Magnetic Resonance Microscopy. Группа авторов

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2 Ceramic Coils for MR Microscopy

       Marine A.C. Moussu1,2, Redha Abdeddaim2, Stanislav Glybovski3, Stefan Enoch2, and Luisa Ciobanu4

      ¹ Multiwave Imaging, Marseille, France

      ² Aix-Marseille Université, CNRS, Centrale Marseille, Institut Fresnel, Marseille, France

      ³ ITMO University, Saint Petersburg, Russia

      ⁴ CEA, DRF, JOLIOT, NeuroSpin, Université Paris-Saclay, Gif-sur-Yvette, France

      2.1 Introduction

      Conventional metallic radiofrequency (RF) coils are made of conductive materials, most often copper, and are designed to resonate at the Larmor frequency thanks to their geometry and carefully disposed lumped elements. The latter impose the coil’s intrinsic noise contributions and are responsible for power losses in the acquisition chain. In magnetic resonance microscopy (MRM), the achievable signal-to-noise ratio (SNR) is significantly degraded by the interaction between the conductive biological sample and the electric field induced by the RF coil. The standard volumetric probe is the solenoid coil, a winding of copper wire closely holding the recipient containing the sample. When driven by current, the coil induces a strong B₁ field in the sample, along the solenoid axis, and, simultaneously, an excessive conservative electric field that limits the SNR by reducing the loaded quality factor.

      An alternative coil design, both in terms of geometry and material, is proposed with ceramic probes. High-permittivity dielectric resonators support a variety of specific field distributions at frequencies defined by the geometry and the constitutive material properties. Each eigenmode of such components is defined by its eigenfrequency and the corresponding field distribution. Developed over the past 10 years, the new generation of RF coils exploits the modal distribution of these high-permittivity dielectric resonators, typically with cylindrical shape.

      For MRM applications, the dimensions of the resonators are typically chosen to match the required field of view, and the material is selected with the specific purpose of reducing the probe’s intrinsic losses while generating a strong magnetic field in the imaged volume. More precisely, the magnetic field configuration created with dielectric resonators inside a sample can be similar to that of a solenoid probe. However, for dielectric probes the conservative electric field is removed, which helps to increase the loaded quality factor for the same field of view. The most important challenge remains to limit the noise from the probe: material losses of the dielectric need to be extremely low, which is only possible to achieve using special kinds of ceramics. In practice, the magnetic field is strongly correlated to the dielectric permittivity of the material: the higher the permittivity, the better the field confinement within the sample volume. Ceramic materials based on calcium and barium/strontium titanates, with their high permittivity values (~100) and possible low loss factors, are therefore good candidates to build dielectric probes.

So far, two eigenmodes of cylindrical resonators have been exploited for MR coils: the first transverse electric (TE) mode named TE_01δ and the first hybrid transverse mode, HEM_11δ. Both modes can be used to replace conventional volume probes. In particular, the first one mimics the solenoid field, while the second one resembles a birdcage field. For both modes, the volume where the generated magnetic field is the strongest is also where the electric field is the weakest. This feature of modal distributions is one reason why dielectric probes have the potential to outperform conventional coils, which, on the contrary, are limited by a systematic coexistence of magnetic and electric fields within the sample.

      We start this chapter by briefly discussing the state of the art of ceramic probes in MR applications. Next, we describe several theoretical tools developed to help design MR probes with optimal performance. Finally, MRM experiments with ceramic probes are detailed and discussed.

      2.2 State of the Art

      Take-home message: Dielectric resonators have been used in microwave engineering for decades. More recently, probes for electron paramagnetic resonance (EPR) and magnetic resonance imaging (MRI) applications at several scales have been designed as well. In EPR, several theoretical tools to quantify the transmit efficiency have been developed. The engineering of high-permittivity, low-loss materials has significantly contributed to the development of these probes.

Dielectric resonators have been used in microwave engineering for decades, since they serve as miniaturized circuit components like antennas and filters, which is of great interest for telecommunication applications [1–6]. More recently, probes for MR applications at several scales have been designed as well.

In EPR, dielectric resonators are used instead of metallic cavities since they have reduced dimensions for the same resonance frequency and much higher quality factors [7,8]. They are built with ferroelectric materials and used at microwave frequencies. Several theoretical tools have been developed to quantify the transmit efficiency of ceramic probes [7,9,10].

Prototypes of dielectric resonators probes for MRI have been proposed at several B₀ field strengths and for various applications, as listed in Table 2.1. Dielectric resonators can be used in transmit/receive mode or as transmitters only. In most cases, the constitutive dielectric material is a high- to very high-permittivity (relative permittivity ϵ_r≥100) ceramic based on calcium and barium/strontium titanates. One exception in this review is the probe for wrist clinical imaging made of water (ϵ_r ~ 80).

      The engineering of high-permittivity, low-loss dielectric materials has contributed significantly to the development of these probes. The high permittivity helps to confine the electromagnetic field within the resonator, while the low losses inside the material limit noise during the signal acquisition. The development of ferroelectric ceramic materials with perovskite crystalline structures has enabled the reachable permittivity to be increased while keeping the losses low [11,12].

Table 2.1 Literature review of dielectric probes for MRI.