ITMC - RWTH-Aachen University, Worringer Weg 1, D52074 Aachen, Germany.
J Magn Reson. 2012 Feb;215:74-84. doi: 10.1016/j.jmr.2011.12.010. Epub 2011 Dec 28.
This paper describes a shimming approach useful to reduce the gradient strength of the magnetic field generated by single-sided sensors simultaneously maximizing its uniformity along the lateral directions of the magnet. In this way, the thickness of the excited sensitive volume can be increased without compromising the depth resolution of the sensor. By implementing this method on a standard U-shaped magnet, the gradient strength was reduced one order of magnitude. In the presence of a gradient of about 2 T/m, slices of 2mm could be profiled with a resolution that ranges from 25 μm at the center of the slice to 50 μm at the borders. This sensor is of particular advantage for applications, where the scanning range is of the order of the excited slice. In those cases, the full profile is measured in a single excitation experiment, eliminating the need for repositioning the excited slice across the depth range to complete the profile as occurs with standard high gradient sensors. Besides simplifying the experimental setup, the possibility to move from a point-by-point measurement to the simultaneous acquisition of the full profile led to the shortening of the experimental time. A further advantage of performing the experiment under a smaller static gradient is a reduction of the diffusion attenuation affecting the signal decay measured with a CPMG sequence, making it possible to measure the T(2) of samples with high diffusivity (comparable to the water diffusivity). The performance of the sensor in terms of resolution and sensitivity is first evaluated and compared with conventional singled-sided sensors of higher gradient strength using phantoms of known geometry and relaxation times. Then, the device is used to profile the structure of human skin in vivo. To understand the contrast between the different skin layers, the distribution of relaxation times T(2) and diffusion coefficients is spatially resolved along the depth direction.
本文描述了一种用于减小单侧面传感器产生的磁场梯度强度的匀场方法，同时最大限度地提高其在磁体横向方向上的均匀性。通过这种方法，可以在不影响传感器深度分辨率的情况下增加激发敏感体积的厚度。在对标准 U 形磁铁实施该方法后，梯度强度降低了一个数量级。在存在约 2 T/m 的梯度的情况下，可以对 2mm 的切片进行轮廓成像，分辨率范围从切片中心的 25μm 到边缘的 50μm。对于扫描范围与激发切片相当的应用，这种传感器具有特别的优势。在这些情况下，通过单次激励实验即可测量完整的轮廓，无需像标准高梯度传感器那样重新定位激发切片以完成整个轮廓。除了简化实验设置外，从逐点测量到同时获取完整轮廓的可能性也缩短了实验时间。在较小的静态梯度下进行实验的另一个优点是，它可以减少扩散衰减对 CPMG 序列测量的信号衰减的影响，从而可以测量具有高扩散率（与水扩散率相当）的样品的 T(2)。该传感器在分辨率和灵敏度方面的性能首先通过使用具有已知几何形状和弛豫时间的体模与更高梯度强度的传统单侧面传感器进行评估和比较。然后，该设备用于对人体皮肤的结构进行体内轮廓成像。为了理解不同皮肤层之间的对比度，沿着深度方向对弛豫时间 T(2)和扩散系数的分布进行空间分辨。
