Dipole Array Design Considerations for Head MRI at 10.5T
Jinfeng Tian1, Russell Lagore1, Lance Delabarre1, and J. Thomas Vaughan1

1U. of Minnesota, Minneapolis, MN, United States

Synopsis

An 8-channel dipole array is a promising structure for human head imaging at 10.5T. In order to optimize the structure for efficiency and homogeneity over the brain, many variations of the dipole were numerically simulated and compared. The variations include varying dipole lengths, warping the dipole, adding shielding, adding dielectric padding or dielectric mirrors and including decoupling capacitors. Compared to a design in use, numerical results predict the RF homogeneity can be greatly improved with a 210 mm dipole array while simultaneously lowering the peak local 1 gram and 10 gram SAR.

Introduction

Preliminary studies have proved the feasibility of head MR imaging with a dipole array on the world’s first whole-body 10.5T (450MHz) MRI1,2,3. But the dipole structure can take various forms. This work reports an extensive research effort to optimize the existing dipole array for head imaging through numeric modeling.

Methods

In order to optimize the dipole array structure for head imaging at 450 MHz, the following factors were taken into consideration: 1) array format: dipole arrays terminated with floating end-ring sections at both ends, dipole array terminated with end-ring sections only at the bottom while the upper half were replaced with straight wires or wires conformal to the head geometry, 2) dipole format: straight wires with meander structure or lumped L/C for frequency tuning, 3) array length: dipole arrays with lengths ranging from 160mm to 237.5mm were taken into consideration, 4) other design elements that may potentially elevate dipole array performance: RF shielding, global dielectric wrapping, dielectric/RF mirror (5mm thick x 240mm diameter) placed 7.5mm above head to limit power leakage, and 5) decoupling methods and their impact on the array performance. Some structures are illustrated in Figure 1.

The starting dipole array (Fig. 1-01) consists of eight 160mm long dipoles terminated with floating copper ring sections (20mm wide x 86mm length) on both ends. The dipoles are evenly distributed on a cylindrical surface (256mm diameter). If used, the RF shielding is 195mm (length) x 312mm (Diameter), Fig. 1-07. To assess dielectric structures (Fig. 1-08,09), the relative permittivity was set to 1, 5, 10, 20, 40 and 80. To evaluate decoupling, variable capacitors are placed among the bottom/upper end-ring sections in Fig. 1-11 to decouple neighboring dipoles.

All simulations were performed with the Finite Difference Time Domain method (XFDTD, Remcom, PA), loaded with a medium male size HUGO head model4. The electrical properties of its 17 tissues were adjusted for 450 MHz. All arrays were driven with quadrature-mode phasing.

All results were normalized to a total of 1 watt RF power dissipated within the head model. RF homogeneity was defined as the percentage of voxels whose |B1+| was within 20% deviation from the averaged |B1+| within the 3D FOV, as denoted by the green box in Fig. 1-08.

Results

The |B1+| distributions for selected array structures are presented in Fig. 2 for the orthogonal central brain slices. Figure 3 presents the current magnitude along coil length for three dipole formats (Fig 1. 04-06). Performance versus coil length is summarized in Figure 4 and Table 1.

Discussions

The following observations are based on the coils being driven in quadrature and the specific loading position as illustrated in Fig. 1-08.

1) The starting design (Fig. 1-01) with quadrature drive generates strong center brightness, resulting in poor RF homogeneity. The addition of RF shielding exaggerates this pattern. Meanwhile, arrays with longer straight (Fig. 1-02) or conformal dipoles (Fig. 1-03) produce much more uniform |B1+|. 2) Dipole arrays of same length have almost identical current distributions, regardless of the tuning elements (meander or lumped inductance or capacitance), as long as the meander size is small compared to the dipole dimensions or the gap among dipoles. 3) As the dipole length increases for structure Fig. 1-02, the mean |B1+| in the FOV also increases, and the peak local 1gram and 10gram SAR decrease. (With one exception: when the dipole was 237.5mm long the peak 1gram SAR location shifted and defied the trend.) But the 210mm long array provided the most uniform |B1+| in the FOV. 4) Global dielectric wrapping or a dielectric/RF mirror does not help elevate dipole array performance as applied. 5) Decoupling with capacitors on the end-ring sections effectively reduces the coupling between nearest elements from around -10dB to -20dB, but it also significantly lowers |B1+| in the FOV. Other decoupling methods are being considered.

Conclusion

The next head dipole array should be 210mm long without upper end-ring sections, similar to Fig 1– 02.

Acknowledgements

NIH-NIBIB 2R01 EB007327, NIH-NIBIB 2R01 EB006835, P41 EB015894, NIH R01 EB011551-01A1, R24 MH105998-01, 2R42EB013543-02, Obama Brain Initiative.

References

1. Vaughan JT, Delabarre L, Tian J, et al. Towards 10.5T MRI. ISMRM 2014: 4822.

2. Raaijmakdrs AJE, Lpek O, Klomp DWJ, et al. . Design of a radiative surface coil array element at 7T: the single side adapted dipole antenna. MRM 2011; 66 : 1488-1497.

3. Tian J, Lagore R, Vaughan JT. Dipole Arrays for MR Head Imaging: 7T vs. 10.5T. ISMRM 2015: . 6171.

4. Collins CM, Li S, Smith MB. SAR and B1 Field Distributions in a Heterogeneous Human Head Model Within a Birdcage Coil. MRM 1998; 40: 847-56.

Figures

Fig.1. Dipole Array Structures, Head Loading Position, and Field of View (defined by the green box in 08)

Fig.2. |B1+| distributions on the central slices for selected array structures. Logarithmic scale, 0dB=1.5µT

Fig.3. Current distributions on the dipoles

Fig.4. Slice-averaged |B1+| along dipole length direction

Table 1. |B1+| and peak local SAR vs. Dipole length



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
3524