Performance Enhancement of an MTL Coil Loaded With High-Permittivity Dielectric Liner for 7 T Brain MRI

In a multi-element microstrip transmission line (MTL) transmit array coil, the transmit field (
$B_{1}^{+}$
) distribution is inhomogeneous due to its standing-wave nature, and the interference effects can severely degrade the
$B_{1}^{+}$
and imaging. Therefore, to improve the homogeneity and strength of
$B_{1}^{+}$
, this study focuses on the development of a multi-element MTL transmit array coil integrated with a dielectric liner (DL) material. Furthermore, the transmission efficiency (
$T_{x,eff}$
) is improved in the head region. An eight-element MTL transmits array head coil is investigated using thinner DLs, and the optimized dimensions of the DL are found from its resonant mode at 7 Tesla (T). Simulations and measurements are performed with an MTL transmit array coil at 7 T, and the performance is analyzed for the DL positions and dimensions. Remarkably, the proposed DLs-integrated transmit array coil system offered significant improvements in the
$T_{x,eff}$
at different DLs positions. Compared to the case without the DL, the
$T_{x,eff}$
is improved by 9% (DL close to the head model), 15% (DL at center), and 39% (DL close to the RF coil). Interestingly, the DL acts as an efficiency tuner element exhibiting a 9% to 39%
$T_{x,eff}$
tuning range of improvement. In addition, the RF-shimming technique improves the
$B_{1}^{+}$
homogeneity and
$T_{x,eff}$
of the coil with DLs by 21% and 42%, respectively, compared to the case without the DL. Moreover, the specific absorption rate (SAR) analysis of the MTL transmit array coil with the DLs is performed. The peak 10g-averaged SAR is reduced from 3.1 W/kg to 2.13 W/kg in the head using DLs and RF-shimming technique. Finally, we used the bench measurement setup to obtain the measured magnetic field. The proposed work exhibits salient features with a smaller DL size and multi-element MTL transmit array coil than other proposed works.