Technical Development of Multinuclear Sodium Magnetic Resonance Imaging

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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.