Sodium long-component T₂* mapping in human brain at 7 tesla

Sodium (²³Na) MRI may provide unique information about the cellular and metabolic integrity of the brain. The quantification of tissue sodium concentration from ²³Na images with nonzero echo time (TE) requires knowledge of tissue-specific parameters that influence the single-quantum sodium signal such as transverse (T₂) relaxation times. We report the sodium (²³Na) long component of the effective transverse relaxation time T₂* 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 N₁ = 5; N₂ = 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 T₂* component increases as a function of the main static field B₀, 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 ²³Na 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.