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

At ultrahigh magnetic field strengths ( B 0  ≥ 7.0 T), potassium ( 39 K) MRI might evolve into an interesting tool for biomedical research. However, 39 K MRI is still challenging because of the low NMR sensitivity and short relaxation times. In this work, we demonstrated the feasibility of 39 K MRI at 21.1 T, determined in vivo relaxation times of the rat head at 21.1 T, and compared 39 K and sodium ( 23 Na) relaxation times of model solutions containing different agarose gel concentrations at 7.0 and 21.1 T. 39 K relaxation times were markedly shorter than those of 23 Na. Compared with the lower field strength, 39 K relaxation times were up to 1.9‐ ( T 1 ), 1.4‐ ( T 2S ) and 1.9‐fold ( T 2L ) longer at 21.1 T. The increase in the 23 Na relaxation times was less pronounced (up to 1.2‐fold). Mono‐exponential fits of the 39 K longitudinal relaxation time at 21.1 T revealed T 1  = 14.2 ± 0.1 ms for the healthy rat head. The 39 K transverse relaxation times were 1.8 ± 0.2 ms and 14.3 ± 0.3 ms for the short ( T 2S ) and long ( T 2L ) components, respectively. 23 Na relaxation times were markedly longer ( T 1  = 41.6 ± 0.4 ms; T 2S  = 4.9 ± 0.2 ms; T 2L  = 33.2 ± 0.2 ms). 39 K MRI of the healthy rat head could be performed with a nominal spatial resolution of 1 × 1 × 1 mm 3 within an acquisition time of 75 min. The increase in the relaxation times with magnetic field strength is beneficial for 23 Na and 39 K MRI at ultrahigh magnetic field strength. Our results demonstrate that 39 K MRI at 21.1 T enables acceptable image quality for preclinical research. Copyright © 2016 John Wiley & Sons, Ltd.