Introduction

Ceramic discs derived from available uHDC composite have been investigated as an approach to increase RF magnetic field at resonance. Recently, uHDC discs were applied to enhance RF coils transmit efficiency and receive sensitivity at non-resonance conditions. In this case, the intrinsic resonance modes of the uHDC material are avoided because of the potential interferences to the coil’s tuning/matching conditions. This research investigated the question as to whether the uHDC materials can be used at the resonance condition along the conventional RF coils in comparison to the non-resonance cases of the same materials.

Method:

To compare the simulation results of previous experimental and analytical work, we consider a well-studied and simple cylindrical dielectric resonator (DR) with known analytical solutions coupled with a circular surface coil (Fig. 1). The design was simulated by using the electromagnetic solver CST Microwave Studio (CST MWS)¹. There are three types of resonant modes of an infinite number of each modes that can be excited in this dielectric resonators: transverse electric (TE), transverse magnetic (TM) or hybrid electromagnetic (HEM) modes². Pure and hybrid modes can be excited and characterized with its magnetic (H) and electric (E) maps, strength, quality factor, and resonance frequency of each mode dependent upon disc geometry, complex permittivity and coupling to the coil; here we are interested in the lower order modes for our applications as only these modes can be strongly coupled to the phantom.

Results:

For explored geometry, the fundamental mode is the first transverse electric mode, TE_01δ, which produces a strong B₁ excitation field, and lower E-field in the phantom. Fig. 2a shows an S11 plot from the coil port, demonstrating the first three lowest resonance modes without and with loading of a phantom. Fig. 2b shows the resonance modes are shifting to lower frequencies when the permittivity increases. Fig. 3 shows the corresponding E and H field distributions of the three modes. The first fundamental mode identified as TE_01δ, has the strongest H-field along the axis of dielectric disc and the surface coil, while the other two modes, HEM (f=152 MHz) and TE_02δ (f=202MHz), introduce weaker magnetic field coupling to the phantom. As shown in Fig. 2b, at ε_r=2486 the fundamental resonance of the DR occurs at the operating frequency of the 3T magnet, i.e. 123MHz while at ε_r=2300 and ε_r=3000 the fundamental first resonance happens respectively 7MHz higher and 11MHz lower than the operating frequency. By increasing the permittivity to 7000, the second mode also reaches below 123MHz, which cause inhomogeneous field distribution and decreases the RX-sensitivity when it is matched by circuit to 123MHz. Subsequently, the surface coupling coils to the DRs were all tuned/matched to 123MHz using the impedance circuit with < -10dB³. The RX-sensitivity of the coil at eight different permittivity values of the DR listed in Table. I, was calculated and compared with the baseline case where no uHDC material were employed. As shown in Fig. 4 and Fig. 5, the Rx-sensitivity value is enhanced as compared to the baseline by increasing the permittivity up to 4000, and it starts decreasing when operating frequency approaches to the second mode (HEM) for higher permittivity cases more than 5000. So, it should work anywhere before reaching the second mode with highest permittivity value.

Discussions and Conclusion:

The RX-sensitivity plots in Fig. 3 indicate that tuning DR by parametric evaluation of permittivity to the operating frequency (123 MHz), although enhancing the SNR dramatically on the surface of the phantom, does not produce the highest average enhancement in whole phantom compared to the baseline. The optimal permittivity of DR for RF field enhancement could be chosen in the way that the operating frequency is between the fundamental (TE_01δ) and higher order resonance frequency (i.e. HEM_11δ, TE_02δ) of DR. The Rx-sensitivity is enhanced significantly (%65) through 3cm inside the phantom compare to the baseline, with Ɛ_r =4000. Computer modelling of dielectric resonator of a cylindrical disc showed that a coupling surface coil can be easily tuned and matched to the resonance frequency of DR. Our current and future work includes the extension of DR modelling to clinical applications incorporating new coil designs, as well as the study of higher resonant modes (like HEM modes).

References

1. CST Microwave Studio, Ver.2020.

2. Kajfez, Darko, and Pierre Guillon. Dielectric Resonators. Dedham (MA.: Artech House, 1986.

3. Roemer, P B et al. “The NMR phased array.” Magnetic resonance in medicine vol. 16,2 (1990): 192-225. doi:10.1002/mrm.1910160203