Dependence of the fractional anisotropy in cervical spine from the number of diffusion gradients, repeated acquisition and voxel size.

The aim of this study is to investigate the consequences of using different gradient schemes, number of repeated measurements and voxel size on the fractional anisotropy (FA) value in a diffusion tensor imaging (DTI) sequence on the cervical tract of the spinal cord. Twenty healthy volunteers underwent a total of 86 DTI axial acquisitions performed by using different voxel size and number of diffusion gradient directions (NDGDs). Three different diffusion gradient schemes were applied, named 6, 15 and 32 according to the NDGD. Furthermore, some acquisitions were repeated to investigate the effects of image averaging on FA value. Our results indicate that the FA value in the cervical spinal cord decreases when increasing the NDGD for a fixed spatial resolution, or when identical acquisitions are repeated, thus, increasing the acquisition time. This effect is observed in all subjects without exceptions, and the differences result statistically significant: the average FA obtained from 6, 15 and 32 NDGD is 0.84 (range, 0.82-0.87), 0.75 (range, 0.68-0.80) and 0.70 (range, 0.65-0.77), respectively, for isotropic 8 mm(3) voxel size. When varying the spatial resolution in a volume range of 2 to 8 mm(3) for a fixed NDGD (6 or 15), the differences in FA values are smaller albeit still statistically significant: the smaller the voxel, the larger the FA. No significant dependence of the FA value from the spatial resolution is observed in the 32 NDGD acquisitions in the studied volume range. In conclusion, our results indicate that the value of the FA in the cervical tract of the spinal cord vary with regularity in intrasubject acquisitions when modifying the NDGD and when repeated acquisitions are used; these observations confirm that the signal-to-noise ratio introduces a systematic error in FA measurements that does not allow simple comparison of quantitative results obtained from separated studies.