FAT AND SODIUM QUANTIFICATION AND CORRELATION BY MRSI

Lipids and sodium (²³Na) are two essential components of the human body. They play a role in almost all biological systems. However, an increase in their levels is associated with metabolic diseases. The elevation of their contents can cause similar health disorders. Examples of prevalent disorders that share an increase of musculoskeletal lipids and ²³Na are hypertension and diabetes. However, the relationship between in vivo lipid and sodium levels in pathophysiology has not been studied enough and therefore is still unclear. Additionally, the available quantification methods to facilitate such a study may not be practical. They are either invasive, not sensitive enough, or require an impractical measurement time.

Therefore, in this work, our aims were to develop practical in vivo methods to quantify the absolute sodium concentration as well as the concentration of each lipid component individually, and to study the correlation between them within the skeletal muscles.

Since lipids and ²³Na have different nuclear magnetic resonance properties, their quantification by magnetic resonance (MR) techniques face different challenges. Thus, we optimized different MR spectroscopic imaging (MRSI) techniques for lipids and ²³Na.

Our proposed proton MRSI was able to provide eight lipid fat fraction (FF) maps representing each musculoskeletal lipid component (fatty acid) detected by our MRSI technique, and demonstrated a superior sensitivity compared to the conventional MR imaging methods.

For ²³Na, our developed ²³Na-MRSI was able to measure and map the absolute ²³Na concentration with values agreeing with those reported previously in biopsy studies, and with a high repeatability (CV < 6 %) within significantly shorter acquisition time compared to other available techniques.

Finally, the ²³Na concentration and the fat fractions of each lipid component within healthy skeletal muscles were measured and correlated using our developed MRSI methods. Our findings suggest a positive regional relationship between ²³Na and lipids and negative correlation between ²³Na and BMI under healthy conditions.