Cardiac Magnetic Resonance Imaging at 7 Tesla
Summary
The sensitivity gain inherent to ultrahigh field magnetic resonance holds promise for high spatial resolution imaging of the heart. Here, we describe a protocol customized for functional cardiovascular magnetic resonance (CMR) at 7 Tesla using an advanced multi-channel radio-frequency coil, magnetic field shimming and a triggering concept.
Abstract
CMR at an ultra-high field (magnetic field strength B0 ≥ 7 Tesla) benefits from the signal-to-noise ratio (SNR) advantage inherent at higher magnetic field strengths and potentially provides improved signal contrast and spatial resolution. While promising results have been achieved, ultra-high field CMR is challenging due to energy deposition constraints and physical phenomena such as transmission field non-uniformities and magnetic field inhomogeneities. In addition, the magneto-hydrodynamic effect renders the synchronization of the data acquisition with the cardiac motion difficult. The challenges are currently addressed by explorations into novel magnetic resonance technology. If all impediments can be overcome, ultra-high field CMR may generate new opportunities for functional CMR, myocardial tissue characterization, microstructure imaging or metabolic imaging. Recognizing this potential, we show that multi-channel radio frequency (RF) coil technology tailored for CMR at 7 Tesla together with higher order B0 shimming and a backup signal for cardiac triggering facilitates high fidelity functional CMR. With the proposed setup, cardiac chamber quantification can be accomplished in examination times similar to those achieved at lower field strengths. To share this experience and to support the dissemination of this expertise, this work describes our setup and protocol tailored for functional CMR at 7 Tesla.
Introduction
Cardiovascular magnetic resonance (CMR) is of proven clinical value with a growing range of clinical indications1,2. In particular, the evaluation of cardiac morphology and function is of major relevance and typically realized by tracking and visualizing the heart motion throughout the entire cardiac cycle using segmented breath-held two-dimensional (2D) cinematograpic (CINE) imaging techniques. While a high spatio-temporal resolution, high blood-myocardium contrast and high signal-to-noise ratio (SNR) are required, the data acquisition is highly constrained by the cardiac and respiratory motion and the use of multiple breath-holds as well as the need for whole heart or left-ventricular coverage often leads to extensive scan times. Parallel imaging, simultaneous multi-slice imaging or other acceleration techniques help to address the motion related constraints3,4,5,6.
Moreover, to benefit from the inherent SNR gain at higher magnetic fields, high field systems with B0 = 3 Tesla are increasingly employed in clinical routine7,8. The development has also encouraged investigations into ultra-high field (B0≥7 Tesla, f≥298 MHz) CMR9,10,11,12,13,14. The gain in SNR and blood-myocardium contrast inherent to the higher field strength holds the promise to be transferrable into enhanced functional CMR using a spatial resolution that exceeds today's limits15,16,17. In turn, new possibilities for magnetic resonance (MR) based myocardial tissue characterization, metabolic imaging and microstructure imaging are expected13. So far, several groups have demonstrated the feasibility of CMR at 7 Tesla and specifically tailored ultra-high field technology has been introduced17,18,19,20,21,22. Regarding these promising developments, the potential of ultra-high field CMR can be considered to be yet untapped13. At the same time, physical phenomena and practical obstacles such as magnetic field inhomogeneities, radio frequency (RF) excitation field non-uniformities, off-resonance artifacts, dielectric effects, localized tissue heating and field strength independent RF power deposition constraints make imaging at ultra-high field challenging10,17. The latter are employed to control RF induced tissue heating and to ensure safe operation. Moreover, electrocardiogram (ECG) based triggering can be significantly impacted by the magneto-hydrodynamic (MHD) effect19,23,24. To address the challenges induced by the short wavelength in tissue, many-element transceiver RF coil arrays tailored for CMR at 7 Tesla were proposed21,25,26,27. Parallel RF transmission provides means for transmission field shaping, also known as B1+ shimming, which allows to reduce the magnetic field inhomogeneities and susceptibility artefacts18,28. While at the current stage, some of these measures might increase the experimental complexity, the concepts have proven helpful and may be translated to the clinical field strengths of CMR 1.5 T or 3 T.
Currently, 2D balanced steady state free precession (bSSFP) CINE imaging is the standard of reference for clinical functional CMR at 1.5 T and 3 T1. Recently, the sequence was successfully employed at 7 Tesla, but a large number of challenges remain19. Patient specific B1+ shimming and extra RF coil adjustments were applied to manage RF power deposition constraints and careful B0 shimming was performed to control the sequence typical banding artifacts. With an average scan time of 93 minutes for left-ventricular (LV) function assessment, the efforts prolonged the examination times beyond clinically acceptable limits. Here, spoiled gradient echo sequences provide a viable alternative. At 7 Tesla, total examination times of (29 ± 5) min for LV function assessment were reported, which corresponds well to clinical imaging protocols at lower field strengths21. Thereby, spoiled gradient echo based CMR benefits from the prolonged T1 relaxation times at ultra-high field that result in an enhanced blood-myocardium contrast superior to gradient echo imaging at 1.5 T. This renders subtle anatomic structures such as the pericardium, the mitral and tricuspid valves as well as the papillary muscles well identifiable. Congruously, spoiled gradient echo based cardiac chamber quantification at 7 Tesla agrees closely with LV parameters derived from 2D bSSFP CINE imaging at 1.5 T20. Apart from that, accurate right-ventricular (RV) chamber quantification was recently demonstrated feasible using a high resolution spoiled gradient echo sequence at 7 Tesla29.
