Abstract: This proposal evaluates sodium MRI and sodium relaxometry as markers of tumor invasiveness and grade of human breast cancer in a pilot study. Concentration of sodium in tissue may be a valuable molecular marker of malignancy. Previous studies have shown elevated sodium levels in large breast cancers. Sodium relaxometry parameters T1 and biexponential T2* have also been shown to be associated with malignancy but have not yet been systematically investigated in breast cancer. Our proposal includes two specific aims: 1) to develop and implement optimized sodium breast MRI hardware and protocols to maximize the accuracy of sodium concentration estimation and relaxometry, and 2) to conduct a pilot study of quantitative sodium MRI in patients scheduled for biopsy of suspicious lesions found by x-ray mammograms, ultrasound, or conventional MRI. This study will explore whether sodium levels and relaxometry data can discriminate between benign and malignant lesions. Data from this pilot study will provide preliminary data for a future proposal for more extensive large scale evaluation of sodium MRI for breast cancer. Our long-term motivation is to improve the specificity of breast MRI for detection of breast cancer. If the specificity of breast MRI can be increased through the use of sodium imaging, this will decrease the number of unnecessary breast biopsies and will provide more accurate information about extent of disease in patients with a new diagnosis of breast cancer to aid in the choice of appropriate therapy. The proposed technology development and pilot study are important steps toward this ultimate goal.