Oxford Centre for Clinical Magnetic Resonance Research, Division of Cardiovascular Medicine, Radcliffe Department of Medicine, University of Oxford, Oxford, United Kingdom.
Perspectum Diagnostics, Oxford, United Kingdom.
Magn Reson Med. 2019 Dec;82(6):2169-2177. doi: 10.1002/mrm.27884. Epub 2019 Jul 17.
The scattering matrix (S-matrix) of a parallel transmit (pTx) coil is sensitive to physiological motion but requires additional monitoring RF pulses to be measured. In this work, we present and evaluate pTx RF pulse designs that simultaneously excite for imaging and measure the S-matrix to generate real-time motion signals without prolonging the image sequence.
Three pTx waveforms for measuring the S-matrix were identified and superimposed onto the imaging excitation RF pulses: (1) time division multiplexing, (2) frequency division multiplexing, and (3) code division multiplexing. These 3 methods were evaluated in healthy volunteers for scattering sensitivity and image artefacts. The S-matrix and real-time motion signals were calculated on the image calculation environment of the MR scanner. Prospective cardiac triggers were identified in early systole as a high rate of change of the cardiac motion signal. Monitoring accuracy was compared against electrocardiogram or the imaged diaphragm position.
All 3 monitoring approaches measure the S-matrix during image excitation with quality correlated to input power. No image artefacts were observed for frequency multiplexing, and low energy artefacts were observed in the other methods. The accuracy of the achieved prospective cardiac gating was 15 ± 16 ms for breath hold and 24 ± 17 ms during free breathing. The diaphragm position prediction accuracy was 1.3 ± 0.9 mm. In all volunteers, good quality cine images were acquired for breath hold scans and dual gated CINEs were demonstrated.
The S-matrix can be measured during image excitation to generate real-time cardiac and respiratory motion signals for prospective gating. No artefacts are introduced when frequency division multiplexing is used.
并行发射（pTx）线圈的散射矩阵（S 矩阵）对生理运动敏感，但需要额外监测 RF 脉冲来进行测量。在这项工作中，我们提出并评估了 pTx RF 脉冲设计，这些设计可同时激发成像并测量 S 矩阵，以生成实时运动信号，而不会延长图像序列。
确定了三种用于测量 S 矩阵的 pTx 波形，并将其叠加到成像激励 RF 脉冲上：（1）时分复用，（2）频分复用和（3）码分复用。在健康志愿者中，对这 3 种方法的散射敏感性和图像伪影进行了评估。S 矩阵和实时运动信号是在 MR 扫描仪的图像计算环境中计算的。前瞻性心脏触发是在早期收缩期作为心脏运动信号的快速变化来识别的。监测准确性与心电图或成像的膈膜位置进行了比较。
所有 3 种监测方法在图像激发期间都测量了 S 矩阵，其质量与输入功率相关。频分复用没有观察到图像伪影，而其他方法观察到低能量伪影。获得的前瞻性心脏门控的准确性为屏气时为 15±16ms，自由呼吸时为 24±17ms。膈膜位置预测的准确性为 1.3±0.9mm。在所有志愿者中，均获得了屏气扫描的高质量电影图像，并演示了双门控电影。
可以在图像激发期间测量 S 矩阵，以生成用于前瞻性门控的实时心脏和呼吸运动信号。使用频分复用时不会引入伪影。
