Investigating respiratory motion induced changes on EM fields and SAR in UHF body MRI

临床MR成像中的大多数磁共振（MR）扫描仪在1.5T和3T的场强下工作。然而，在研究中，在≥7T的场强下工作的所谓超高场（UHF）MR扫描仪越来越多地用于实现更高的分辨率和更强的图像对比度。UHF MRI已成功应用于针对人体头部的几种应用中，但由于各种挑战，其成功受到身体目标的强烈抑制。MR成像过程的一个重要部分在于通过MR线圈产生的时间依赖性射频（RF）场对目标体积内的自旋进行相干激发。然而，主场强的增加导致该RF场的强度的空间变化增加，这导致空间信号和对比度调制，并且最终生成非诊断质量的图像。同时，RF场的空间变化增加导致具有高比吸收率（SAR）的局部区域，从而导致局部组织加热。为了解决这样的问题，已经成功地应用了利用由多个RF线圈元件组成的RF线圈的被称为“并行传输”（pTX）的技术。这里，元件被同时驱动，但是每个元件由独立的RF波形驱动，使得叠加的RF场在降低SAR的同时产生期望的空间信号强度。最近，几个独立的报告已经证明，这种RF场的形状在整个呼吸周期中也强烈变化。因此，在呼气阶段期间具有均匀信号的pTX激励可以导致在吸气阶段期间具有局部信号丢失的图像。这种影响，特别是影响超高频采集过程中进行自由呼吸，这是目前一个活跃的研究领域。第一个子项目通过使用包含呼吸相关变形的虚拟身体模型，系统地研究呼吸对模拟中RF场的影响。将针对不同的生理参数（呼吸类型、性别、体型）和技术参数（RF线圈类型、场强）分析呼吸诱导场变化的强度。将在体模和体内在3T和7T下验证模拟。在第二子项目中，将开发新的pTX技术，该技术在整个呼吸周期中产生均匀的、与呼吸无关的信号以及低SAR。这些脉冲将在体模和体内以7T和10.5T进行测试。后一种扫描将与美国明尼苏达大学的研究人员合作进行，该大学拥有目前全球最高场强的全身MRI系统。我们将首次在自由呼吸下使用上述pTX技术获取心脏的高分辨率3D数据集。

The majority of magnetic resonance (MR) scanners in clinical MR imaging operate at a field strength of 1.5T and 3T. In research, however, so-called ultra-high field (UHF) MR scanners operating at field strengths of ≥7T are increasingly being used to achieve higher resolutions and stronger image contrasts. UHF MRI has successfully been applied to several applications targeting the human head, but its success is strongly damped for body targets due to various challenges.An essential part of the MR imaging process consists in the coherent excitation of the spins within the target volume by a time-dependent radiofrequency (RF) field that is generated by the MR coil. An increase of the main field strength, however, results in increasing spatial variations of the intensity of this RF field, which causes spatial signal and contrast modulations and, ultimately, generates images of a non-diagnostic quality. At the same time, the increased spatial variations of the RF fields lead to localized areas with high specific absorption rate (SAR) that result in localized tissue heating. To address such problems, a technique termed 'parallel transmission' (pTX) that makes use of an RF coil consisting of multiple RF coil elements has successfully been applied. Here, the elements are driven simultaneously but each element is driven by an independent RF waveform such that the superposing RF field generates the desired spatial signal intensity while reducing the SAR.Recently, several independent reports have demonstrated that the shape of such RF fields also strongly varies throughout the respiratory cycle. Thus, pTX excitations with a homogeneous signal during the exhale phase can lead to images during the inhale phase with local signal dropouts. This effect in particular affects UHF acquisitions performed during free-breathing, which presently is an active research field.The present work is divided into two sub-projects. The first sub-project systematically investigates the impact of respiration on the RF fields in simulations by using virtual body models that contain respiration-dependent deformations. The strength of the respiration-induced field variations will be analyzed for different physiological parameters (type of respiration, gender, body size) and technical parameters (type of RF coil, field strength). Simulations will be verified in phantoms and in-vivo at 3T and 7T. In the second sub-project novel pTX techniques will be developed that generate homogeneous and respiration-independent signal as well as low SAR throughout the entire respiratory cycle. Such pulses will be tested in phantoms and in-vivo at 7T and 10.5T. The latter scans will be performed in collaboration with researchers from the University of Minnesota, USA, that hosts a whole-body MRI system with presently the highest field strength world-wide. For the first time, we will acquire highly resolved 3D datasets of the heart using above-mentioned pTX techniques under free-breathing.