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优势，包括提高灵敏度以及在许多情况下提高对比度。专门研究证明了这种潜力，导致 7T 甚至 9.4T 人体扫描仪的激增。然而，将这些系统从研究转移到临床领域存在突出的方法学挑战。最大的限制问题之一是在整个身体上实现均匀图像对比度的能力。这种混淆的根源是生物组织中 RF 波长的缩短，导致发射 B1 场的空间不均匀性，进而导致许多序列中的对比度变化。在传统的 T1 加权体积采集中，图像对比度可以在 7T 的头部上发生一个数量级的变化。存在时变梯度的射频激励提供了多维选择性激励的潜力。在此方案中，激励的形状可减轻头部测量的 B1 模式。主要挑战是在足够短的时间内对 3D 激励进行编码，以便可用于各种序列，包括 2D 切片选择性序列以及 3D（非选择性）采集。我们提出了一个开发计划来测试空间整形脉冲减轻 7T 头部 B1 不均匀性的能力。我们的首要目标是开发所需的射频发射阵列，以减少使用发射 SENSE 方法对这些脉冲进行编码所花费的时间。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 7T 头部中发射 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安全、准确地监测发射阵列的本地 SAR 水平所需的方法，并评估与多个发射通道的 E 场的相长干扰相关的 SAR 损失。

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.