Frequency Selective Surface (FSS) Radiofrequency Shield of Solenoid Coil
for Low-Field Portable MRI
Abstract
A lightweight cylindrical Frequency Selective Surface (FSS) using
high-pass inductive mesh grid is proposed as a radiofrequency (RF)
shield for solenoid coils at 2.84 MHz working in a portable MRI system
with a transversal B0-field averaged at 67 mT. Using a solenoid coil of
60 mm diameter as an example, the proposed FSS shield shows good
shielding effectiveness at different wall-to-wall distances between the
coil and the shield (5-25 mm). It also shows less compromise in
B1-sensitivity when compared to the coil with a copper shield. The
effects of the FSS shield were examined numerically and validated
experimentally. When the wall-to-wall distance is 5 mm, the solenoid
coil with the proposed FSS shield is compact, lightweight, and shows
B1-sensitivity of more than 50% higher than the copper counterparts
with comparable shieding effectiveness. Compared to an unshielded coil,
it shows a wider bandwidth which benefits the excitation of an MRI
system that has less homogeneous fields and a smaller inductance which
indicates less trapping of energy and thus can enable different pulse
sequence programming by taking this approach. This study shows that an
FSS shield with inductive mesh grids around a solenoid coil is a
promising approach for compact shielding without a big shielding box. It
will contribute to the compactness of a portable MRI system.