”; aortic valve, “AV”; left ventricle, “LV”; right ventricle, “RV”; interventricular septum, “S”.

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  Figure 4. Schematic illustration of a GRE pulse sequence used for cardiac MRE. The diagram shows the temporal relationship between cardiac motion (RR-interval) and the sequence components (red indicator below the ECG-curve). Continuous mechanical excitation is applied throughout the cardiac cycle, while a radiofrequency (RF) excitation pulse initiates signal generation. Imaging gradients, Gx, Gy, and Gz, encode unique information for each of their corresponding geometrical directions. The first-moment nulled MEG, which can be applied on any axis, is applied after the RF excitation of the sample and before the measurement of the induced signal. The fractionally encoded signal is assessed eight times per mechanical vibration, and the process is repeated throughout the cardiac cycle. TR represents the repetition time between RF excitations in a GRE sequence [1,60]. Figure created based on data from Ref. [60]. Abbreviations: electrocardiogram, “ECG”; imaging gradient in x-direction, “Gx”; imaging gradient in y-direction, “Gy”; imaging gradient in z-direction, “Gz”; gradient-recalled echo, “GRE”; motion encoding gradient, “MEG”; magnetic resonance elastography, “MRE”; radiofrequency, “RF”;…