CommunicationNew high dielectric constant materials for tailoring the distribution at high magnetic fields
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:where H is the magnetic field, Jc the conductive current, JD the displacement current, σ the conductivity, E the electric field, εr 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 (Ba0.04 Sr0.96 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 values, but also to a reduced water content, and (iii) a geometrically deformable but stable suspension.
Section snippets
Material characterization
Calcium titanate (CaTiO3) (Alfa Aesar, Ward Hill, MA), specified εr = 156 when sintered, is available as a fine powder with a density of 4.1 g/cc. CaTiO3 was mixed with distilled, de-ionized water in volume/volume ratios up to 40%, at which point the suspension becomes saturated. The dielectric constants and loss tangents were measured as a function of frequency between 100 and 400 MHz using a dielectric probe kit (85070E, Agilent Technologies, Santa Clara, CA) and S11 measurements on a network
Material characterization
Fig. 1(a) and (b) shows measured values of the relative dielectric constant (real and imaginary components) and loss tangent, as a function of frequency, for different volume fractions of CaTiO3. The dielectric constant is complex, and is given by:
The loss tangent (tan δ) is defined as:
As can be seen from Fig. 1, for pure water the real component of the dielectric constant does not vary with frequency, and the imaginary component is much smaller than the real
Discussion and conclusions
We have characterized a new high-dielectric material using calcium titanium oxide which allows tailoring of the RF spatial distribution at high field. In the head, the high dielectric constant is particularly effective at improving image quality in areas, such as the temporal lobe, usually associated with low signal intensity. In addition to the high dielectric constant, the new material produces a very low background signal intensity, without the need for paramagnetic doping. The 40% volume
Acknowledgment
The authors thank Dr. Michael Lanagan at Penn State University’s Materials Research Institute for valuable advice on dielectric measurements and mixing rules.
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