Long component time constant of 23Na T*2 relaxation in healthy human brain

Signal intensity in Na-23 images is altered in pathologic conditions such as ischemia and may provide information regarding tissue viability complementary to MR diffusion and perfusion imaging. However, the multicomponent transverse relaxation of Na-23 (spin 3/2) complicates the determination of tissue sodium concentration from Na-23 images with nonzero echo-time. The purpose of this study was to measure the long component time constant of tissue sodium T-2(*) relaxation in the healthy human brain at 4 T. Multiecho gradient-echo Na-23 images (10 echo-times ranging from 3.8-68.7 ms) were acquired in five healthy human volunteers. T-2(*) was quantified on a pixel-by-pixel basis using a nonnegative least squares fitting routine using 100 equally spaced bins between 0.5-99.5 ms and parametric maps were produced representing components between 0.5-3, 3.1-50, 50.1-98, and 98.1-99.5 ms. The long T-2(*) component of tissue sodium (average +/- standard deviation) varied between cortex (occipital = 22.0 +/- 2.4 ms), white matter (parietal 18.2 +/- 1.9 ms), and subcortical gray matter (thalamus = 16.9 +/- 2.4 ms). These results demonstrate considerable regional variability and establish a foundation for future characterization of Na-23 T-2(*). in 2 conditions such as cerebral ischemia and cancer.