Synopsis

In this presentation I will give an brief overview of X-nuclei MRI/MRS, of its challenges and potential clinical applications. I will mainly focus on ²³Na MRI and ³¹P MRS/MRI as examples of potentially useful non-proton imaging methods that could give interesting new metabolic information in vivo in a non-invasive and quantitative manner.

Introduction

Non-proton nuclei MRI (also called X-nuclei MRI) can provide new information on living tissues that is not available with proton (¹H) MRI. Standard ¹H MRI can generate images of the human body with many different contrasts, such as T1-weighted, T2-weighted, proton density-weighted, T1 and T2 maps, diffusion weighted imaging (DWI), diffusion tensor imaging (DTI), susceptibility weighted imaging (SWI), diffusion kurtosis imaging (DKI), dynamic contrast enhanced (DCE) MRI, and many more, that can provide structural/anatomical information on the tissues under investigation, to detect and monitor diseases. Proton MRI can also be adapted to be more sensitive to 'functions' in the body (mostly in brain, but also in muscle), generally through the technique of blood-oxygen dependent level (BOLD) MRI, which detects changes in signal which are mainly due to transient increases of blood flow to areas with increased neuronal activation, which alter the local ratio between oxy- and deoxyhemoglobin. Certain X-nuclei such as sodium (²³Na), phosphorus (³¹P), potassium (³⁹K), chlorine (³⁵Cl), and others, play an important role in the body metabolism (such as ion homeostasis or propagation of action potential) and can also be detected with magnetic resonance. Although these ions are in very low concentration in the body compared to the protons from water molecules (about 62% of the body atoms are hydrogen), and have different magnetic properties that can make them difficult to detect, X-nuclei MRI in a clinical environment is now possible due the recent technological advances of high-field MR systems (3 T, 7 T, 9.4 T), multichannel-dual-tuned RF coils, efficient acquisition pulse sequences and image reconstruction algorithms.

X-Nuclei MRI/MRS

X-nuclei in biological tissues

• ion homeostasis, natural abundance

NMR properties of X-nuclei

• spin, NMR sensitivity, frequencies, relaxation times, signal strentgh

Sodium ²³Na

• ²³Na NMR: spin 3/2, quadrupolar relaxation, biexponential relaxation
• Data acquisition: Ultrashort TE, non-Cartesian trajectories, resolution, SNR, multiple quantum filtering
• Data quantification: calibration, intracellular sodium concentration, cartilage sodium content

Phosphorus ³¹P

• ³¹P NMR: spin 1/2, metabolites, relaxation, chemical exchange, pH
• Data acquisition: CSI, UTE MRI, pulse selective, dynamic acquisition during exercise, saturation transfer
• Data quantification: calibration, pH, PCr, ATP, Pi

Other nuclei

• Chlorine ³⁵Cl
• Potassium ³⁹K
• Oxygen ¹⁷O
• Lithium ⁷Li
• Fluorine ¹⁹F
• Other hyperpolarized nuclei: ³He, ¹²⁹Xe, ¹³C

Potential clinical applications

Neurology

• Neurodegeneration in Alzheimer's disease
• Tumors
• Traumatic brain injury (TBI)
• Multiple Sclerosis (MS)

Musculosketal (MSK)

• Cartilage: Osteoarthritis, cartilage repair
• Muscle: Diabetes, muscular dystrophy, muscular channelopathy, myotonic dystrophy, hypertension
• Bone

Breast cancer

Heart

Kidney

Therapy monitoring

Limitations and prospects

Limitations

• SNR, acquisition time, resolution, sensitivity/specificity, accuracy, repeatability and reproducibility

Prospects

• Compressed sensing, MR fingerprinting (MRF), denoising, high-field MR systems, new acquisition sequences

Acknowledgements

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