QUANTITATIVE MAGNETIZATION TRANSFER BOUND POOL MAPPING AT 3T

本子项目是利用由NIH/NCRR资助的中心赠款提供的资源的众多研究子项目之一。子项目和研究者（PI）可能已经从另一个NIH来源获得了主要资金，因此可以在其他CRISP条目中表示。列出的机构是中心的，不一定是研究者的机构。导语：虽然磁化转移（MT）在检测白质疾病的细微变化方面很有用，但它在高场强下的使用受到SAR关注的限制。大多数MT序列使用sar密集的RF脉冲，或者使用复杂的参数模型，需要从RF干扰梯度回波（GRE）采集的大型数据集。白质池（WIMP）映射使用Ropele等人的受激回波（STE）方法，结合可变密度（VD）螺旋读数[3]，直接测量束缚质子分数（BPF）。然而，WIMP使用两个相似对比度的STE图像之间的差异，这会导致计算的BPF图像中的低信号（从而导致信噪比）。在1.5T的实验中，图像之间的平均差异仅<3%。虽然多次平均是提高信噪比的简单解决方案，但过度平均运动成为日益严重的问题。通过使用更高的场强（例如3T），可以获得额外的信号和信号差异。Ropele的方法对于高场应用特别有趣，因为除了非常低的SAR外，它既不需要使用MT RF脉冲，也不需要了解T1或T2值的基础。因此，这种方法在高场强下是可行的和实用的。方法：WIMP序列类似于受激回波准备，有三个？/2个射频脉冲。第二脉冲和第三脉冲由一个时间TM分开，其中包括两个相邻的2.4 ms ？/ 2脉冲。第二脉冲的相位可以被调制，从而产生0°或180°的总复合翻转角。化学选择性射频脉冲和梯度腐蚀立即在最终射频脉冲之前执行脂肪饱和，如图1所示。然后VD螺旋读数跟随这个bpf映射的WIMP准备。在GE Signa 3.0T扫描仪（GEHC, Waukesha， WI）上进行采集，梯度为40mT/m，使用标准正交鸟笼头线圈和8通道相控阵头线圈（MRI Devices, Milwaukee， WI）。扫描参数为：FOV =24cm, matrix=256x256， TR=3s， TE=6ms， TM=200ms， NEX=10，厚度均为5mm。WIMP标记梯度为15 mT/m，持续时间为250秒。变密度螺旋采用16条交织线，螺距系数2.5。为了计算相对绑定池大小（SWIMP）和绑定池分数（BPFWIMP），该序列运行两次。用正交头线圈获得的BPF图像如图2的顶部所示；而使用仅接收相控阵线圈获得的那些在底部一行。由于正交头线圈用于B1传输，而不是体线圈用于仅接收线圈，其较小的直径，即使在3T，也会在感兴趣的体积上产生更明显的B1变化。在图2右上方的最终定量BPF图中，残留的B1伪影很明显。在图2的右下方，本体线圈更均匀的激发轮廓有助于提高图像质量。尽管图像来自B1场，但图像质量显着，呈现出出色的灰/白对比度。白质中的BPF值与先前报道的一致[4-6]。

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Introduction: While magnetization transfer (MT) is useful in detecting subtle changes in white matter diseases, its use at high field strengths has been limited by SAR concerns [1]. Most MT sequences use SAR-intensive RF pulses, or use complex parametric models requiring large datasets from RF-spoiled gradient-echo (GRE) acquisitions. White matter pool (WIMP) mapping uses a stimulated echo (STE) method by Ropele et al. [2] combined with a variable density (VD) spiral readout [3] that directly measures the bound proton fraction (BPF). However, WIMP uses the difference between two similar-contrast STE images, which results in low signal (and thus SNR) in the computed BPF images. In experiments at 1.5T, an average of only <3% difference between the images was noted. While several averages are simple solution to increase SNR, with excessive averaging motion becomes an increasing issue. Both additional signal and signal difference may be possible via the use of higher field strength, e.g. 3T. Ropele¿s method is particularly interesting for high field applications since, aside from exceptionally low SAR, it requires neither the use of MT RF pulses nor the knowledge of underlying the T1 or T2 values. This method is therefore both feasible and practical at high field strength. Methods: A WIMP sequence is similar to a stimulated echo preparation, with three ?/2 RF pulses. The second and third pulses are separated by a time TM, which includes a composite refocusing pulse of two abutted 2.4 ms ?/2 pulses. The phase of the second pulse can be modulated, resulting in a total composite flip angle of 0¿ or 180¿. A chemically-selective RF pulse and gradient spoiling immediately precede the final RF pulse to perform fat saturation, as in Fig. 1. VD spiral readouts then follow this BPF-mapping WIMP preparation. Acquisitions in a volunteer were performed on a GE Signa 3.0T scanner (GEHC, Waukesha, WI) with 40mT/m gradients using the standard quadrature birdcage head coil, and an 8 channel phased array head coil (MRI Devices, Milwaukee, WI). Scan parameters are: FOV =24cm, matrix=256x256, TR=3s, TE=6ms, TM=200ms, NEX=10, thickness=5mm for both examinations. The WIMP labeling gradients were 15 mT/m and lasted 250 ?sec. The variable density spiral used 16 interleaves with pitch factor 2.5. In order to calculate the relative bound pool size (SWIMP) and the bound pool fraction (BPFWIMP), the sequence is run twice. BPF images attained with the quadrature head coil are presented in the top row of Fig. 2; while those acquired with the receive-only phased-array coil are in the bottom row. As the quadrature head coil is used for B1 transmission, rather than the body coil for the receive-only coil, its smaller diameter, even at 3T, gives a more marked B1 variation across the volume of interest. The residual B1 artifacts are evident in the final quantitative BPF maps on the top right in Fig. 2. The more even excitation profile of the body coil aids the image quality in the bottom right of Fig. 2. Despite the image apodization from the B1 field, image quality is remarkable, showing excellent grey/white contrast. BPF values in white matter are consistent with those previously reported [4-6].