First In Vivo Potassium-39 $(^{\bf 39}$ K) MRI at 9.4 T Using Conventional Copper Radio Frequency Surface Coil Cooled to 77 K

Potassium-39 ( ³⁹K) magnetic resonance imaging (MRI) is a noninvasive technique which could potentially allow for detecting intracellular physiological variations in common human pathologies such as stroke and cancer. However, the low signal-to-noise ratio (SNR) achieved in ³⁹K-MR images hampered data acquisition with sufficiently high spatial and temporal resolution in animal models so far. Full wave electromagnetic (EM) simulations were performed for a single-loop copper (Cu) radio frequency (RF) surface resonator with a diameter of 30 mm optimized for rat brain imaging at room temperature (RT) and at liquid nitrogen (LN₂) with a temperature of 77 K. A novel cryogenic Cu RF surface resonator with home-built LN₂ nonmagnetic G10 fiberglass cryostat system for small animal scanner at 9.4 T was designed, built and tested in phantom and in in vivo MR measurements. Aerogel was used for thermal insulation in the developed LN₂ cryostat. In this paper, we present the first in vivo ³⁹K-MR images at 9.4 T for both healthy and stroke-induced rats using the developed cryogenic coil at 77 K. In good agreement with EM-simulations and bench-top measurements, the developed cryogenic coil improved the SNR by factor of 2.7 ± 0.2 in both phantom and in in vivo MR imaging compared with the same coil at RT.