Parallel Excitation Methods for High Field MRI

描述（由申请人提供）：高视场（7T） MRI为临床和科学研究提供了许多潜在的优势，包括提高灵敏度和在许多情况下提高对比度。这种潜力在专门研究中的证明已经导致7T甚至9.4T人体扫描仪的扩散。然而，将这些系统从研究带到临床领域存在着突出的方法挑战。其中一个最受限制的问题是在身体上实现均匀图像对比度的能力。这种混淆的来源是生物组织中缩短的射频波长导致发射B1场的空间不均匀性，这反过来导致许多序列的对比度变化。在传统的T1加权体积采集中，在7T时，图像对比度会在头部上方发生一个数量级的变化。时变梯度下的射频激励提供了多维选择激励的潜力。在该方案中，激励的形状是为了减轻在头部测量到的B1模式。主要的挑战是在足够短的时间内对三维激励进行编码，以适用于各种序列，包括二维切片选择性序列以及三维（非选择性）采集。我们提出了一个开发程序来测试空间形状脉冲在7T时减轻头部B1不均匀性的能力。我们的第一个目标是开发射频发射阵列，以减少使用发送SENSE方法编码这些脉冲所花费的时间长度。我们的第二个目标是改进用于计算加速脉冲的方法，并测试在7T时头部发送SENSE方法的极限，包括高翻转角激励。我们的第三个目标是建立一种方法，该方法将需要安全准确地监测发射阵列的本地SAR水平，并评估与来自多个发射信道E场的建设性干扰相关的SAR惩罚。

DESCRIPTION (provided by applicant): Higher field (7T) MRI offers many potential advantages to clinical and scientific studies, including increased sensitivity and in many cases improved contrast. Demonstration of this potential in specialized studies has resulted in the proliferation of 7T and even 9.4T human scanners. There are, however, outstanding methodological challenges to bringing these systems from the research to the clinical arena. One of the most limiting issues is the ability to achieve uniform image contrast over the body. The source of this confound is the shortened RF wavelength in biological tissues leading to spatial inhomogeneities of the transmit B1 field, which, in turn lead to contrast variations in many sequences. In a conventional T1 weighted volume acquisition, the image contrast can change over an order of magnitude over the head at 7T. RF excitation in the presence of time-varying gradients offers the potential of multi-dimensional selective excitation. In this scheme, the excitation is shaped to mitigate the B1 pattern measured in the head. The principal challenge is to encode the 3D excitation in a short enough time to be useful for a wide variety of sequences, including 2D slice selective sequences as well as 3D (non-selective) acquisitions. We propose a development program to test the ability the spatially shaped pulses to mitigate B1 inhomogeniety in the head at 7T. Our first aim is to develop the RF transmit arrays needed to decrease the length of time spent encoding these pulses using the transmit SENSE method. Our second aim is to improve the methods used for calculating the accelerated pulses and test the limits of the transmit SENSE approach in the head at 7T, including high flip angle excitations. Our third aim is to build the methodology that will be needed to safely and accurately monitor local SAR levels for transmit arrays and assess the SAR penalty associated with constructive interference from the E fields of the multiple transmit channels.