Читать книгу Magnetic Resonance Microscopy - Группа авторов - Страница 43

Since ceramic probes are based on dielectric resonators, it is possible to exploit the coupling phenomenon between such objects positioned close to each other. Figure 2.16 depicts the coupling of the TE_01δ modes of two ring resonators each holding a sample, but the principle is the same for empty rings or disks. When the resonators are placed one above the other, the electromagnetic coupling of the individual modes (at the same frequency f₀) gives rise to two coupled modes: a lower-frequency mode denoted (++), with a field distribution such that the magnetic field between the resonators is enhanced; and a higher-frequency mode (+−) such that the magnetic field in between is decreased. In [32] the (++) mode of two coupled disks is exploited to image a sample held between the disks. In [33] two samples are imaged simultaneously by using the (++) mode of two ring resonators, each of them holding a sample. The corresponding magnetic field distribution is depicted in Figure 2.16 (right field maps, second line).

Figure 2.16 Coupling model of the first TE modes of two ring resonators. From [33].

Due to coupling, for the same input power, the SNR in each sample of the coupled ceramic probe is degraded by a factor of compared with the single ceramic probe performance. However, in this study exploiting a prototype that had already overcome the reference solenoid probe by a factor of more than two, the coupled ceramic probe still provided 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] .