Real-time MRI

Real-time magnetic resonance imaging (MRI) refers to the continuous monitoring ("filming") of moving objects in real time. Because MRI is based on time-consuming scanning of k-space, real-time MRI was possible only with low image quality or low temporal resolution. Using an iterative reconstruction algorithm these limitations have recently been removed: a new method for real-time MRI achieves a temporal resolution of 20 to 30 milliseconds for images with an in-plane resolution of 1.5 to 2.0 mm.[2] Real-time MRI promises to add important information about diseases of the joints and the heart. In many cases MRI examinations may become easier and more comfortable for patients.

Physical basis

While early applications were based on echo planar imaging, which found an important application in real-time functional MRI (rt-fMRI),[3] recent progress is based on iterative reconstruction and FLASH MRI.[4][5] The real-time imaging method proposed by Uecker and colleagues[2] combines radial FLASH MRI,[6] which offers rapid and continuous data acquisition, motion robustness, and tolerance to undersampling, with an iterative image reconstruction method based on the formulation of image reconstruction as a nonlinear inverse problem.[7][8] By integrating the data from multiple receive coils (i.e. parallel MRI) and exploiting the redundancy in the time series of images with the use of regularization and filtering, this approach enhances the possible degree of data undersampling by one order of magnitude, so that high-quality images may be obtained out of as little as 5 to 10% of the data required for a normal image reconstruction.

Because of the very short echo times (e.g., 1 to 2 milliseconds), the method does not suffer from off-resonance effects, so that the images neither exhibit susceptibility artifacts nor rely on fat suppression. While spoiled FLASH sequences offer spin density or T1 contrast, versions with refocused or fully balanced gradients provide access to T1/T2 contrast. The choice of the gradient-echo time (e.g., in-phase vs opposed-phase conditions) further alters the representation of water and fat signals in the images and will allow for separate water/fat movies.

Applications

Although applications of real-time MRI cover a broad spectrum ranging from non-medical studies of turbulent flow[9] to the noninvasive monitoring of interventional (surgical) procedures, the most important application making use of the new capabilities is cardiovascular imaging.[1] With the new method it is possible to obtain movies of the beating heart in real time with up to 50 frames per second during free breathing and without the need for a synchronization to the electrocardiogram.[10]

Apart from cardiac MRI other real-time applications deal with functional studies of joint kinetics (e.g., temporomandibular joint,[11] knee and the wrist[12]) or address the coordinated dynamics of the articulators such as lips, tongue, soft palate and vocal folds during speaking (articulatory phonetics)[13] or swallowing.[14] Applications in interventional MRI, which refers to the monitoring of minimally invasive surgical procedures, are possible by interactively changing parameters such as image position and orientation.

References
  1. S Zhang, M Uecker, D Voit, KD Merboldt, J Frahm (2010a) Real-time cardiovascular magnetic resonance at high temporal resolution: radial FLASH with nonlinear inverse reconstruction. J Cardiovasc Magn Reson 12, 39, doi:10.1186/1532-429X-12-39
  2. M Uecker, S Zhang, D Voit, A Karaus, KD Merboldt, J Frahm (2010a) Real-time MRI at a resolution of 20 ms. NMR Biomed 23: 986-994, doi:10.1002/nbm.1585
  3. Cohen MS (2001). "Real-time functional magnetic resonance imaging". Methods. 25 (2): 201–220. doi:10.1006/meth.2001.1235. PMID 11812206.
  4. J Frahm, A Haase, W Hänicke, KD Merboldt, D Matthaei (1985) Hochfrequenz-Impuls und Gradienten-Impuls-Verfahren zur Aufnahme von schnellen NMR-Tomogrammen unter Benutzung von Gradientenechos. German Patent Application P 35 04 734.8, February 12, 1985
  5. J Frahm, A Haase, D Matthaei (1986) Rapid NMR imaging of dynamic processes using the FLASH technique. Magn Reson Med 3:321-327 doi:10.1002/mrm.1910030217
  6. S Zhang, KT Block KT, J Frahm (2010b) Magnetic resonance imaging in real time: Advances using radial FLASH. J Magn Reson Imag 31: 101-109, doi:10.1002/jmri.21987
  7. M Uecker, T Hohage, KT Block, J Frahm (2008) Image reconstruction by regularized nonlinear inversion – Joint estimation of coil sensitivities and image content. Magn Reson Med 60: 674-682, doi:10.1002/mrm.21691
  8. M Uecker, S Zhang, J Frahm (2010b) Nonlinear inverse reconstruction for real-time MRI of the human heart using undersampled radial FLASH. Magn Reson Med 63: 1456-1462, doi:10.1002/mrm.22453
  9. V Wedeen; A Crawley; R Weisskoff; G Holmvang & MS Cohen (1990). "Real time MR imaging of structured fluid flow". Society for Magnetic Resonance in Medicine: 164.
  10. I Uyanik, P Lindner, D Shah, N Tsekos I Pavlidis (2013) Applying a Level Set Method for Resolving Physiologic Motions in Free-Breathing and Non-gated Cardiac MRI. FIMH, 2013, "Computational Physiology Lab" (PDF). Retrieved 2013-10-01.
  11. S Zhang, N Gersdorff, J Frahm (2011) Real-Time Magnetic Resonance Imaging of Temporomandibular Joint Dynamics. The Open Medical Imaging Journal, 2011, 5, 1-7, "Archived copy" (PDF). Archived from the original (PDF) on 2012-09-26. Retrieved 2011-09-16.CS1 maint: archived copy as title (link)
  12. Boutin RD, Buonocore MH, Immerman I, Ashwell Z, Sonico GJ, Szabo RM and Chaudhari AJ (2013) Real-Time Magnetic Resonance Imaging (MRI) during Active Wrist Motion—Initial Observations. PLoS ONE 8(12): e84004. doi:10.1371/journal.pone.0084004
  13. Niebergall A, Zhang S, Kunay E, Keydana G, Job M, et al. Real-time MRI of Speaking at a Resolution of 33 ms: Undersampled Radial FLASH with Nonlinear Inverse Reconstruction. Magn Reson Med 2010, doi:10.1002/mrm.24276.
  14. Zhang S, Olthoff A and Frahm J. Real-time magnetic resonance imaging of normal swallowing. J Magn Reson Imaging 2011;35:1372-1379. doi:10.1002/jmri.23591.
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