High-field MR microscopy using fast spin-echoes.

Fast spin-echo imaging has been investigated with attention to the requirements and opportunities for high-field MR microscopy. Two- and three-dimensional versions were implemented at 2.0 T, 7.1 T, and 9.4 T. At these fields, at least eight echoes were collectable with a 10 ms TE from fixed tissue specimens and living animals, giving an eightfold improvement in imaging efficiency. To reduce the phase-encoding gradient amplitude and its duty cycle, a modified pulse sequence with phase accumulation was developed. Images obtained using this pulse sequence exhibited comparable signal-to-noise (SNR) to those obtained from the conventional fast spin-echo pulse sequences. Signal losses due to imperfections in RF pulses and lack of phase rewinders were offset in this sequence by reduced diffusion losses incurred with the gradients required for MR microscopy. Image SNR, contrast, edge effects and spatial resolution for three k-space sampling schemes were studied experimentally and theoretically. One method of sampling k-space, 4-GROUP FSE, was found particularly useful in producing varied T2 contrast at high field. Two-dimensional images of tissue specimens were obtained in a total acquisition time of 1 to 2 min with in-plane resolution between 30 to 70 microns, and 3D images with 256(3) arrays were acquired from fixed rat brain tissue (isotropic voxel = 70 microns) and a living rat (isotropic voxel = 117 microns) in approximately 4.5 h.