Dielectric Materials & Resonators
1Leiden University Medical Center, Leiden, Netherlands, 2Neurobiology, Weizmann Institute of Science, Rehovot, Israel
Synopsis
This talk will review and explain effects in dielectric materials relevant to MRI. It will cover effects due to the body tissues properties, tailoring of the RF field using high permittivity dielectric materials as well as resonant structure implementations. Applications of dielectrics for MRI in a range of magnetic fields will be shown.
Purpose
A dielectric material is characterized by its electrical permittivity (also called dielectric constant), which describes the generated electrical polarization in the presence of an electric field and is responsible for storing RF energy1-2 (related to RF transmit efficiency - see Figure 1). Most of the body tissue is made up of dielectric materials with relative to vacuum permittivity (εr) which is frequency dependent2-3. At 128MHz the εr is in the range of 1-100 (bone ~14, fat ~12, muscle ~63, brain gray matter ~73, and CSF ~84)3. As the magnetic field increases the RF wavelength inside the body decreases. Once the wavelength is of the order of the object/region of interest, local interferences appear, also called wavelength effects4-5. These interferences generate local shading and an inhomogeneous RF distribution, which can be observed at 3 T when imaging the abdomen region6 and at 7 T in almost any part of the body7. This is the motivation to study and understand the effects of dielectric materials. Another motivation is the potential use of the dielectric materials for tailoring the RF field distribution. Such RF tailoring is useful in eliminating undesired shading8-9, better SAR management10 and enhancing local efficiency and sensitivity11-12. Dielectric materials are already in common use as pads attached to the region of interest, for example for brain imaging in the Human Connectome Project, when scanning at 7 T13. Dielectric materials are also designed as resonators14, serving as an alternative to common surface or volume coils. Due to the required dimensions, such implementations are more relevant for MRI scanners at fields ≥7 T15. One of the benefits is the simple implementation of such resonators. Better understanding of the effects of dielectric materials can provide new insights in controlling the RF field in MRI.Methods
Full estimation of the RF field distribution requires solving the Maxwell set of equations, commonly using dedicated software (simulations shown here were performed using CST Microwave Studio, Darmstadt, Germany). The design of the dielectric material dimensions and properties is mainly based on a forward solution16-17 (calculating the RF field - B1+- for a specific setup), which is repeated to optimize parameters (see for example Figure 2). Published studies include setups of pads made of water10, CaTiO3, and/or BaTiO3 suspensions18-20– having a relative permittivity in the range of 80-300. Mixing different fractions of CaTiO3 or BaTiO3 powder with water will result in different relative permativity εr. A slurry of BaTiO3 beads21 has also been used to reach a higher value of εr (εr = 515). Pad preparation with deionized water is practical in eliminating the signal from the pad itself. Several studies were conducted to analyze the required mixing ratios for generating suspensions with any target permittivity22-23. Recent works also show applications using ultra-high permittivity (500-1000), based on PZT ceramic material24-26. Dielectric materials also support resonant electromagnetic (EM) modes, which depend on the specific shape of the dielectric (see for example Figure 3)27. These are in common use in RF engineering, with a growing interest in MRI. For example, a hollow cylinder was demonstrated to serve as a volumetric resonator15, with implementations at 1H imaging of the calf and wrist and 31P spectroscopy of the calf28. Thin dielectrics can act as resonators similar to surface coil - such resonators were implemented at both 7T and 10.5T29-31.Results
Several groups have demonstrated applications of dielectric materials for improved transmit efficiency, local SNR enhancement, and SAR management. A range of studies was performed with dielectric pads as part of the setup for functional and diffusion MRI at 7T 8,12,32. Studies using dielectric pads at 3 T include abdomen9, cardiac33, and other applications, resulting in better RF distribution. Several works also used dielectric materials as part of the coil assembly, improving the coil efficiency34-36. While most of the work has been done for proton images, feasibility studies included other nuclei as well (13C, 23Na, and 31P). Recent work has shown an enhancement using ultra high permittivity blocks in 1H imaging at 1.5 and 3T24 and in 31P brain spectroscopy at 7T23. Dielectric and cavity resonators using just water show promising results at 7T MRI implemented for limited regions of interest, including: calf, knee, wrist, foot, fingers, and neck 15,37-39. In addition, a multi-transmit resonator was implemented using PZT blocks at 7 T 30,40. A BaTiO3 suspension was also used to show feasibility of a 31P volume resonator at 7 T28.Discussion
Although a range of studies has shown useful applications for dielectric materials, many open questions and challenges remain. Realization of an arbitrary target RF magnetic or electric field is still a challenge. Designs of pads with more complicated geometric shapes are limited, since no simple inverse solution exist. Such studies include pad design with a central hole22 and a work that examines empirically more complex shape for local signal improvement40. Several studies show implementations of dielectric resonators and cavity resonators15,37-39. One of the challenges of such resonators is their detuning, i.e., how to turn off the resonator during receive. One such implementation was shown using a structure of conducting strips and PIN diodes41. Using ceramic blocks (for example uHDC) as dielectric materials has an important advantage of having low conductivity/losses. However, shaping such blocks and achieving flexible setups is challenging. One “workaround” is to connect the blocks using conducting strips. Such connections can generate effectively larger blocks, which can have significant effect on the enhancement42,43. Artificial structures combining dielectric material and conducting material can be the next step in development44,45. Such combinations can achieve structures that are more compact and allow new prospects for RF field control.Acknowledgements
No acknowledgement found.References
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