New high dielectric constant materials for tailoring the $B_1^{+}$ distribution at high magnetic fields

Abstract

The spatial distribution of electromagnetic fields within the human body can be tailored using external dielectric materials. Here, we introduce a new material with high dielectric constant, and also low background MRI signal. The material is based upon metal titanates, which can be made into a geometrically-formable suspension in de-ionized water. The material properties of the suspension are characterized from 100 to 400 MHz. Results obtained at 7 T show a significant increase in image intensity in areas such as the temporal lobe and base of the brain with the new material placed around the head, and improved performance compared to purely water-based gels.

Introduction

Sample-induced inhomogeneities in the distribution of electromagnetic (EM) energy for human MRI at high magnetic fields are well documented [1], [2], [3], [4], [5], [6], [7]. These inhomogeneities arise primarily from the dielectric properties of tissue, which result in partial constructive and destructive RF interactions from complex wave behavior. The effective wavelength of EM energy in tissue is ∼13 cm for white and gray matter at 7 T and ∼11 cm at 9.4 T. The brain has dimensions similar to, and the abdomen dimensions considerably larger than, the EM wavelength at these field strengths. Standard volume resonators geometries, such as the birdcage [8] and the transverse electromagnetic mode (TEM) [9], which produce homogeneous transmit and receive fields at low magnetic fields, cannot produce the same homogeneous fields as the static magnetic field increases [1], [2]. These geometries are designed such that the current elements on opposite sides of the resonator are equal in magnitude but opposite in phase. If the sample within the resonator is greater than a wavelength, areas of destructive interference occur. Manipulating the geometric distribution of EM fields within the human body has a long history of both theoretical and experimental development, driven initially by the field of therapeutic hyperthermia [10], [11], [12], [13]. Using an approach derived from such developments, one solution to at least partially solving the inhomogeneous EM distribution for high-field MRI is to drive each element of a resonator separately with a variable magnitude and phase, the so-called transmit-array approach [2], [14], [15], [16], [17], [18]. Other approaches involve the design of specialized radiofrequency (RF) pulses [19], [20], or the combination of these two approaches of RF pulse design and transmit arrays [21], [22], [23], [24], [25].

An alternative, though ultimately also complementary, and very simple approach to “directing” the transmit RF field has been demonstrated by Yang et al. [26] at 7 T using water bags placed around the head. The use of pads with high dielectric constant has also been demonstrated at 3 T and is in common clinical practice at many institutions for routine body imaging at 3 T. For example, Sreenivas et al. [27] used 4-L bags filled with water, doped with between 20 and 50 mM manganese salts. Similar work has been reported in phantoms by Takayama et al. [28] and Sunaga et al. [29], who designed a gel with dielectric constant similar to tissue for improved impedance matching to human skin. The basic mechanism by which the RF spatial distribution can be altered using such external high-dielectric structures, summarized by Yang et al. [26], follows from one of Maxwell’s equations:

$$\nabla \times \mathit{H}={\mathit{J}}_c+{\mathit{J}}_D=\sigma \mathit{E}+i{\epsilon }_{\text{r}}{\epsilon }_0\omega \mathit{E}$$

where H is the magnetic field, J_c the conductive current, J_D the displacement current, σ the conductivity, E the electric field, ε_r the relative dielectric constant of the material and ε₀ that of vacuum. The inclusion of an external high-dielectric material results in high displacement currents within this material, which in turn produces a separate RF field, in addition to that of the RF coil, close to the material.

Pads in current use have a relative dielectric constant of ∼60, and since they contain significant paramagnetic concentrations, can cause significant signal loss in adjacent tissue in single-shot or long echo-time gradient-echo sequences due to strong magnetic susceptibility effects. Particularly for studies at high magnetic fields, one would ideally like a material which has a dielectric constant higher than water (but which also offers the possibility to be tailored in value), a low background signal without paramagnetic doping, and one which can be geometrically formed in order to adapt to different patient sizes.

With respect to obtaining higher dielectric constants, our group has recently designed RF coils for microimaging studies at 600 MHz using ceramic dielectric resonators [30], [31]. These resonators are constructed from high-temperature-sintered and cold-pressed samples of barium strontium titanate (Ba_0.04 Sr_0.96 TiO₃) [31], which has a very high relative permittivity of 323, and calcium titanate [30], which has a relative permittivity of 156. Although these materials have a very high dielectric constant when they are sintered and pressed, the values for the native powders are much lower, since the volume ratio of the powder is only ∼40%, i.e., 60% of the volume of a given sample is air, which has a very low dielectric constant. However, by creating a suspension of the powders with de-ionized water, the dielectric constant can be increased, with a value that can be tailored by the appropriate composition of the suspension. Initially, we have concentrated on the use of calcium titanate due to its wide availability and low cost. The properties of these suspensions, as expanded in later sections, are: (i) a high dielectric constant between 100 and 120, (ii) a low background signal due primarily to short T₂ and $T_2^{\ast }$ values, but also to a reduced water content, and (iii) a geometrically deformable but stable suspension.