Sohn Sung-Min, DelaBarre Lance, Gopinath Anand, Vaughan John Thomas
Department of Electrical and Computer Engineering, University of Minnesota. He is now with Center for Magnetic Resonance Research (CMRR), University of Minnesota, Minneapolis, MN 55455 USA.
Center for Magnetic Resonance Research (CMRR), University of Minnesota, Minneapolis, MN 55454.
IEEE Trans Microw Theory Tech. 2014 Aug;62(8):1784-1789. doi: 10.1109/tmtt.2014.2331621.
Higher magnetic field strength in magnetic resonance imaging (MRI) systems offers higher signal-to-noise ratio (SNR), contrast, and spatial resolution in MR images. However, the wavelength in ultra-high fields (7 tesla and beyond) becomes shorter than the human body at the Larmor frequency with increasing static magnetic field (B) of MRI system. At short wavelengths, interference effect appears resulting in non- uniformity of the RF magnetic near-field (B) over the subject and MR images may have spatially anomalous contrast. The B near-field generated by the transverse electromagnetic (TEM) RF coil's microstrip line element has a maximum near the center of its length and falls off towards both ends. In this study, a double trapezoidal shaped microstrip transmission line element is proposed to obtain uniform B field distribution by gradual impedance variation. Two multi-channel RF head coils with uniform and trapezoidal shape elements were built and tested with a phantom at 7T MRI scanner for comparison. The simulation and experimental results show stronger and more uniform B near-field with the trapezoidal shape.
磁共振成像(MRI)系统中更高的磁场强度能在MR图像中提供更高的信噪比(SNR)、对比度和空间分辨率。然而,随着MRI系统静磁场(B)的增加,在超高场(7特斯拉及以上)中,拉莫尔频率下的波长变得比人体短。在短波长时,会出现干涉效应,导致射频磁场近场(B)在受试者身上不均匀,并且MR图像可能会出现空间异常对比度。由横向电磁(TEM)射频线圈的微带线元件产生的B近场在其长度中心附近最大,并向两端衰减。在本研究中,提出了一种双梯形微带传输线元件,通过逐渐改变阻抗来获得均匀的B场分布。构建了两个分别具有均匀形状元件和梯形形状元件的多通道射频头部线圈,并在7T MRI扫描仪上用体模进行测试以作比较。模拟和实验结果表明,梯形形状的B近场更强且更均匀。