Systematic Analysis of the Improvements in Magnetic Resonance Microscopy with Ferroelectric Composite Ceramics

Abstract

small specimens as single cells, [2] single neurons, [3][4][5] and myofibers, [6] and even functional investigation of such samples. [7] Although the ultimate limit to spatial resolution in MRM is the diffusion of water within the finite measuring time, the practical limit is the achievable signal-tonoise ratio (SNR) per unit voxel in a given data acquisition time. [1,8] MRM is performed at very high B 0 with compact radiofrequency coils (called probes) typically operating both in transmission and reception modes. The probes should be made as close-fitting as possible for a given sample to maximize the SNR and, therefore, attainable image spatial resolution. [8] As the noise is generated by both the probe and the sample, reducing the probe losses and the electric-field-mediated coil-sample interactions is very desirable. [9] Cooling the probe down to cryogenic temperatures allows operation in a mode in which the noise is dominated by the sample. Several authors reported an approximately twofold increase in SNR when studying biological samples compared to using conventional copper coils. [10,11] The disadvantage of all cryogenic probes is that they are much more difficult to fabricate and provide limited access to the sample as well as a lower filling factor.The spatial resolution and signal-to-noise ratio (SNR) attainable in magnetic resonance microscopy (MRM) are limited by intrinsic probe losses and probe-sample interactions. In this work, the possibility to exceed the SNR of a standard solenoid coil by more than a factor-of-two is demonstrated theoretically and experimentally. This improvement is achieved by exciting the first transverse electric mode of a low-loss ceramic resonator instead of using the quasi-static field of the metal-wire solenoid coil. Based on theoretical considerations, a new probe for microscopy at 17 T is developed as a dielectric ring resonator made of ferroelectric/dielectric low-loss composite ceramics precisely tunable via temperature control. Besides the twofold increase in SNR, compared with the solenoid probe, the proposed ceramic probe does not cause static-field inhomogeneity and related image distortion. Ceramics for Improved Microscopy

A Semi-Analytical Model of High-Permittivity Dielectric Ring Resonators for Magnetic Resonance Imaging

“…This semi-analytical approach is then used to derive a quantitative estimation of the reachable SNR performance of ceramic probes. The practical example used is designed for a Larmor frequency f L = 730 MHz and previously presented experimentally in [23], with a ring of a given height of 10 mm, outer diameter 18 mm, and inner diameter 5.6 mm, and a dielectric material of relative permittivity 536 and loss tangent 8 × 10 −4 .…”

Imaging of two samples with a single transmit/receive channel using coupled ceramic resonators for MR microscopy at 17.2 T

“…The TE 01 δ mode of a single ring ceramic resonator has already been exploited in MRM 13 . This mode is easily excited using a simple copper loop, placed outside the resonator and perpendicular to its axis.…”

“…To be precise, besides the sample, only the ceramic ring contributes to losses, and this contribution is low, due to the low dissipation factor tan δ of the material itself. Another advantage of using ceramic rings to provide the excitation field B 1 is that the ratio E / H in the sample is reduced compared with a solenoid 13,15 . It follows that, for a given sample diameter, the electric field being minimum in the FOV region, a change in sample permittivity has a limited impact on the mode frequency and on the field distribution.…”

“…All the multiple‐coil probes described in the literature are based on solenoids, as the solenoidal coil is the reference volume coil for MRM 10 . In the past, an alternative microscopy probe design was proposed, based on the resonance of ceramic disks or rings 11–13 . Dielectric resonators support a variety of eigenmodes, for which the frequency distribution depends on the resonator geometry and the electromagnetic properties of the constitutive material.…”

“…The (++) coupled mode frequency is set at the Larmor frequency of a 17.2 T scanner (730 MHz) by adjusting the relative positions of the two identical rings. Such rings were previously designed for MRM at 17.2 T and they demonstrated, when used individually, greater than 2‐fold SNR enhancement compared with the optimal solenoid 13 . With the dual‐ring probe, it is therefore expected to degrade the single ring SNR by a factor ξ close to , and to overcome the optimal solenoid SNR by a factor close to .…”