Sodium Long-Component T2* Mapping in Human Brain at 7 Tesla

Sodium (Na-23) MRI may provide unique information about the cellular and metabolic integrity of the brain. The quantification of tissue sodium concentration from Na-23 images with nonzero echo time (TE) requires knowledge of tissue-specific parameters that influence the single-quantum sodium signal such as transverse (T-2) relaxation times. We report the sodium ((23) Na) long component of the effective transverse relaxation time T-2* values obtained at 7 T in several brain regions from six healthy volunteers. A two-point protocol based on a gradient-echo sequence optimized for the least error per given imaging time was used (TE1 = 12 ms; TE2 = 37 ms; averaged N-1 = 5; N-2 = 15 times; pulse repetition time = 130 ms). The results reveal that long T2* component of tissue sodium (mean +/- standard deviation) varied between cerebrospinal fluid (54 +/- 4 ms) and gray (28 +/- 2 ms) and white (29 +/- 2 ms) matter structures. The results also show that the long T-2* component increases as a function of the main static field B-0, indicating that correlation time of sodium ion motion is smaller than the time-scale defined by the Larmor frequency. These results are a prerequisite for the quantification of tissue sodium concentration from Na-23 MRI scans with nonzero echo time, will contribute to the design of future measurements (such as triple-quantum imaging), and themselves may be of clinical utility. Magn Reson Med 62: 1338-1341, 2009. (C) 2009 Wiley-Liss, Inc.