Towards Patient-Specific and Reliable SAR Management for Parallel-Transmit Technology on Ultra-High Field MRI

人类大脑中的神经电路可能是任何电子工程师所能想到的最复杂的大规模电路：它包含大约1000亿个神经元，这些神经元通过数万亿条路径瞬间连接起来，产生思想、记忆、行动或情感。当神经回路中出现故障时，可能会出现不同的大脑紊乱。英国有1000万人患有神经疾病，相关的经济负担估计为每年1160亿英镑。随着欧洲和美国最近在脑成像项目上的重大投资，神经科学家需要开发具有超高信噪比(SNR)的新成像技术，如超高场磁共振成像，以展示复杂的神经电路如何相互作用，并最终产生预防和治疗神经疾病的方法。然而，7T磁共振图像上出现的各种伪影增加了MRI测量的不确定性，这是因为磁激发场的不均匀性增加了。解决这一问题的最新技术是利用一种新的射频技术：并行传输(PTX)，通过具有独特空间轮廓的多个射频激励线圈来实现均匀的磁化。要将这一工程解决方案应用于医学成像应用，迫切需要解决一个技术挑战：开发一种可靠的射频安全管理，通过比吸收率(SAR)来衡量，以确保该技术将满足射频暴露的法律限制。自从用于7T MRI的PTX出现以来，精确描述局部SAR的困难，特别是针对特定对象优化的PTX脉冲，极大地阻碍了其临床应用。目前在全球范围内采用电磁(EM)模拟进行的SAR估计需要昂贵的安全裕度(至少高出40%)来适应不确定性。最近由我们的项目合作伙伴(飞利浦医疗)首创的另一种SAR计算方法，基于B1-MAP的超快SAR计算，由于几个不切实际的默认假设，精度仍然很低，并且尚未与超高场MRI的定量实验测量进行验证。在这个项目中，我们将开发一种可靠和超快的SAR计算方法，通过首次将电磁数值解与B1映射相结合来解决这一工程挑战，以实现从使用通用模型的数值模拟到针对特定研究对象的主动管理SAR的范式转变。特别是，我们将解决与最新的基于MRI B1-MAP的超快SAR计算相关的精度较低的问题，使用互补的EM知识来补充缺失的信息。这将是首次尝试采用这种混合方法来解决这一全球研究前沿领域的MRI SAR问题。该项目被认为是及时的，因为它将解决2012年EPSRC/MRC医学成像技术工作组报告中提出的两个挑战：“更安全、更低成本和更高吞吐量的系统，并提高现有医学成像技术的价值”。该研究项目还将受益于与学术界(诺丁汉大学和昆士兰大学)和业界(飞利浦医疗保健和Pulseteq Ltd)在磁共振射频技术领域的领先国内和国际合作伙伴的密切合作，以确保从一开始就对英国的医疗行业产生直接影响。为了加速我们研究的影响，这里开发的SAR管理软件将向所有学术和临床MR研究人员提供免费许可。申请者还将积极参与卫生保护局的咨询小组，英国放射学会和ISMRM的安全委员会，他们审查我们的研究成果。

Neural circuitry inside a human brain is probably the most complicated large scale circuit that any electronic engineer could think of: it contains around100 billion neurons, instantly linked through trillions of pathways to generate thought, memory, action or emotion. When faults rise within the neural circuitry, different brain disorders may arise. There are 10 million people in the UK living with a neurological condition, and the associated economic burden is estimated to be 116 billion pounds per year. With the recent significant investments in Brain Mapping projects in both Europe and the US, Neuroscientists need to develop new imaging technologies with superior signal-to-noise ratio (SNR), such as Ultrahigh field Magnetic Resonance Imaging, to show how complex neural circuits interact and eventually yield methods of preventing and treating neurological disorders. However various image artifacts presented at 7T MR images raise uncertainty of MRI measurements, because of increased inhomogeneity of the magnetic excitation field. The latest technology to solve this problem is to utilise a novel RF technology: Parallel Transmission (pTX), through multiple RF excitation coils with unique spatial profiles to achieve the homogeneous magnetization. To adopt this engineering solution for medical imaging application, there is a preeminent need to address one technical challenge: to develop a reliable RF safety management, measured by Specific Absorption Rate (SAR), to ensure this technique will meet the RF exposure legal constraint. Since the advent of pTX for 7T MRI, the difficulty in accurately characterising the local SAR, especially for the pTX pulse optimised for a specific subject, greatly hampers its clinical application. The current practice for SAR estimation in use worldwide by adopting electromagnetic (EM) simulation requires an expensive safety margin (at least 40% higher) to accommodate the uncertainties. The other SAR calculation approach recently pioneered by our project partner (Philips Healthcare), B1-map based ultrafast SAR calculation, still suffers from poor accuracy due to several unrealistic default assumptions, and has not been validated against quantitative experimental measurements at Ultrahigh field MRI yet. Within this project, we will develop a trustworthy and ultrafast SAR calculation approach to address this Engineering challenge by combining the electromagnetic numerical solution with B1 mapping for the first time, to enable a paradigm shift from using numerical simulation with generic models to proactively managing SAR for specific subjects under examination. In particular we will tackle the poor accuracy problem associated with the latest MRI B1-map based ultrafast SAR calculation by using complementary EM knowledge to supplement the missing information. It will be the first attempt to adopt such a hybrid approach, which could draw the essence of each technique, to tackle the MRI SAR issue in this global research forefront area.The project is seen as timely because it will address two challenges raised in the 2012 EPSRC/MRC Medical Imaging Technology Working Group Report: "Safer, lower cost, and higher throughput systems and Improving the value of current medical imaging technologies". The research project will also benefit from close collaboration with leading national and international partners in MRI RF technology from both academia (Nottingham University and Queensland University) and industry (Both Philips Healthcare and Pulseteq Ltd) to ensure its immediate impact on Healthcare Industry in the UK from the outset. To accelerate the impact of our research, the SAR management software developed here will provide free licences to all academic and clinical MR researchers. The applicant will also engage actively with the Health Protection Agency's Advisory Group, British Institute of Radiology and Safety Committee of the ISMRM for their reviews on our research output.