「Development of ultra-fast imaging (EPI) for a permanent magnet MRI system」
要旨
To implement the echo planar imaging (EPI) sequence in a 1.0 T permanent magnet MRI system, a temperature control system, a second order shim coil system, and a multiple gradient echo sequence were developed. The temperature control system consisting of a greenhouse and a PID controlled fan heater improved the stability of the magnet temperature about 4 times and was kept ±0.25 °C over 24 hours. The second order shim coil system reduced the inhomogeneity of the magnetic field in a 2 mm thick and 14.4 mm diameter disc region from 28.9 ppm to 11.4 ppm. This is 2.5 times improvement of the magnetic field homogeneity. The multiple gradient echo sequence produced 64 gradient echoes used for evaluation of the quality of the echoes. The quality of 7~8 echoes were sufficient for EPI but the other echoes were insufficient. Therefore, we concluded that further improvement of magnetic field homogeneity is required for the implementation of the EPI in our system.