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)
MRI
1,2,3. 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
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1488-1497.
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