Introduction

Dipole antennas can provide significant transmit field (B₁⁺) efficiency and SNR gains in UHF-MRI^1,2. One promising approach to shorten dipole antennas (~50cm at 297.2MHz in free space) is to use a high-ε_r medium^3-9. The dimensions of dielectric blocks used previously were rather large, which made it difficult to use them in large dipole antenna arrays. Those studies followed what Raaijmakers et al. suggested³, and what they investigated later⁴ showing that optimal B₁⁺ efficiency can be achieved for a block with (150x50x50)mm³ and ε_r between 90 and 110. Both of these reports^3,4 focused on the special case where there was direct contact between the block and the human body. Yet such contact is rather difficult to achieve for a solid, rectangular geometry, and it may not always be feasible in clinical settings (especially for e.g. human head). Therefore, it is reasonable to assume that the dielectric block and the anatomical structure are physically separated. In previous work, we observed that certain types of dielectrically-shortened dipole antennas produced an efficient B₁⁺ in the presence of a small air gap while others did not¹⁰. We hypothesized that different dielectric modes^11,12 can be induced within the rectangular block, thereby affecting antenna performance. Therefore, the aim of the present study was to investigate the influence of rectangular block geometry, dielectric permittivity and antenna/subject physical separation on the B₁⁺ produced by dielectrically-shortened dipole antennas suitable for MRI at 7T and to determine how they influence in vivo experiments.

Methods

Electromagnetic field simulations were performed using Sim4Life (Sim4Life,Switzerland). Two phantoms were used: a spherical one (radius=85mm, ε_r=50.6, σ=0.66S/m), and a cuboid one ((300x300x300)mm³, ε_r=34, σ=0.47S/m). We used B₁⁺ efficiency defined as: B₁⁺/√P_in , where P_in is input power, and SAR efficiency defined as B₁⁺/√SAR_10g, where SAR_10g is maximum SAR averaged over 10g. B₁⁺ distribution within the spherical phantom was studied for different rectangular block geometries (Fig.1) and different values of ε_r (35,50,80,100,150,200,300,500), assuming a 5-mm air gap between the block and the phantom. The dimension a (a₀) was constant for each ε_r value: 35(242mm), 50(202mm), 80(160mm), 100(144mm), 150(118mm), 200(102mm), 300(84mm) and 500(64mm). One constant geometry (160x60x7.5)mm³ of the block was used to compare B₁⁺ efficiency for different ε_r. The effect of an air gap (1,2,3,4,5mm) on B₁⁺ distribution in a cuboid phantom was studied for one larger block (0.75b₀, d/b=0.75) for each ε_r. The cutoff frequency of each quasi-transverse electric (TE) mode was determined from: $$$f_{cutoff}=\frac{c}{2\pi\epsilon_{r}}\sqrt{\left(\frac{m\pi}{a}\right)^{2}+\left(\frac{n\pi}{b}\right)^{2}+\left(\frac{l\pi}{d}\right)^{2}}$$$ where c is the velocity of light; m,n,l are the dielectric mode indices (Fig.1). Two dielectrically-shortened dipole antennas (ε_r=80) were designed, built and evaluated in MR experiments: (160x70x52.5)mm³ and (160x70x17.5)mm³. MR experiments were conducted in one male subject in three different regions of interest (head,calf,wrist) using a 7T-68cm bore scanner (Siemens,Germany). MR experiments were conducted using gradient echo (GRE) imaging. For in vivo experiments, deionized water was replaced by D₂O.

Results

B₁⁺ distribution within the spherical phantom for d=0.125b (b=0.5b₀ and b=0.75b₀) resulted in the highest B₁⁺ efficiency in the center of the phantom for all ε_r. The most apparent change in B₁⁺ and decreased efficiency was observed for d=0.75b (b=b₀, b=0.75b₀ and b=0.5b₀) for all ε_r and for blocks with d=0.5b (especially for higher ε_r -Fig.1). For the constant block geometry (160x60x7.5)mm³ the highest transmit field efficiency in the center of the spherical phantom was obtained with ε_r=300 and ε_r=200 (Fig.2). For lower ε_r (35,50), despite the air gap, the B₁⁺ pattern was very similar to the one obtained with direct contact between the block and the cuboid phantom, while for higher ε_r a significant difference in B₁⁺ pattern and efficiency could be observed (Fig.3). The mode excited in the larger block (d=0.75b) was interpreted as $$$TE_{1\delta\delta}^y$$$ and the one in the smaller block (d=0.25b) as $$$TE_{11\delta}^z$$$ (Fig.4). The thinner block provided highly superior in vivo image quality for all tissues compared to its thicker counterpart, which yielded very noisy images (very low to no SNR - Fig.5).

Discussion and Conclusion

This study demonstrates for the first time why different types of dielectrically-shortened dipole antennas can produce (in)efficient B₁⁺ in an in vivo UHF-MRI experiment. We showed that the different types of dielectric modes ($$$TE_{11\delta}^z$$$ and $$$TE_{1\delta\delta}^y$$$) that can be excited within the block had a crucial impact on the resulting B₁⁺. Dielectric blocks in which the $$$TE_{11\delta}^z$$$ mode was allowed to propagate performed better in terms of B₁⁺ and SAR efficiency than their smaller counterparts, for which the $$$f_{cutoff}$$$ was well above the resonance frequency of protons at 7T. Reducing the block thickness (d=0.125b) is advantageous in the context of B₁⁺ distribution and efficiency for all ε_r values. Higher-order mode $$$TE_{12\delta}^z$$$, which was induced in higher-ε_r blocks, provided significantly higher B₁⁺ efficiency than $$$TE_{11\delta}^z$$$, which propagated in lower-ε_r blocks for a given block geometry (160x60x7.5)mm³. We observed that the block/subject physical separation and loading geometry are critical to understand why some dielectric blocks in which $$$TE_{1\delta\delta}^y$$$ is expected to propagate produced rather efficient B₁⁺ while others did not. In vivo experiments showed that $$$TE_{1\delta\delta}^y$$$ mode, unlike $$$TE_{11\delta}^z$$$, led to substantially degraded image quality, highlighting the influence of dielectric block geometry and propagation of dielectric modes on the performance of dielectrically-shortened dipole antennas.

Acknowledgements

We acknowledge access to the facilities and expertise of the CIBM Center for Biomedical Imaging, a Swiss research center of excellence founded and supported by Lausanne University Hospital (CHUV), University of Lausanne (UNIL), Ecole polytechnique fédérale de Lausanne (EPFL), University of Geneva (UNIGE) and Geneva University Hospitals (HUG).

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