  It is well known that magnetic resonance imaging (MRI) at 7 Tesla (7T) and
higher magnets can provide much better signal sensitivity compared with lower
field strengths. However, variety of commercially available ultra-high-field
MRI coils are still limited, due to the technical challenges associated with
wavelength effect, such as flip angle inhomogeneity and asymmetric transmit and
receive RF field patterns. We aimed to develop a passive RF shimming technique
using an inductively coupled RF resonator array to improve brain MRI at 7T,
focusing of cerebellum. Method: an inductively coupled RF resonator array was
designed for placing inside the commercial head coil to enhance the transmit
field homogeneity and to improve the receive signal sensitivity. Each element
of the array is a coupled-split-ring resonator (CSRR), which they are decoupled
for each other using critical overlap technique. Phantom and ex-vivo MRI
experiments were performed to evaluate the transmit efficiency and signal
sensitivity in the presence of the array. Results: MRI experiments showed 2 to
4-fold improvement in Signal-to-noise ratio (SNR) and 2-fold improvement in
contrast-to-noise ratio (CNR) in cerebellum. Conclusion: We modeled a wireless
RF resonator array that can improve the transmit efficiency of the standard
head coil and enhance the signal sensitivity at brain MRI without compromising
RF safety.
