39K and 23Na relaxation times and MRI of rat head at 21.1 T

At ultrahigh magnetic field strengths (B0 ≥ 7.0 T), potassium (39K) MRI might evolve into an interesting tool for biomedical research. However, 39K MRI is still challenging because of the low NMR sensitivity and short relaxation times. In this work, we demonstrated the feasibility of 39K MRI at 21.1 T, determined in vivo relaxation times of the rat head at 21.1 T, and compared 39K and sodium (23Na) relaxation times of model solutions containing different agarose gel concentrations at 7.0 and 21.1 T. 39K relaxation times were markedly shorter than those of 23Na. Compared with the lower field strength, 39K relaxation times were up to 1.9‐ (T1), 1.4‐ (T2S) and 1.9‐fold (T2L) longer at 21.1 T. The increase in the 23Na relaxation times was less pronounced (up to 1.2‐fold). Mono‐exponential fits of the 39K longitudinal relaxation time at 21.1 T revealed T1 = 14.2 ± 0.1 ms for the healthy rat head. The 39K transverse relaxation times were 1.8 ± 0.2 ms and 14.3 ± 0.3 ms for the short (T2S) and long (T2L) components, respectively. 23Na relaxation times were markedly longer (T1 = 41.6 ± 0.4 ms; T2S = 4.9 ± 0.2 ms; T2L = 33.2 ± 0.2 ms). 39K MRI of the healthy rat head could be performed with a nominal spatial resolution of 1 × 1 × 1 mm3 within an acquisition time of 75 min. The increase in the relaxation times with magnetic field strength is beneficial for 23Na and 39K MRI at ultrahigh magnetic field strength. Our results demonstrate that 39K MRI at 21.1 T enables acceptable image quality for preclinical research. Copyright © 2016 John Wiley & Sons, Ltd. 39K and 23Na relaxation times show a distinct increase with magnetic field strength, which is beneficial for MRI at ultrahigh magnetic field strengths (e.g. 21.1 T). This increase is more pronounced for 39K MRI. 39K MRI largely benefits from ultrahigh magnetic fields, and the acquired in vivo 39K MR images of healthy rat brain show markedly improved resolution and image quality compared with previous work performed at 9.4 T.