The Next Chapter in MRI: Back to the Future?
2019; Radiological Society of North America; Volume: 293; Issue: 2 Linguagem: Inglês
10.1148/radiol.2019192011
ISSN1527-1315
Autores Tópico(s)Atomic and Subatomic Physics Research
ResumoHomeRadiologyVol. 293, No. 2 PreviousNext Reviews and CommentaryFree AccessEditorialThe Next Chapter in MRI: Back to the Future?Thomas M. Grist Thomas M. Grist Author AffiliationsFrom the Department of Radiology, University of Wisconsin–Madison, School of Medicine and Public Health, 600 Highland Ave, Madison, WI 53705.Address correspondence to the author (e-mail: [email protected]).Thomas M. Grist Published Online:Oct 1 2019https://doi.org/10.1148/radiol.2019192011MoreSectionsPDF ToolsImage ViewerAdd to favoritesCiteTrack CitationsPermissionsReprints ShareShare onFacebookTwitterLinked In See also the article by Campbell-Washburn et al in this issue.Thomas M. Grist, MD, is the John H. Juhl professor of radiology and medical physics and chair of the Department of Radiology at the University of Wisconsin-Madison. He is past president of the International Society for Magnetic Resonance in Medicine and chair of the Radiological Society of North America R&E foundation.Download as PowerPointOpen in Image Viewer MRI is often cited as one of the most important medical developments in the 20th century, and its impact continues to grow in the 21st century (1).Since the earliest reports of the clinical application of MRI at low fields, a steady trajectory toward ever-higher magnetic field strengths and more advanced acquisition technologies has facilitated the development of MRI as a powerful clinical tool (2,3).Most of the developed world has benefited from the technical advances in MRI hardware and the software. Much of the focus has been on building higher strength magnets to improve the polarization of protons, therefore increasing the signal available to create better images.Advances in MRI hardware have led to increasingly more powerful magnetic field strength, now up to 7 T for clinical use. In addition to magnetic field strength, other hardware improvements have contributed greatly to improvements in MRI. Critical developments include stronger imaging gradients, better receiver coil arrays, digital receiver technology, more powerful reconstruction computers, and improved patient comfort systems. Simultaneously, improved software-based image acquisition and reconstruction methods have led to newer MRI contrast mechanisms, faster image acquisition, and improved image quality. The synergy of these hardware and software improvements creates an outcome where the sum is much greater than each of the incremental improvements. Despite the impressive developments that have been applied to MRI systems, many of the advances in image formation at 1.5 T and higher have not been used to improve image quality at lower field strengths.In this issue of Radiology, Campbell-Washburn et al (4) describe their results of applying advanced MRI acquisition hardware and software methods from state-of-the-art high-field-strength MRI (1.5-T) systems, to a lower field-strength (0.55-T) MRI system. At present, low-field-strength (and low-cost) MRI systems are developed for niche applications (eg, so-called open MRI, extremity imaging, and brain imaging). To my knowledge, the application of modern acquisition technology to lower field strength magnets has not previously been assessed. The authors make a compelling argument for further development and potential dissemination of high-performance, low-field-strength MRI systems.Campbell-Washburn et al explore opportunities where the lower field strength may have advantages. These include imaging in areas of the body with high magnetic susceptibility (chest MRI), leveraging advanced image reconstruction methods, greater effectiveness of contrast agents, and image-guided catheter procedures. The authors' effort represents a fresh look at an old field strength, and hence they are traveling "back to the future." This is not unlike the trip taken by Marty McFly in the movie Back to the Future, when he used a technology-enhanced DeLorean to travel back in time, where he made discoveries about his life (5). In this exciting preliminary work, the authors demonstrate several reasons to explore the combination of high-performance imaging hardware with a lower static (B0) magnetic field than is commonly used today. Examples of rediscovering low-field-strength MRI opportunities include improving performance in areas where the static magnetic field is distorted, leveraging recent improvements to enhance the image signal and lower noise compared with original applications at low field, and creating value through better access to diagnostic and interventional MRI at the lower magnetic field strength.First, MRI signal loss associated with variations in magnetic susceptibility creates artifacts