High Field Imaging
Harald H. Quick1

1High-Field and Hybrid MR Imaging, Erwin L. Hahn Institute for MRI, University Essen-Duisburg, Essen, Germany

Synopsis

With more than 80 installed MRI systems worldwide operating at a magnetic field strength of 7 Tesla or higher, ultra-high field (UHF) MRI has been established as a platform for clinically oriented research in recent years. Profound technical and methodological developments have helped to overcome the inherent physical challenges of UHF radiofrequency (RF) signal homogenization in the human body. The ongoing development of dedicated transmit/receive RF coil arrays was pivotal in realizing UHF body MRI, beyond mere brain imaging applications. Against this backdrop, UHF MRI recently has demonstrated capabilities and potentials for clinical diagnostics in a variety of studies.

Target Audience

Clinicians and physicists interested in ultra-high field (UHF) MRI.

Objectives

To understand the motivation, physical challenges and basic technical solutions to perform UHF MRI. To review recent clinical applications of UHF MRI in neurovascular, abdominal and pelvic MR imaging.

High Field Imaging

With more than 80 installed magnetic resonance imaging (MRI) systems worldwide operating at a magnetic field strength of 7 Tesla (7 T) or higher, ultra-high field (UHF) MRI has been established as a platform for clinically oriented research in recent years [1]. The driving force behind all hardware and methods developments for UHF MRI is the exploitation of the inherently higher signal-to-noise ratio (SNR), the pivotal currency in MR. In a recent study on SNR and MR tissue parameters in human brain imaging at 3, 7, and 9.4 T experimental evidence of a supralinear increase of SNR with field strength (SNR ∼ B01.65) was shown [2]. Profound technical and methodological developments have helped to overcome the inherent physical challenges of UHF radiofrequency (RF) signal homogenization in the human body. The general advantages and challenges for imaging at ultra-high field strength have been expertly described in many previous reviews [1, 3-9]. The ongoing development of dedicated transmit/receive RF coil arrays was pivotal in realizing UHF body MRI, beyond mere brain imaging applications [10]. Against this backdrop, UHF MRI recently has demonstrated capabilities and potentials for clinical diagnostics in a variety of studies. Owing to numerous methodological and hardware developments, the UHF MRI community has witnessed the expansion of the application range from brain [11] and musculoskeletal imaging [12] to first body MR imaging applications [13]. For neuro applications, the increase in SNR and the enhanced sensitivity to magnetic susceptibility have allowed a more detailed depiction of microvascularity, and high-resolution imaging of cerebral vessel pathologies, which offer potential benefits for patient treatment in terms of diagnostics, surgical planning, and therapy monitoring (Fig. 1, 2) [14, 15]. For UHF body MRI applications, recent methodological developments regarding multi-channel body RF coils, RF shimming and parallel transmit strategies, and SAR supervision are just now enabling the exploitation of the potential of UHF MR imaging. Despite these significant physical and methodological challenges, impressive body UHF MRI applications have been demonstrated for abdominal and pelvic imaging (Fig. 3, 4) [13, 16, 17, 18]. High-resolution, high contrast cancer imaging of the breast, the prostate, the cervix and rectum at 7 T magnetic field strength may provide a diagnostic advantage in oncologic imaging over lower field strength MRI [18].

Acknowledgements

No acknowledgement found.

References

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2. Pohmann R, Speck O, Scheffler K. Signal-to-noise ratio and MR tissue parameters in human brain imaging at 3, 7, and 9.4 Tesla using current receive coil arrays. Magn Reson Med 2016;75:801–809.

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9. Ladd ME, Bachert P, Meyerspeer M, et al. Pros and cons of ultra-high-field MRI/MRS for human application. Prog Nucl Magn Reson Spectrosc. 2018 Dec;109:1-50.

10. Rietsch SHG, Orzada S, Maderwald S, Brunheim S, Philips BWJ, Scheenen TWJ, Ladd ME, Quick HH. 7T ultra-high field body MR imaging with an 8-channel transmit/32-channel receive radiofrequency coil array. Med Phys. 2018 Jul;45(7):2978-2990.

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13. Wrede KH, Matsushige T, Goericke SL, et al. Non-enhanced magnetic resonance imaging of unruptured intracranial aneurysms at 7 Tesla: Comparison with digital subtraction angiography. Eur Radiol 2017;27: 354–364.

14. Wrede KH, Dammann P, Johst S, et al. Non-enhanced MR imaging of cerebral arteriovenous malformations at 7 Tesla. Eur Radiol 2016;26: 829–839.

15. Hahnemann ML, Kraff O, Orzada S, et al. T1-weighted contrast enhanced magnetic resonance imaging of the small bowel: comparison between 1.5 and 7 T. Invest Radiol 2015;50:539–547.

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18. Philips BWJ, Fortuin AS, Orzada S, Scheenen TWJ, Maas MC. High resolution MR imaging of pelvic lymph nodes at 7 Tesla. Magn Reson Med. 2017 Sep;78(3):1020-1028.

Figures

Fig. 1: Basilar tip aneurysm in a 47-year-old male patient (arrow indicating the aneurysm dome). A: Coronal non-enhanced MPRAGE sequence visualizes aneurysm architecture and adjacent brain tissue. B: High-resolution (0.02 x 0.02 x 0.04 mm3) transverse TOF sequence depicting the parent vessel, aneurysm neck, and aneurysm dome. C: Volume rendering of high-resolution TOF sequence delineating the angio-architecture of the circle of Willis and the aneurysm morphology; missing left posterior communicating artery indicated by asterisk. (Courtesy of K.H. Wrede and B. Chen, Erwin L. Hahn Institute for MRI, University of Duisburg-Essen, Essen, Germany.)

Fig. 2: Left occipital arteriovenous malformation (AVM) in a 49-years-old female patient (asterisk next to the AVM nidus). A: PD-weighted TSE sequence. B: Non-enhanced MPRAGE sequence. C: Volume rendering of high-resolution TOF sequence. (Courtesy of K.H. Wrede and B. Chen, Erwin L. Hahn Institute for MRI, University of Duisburg-Essen, Essen, Germany.)

Fig. 3: Side-by-side comparison between 1.5 vs. 7T MRI of the small bowel. Upper row (A,B) shows contrast-enhanced T1-weighted coronal 3D FLASH sequence 20 seconds after contrast application, revealing comparable tissue contrast and image detail of the small bowel. Lower row (C,D) shows coronal TrueFISP sequence, revealing equivalent tissue contrast of the small bowel, whereas image details of the jejunum and ileum are better visible at 7T due to improved spatial resolution. Note better definable plicae circulares of the jejunum (encircled area) and ileum (arrow in D) at 7T. (Courtesy of M. Hahnemann, Erwin L. Hahn Institute for MRI, University of Duisburg-Essen, Germany.)

Fig. 4: High-resolution imaging of rectal cancer. 67-year-old patient with rectal cancer with perirectal stranding as reflection of a perirectal infiltration and a positive lymph node. In this 1.5 vs. 7T side-by-side comparison, a contrast enhanced FLASH sequence was used at both field strengths. Spatial resolution in (A, 1.5T) was 0.8 x 0.8 x 5.0 mm3, while spatial resolution in (B, 7T) was significantly higher with 0.6 x 0.6 x 2.0 mm3, allowing for more detailed depiction of the pelvic anatomy as well as improved soft-tissue contrast. (Courtesy of K. Beiderwellen and L. Umutlu, Erwin L. Hahn Institute for MRI, University of Duisburg-Essen, Germany.)

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)