Technical Development of Multinuclear Sodium Magnetic Resonance Imaging

在过去的30年里，工程技术的进步使磁共振成像（MRI）成为一种强大的非侵入性人体研究的成像方式。常规临床MRI测量水中氢核（1H）信号。然而，通过适当的硬件和成像技术，MRI也可以获得来自其他核的信号，如钠（23Na）和钾（39K），这些离子可能比无处不在的水分子更具体地反映组织损伤和疾病的各个方面。钠的成像能力可以为骨关节炎的软骨退化提供一种测量方法，骨关节炎是一种影响数百万加拿大人的衰弱性疾病。钾是影响脑功能的关键离子，但目前还缺乏测量人脑钾含量的方法。这项拨款的目的是开发高灵敏度的射频（RF）线圈硬件（例如敏感相控阵）和成像方法，以便对软骨（23Na）或大脑（39K）中的这些核进行定量成像。这项提议建立在我们十年来钠核磁共振研究的基础上，该研究产生了世界领先的人类大脑和膝盖图像（由NSERC在过去5年资助）。

Engineering advances over the last 30 years have made magnetic resonance imaging (MRI) a powerful imaging modality for the non-invasive investigation of the human body. Routine clinical MRI measures signal from the hydrogen (1H) nuclei in water. However, with the appropriate hardware and imaging techniques, MRI can also acquire signal from other nuclei such as sodium (23Na) and potassium (39K), ions which may be more specific to aspects of tissue injury and disease than the ubiquitous water molecule. The ability to image sodium can provide a measure of cartilage degradation in osteoarthritis, a debilitating condition affecting millions of Canadians. Potassium is a key ion in brain function, but methods to measure it in human brain are lacking. The purpose of this grant is to develop highly sensitive radiofrequency (RF) coil hardware (e.g. sensitive phased arrays) and imaging methodology to enable quantitative imaging of these nuclei either in cartilage (23Na) or brain (39K). This proposal builds on our decade’s worth of sodium MRI research, which produced world-leading images of human brain and then knee (funded over last 5 years by NSERC). Sodium (and potassium) MRI is very challenging because of low concentration in tissue, small magnetogyric ratio, rapid signal decay, complex spin physics, and the need for nucleus-specific hardware and optimal methods. Our previous sodium MRI research on cartilage of the knee focused on the design of optimal acquisition strategies, but we used only standard volume RF coils. However, more complex and sensitive phased-array RF coils, which consist of many small localized elements, can dramatically increase signal-to-noise ratio and thus yield major gains in image resolution and quantification. While phased-array technology is now standard for regular 1H MRI, it is in its infancy for sodium MRI and needs to be explored not only for knee, but also for other body regions such as wrist, ankle, hip, and spine (where cartilage is also affected by osteoarthritis). Potassium has even greater technical challenges to overcome than sodium, and there are only two recent preliminary reports of potassium MRI in human brain. The novel MRI hardware and methods will be designed, simulated, constructed, programmed, tested, and published by the trainees. The developments will be on a ‘triple strength’ high field 4.7T MRI to enable major gains in signal. We hypothesize that our technical MRI advances will enable the accurate and precise measurement of sodium in cartilage and potassium in brain. Specific Aims: 1) To develop phased-array receive-only sodium RF coils and dual-frequency (23Na/1H) detunable, concentric transmit volume coils for imaging the knee, ankle, and wrist. In this case the volume coil provides uniform excitation and the phased-array provides high receive sensitivity. The capability for simultaneous reception from both the phased-array and volume coils using counter-rotating-current coil elements will be explored to facilitate accelerated image intensity correction related to the spatially varying RF sensitivity profiles of the phased-array elements. 2) To develop transceive dual-frequency phased-array RF coils for sodium MRI of the spine and hip, where uniform volume excitation is not feasible for sodium. An associated aim is to develop and optimize multiple coil element transmission for uniform excitation. 3) To develop quantitative potassium MRI of human brain. After 5 years, we will have developed novel multi-nuclear radiofrequency coil hardware and optimized MRI acquisition to enable imaging of (i) sodium in cartilage of the extremities, hip, and spine which are key structures affected in osteoarthritis, and (ii) potassium in the brain, a novel biomarker of brain metabolism.