Magnetic Resonance Microscopy. Группа авторов
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a better SNR than the reference, with an SNR improvement of approximately 1.4, according to the theoretical modeling of the electromagnetic coupling and to numerical simulations. Experimental results from this work are shown in Figure 2.17, which displays plant petiole images obtained with the reference probe and the developed coupled ceramic probe. In the latter, two different samples are imaged, one in each dielectric resonator (DR). For sample 1, imaged with the solenoid and one of the coupled DRs, the maximum achieved SNR gain (dual ceramic probe over reference solenoid) is 1.2 (Figure 2.17c). Coupling these two resonators helps reduce the total acquisition time, since two samples can be imaged simultaneously.
Figure 2.17 MR images of plant petioles using a solenoid coil and a ceramic probe exploiting the coupling of two dielectric ring resonators. In the latter, petioles from two different plants are imaged: sample 1 in (b) and sample 2 in (d), during the same acquisition time slot. Images of sample 1 are also acquired with the solenoid coil for comparison (a). A quantitative comparison of the two coils performance is provided in (c) with the signal-to-noise ratio (SNR) distribution of given regions of interest in sample 1. From [33] .
2.5 Conclusion and Future Prospects
While used for years in the field of microwave engineering, ceramic probes dedicated to MRI have only recently been developed for other various applications, from clinical imaging to microscopy. The principle is to excite one specific eigenmode of a high-permittivity resonator like a disk or ring, and to exploit the stimulated mode magnetic field as a transmit and/or receive field for MRI. The particularity of the eigenmodes used is the spatial coincidence of the maximum magnetic field with low electric field levels as well as the purely magnetically induced (nonconservative) nature of the electric field in the sample. Positioning the sample in this specific region therefore limits the losses due to the electric field interaction with the conductive biological materials, while keeping B1 values sufficiently high to perform imaging. The recent development of low-loss, high-permittivity ceramic materials has had a significant impact on the performance of dielectric probes, limiting the intrinsic losses.
Most probes proposed for MRM exploit the first TE mode of cylindrical high-permittivity resonators. Future work on developing probes exploiting the first hybrid mode would be beneficial to widen the possibilities of using ceramic probes. Another point of interest would be to develop arrays of ceramic probes to significantly reduce the acquisition time, which is a critical experimental parameter when performing MRI.
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