IEEE Trans Med Imaging. 2015 Apr;34(4):836-45. doi: 10.1109/TMI.2014.2370533. Epub 2014 Nov 13.
Ultra-high field magnetic resonance imaging (MRI) scanners ( ≥ 7T) require radio-frequency (RF) coils to operate in the range of the electromagnetic spectrum where the effective wavelength in the tissue approaches the patient dimensions. Multi-channel transmit arrays, driven in parallel, have been developed to increase the transmit field (B1(+)) uniformity in this wavelength regime. However, the closely packed array elements interact through mutual coupling. This paper expands on the ability of a distributed planar filter (the "magnetic wall") to decouple individual elements in an entire array. A transmit RF coil suitable for neuroimaging at 7T was constructed. The transmit coil, composed of 10 individual surface coil elements, was decoupled with magnetic walls. A separate receive coil array was used for signal reception. The hardware and imaging performance of the transmit coil was validated with electromagnetic simulation, bench-top measurements, and in vivo MRI experiments. Analysis and measurements confirmed that the magnetic wall decoupling method provides high isolation between transmit channels, while minimally affecting the B1(+) field profiles. Electromagnetic simulations confirmed that the decoupling method did not correlate to local specific absorption rate (SAR) "hot spots" or increase local-to-global SAR fractions in comparison to previously reported 7T multi-channel transmit arrays employing different decoupling methods.
超高场磁共振成像（MRI）扫描仪（≥7T）需要射频（RF）线圈在电磁频谱范围内运行，其中组织中的有效波长接近患者尺寸。多通道发射阵列，以并行方式驱动，已被开发用于增加在该波长范围内的发射场（B1（+））均匀性。然而，紧密排列的阵列元件通过互耦相互作用。本文扩展了分布式平面滤波器（“磁墙”）在整个阵列中分离各个元件的能力。构建了适用于 7T 神经成像的发射 RF 线圈。发射线圈由 10 个单独的表面线圈元件组成，并用磁墙进行去耦。使用单独的接收线圈阵列进行信号接收。发射线圈的硬件和成像性能通过电磁模拟、台式测量和体内 MRI 实验进行了验证。分析和测量结果证实，磁墙去耦方法在最小影响 B1（+）场分布的情况下，在发射通道之间提供了高隔离度。电磁模拟证实，与以前报道的使用不同去耦方法的 7T 多通道发射阵列相比，该去耦方法与局部特定吸收率（SAR）“热点”无关，也不会增加局部到全局 SAR 分数。
