HIGH-FIELD MR MICROSCOPY USING FAST SPIN-ECHOES

Fast spin-echo imaging has been investigated with attention to the req uirements and opportunities for high-field MR microscopy. Two- and thr ee-dimensional versions were implemented at 2.0 T, 7.1 T, and 9.4 T. A t these fields, at least eight echoes were collectable with a 10 ms TE from fixed tissue specimens and living animals, giving an eightfold i mprovement in imaging efficiency. To reduce the phase-encoding gradien t amplitude and its duty cycle, a modified pulse sequence with phase a ccumulation was developed. Images obtained using this pulse sequence e xhibited comparable signal-to-noise (SNR) to those obtained from the c onventional fast spin-echo pulse sequences. Signal losses due to imper fections in RF pulses and lack of phase rewinders were offset in this sequence by reduced diffusion losses incurred with the gradients requi red for MR microscopy. Image SNR, contrast, edge effects and spatial r esolution for three k-space sampling schemes were studied experimental ly and theoretically. One method of sampling k-space, 4-GROUP FSE, was found particularly useful in producing varied T2 contrast at high fie ld. Two-dimensional images of tissue specimens were obtained in a tota l acquisition time of 1 to 2 min with in-plane resolution between 30 t o 70 pm, and 3D images with 256(3) arrays were acquired from fixed rat brain tissue (isotropic voxel = 70 mum) and a living rat (isotropic v oxel = 117 mum) in approximately 4.5 h.