Contrast at high field: relaxation times, magnetization transfer and phase in the rat brain at 16.4 T.

As field strength increases, the magnetic resonance imaging contrast parameters like relaxation times, magnetization transfer or image phase change, causing variations in contrast and signal-to-noise ratio. To obtain reliable data for these parameters at 16.4 T, high-resolution measurements of the relaxation times T(1), T(2) and T(2)*, as well as of the magnetization transfer ratio and the local frequency in the rat brain were performed. Tissue-specific values were obtained for up to 17 brain structures to assess image contrast. The measured parameters were compared to those found at different field strengths to estimate contrast and signal behavior at increasing field. T(1) values were relatively long with (2272 ± 113) ms in cortex and (2073 ± 97) ms in white matter, but did not show a tendency to converge, leading to an almost linear increase in signal-to-noise ratio and still growing contrast-to-noise ratio. T(2) was short with (25 ± 2) ms in cortex and (20 ± 1) ms in white matter. Magnetization transfer effects increase by around 25% compared to published 4.7 T data, which leads to improved contrast. The image phase, as novel and high-field specific contrast mechanism, is shown to obtain good contrast in deep brain regions with increasing signal-to-noise ratio up to high field strengths.