The proposed solution combines two techniques: (1) concomitant saturation bands; (2) snapshot k-ordering. In the first technique, the gap between two adjacent slices is made equal to or greater than the slice thickness. The excitation thickness is greater than the slice thickness, and the refocusing thickness is equal to the slice thickness. In-flow CSF sees the excitation pulse first, but no refocusing pulses, therefore it accumulates de-phasing from encoding and crushing gradients before arriving at the imaged plane. FSE sequence evolves into two concomitant saturation bands at both sides of each imaged slice, which suppress any rapidly flowing CSF, and reduce data inconsistency. Full spatial coverage is achieved by shifting the position of excited slices to un-acquired regions after the acquisition of a slice group. Though the concomitant saturation bands can significantly reduce the data inconsistency due to CSF flow, they cannot totally avoid such occurrence of in extreme cases such as when central k-space data are acquired at the peak flow of CSF ( e.g. in systolic phase), or when patient swallows. To further improve the success rate of such examination, a novel k-space view ordering is proposed (referred to as snapshot k-ordering). As demonstrated in Figure 2, data from a train of N echoes are sequentially filled into N adjacent k-space locations. A subsequent echo train is filled into the next N adjacent k-space locations with an inversed order and the procedure is repeated until all data are collected. The periodic modulation of the signal intensity due to T2 attenuation can be alleviated by averaging. In case of two averages, for example, all data are acquired twice with inversed order, and averaged during reconstruction. Alternatively, variable refocusing flip angles can be utilized to achieve smooth signal evolution over the echo train⁴. The proposed solution was implemented into a 3T clinical MR scanner. Over 20 volunteers were imaged with consent. For comparison, conventional and proposed FSE imaging were interleaved for each examination. Inversion recovery was used for fat suppression in all cases.

In the image acquired with conventional FSE sequence (Figure 3a), both the vertebra and the spinal cord were often seriously contaminated by pulsatile artifacts of CSF. In contrast, CSF artifacts were significantly reduced in the image acquired with the proposed FSE sequence with concomitant saturation bands (Figure 3b). The advantage of the snapshot k-ordering is its robustness against volunteers’ uncooperative behaviors, e.g. bulk motion, swallowing etc. Figure 4 shows the difference in image quality between the conventional and proposed FSE imaging when the volunteer became impatient and mobile.

By employing concomitant saturation bands, artifacts of rapidly moving CSF can be significantly reduced. Compared to conventional saturation band, concomitant saturation band is superior because it does not lengthen the minimum TR, and has not gap between the saturation band and the imaged slice. Much faster sampling of k-space central region is achieved with the snapshot k-ordering, which explains why the image quality of snapshot k-ordering was always more stable in the interleaved comparison with the conventional k-ordering. By combining these techniques, c-spine imaging with reliably high quality is made feasible even with normal Cartesian k-space sampling.