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

研究 UHF 身体 MRI 中呼吸运动引起的电磁场和 SAR 变化

• 批准号: 405363511
• 负责人: Dr. Sebastian Schmitter
• 金额: --
• 依托单位: Fachbereich 8.1 - Biomedizinische Magnetresonanz
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2018
• 资助国家: 德国
• 起止时间: 2017-12-31 至 2022-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/405363511?language=en
• 关键词: Investigating; respiratory; motion; induced; changes

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.

临床磁共振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，工作场强≥7T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。超高频MRI已经成功地应用于针对人类头部的几种应用，但由于各种挑战，它的成功在身体目标上受到强烈的阻碍。磁共振成像过程的一个重要组成部分是由磁共振线圈产生的随时间射频（RF）场对目标体积内的自旋进行相干激发。然而，主场强的增加会导致射频场强度的空间变化增加，从而导致空间信号和对比度调制，最终产生非诊断质量的图像。同时，射频场的空间变异性增加，导致高比吸收率（SAR）的局部区域，从而导致局部组织加热。为了解决这些问题，一种称为“并行传输”（pTX）的技术已经成功应用，该技术利用由多个RF线圈元件组成的RF线圈。在这里，元件是同时驱动的，但每个元件是由一个独立的射频波形驱动的，这样叠加的射频场在降低sar的同时产生所需的空间信号强度。最近，几个独立的报告表明，这种射频场的形状在整个呼吸周期中也有很大的变化。因此，在呼气阶段具有均匀信号的pTX激励可以导致在吸气阶段具有局部信号缺失的图像。这种影响特别影响在自由呼吸期间进行的超高频采集，这目前是一个活跃的研究领域。目前的工作分为两个子项目。第一个子项目通过使用包含呼吸依赖变形的虚拟身体模型，系统地研究了呼吸对模拟射频场的影响。将根据不同的生理参数（呼吸类型、性别、体型）和技术参数（射频线圈类型、场强）分析呼吸引起的场变化强度。模拟将在3T和7T的模型和活体中进行验证。在第二个子项目中，将开发新的pTX技术，在整个呼吸周期中产生均匀且与呼吸无关的信号以及低SAR。这种脉冲将在7T和10.5T的模拟和活体中进行测试。后一种扫描将与美国明尼苏达大学的研究人员合作进行，该大学拥有目前世界上最高场强的全身MRI系统。我们将首次使用上述自由呼吸下的pTX技术获得高分辨率的心脏三维数据集。