In magnetic resonance imaging, a birdcage coil is the most commonly used volumetric resonator creating a highly homogeneous radiofrequency magnetic field in a conductive sample. An artificial magnetic radiofrequency shield was recently shown to improve the magnetic field amplitude per unit power (transmit efficiency) of a preclinical birdcage coil by reducing the intrinsic losses in the coil and increasing power absorbed by the sample. In this paper, we propose a new application of an artificial shield in clinical MRI. Thanks to the proposed artificial shield a birdcage coil for human brain imaging operating at 300 MHz (Larmor frequency of protons at static fields of 7 T) can be expanded to increase free space. As a result, the coil becomes more comfortable for the patient and keeping comparable transmit efficiency. The same extended coil with a conventional copper shield would have at least 10% lower efficiency. The proposed artificial shield is implemented as an annular-ring cavity-backed slot in a copper cylinder that tightly surrounds the birdcage. To demonstrate the effect, radiofrequency magnetic field and specific absorption rate distributions were compared numerically and experimentally for the initial and extended coils with different shields.

在磁共振成像中，鸟笼线圈是最常用的体积共振器，可在导电样品中产生高度均匀的射频磁场。最近显示出一种人造电磁射频屏蔽罩，可通过减少线圈中的固有损耗并增加样品吸收的功率来提高临床前鸟笼线圈的单位功率磁场强度（传输效率）。在本文中，我们提出了一种人工屏蔽在临床MRI中的新应用。多亏了提议的人工屏蔽，可以扩展用于人脑成像的鸟笼线圈，以300 MHz（在7 T静态场上的质子的Larmor频率）工作，以增加自由空间。结果，线圈对患者而言变得更加舒适并保持相当的传输效率。具有常规铜屏蔽的相同扩展线圈的效率至少降低10％。拟议的人工屏蔽被实现为紧紧围绕鸟笼的铜圆柱体中的背靠环形腔的槽。为了证明这一效果，对具有不同屏蔽的初始线圈和扩展线圈的数值磁场和实验结果进行了射频磁场和特定吸收率分布的比较。