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Jacobs PS, Wilson N, Brink W, Swain A, Armbruster R, Hanumapur A, Tisdall MD, Detre J, Nanga RPR, Elliott MA, Reddy R. In vivo B 1 + enhancement of calf MRI at 7 T via optimized flexible metasurfaces. Magn Reson Med 2024; 92:1277-1289. [PMID: 38469893 PMCID: PMC11209820 DOI: 10.1002/mrm.30060] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/16/2023] [Revised: 01/18/2024] [Accepted: 02/05/2024] [Indexed: 03/13/2024]
Abstract
PURPOSE Ultrahigh field (≥7 T) MRI is at the cutting edge of medical imaging, enabling enhanced spatial and spectral resolution as well as enhanced susceptibility contrast. However, transmit (B 1 + $$ {\mathrm{B}}_1^{+} $$ ) field inhomogeneity due to standing wave effects caused by the shortened RF wavelengths at 7 T is still a challenge to overcome. Novel hardware methods such as dielectric pads have been shown to improve theB 1 + $$ {\mathrm{B}}_1^{+} $$ field inhomogeneity but are currently limited in their corrective effect by the range of high-permittivity materials available and have a fixed shelf life. In this work, an optimized metasurface design is presented that demonstrates in vivo enhancement of theB 1 + $$ {\mathrm{B}}_1^{+} $$ field. METHODS A prototype metasurface was optimized by an empirical capacitor sweep and by varying the period size. Phantom temperature experiments were performed to evaluate potential metasurface heating effects during scanning. Lastly, in vivo gradient echo images andB 1 + $$ {\mathrm{B}}_1^{+} $$ maps were acquired on five healthy subjects on a 7 T system. Dielectric pads were also used as a comparison throughout the work as a standard comparison. RESULTS The metasurfaces presented here enhanced the average relative SNR of the gradient echo images by a factor of 2.26 compared to the dielectric pads factor of 1.61. AverageB 1 + $$ {\mathrm{B}}_1^{+} $$ values reflected a similar enhancement of 27.6% with the metasurfaces present versus 8.9% with the dielectric pads. CONCLUSION The results demonstrate that metasurfaces provide superior performance to dielectric padding as shown byB 1 + $$ {\mathrm{B}}_1^{+} $$ maps reflecting their direct effects and resulting enhancements in image SNR at 7 T.
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Affiliation(s)
- Paul S Jacobs
- Center for Advanced Metabolic Imaging in Precision Medicine, Department of Radiology, University of Pennsylvania, Philadelphia, PA, United States
| | - Neil Wilson
- Center for Advanced Metabolic Imaging in Precision Medicine, Department of Radiology, University of Pennsylvania, Philadelphia, PA, United States
| | - Wyger Brink
- Magnetic Detection and Imaging group, TechMed Centre, University of Twente, Enschede, The Netherlands
| | - Anshuman Swain
- Center for Advanced Metabolic Imaging in Precision Medicine, Department of Radiology, University of Pennsylvania, Philadelphia, PA, United States
| | - Ryan Armbruster
- Center for Advanced Metabolic Imaging in Precision Medicine, Department of Radiology, University of Pennsylvania, Philadelphia, PA, United States
| | - Aniketh Hanumapur
- Center for Advanced Metabolic Imaging in Precision Medicine, Department of Radiology, University of Pennsylvania, Philadelphia, PA, United States
| | - M. Dylan Tisdall
- Department of Radiology, University of Pennsylvania, Philadelphia, PA, United States
| | - John Detre
- Center for Advanced Metabolic Imaging in Precision Medicine, Department of Radiology, University of Pennsylvania, Philadelphia, PA, United States
- Department of Neurology, University of Pennsylvania, Philadelphia, PA, United States
| | - Ravi Prakash Reddy Nanga
- Center for Advanced Metabolic Imaging in Precision Medicine, Department of Radiology, University of Pennsylvania, Philadelphia, PA, United States
| | - Mark A. Elliott
- Center for Advanced Metabolic Imaging in Precision Medicine, Department of Radiology, University of Pennsylvania, Philadelphia, PA, United States
| | - Ravinder Reddy
- Center for Advanced Metabolic Imaging in Precision Medicine, Department of Radiology, University of Pennsylvania, Philadelphia, PA, United States
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Jacobs PS, Brink W, Reddy R. A review of recent developments and applications of high-permittivity dielectric shimming in magnetic resonance. NMR IN BIOMEDICINE 2024; 37:e5094. [PMID: 38214202 DOI: 10.1002/nbm.5094] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/16/2023] [Revised: 10/20/2023] [Accepted: 11/29/2023] [Indexed: 01/13/2024]
Abstract
We present a review outlining the basic mechanism, background, recent technical developments, and clinical applications of aqueous dielectric padding in the field of MRI. Originally meant to be a temporary solution, it has gained traction as an effective method for correcting B1 + inhomogeneities due to the unique properties of the calcium titanate and barium titanate perovskites used. Aqueous dielectric pads have used a variety of high-permittivity materials over the years to improve the quality of MRI acquisitions at 1.5 and 3 T and more recently for 7 T neuroimaging applications. The technical development and assessment of these pads have been advanced by an increased use of mathematical modeling and electromagnetic simulations. These tools have allowed for a more complete understanding of the physical interactions between dielectric pads and the RF coil, making testing and safety assessments more accurate. The ease of use and effectiveness that dielectric pads offer have allowed them to become more commonplace in tackling imaging challenges in more clinically focused environments. More recently, they have seen usage not only in anatomical imaging methods but also in specialized metabolic imaging sequences such as GluCEST and NOEMTR . New colossally high-permittivity materials have been proposed; however, practical utilization has been a continued challenge due to unfavorable frequency dependences as well as safety limitations. A new class of metasurfaces has been under development to address the shortcomings of conventional dielectric padding while also providing increased performance in enhancing MRI images.
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Affiliation(s)
- Paul S Jacobs
- Center for Advanced Metabolic Imaging in Precision Medicine, Department of Radiology, University of Pennsylvania, Philadelphia, Pennsylvania, USA
| | - Wyger Brink
- Magnetic Detection and Imaging group, TechMed Centre, University of Twente, Enschede, The Netherlands
| | - Ravinder Reddy
- Center for Advanced Metabolic Imaging in Precision Medicine, Department of Radiology, University of Pennsylvania, Philadelphia, Pennsylvania, USA
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