MR ElastographyHow does MR Elastography (MRE) work? What is it used for?
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MR Elastography (MRE) allows estimation of the mechanical properties of tissue in response to compression or vibration. Available as a commercial product from at least three MR vendors, its current principal application is for staging hepatic fibrosis. MRE also shows promise for characterizing lesions of the brain, breast, and heart.
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A typical liver MRE exam begins by positioning a plastic diaphragm ("passive driver") over the liver held in place by an elastic band. This is connected by a long hose to a pneumatic pump ("active driver") outside the MRI room that inflates and deflates the diaphragm about 60 times per second. These mechanical impulses induce "seismic" shear waves in the liver that can be detected with phase-sensitive MR sequences. Image acquisition is very fast, a single 2D slice obtained within a 20-25 sec breath hold.
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The Resoundant system for MRE developed by Ehman and colleagues at the Mayo Clinic (Figure adapted from promotional news/marketing materials distributed by resoundant.com)
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MRE wave and elastogram images compared in a normal subject (upper row) and a patient with hepatic fibrosis (lower row). Figure adapted from Li et al (2015) under CC BY.
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As tissues become stiffer, waves propagate more rapidly and have longer wavelengths. This can be appreciated by observing the spacing between peak displacements in MRE wave images as shown in the figure. Note the wider wave spacing in the fibrotic liver (lower left) compared to normal liver (upper left). MRE also allows estimation of tissue shear stiffness, typically reported in kiloPascals (kPa). The kPa is an SI unit for pressure and equals 1000 newtons per square meter. Shear stiffness calculations are displayed as color overlays on anatomic images known as an elastogram.
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The pulse sequence used for liver MRE is typically a 2D GRE with short TR, short TE, and low flip angle. Bipolar motion-sensitizing gradients are applied similar to those used for phase-contrast MR angiography. As described in a prior Q&A, moving tissues accumulate a phase-shift when bipolar gradients are applied, but stationary tissues do not. MR image acquisition is synchronized with the mechanical compressions, with both magnitude and phase data reconstructed using 4−8 different time offsets.
References
Babu AS, Wells ML, Teytelboym OM, et al. Elastography in chronic liver disease: modalities, techniques, limitations, and future directions. RadioGraphics 2016; 36:1987-2006. [DOI Link] (Good review of MRE and US elastography in the liver).
Dong H, White RD, Kolipaka A. Advances and future direction of magnetic resonance elastography. Top Magn Reson Imaging 2018; 27:363-384. [DOI Link] (excellent recent review with potential new MRE techniques and applications outside the liver)
Glaser KJ, Manduca A, Ehman RL. Review of MR elastography applications and recent developments. J Magn Reson Imaging 2012; 36:757-774. [DOI Link] (Good review by RL Ehman's group at Mayo, who are responsible for many current advances in MRE)
Guglielmo FF, Venkatesh, Mitchell DG. Liver MR elastography technique and image interpretation: pearls and pitfalls. RadioGraphics 2019; 39:1983-2002. [DOI Link] (Good recent review including how to evaluate image quality)
Li Z, Sun J, Yang X. Recent advances in molecular magnetic resonance imaging of liver fibrosis. Biomed Res Internat 2015; ID 5954767:1-12. [DOI Link]
Low G, Kruse SA, Lomas DJ. General review of magnetic resonance elastography. World J Radiol 2016; 8:59-72. [DOI Link] (describes MRE applications outside the liver).
Sarvazyan A, Hall TJ, Urban MW, et al. An overview of elastography — an emerging branch of medical imaging. Curr Med Imaging Rev 2011; 7:255-282. [DOI Link] (Good review of physics of elastography, including both MRE and US applications)
Venkatesh SK, Yin M, Ehman RL. Magnetic resonance elastography of liver: technique, analysis, and clinical applications. J Magn Reson Imaging 2013; 37:544-555. [DOI Link] (Another review by the Ehman group)
Babu AS, Wells ML, Teytelboym OM, et al. Elastography in chronic liver disease: modalities, techniques, limitations, and future directions. RadioGraphics 2016; 36:1987-2006. [DOI Link] (Good review of MRE and US elastography in the liver).
Dong H, White RD, Kolipaka A. Advances and future direction of magnetic resonance elastography. Top Magn Reson Imaging 2018; 27:363-384. [DOI Link] (excellent recent review with potential new MRE techniques and applications outside the liver)
Glaser KJ, Manduca A, Ehman RL. Review of MR elastography applications and recent developments. J Magn Reson Imaging 2012; 36:757-774. [DOI Link] (Good review by RL Ehman's group at Mayo, who are responsible for many current advances in MRE)
Guglielmo FF, Venkatesh, Mitchell DG. Liver MR elastography technique and image interpretation: pearls and pitfalls. RadioGraphics 2019; 39:1983-2002. [DOI Link] (Good recent review including how to evaluate image quality)
Li Z, Sun J, Yang X. Recent advances in molecular magnetic resonance imaging of liver fibrosis. Biomed Res Internat 2015; ID 5954767:1-12. [DOI Link]
Low G, Kruse SA, Lomas DJ. General review of magnetic resonance elastography. World J Radiol 2016; 8:59-72. [DOI Link] (describes MRE applications outside the liver).
Sarvazyan A, Hall TJ, Urban MW, et al. An overview of elastography — an emerging branch of medical imaging. Curr Med Imaging Rev 2011; 7:255-282. [DOI Link] (Good review of physics of elastography, including both MRE and US applications)
Venkatesh SK, Yin M, Ehman RL. Magnetic resonance elastography of liver: technique, analysis, and clinical applications. J Magn Reson Imaging 2013; 37:544-555. [DOI Link] (Another review by the Ehman group)
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