Recognizing the challenges and opportunities of CMR at ultra-high field, this work presents a setup and protocol customized for functional CMR acquisitions on an investigational 7 Tesla research scanner. The protocol outlines the technical underpinnings, shows how impediments can be overcome, and provides practical considerations that help to keep the extra experimental overhead at a minimum. The proposed imaging protocol constitutes a four-fold improvement in the spatial resolution versus today's clinical practice. It is meant to provide a guideline for clinical adaptors, physician scientists, translational researchers, application experts, MR radiographers, technologists and new entrants into the field.
Protocol
Representative Results
Discussion
Functional CMR examinations could be conducted successfully at 7 Tesla. Based on the field strength driven SNR gain, CINE images of the human heart could be acquired with significantly higher spatial resolution compared to 1.5 or 3 T. While a slice thickness of 6 to 8 mm and in-plane voxel edge lengths of 1.2 to 2.0 mm are commonly used at lower clinical field strengths1,30, the measurements at 7 Tesla could be conducted with a slice thickness of 4 mm and an isot…
Disclosures
The authors have nothing to disclose.
Acknowledgements
The authors acknowledge the facilities, and the scientific and technical assistance of the National Imaging Facility at the Centre for Advanced Imaging, University of Queensland. We would also like to thank Graham Galloway and Ian Brereton for their help to obtain a CAESIE grant for Thoralf Niendorf.
Materials
| 7 Tesla MRI system | Siemens | Investigational Device | |
| 32-Channel -1H-Cardiac Coil | MRI.Tools GmbH | Transmit/Receive RF Coil for MR Imaging and Spectroscopy at 7.0 Tesla | |
| ECG Trigger Device | Siemens | ||
| Pulse Trigger Device | Siemens |
References
- Kramer, C. M., et al. Standardized cardiovascular magnetic resonance (CMR) protocols 2013 update. Journal of Cardiovascular Magnetic Resonance. 15 (1), 1 (2013).
- Earls, J. P., Ho, V. B., Foo, T. K., Castillo, E., Flamm, S. D. Cardiac MRI: Recent progress and continued challenges. Journal of Magnetic Resonance Imaging. 16 (2), 111-127 (2002).
- Wintersperger, B. J., et al. Cardiac CINE MR imaging with a 32-channel cardiac coil and parallel imaging: Impact of acceleration factors on image quality and volumetric accuracy. Journal of Magnetic Resonance Imaging. 23 (2), 222-227 (2006).
- Schmitt, M., et al. A 128-channel receive-only cardiac coil for highly accelerated cardiac MRI at 3 Tesla. Magnetic Resonance in Medicine. 59 (6), 1431-1439 (2008).
- Wech, T., et al. High-resolution functional cardiac MR imaging using density-weighted real-time acquisition and a combination of compressed sensing and parallel imaging for image reconstruction. RöFo: Fortschritte Auf Dem Gebiete Der Röntgenstrahlen Und Der Nuklearmedizin. 182 (8), 676-681 (2010).
- Stäb, D., et al. CAIPIRINHA accelerated SSFP imaging. Magnetic Resonance in Medicine. 65 (1), 157-164 (2011).
- Gutberlet, M., et al. Influence of high magnetic field strengths and parallel acquisition strategies on image quality in cardiac 2D CINE magnetic resonance imaging: comparison of 1.5 T vs. 3.0 T. European Radiology. 15 (8), 1586-1597 (2005).
- Gutberlet, M., et al. Comprehensive cardiac magnetic resonance imaging at 3.0 Tesla: feasibility and implications for clinical applications. Investigative radiology. 41 (2), 154-167 (2006).
- Kraff, O., Fischer, A., Nagel, A. M., Mönninghoff, C., Ladd, M. E. MRI at 7 tesla and above: Demonstrated and potential capabilities: Capabilities of MRI at 7T and Above. Journal of Magnetic Resonance Imaging. 41 (1), 13-33 (2015).
- Moser, E., Stahlberg, F., Ladd, M. E., Trattnig, S. 7-T MR-from research to clinical applications?. NMR in Biomedicine. 25 (5), 695-716 (2012).
- Hecht, E. M., Lee, R. F., Taouli, B., Sodickson, D. K. Perspectives on Body MR Imaging at Ultrahigh Field. Magnetic Resonance Imaging Clinics of North America. 15 (3), 449-465 (2007).
- Niendorf, T., et al. W(h)ither human cardiac and body magnetic resonance at ultrahigh fields? technical advances, practical considerations, applications, and clinical opportunities: Advances in ultrahigh field Cardiac and Body Magnetic Resonance. NMR in Biomedicine. 29 (9), 1173-1179 (2016).