at areas of interface between soft tissue and air or bone. Magnetic susceptibility increases with increasing field strength. Many investigators have developed corrections for these MRI susceptibility artifacts at higher fields; however, the success of correction techniques is limited by the fundamental physics associated between the interaction of the static magnetic field and the human body (6). For example, MRI in the lung at high field strength requires high-performance gradients and innovative acquisition strategies (6). However, Campbell-Washburn et al demonstrated excellent images of the lung parenchyma with conventional T2-weighted spin-echo MRI sequences. This development may have important implications for radiation-free evaluation of lung disease with MRI (eg, reduced radiation dose for evaluating lung disease in children with cystic fibrosis).Likewise, diffusion MRI requires strong gradients and uniform magnetic fields for the best possible image quality. However, in areas of nonuniform magnetic fields (eg, at the skull base and in the upper abdomen), diffusion MRI remains limited by artifacts associated with magnetic susceptibility (7). The authors show improvements in diffusion imaging at the skull base and in the upper abdomen with low-field-strength MRI, an encouraging preliminary result.MRI signal is linearly proportional to the applied B0 magnetic field strength. This limit of physics has motivated development in MRI toward higher field strengths, despite an early warning in 1978 that imaging above about 0.7 T may not be possible (8). Interestingly, the industry arrived at the standard of 1.5 T because of a somewhat random historical event when the research team at GE ramped a 2.0-T magnet to 1.5 T, undershooting the original target B0 goal by 25% (9). Unlike MRI signal, image quality is related to the signal-to-noise ratio. During the first 3 decades of MRI, higher B0 magnetic fields also created a situation where the perceived noise in the image was dominated by the noise generated from the body rather than the noise generated from the electronics in the receiver system. At lower magnetic field strengths, the noise from the body is less than that at higher fields. In recent years, there has been steady improvement in the quality of the electronics, including the receiver systems, that has resulted in less noise introduced by the MRI system itself. The authors (4) are clever to recognize that the newer technology makes it possible to reconsider the benefits of imaging at a lower field strength because the improved electronics allow the noise from the body to dominate the signal-to-noise ratio in the image.Contrast-to-noise ratio at MRI is also a consideration when assessing image quality. In Table 1 of their article, the authors (4) demonstrate that T1 relaxation times are shorter and T2 relaxation times longer at 0.55 T. The shorter T1 times make it possible to image faster, and the longer T2 times make it possible to sample the MRI echoes for a longer duration. Both of these phenomena favor MRI pulse sequences like multiecho, steady-state free precession and echo-planar image acquisition methods that are commonly used in advanced imaging applications today.The contrast-to-noise ratios of MRI examinations are often enhanced by using exogenous contrast agents, especially gadolinium-based contrast agents. As expected, the T1 and T2 relaxivity of small molecular weight contrast agents are similar at 0.55 T and 1.5 T. However, the authors (4) note substantial improvement (23%–98%) in relaxivity for larger molecular weight agents like ferumoxytol, gadofosveset, and gadolinium dendrimers, opening the possibility for marked image contrast improvement or enhanced applications like MR angiography (10).Image quality and signal strength depend not only on magnetic field strength but also on the efficiency of the MRI pulse sequence at acquiring data. The efficiency is the ratio between the time that MRI signals are acquired relative to the time in the pulse sequence used to prepare the signals to be acquired.Campbell-Washburn et al (4) used improved hardware and innovative spiral acquisition pulse sequences with high-performance hardware at the lower field strength to nearly double acquisition efficiency (70% vs 45% for steady-state free precession vs Cartesian acquisition, respectively; spiral spin echo, 32% vs 15%) This improved efficiency can recover some of the signal loss attributed to the lower field imaging, creating images that have 57% of the signal-to-noise ratio as those at three times the magnetic field strength.The authors (4) also explored the potential application of the lower field MRI system in conjunction with real-time imaging tools to visualize devices at MRI guidance. Whereas the promise of interventional MRI systems has been promoted for many years, the reality is that interventional MRI