Sodium ((23)Na) MRI may provide unique information about the cellular and metabolic integrity of the brain. The quantification of tissue sodium concentration from (23)Na 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 (TE(1) = 12 ms; TE(2) = 37 ms; averaged N(1) = 5; N(2) = 15 times; pulse repetition time = 130 ms). The results reveal that long T(2)(*) 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 (23)Na 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.
(c) 2009 Wiley-Liss, Inc.