| 摘要 |
Fast magnetic resonance imaging uses combined gradient echoes and spin echoes. In each of one or more TR intervals, after an initial NMR RF nutation pulse, a sequence of 180 DEG RF nutation pulses is used to refocus the RF response into corresponding string of spin echoes. However, in addition, during the time that such spin echo would normally occur after each such 180 DEG RF nutation pulse, a plurality of alternating polarity read-out magnetic gradient pulses is utilized so as to very rapidly form a sub-sequence of gradient echoes. This fast multi-section MRI sequence utilizes the speed advantages of gradient refocusing while overcoming the image artifacts arising from static field homogeneity and chemical shift. Image contrast is still determined by the T2 contrast in Hahn spin echoes. A novel k-space trajectory temporally modulates signals and demodulates artifacts. The echo responses are selectively phase-encoded and time shifted in occurrence so as to smoothly distribute unwanted phase shift from field inhomogeneity and/or chemical phase shift effects over the entire phase encoded dimension in k-space. The technique can also be extended so as to provide T2-weighted multi-slab three-dimensional volume images. |