programs have been implemented at only a few academic medical centers. Two factors limiting the application include access to MRI-compatible devices and relatively poor image quality because of the system design compromises necessary to provide access to the patient during MRI-guided procedures. Whereas the patient access issue is not addressed by the closed-bore MRI system used for this study, the demonstrated ability to use off-the-shelf metal guidewire devices without introducing substantial heating (0.8°C at 0.55 T vs 7.7°C at 1.5 T) may enable MRI guidance in invasive procedures.Finally, and perhaps most importantly, the lower field strength magnet may allow the dissemination of lower cost and more comfortable MRI systems, thus increasing the value of MRI. The lower helium supply that is necessary to cool a lower field imager may conserve this limited and valuable resource. The demands on the gradient system may also lower the cost and make it possible to manufacture quieter imagers, thus enhancing patient comfort. And how much setup time is wasted by adjusting the electrocardiography leads to obtain suitable gating at high field? The magnetohydrodynamic effect that interferes with the electrocardiographic signal is substantially less at lower fields.Whereas these preliminary results are encouraging, the study is limited by the number of patients and disease states that are presented in this research effort. In addition, certain aspects of the authors' approach are not optimized for use at 0.55 T; for example, 1.5-T coils were simply retuned to 0.55 T, so some favorable results may be underestimated. Therefore, further work is necessary to refine and demonstrate the impact of this new MRI approach in larger groups of patients with disease.In summary, Campbell-Washburn et al (4) have taken a step forward in MRI by taking a step back in time to examine the potential impact of the latest imaging technology applied at a magnetic field strength that was left behind many years ago. In this way, the investigators have perhaps given us all good reason to buckle ourselves into a technology-enhanced DeLorean at lower field and travel back in time to our MRI future.Disclosures of Conflicts of Interest: T.M.G. Activities related to the present article: disclosed no relevant relationships. Activities not related to the present article: disclosed money to author's institution for grants/grants pending from GE Healthcare, Siemens, Hologic, and Bracco. Other relationships: disclosed no relevant relationships.References1. Fuchs VR, Sox HC Jr. Physicians' views of the relative importance of thirty medical innovations. Health Aff (Millwood) 2001;20(5):30–42. Crossref, Medline, Google Scholar2. Hinshaw WS, Bottomley PA, Holland GN. Radiographic thin-section image of the human wrist by nuclear magnetic resonance. Nature 1977;270(5639):722–723. Crossref, Medline, Google Scholar3. Crooks L, Arakawa M, Hoenninger J, et al. Nuclear magnetic resonance whole-body imager operating at 3.5 KGauss. Radiology 1982;143(1):169–174. Link, Google Scholar4. Campbell-Washburn AE, Ramasawmy R, Restivo MC, et al. Opportunities in interventional and diagnostic imaging by using high-performance low-field-strength MRI. Radiology 2019;293:384–393. Link, Google Scholar5. Zemeckis R, Gale B. Back to the Future. 1985. Google Scholar6. Bergin CJ, Glover GH, Pauly JM. Lung parenchyma: magnetic susceptibility in MR imaging. Radiology 1991;180(3):845–848. Link, Google Scholar7. Le Bihan D, Poupon C, Amadon A, Lethimonnier F. Artifacts and pitfalls in diffusion MRI. J Magn Reson Imaging 2006;24(3):478–488. Crossref, Medline, Google Scholar8. Bottomley PA, Andrew ER. RF magnetic field penetration, phase shift and power dissipation in biological tissue: implications for NMR imaging. Phys Med Biol 1978;23(4):630–643. Crossref, Medline, Google Scholar9. Bottomley P. Mansfield Lecture. In: Proceedings of the Twentieth Meeting of the International Society for Magnetic Resonance in Medicine. Berkeley, Calif: International Society for Magnetic Resonance in Medicine, 2012. Google Scholar10. Grist TM, Korosec FR, Peters DC, et al. Steady-state and dynamic MR angiography with MS-325: initial experience in humans. Radiology 1998;207(2):539–544. Link, Google ScholarArticle HistoryReceived: Sept 4 2019Revision requested: Sept 11 2019Revision received: Sept 16 2019Accepted: Sept 17 2019Published online: Oct 01 2019Published in print: Nov 2019 FiguresReferencesRelatedDetailsCited ByMRI of acute neck infections: evidence summary and pictorial reviewJussiHirvonen, JaakkoHeikkinen, MikkoNyman, TatuHapponen, JarnoVelhonoja, HeikkiIrjala, TeroSoukka, KimmoMattila, JanneNurminen2023 | Insights into Imaging, Vol. 14, No. 1A Robust edge detection technique for bone extraction from X-ray images based on image processing techniquesNashaat M. 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