High Permittivity Dielectric Pads

The generation of a high quality MR image is extremely challenging when imaging body parts that are large compared with the wavelength of the Radio-Frequency (RF) field. One approach to improve image quality in these situations is to use High Permittivity Dielectric Pads, which offer a very practical solution to this problem.

Andrew Webb, Wouter Teeuwisse, Thomas O’Reilly, Paul de Heer, Kirsten Koolstra,
Peter Börnert, Rob Remis, Jeroen van Gemert and Wyger Brink
Funding: STW 13375, NWO TOPGO
 

Background

Since its introduction, Magnetic Resonance Imaging (MRI) has become an indispensable tool in clinical medicine. Stronger MRI systems offer an increased signal strength, allowing to generate substantially clearer images of body structure and physiology. This has led to MRI systems operating at 3T for clinical use and systems exceeding 9.4T for research use.

MR imaging at these field strengths is extremely challenging, however, as the human body distorts the radiofrequency (RF) fields that are used to generate images. This leads to areas of constructive and destructive interference, as shown in figure 1, which can severely degrade image quality and diagnostic value. These interference effects may be particularly visible in abdominal and cardiac imaging at 3T, and neurological and body imaging at 7T.

Figure 1. Schematic illustration of moving neuroimaging from 3T to 7T, where constructive and destructive interferences in the radiofrequency field becomes more pronounced.

Our approach to improve this is to position cushions with high permittivity materials, often referred to as “dielectric pads”, next to the human body. These cushions, illustrated in Figure 2, distort the RF fields allowing us to modify the interferences present in the RF field. This has improved MR image quality in various applications, as shown on the bottom of this page.


Figure 2. Fabricated dielectric pads and their positioning in the 7T MRI system.

Design software

Each application requires a specific pad design in terms of its geometry and material properties. This process can take many days to weeks of engineering using conventional simulation software, limiting the utilization of high permittivity materials in MRI. In our recent research we have developed a more efficient simulation technique, which allows to design these dielectric materials within minutes. This software has been made available for download to enable MR researchers and clinicians throughout the world to make more effective use of this technology.


Figure 3. In-house developed software tool which allows any MR operator to design dielectric pads within minutes.

Recent papers

Electromagnetic modelling

  • JHF van Gemert, WM Brink, AG Webb, and RF Remis. High-Permittivity Pad Design for Dielectric Shimming in Magnetic Resonance Imaging using Projection Based Model Reduction and a Nonlinear Optimization Scheme. IEEE Transactions in Medical Imaging 2018;37(4):1035-1044. DOI: 10.1109/TMI.2018.2791179
  • JHF van Gemert, WM Brink, AG Webb, and RF Remis. An Efficient Pad Design Methodology for Dielectric Shimming in Magnetic Resonance Imaging. IEEE Transactions in Medical Imaging 2017;36(2):666-673. DOI: 10.1109/TMI.2016.2624507
  • WM Brink, RF Remis, and AG Webb. A theoretical approach based on electromagnetic scattering for analysing dielectric shimming in high-field MRI. Magnetic Resonance in Medicine 2016;75(5):2185-94. DOI: 10.1002/mrm.25783
7 Tesla development and applications
  • MA van der Jagt, WM Brink, MJ Versluis, S.C.A Steens, JJ Briaire, AG Webb, JHM Frijns, and BM Verbist. Visualization of Human Inner Ear Anatomy with High Resolution Magnetic Resonance Imaging at 7 Tesla: first initial clinical assessment and application. American Journal of Neuroradiology 2015;36(2):378-83. DOI: 10.3174/ajnr.A4084
  • WM Brink, MA van der Jagt, MJ Versluis, BM Verbist, and AG Webb. Dielectric pads improve high resolution imaging of the inner ear at 7 Tesla. Investigative Radiology 2014;49(5):271-7. DOI: 10.1097/RLI.0000000000000026
  • WM Teeuwisse, WM Brink, and AG Webb. A quantitative assessment of the effects of high permittivity pads in 7 Tesla MRI of the brain. Magnetic Resonance in Medicine 2012;67:1285-1293. DOI: 10.1002/mrm.23108
  • K Haines, NB Smith, and AG Webb. New high dielectric constant materials for tailoring the B1+ distribution at high magnetic fields. Journal of Magnetic Resonance 2010;203(2):323-7. DOI: 10.1016/j.jmr.2010.01.003
3 Tesla applications

  • WM Brink, MJ Versluis, JM Peeters, P Börnert, and AG Webb. Passive radiofrequency shimming in the thighs at 3 Tesla using high permittivity materials and body coil receive uniformity correction. Magnetic Resonance in Medicine 2015;76(6):1951-1956. doi: 10.1002/mrm.26070.
  • WM Brink, JS van den Brink, and AG Webb. The effect of high-permittivity pads on SAR behaviour in RF-shimmed cardiac imaging at 3 Tesla. Journal of Cardiovascular Magnetic Resonance 2015 Sep 19;17:82. DOI: 10.1186/s12968-015-0188-z
  • WM Brink, and AG Webb. High permittivity pads reduce specific absorption rate, improve B1 homogeneity, and increase contrast-to-noise ratio for functional cardiac MRI at 3T. Magnetic Resonance in Medicine 2014;71(4):1632-40. DOI: 10.1002/mrm.24778
  • P de Heer, WM Brink, BJ de Kooij, and AG Webb. Increasing signal homogeneity and image quality in abdominal imaging at 3 T with very high permittivity materials. Magnetic Resonance in Medicine 2012;68(4):1317-24. DOI: 10.1002/mrm.24438
Review articles

  • WM Brink, V Gulani, and AG Webb. Clinical applications of dual-channel transmit MRI: A review. Journal of Magnetic Resonance Imaging 2015;42(4):855-69. DOI: 10.1002/jmri.24791
  • A.G.Webb. Dielectric materials in magnetic resonance, Concepts in Magnetic Resonance 2011;38A:148-184. DOI: 10.1002/cmr.a.20219

Application examples

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Media https://www.lumc.nl/over-het-lumc/nieuws/2016/januari/mri-kussens-van-klei/