Parallel Excitation Methods for High Field MRI

描述(由申请人提供)：高场强(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，在许多情况下改善对比度。在专门研究中对这一潜力的展示导致了7T甚至9.4T人体扫描仪的普及。然而，将这些系统从研究带到临床舞台上存在着突出的方法学挑战。最受限制的问题之一是在身体上实现统一图像对比度的能力。这种混乱的来源是生物组织中RF波长的缩短导致B1场的空间不均匀，这反过来又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，在7T时，图像对比度可能会在头部上方变化一个数量级。在存在时变梯度的情况下，射频激发提供了多维选择性激发的可能性。在该方案中，对激励进行整形以减轻在头部测量的B1模式。主要的挑战是在足够短的时间内对3D激励进行编码，以便对各种序列有用，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程序来测试空间形状脉冲在7T时缓解头部B1不均匀性的能力。我们的第一个目标是开发所需的RF发射阵列，以减少使用发射感测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7T时头部发射感测方法的极限，包括大翻转角激励。我们的第三个目标是建立所需的方法，以安全和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通道的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.