Abstract
In a multimodal volume coil, only one mode can generate homogeneous Radiofrequency (RF) field for Magnetic Resonance Imaging. The existence of other modes may increase the volume coil design difficulties and potentially decreases coil performance. In this study, we introduce common-mode resonator technique to high and ultrahigh field volume coil designs to reduce the resonant mode while maintain the homogeneity of the RF field. To investigate the design method, the common-mode resonator was realized by using a microstrip line which was split along the central to become a pair of parallel transmission lines within which common-mode currents exist. Eight common-mode resonators were placed equidistantly along the circumference of a low loss dielectric cylinder to form a volume coil. Theoretical analysis and comparison between the 16-strut common-mode volume coil and a conventional 16-strut volume coil in terms of RF field homogeneity and efficiency was performed using Finite-Difference Time-Domain (FDTD) method at 298.2 MHz. MR imaging experiments were performed by using a prototype of the common-mode volume coil on a whole body 7 Tesla scanner. FDTD simulation results showed the reduced number of resonant modes of the common-mode volume coil over the conventional volume coil, while the RF field homogeneity of the two type volume coils was kept at the same level. MR imaging of a water phantom and a kiwi fruit showing the feasibility of the proposed method for simplifying the volume coil design is also presented.
Highlights
In Magnetic Resonance Imaging (MRI), volume coil is a well-established RF coil type which is capable of creating a nearly homogeneous Radiofrequency (RF) field and clinically-acceptable signal-to-noise ratio (SNR) [1]
We present a novel volume coil design method using common-mode resonator technique to reduce the resonant modes without sacrificing the RF field homogeneity
The advantage of the common-mode volume coil is with the same leg number as a typical volume coil, the resonant mode number can be reduced by half almost without deteriorating the RF field homogeneity or decreasing the coil efficiency
Summary
In Magnetic Resonance Imaging (MRI), volume coil is a well-established RF coil type which is capable of creating a nearly homogeneous Radiofrequency (RF) field and clinically-acceptable signal-to-noise ratio (SNR) [1]. A multimodal volume coil with N rungs or struts resonates at N/2 intrinsic modes [1,43-45] or N/2 + 1 modes [46,47]. Among these resonant modes, there is only one mode which generates a transverse homogeneous RF field for MR imaging in a horizontal static magnetic field. The existence of the other modes may overlap with the homogeneous resonant mode, which makes it difficult to differentiate them and to design the coil [48] Another practical issue is the coupling among the elements. In order to yield homogeneous RF field distribution, sufficient coupling among the resonant elements (rungs or struts) of a multimodal volume coil is necessary. The number of resonant elements in a volume coil has to be chosen to reach a good compromise between field homogeneity degree and quality factor for the resonators
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