Compression sensitive elastography
Ledia Lilaj, Thomas Fischer, Jing Guo, Jürgen Braun, Ingolf Sack, Sebastian Hirsch
Background: Many diseases are associated with imbalanced fluid pressure regulation mechanisms. For example, normal pressure hydrocephalus or hepatic hypertension impose permanent or transient parenchymal pressure alterations, which are hard to detect by conventional imaging methods. MRE, sensitive to mechanical constants of living tissue, may offer a way for the early and noninvasive detection of such pressure-related diseases.
Problem: Being composed of about 75% water, biological tissue is normally regarded incompressible. Classical MRE quantifies the shear elasticity of biological tissue, which is independent of the bulk modulus and compressibility of the tissue. Considering biological tissue as a bi-phasic medium composed of incompressible parenchyma permeated by fluid-filled pores provides the link between compressibility and tissue pressure in biological soft matter. Restrictions to the fluid motion due to fast dynamics (e.g. in ultrasound) or by confined boundary conditions yield a compression modulus of soft biological tissue in the range of gigapascals (about thousand times higher than the shear modulus).
Proposed solution: 3D displacement fields as usually induced in MRE consist of both shear and compression wave fields. We measure the compression wave field utilizing an adapted fast MR imaging sequence and apply the divergence operator to the field. The irrotational field displays pure compression, which is not zero in living biological tissue due to the aforementioned poroelastic properties [1,2].
In human lung, we observed significantly higher strain amplitudes in inspiration than in expiration, reflecting the influence of the intrapulmonary air volume on the mechanics of lung parenchyma . Moreover, induced volumetric strain was quantified in the human brain. Volunteers were asked to sustain contraction of the abdominal muscles during the first experiment, thus hindering venous outflow of blood from the brain and hence increasing intra-cranial fluid pressure. This state was compared to the physiologically normal, relaxed state. Strain amplitudes in the high-pressure state were clearly elevated. Similarly, the influence of cardiac pulsation on brain mechanics was analyzed by means of ECG-gated data acquisition .
Since MRI measures a combined signal of the two soft tissue compartment, we developed a method that combines inversion recovery MR imaging (IR-MRI) and a biphasic MR signal model to measure, independently from MRE, the contribution of each phase to the total signal and their volume ratio. Therefore, porosity, which is defined as the volume of the fluid phase divided by the entire volume of the medium, is quantified in each voxel leading to porosity maps of the tissue. This method was validated in tissue-mimicking phantoms and applied to in-vivo healthy brain [5, 6].
Further studies will be performed in the context of different pathologies to assess the diagnostic potential of pressure-sensitive MRE with the additional knowledge of porosity as a characteristic parameter of the poroelastic model.
Figure 1: Spatial maps of induced volumetric strain (dimensionless) in the lungs of one volunteer. Inhalation (left) shows a higher proportion of high amplitudes than expiration (right).
Figure 2: Averaged volumetric strain (vertical axis) in the relaxed state (white box) and under abdominal muscle contraction (AMC, right). The significantly increased volumetric strain amplitude for the latter state is also clearly perceivable in the spatial strain maps.
1. Hirsch, S., Sack, I. & Braun, J. Magnetic Resonance Elastography: Physical Background And Medical Applications. John Wiley & Sons 131-144 (2017). DOI:10.1002/9783527696017
2. Sack, I. & Schaeffter, T. Quantification of Biophysical Parameters in Medical Imaging. Quantification of Biophysical Parameters in Medical Imaging. Springer 71-88 (2018). DOI:10.1007/978-3-319-65924-4
3. Hirsch, S., Posnansky, O., Papazoglou, S. & Elgeti, T. Measurement of Vibration-Induced Volumetric Strain in the Human Lung. Magn. Reson. Med. 69:667–674 (2013). DOI: 10.1002/mrm.24294
4. Hirsch, S., Klatt, D., Freimann, F., Scheel, M., Braun, J. & Sack, I. In vivo measurement of volumetric strain in the human brain induced by arterial pulsation and harmonic waves. Magn. Reson. Med. 70:671–683 (2013). DOI:10.1002/mrm.24499
5. Lilaj, L., Braun, J., Fischer, T., Sack I. & Hirsch, S. Inversion-recovery MRI based biphasic analysis of porous media: simulations, phantom experiments and in vivo brain study [abstract]. ISMRM 2019, Montreal, Canada. Abstract nr 4977.
6. Lilaj, L., Braun, J., Fischer, T., Sack I. & Hirsch, S. Inversion Recovery Magnetic Resonance Poro-Elastography for Encoding Solid and Fluid Motion in Biphasic Media [abstract]. WCB 2018, Dublin, Ireland. Abstract nr 00652.