Division of MR Research, Department of Radiology, Johns Hopkins University, School of Medicine, Baltimore, Maryland 21287, USA.
Magn Reson Med. 2010 Mar;63(3):797-802. doi: 10.1002/mrm.22152.
The "loopless antenna" is an interventional MRI detector consisting of a tuned coaxial cable and an extended inner conductor or "whip". A limitation is the poor sensitivity afforded at, and immediately proximal to, its distal end, which is exacerbated by the extended whip length when the whip is uniformly insulated. It is shown here that tapered insulation dramatically improves the distal sensitivity of the loopless antenna by pushing the current sensitivity toward the tip. The absolute signal-to-noise ratio is numerically computed by the electromagnetic method-of-moments for three resonant 3-T antennae with no insulation, uniform insulation, and with linearly tapered insulation. The analysis shows that tapered insulation provides an approximately 400% increase in signal-to-noise ratio in trans-axial planes 1 cm from the tip and a 16-fold increase in the sensitive area as compared to an equivalent, uniformly insulated antenna. These findings are directly confirmed by phantom experiments and by MRI of an aorta specimen. The results demonstrate that numerical electromagnetic signal-to-noise ratio analysis can accurately predict the loopless detector's signal-to-noise ratio and play a central role in optimizing its design. The manifold improvement in distal signal-to-noise ratio afforded by redistributing the insulation should improve the loopless antenna's utility for interventional MRI.
“无环天线”是一种介入式 MRI 探测器，由调谐同轴线和延伸的内导体或“鞭状天线”组成。其局限性在于其远端和紧邻处的灵敏度较差，当鞭状天线均匀绝缘时，这种情况会因鞭状天线的延伸而加剧。本文表明，通过将电流灵敏度推向尖端，锥形绝缘可显著提高无环天线的远端灵敏度。通过电磁矩量法对三个无绝缘、均匀绝缘和线性渐变绝缘的谐振 3-T 天线进行了数值计算，得到了绝对信噪比。分析表明，与等效的均匀绝缘天线相比，锥形绝缘在距尖端 1 厘米的轴平面上提供了大约 400%的信噪比提高，并使敏感区域增加了 16 倍。这些发现直接通过体模实验和主动脉标本的 MRI 得到证实。结果表明，数值电磁信噪比分析可以准确预测无环探测器的信噪比，并在优化其设计中发挥核心作用。通过重新分配绝缘材料来提高远端信噪比，可以提高无环天线在介入性 MRI 中的实用性。
