Improving homogeneity in abdominal imaging at 3 T with light, flexible, and compact metasurface

Abstract

Purpose: Radiofrequency field inhomogeneity is a significant issue in imaging large fields of view in high-and ultrahigh-field MRI. Passive shimming with coupled coils or dielectric pads is the most common approach at 3 T. We introduce and test light and compact metasurface, providing the same homogeneity improvement in clinical abdominal imaging at 3 T as a conventional dielectric pad. Methods: The metasurface comprising a periodic structure of copper strips and parallel-plate capacitive elements printed on a flexible polyimide substrate supports propagation of slow electromagnetic waves similar to a high-permittivity slab. We compare the metasurface operating inside a transmit body birdcage coil to the stateof-the-art pad by numerical simulations and in vivo study on healthy volunteers. Results: Numerical simulations with different body models show that the local minimum of B + 1 causing a dark void in the abdominal domain is removed by the metasurface with comparable resulting homogeneity as for the pad with decreasing maximum and whole-body SAR values. In vivo results confirm similar homogeneity improvement and demonstrate the stability to body mass index. Conclusion:The light, flexible, and inexpensive metasurface can replace a relatively heavy and expensive pad based on the aqueous suspension of barium titanate in abdominal imaging at 3 T.

The restricted SAR protocol: A method to assess MRI coil prototypes in an unconditionally safe manner

“…The B 1 + CV describes the spread of the transmit magnetic-field inhomogeneity within the masked region. Coil transmit efficiency (TrE) is defined as B 1 + inv.targ / √ P. Here, P denotes the transmitted power equal to V ref 2 /R with coil impedance R = 50 Ω, and B 1 + inv.targ is a constant coefficient corresponding to the amplitude of a rectangular inversion RF pulse with the length of 𝜏 = 1 ms: B 1 + inv.targ = π/(2πγ⋅τ) where γ = 42.57⋅10 6 Hz/T -gyromagnetic ratio. These values characterizing the Nova and MW1 coils obtained for the 2 volunteers are provided in Table 4.…”

“…The B 1 + inhomogeneity leads to changes in the signal intensity and contrast, complicating image evaluation and diagnosis. To face this problem, some works have considered antennas incorporating new materials such as ceramics or metamaterials that modify the field distribution in the subject 5–7 . Another state‐of‐art solution is parallel transmission (pTx), 8–10 providing control over the B 1 + of several individual transmit elements to achieve more uniform excitation under given power and SAR constraints.…”

Novel materials in magnetic resonance imaging: high permittivity ceramics, metamaterials, metasurfaces and artificial dielectrics

“…This artificial dielectric mimicked the action of an equivalently-sized dielectric pad with ε r = 120, but was not ideal in terms of the orientation of the secondary B-field created, and the fact that several PCB layers were needed which gave rise to a very rigid structure. They recently showed an improved design approach for abdominal imaging at 3 T [ 82 ] using an artificial dielectric produced on a thin single double-sided low-loss PCB with capacitors formed by the overlap of conductive patches on either side of the flexible PCB connected by thin copper strips. Results, shown in Fig.…”

Enhancing the brain MRI at ultra‐high field systems using a meta‐array structure

“…The use of high permittivity layer (HPL) under neath of an array of resonators is the main difference between the current device and our previous design 35 . Many research groups have shown that the use of DPs (high permittivity materials) can increase the homogeneity of the transmit field in MRI 12,28,38,39 . In our design, we used the HPL to further improve the transmit efficiency due to the induced displacement current and decrease the specific absorption rate (SAR) 39 .…”