Rapid multi-frequency steady-state MR elastography for quantification of short-term alterations of viscoelasticity in biological soft tissue
Felix Schrank, Carsten Warmuth, Lars-Arne Schaafs, Thomas Elgeti, Ingolf Sack
While MR elastography (MRE) is an established imaging modality for mapping viscoelastic properties of the human liver and brain , there is a growing need to apply MRE in situations when the organ moves or viscoelasticity rapidly changes. One of our research interests is to quantify short-term alterations of viscoelasticity of soft tissue as a response to muscle function, perfusion, or cardiac work. Such MRE applications require rapid wave field encoding with precise synchronization of imaging sequence and wave dynamics.
To address these challenges, we developed a cardiac-gated steady-state gradient echo MRI sequence with segmented spiral k-space acquisition, respiratory navigation, and stroboscopic wave field sampling. We applied this method on the human brain to show for the first time how cerebral arterial pulsation influences brain viscoelasticity. Intriguingly, we showed that the brain becomes softer and more viscous during cerebral systole, possibly due to an effect of CAP-induced arterial expansion (see Fig.1) . We are further developing cardiac MRE to quantify the temporal variation of myocardial stiffness over the heart cycle. Preliminary results in healthy volunteers show a periodic stiffening and softening of the left ventricular wall during systole and diastole, reflecting the mechanical work performed by the beating heart (see Fig.2) .
- Hirsch, S., Sack, I., & Braun, J. (2017). Magnetic resonance elastography: physical background and medical applications. John Wiley & Sons.
- Schrank, F., Warmuth, C., Tzschätzsch, H., Kreft, B., Hirsch, S., Braun, J., Elgeti, T., Sack, I. (2019). Cardiac-gated steady-state multifrequency magnetic resonance elastography of the brain: Effect of cerebral arterial pulsation on brain viscoelasticity. Journal of Cerebral Blood Flow & Metabolism. https://www.ncbi.nlm.nih.gov/pubmed/31142226.
- Schrank, F., Warmuth, C., Schaafs, L., Tzschätzsch, H., Elgeti, T., Braun, J., Sack, I. (2019). C Multi-frequency magnetic resonance elastography with spiral readout, respiratory navigator and stroboscopic wave sampling for cardiac stiffness mapping with high spatial and temporal resolution, digital poster #3975.
Fig 1. A: Temporal averaged magnitude of the complex shear modulus |G*|. Red lines demarcate the whole-parenchyma region of interest while the blue circle indicates the region of the temporal artery from which the blood flow signal was derived in the magnitude of the complex MRI signal. B: Derived time courses show the normalized blood flow (blue) and stiffness (red) alternations.
Fig 2. Representative cardiac MRE results from one volunteer. A: Shear-wave speed maps in short-axis view at diastole, systole, and diastole, show a marked increase in stiffness within the left ventricular (LV) wall. B: Corresponding time course of spatially-averaged LV-stiffness and volume. C: MRE-derived stiffness-volume diagram reflecting the mechanical work of the heart. Stiffness precedes volume changes as indicated by the panel C.