- Niendorf, T., Sodickson, D. K., Krombach, G. A., Schulz-Menger, J. Toward cardiovascular MRI at 7 T: clinical needs, technical solutions and research promises. European Radiology. 20 (12), 2806-2816 (2010).
- Niendorf, T., et al. Progress and promises of human cardiac magnetic resonance at ultrahigh fields: A physics perspective. Journal of Magnetic Resonance. 229, 208-222 (2013).
- Hinton, D. P., Wald, L. L., Pitts, J., Schmitt, F. Comparison of Cardiac MRI on 1.5 and 3.0 Tesla Clinical Whole Body Systems. Investigative Radiology. 38 (7), 436-442 (2003).
- Ohliger, M. A., Grant, A. K., Sodickson, D. K. Ultimate intrinsic signal-to-noise ratio for parallel MRI: Electromagnetic field considerations. Magnetic resonance in medicine. 50 (5), 1018-1030 (2003).
- Vaughan, J. T., et al. Whole-body imaging at 7T: Preliminary results. Magnetic Resonance in Medicine. 61 (1), 244-248 (2009).
- Hezel, F., Thalhammer, C., Waiczies, S., Schulz-Menger, J., Niendorf, T. High Spatial Resolution and Temporally Resolved T2* Mapping of Normal Human Myocardium at 7.0 Tesla: An Ultrahigh Field Magnetic Resonance Feasibility Study. PLOS ONE. 7 (12), e52324 (2012).
- Suttie, J. J., et al. 7 Tesla (T) human cardiovascular magnetic resonance imaging using FLASH and SSFP to assess cardiac function: validation against 1.5 T and 3 T. NMR in biomedicine. 25 (1), 27-34 (2012).
- von Knobelsdorff-Brenkenhoff, F., et al. Cardiac chamber quantification using magnetic resonance imaging at 7 Tesla-a pilot study. European Radiology. 20 (12), 2844-2852 (2010).
- Winter, L., et al. Comparison of three multichannel transmit/receive radiofrequency coil configurations for anatomic and functional cardiac MRI at 7.0T: implications for clinical imaging. European Radiology. 22 (10), 2211-2220 (2012).
- Schmitter, S., et al. Cardiac imaging at 7 tesla: Single- and two-spoke radiofrequency pulse design with 16-channel parallel excitation: Cardiac Imaging at 7T. Magnetic Resonance in Medicine. 70 (5), 1210-1219 (2013).
- Krug, J., Rose, G., Stucht, D., Clifford, G., Oster, J. Limitations of VCG based gating methods in ultra high field cardiac MRI. Journal of Cardiovascular Magnetic Resonance. 15 (Suppl 1), W19 (2013).
- Stäb, D., Roessler, J., O’Brien, K., Hamilton-Craig, C., Barth, M. ECG Triggering in Ultra-High Field Cardiovascular MRI. Tomography. 2 (3), 167-174 (2016).
- Gräßl, A., et al. Design, evaluation and application of an eight channel transmit/receive coil array for cardiac MRI at 7.0T. European Journal of Radiology. 82 (5), 752-759 (2013).
- Graessl, A., et al. Modular 32-channel transceiver coil array for cardiac MRI at 7.0T. Magnetic Resonance in Medicine. 72 (1), 276-290 (2014).
- Snyder, C. J., et al. Initial results of cardiac imaging at 7 tesla. Magnetic Resonance in Medicine. 61 (3), 517-524 (2009).
- Meloni, A., et al. Detailing magnetic field strength dependence and segmental artifact distribution of myocardial effective transverse relaxation rate at 1.5, 3.0, and 7.0 T: Magnetic Field Dependence of Myocardial R 2 *. Magnetic Resonance in Medicine. 71 (6), 2224-2230 (2014).
- von Knobelsdorff-Brenkenhoff, F., et al. Assessment of the right ventricle with cardiovascular magnetic resonance at 7 Tesla. Journal of Cardiovascular Magnetic Resonance. 15, 23 (2013).
- Petersen, S. E., et al. Reference ranges for cardiac structure and function using cardiovascular magnetic resonance (CMR) in Caucasians from the UK Biobank population cohort. Journal of Cardiovascular Magnetic Resonance. 19 (1), (2017).
- Frauenrath, T., et al. Feasibility of cardiac gating free of interference with electro-magnetic fields at 1.5 Tesla, 3.0 Tesla and 7.0 Tesla using an MR-stethoscope. Investigative radiology. 44 (9), 539-547 (2009).
- Frauenrath, T., et al. Acoustic cardiac triggering: a practical solution for synchronization and gating of cardiovascular magnetic resonance at 7 Tesla. Journal of Cardiovascular Magnetic Resonance. 12 (1), 67 (2010).
- Schroeder, L., et al. A Novel Method for Contact-Free Cardiac Synchronization Using the Pilot Tone Navigator. Proceedings of the International Society for Magnetic Resonance in Medicine. 24, 3103 (2016).
