Effect of Different Number of Diffusion Gradients on Dispersion Degree of FA Values and Its SNR

To study the effect of different number of diffusion gradient directions (NDGD) of diffusion tensor imaging (DTI) on dispersion degree of fractional anisotropy (FA) values and its signal noise ratio (SNR) for adult brain tissues. Eight health volunteers were imaged by a 1.5T magnetic resonance scanner with different NDGD (6, 9, 12, 15, 20, 25, and 30 noncollinear) respectively, and seven FA maps associated with the different NDGD were obtained. Four region of interest (ROI) (genu and splenial of corpus callosum, genu and posterior limb of internal capsule) were chose in white matter of FA maps, and FA values and its SNR of the ROIs were computed and compared. Analysis of variance and independent-samples t-test were performed with a p value less than 0.05 regarded as statistical significance. Variance of FA values within the ROIs with stronger signals (genu and splenial of corpus callosum) fluctuated randomly and had no linear relationship with NDGD, and SNR increased slightly with NDGD increasing. But variance of FA values within the ROIs with weaker signals (genu and posterior limb of internal capsule) diminished significantly with NDGD increasing from 6 to 20, and slowly in the range from 20 to 30. SNR of FA values within the ROIs with weaker signals was improved significantly with NDGD increasing from 6 to 20 (P<;0.05), and had no significant change while NDGD increasing from 20 to 30 (P>0.05). To detect FA values and its fluctuation and SNR of the ROIs with stronger signals, NDGD = 6 is enough. for detecting those of the ROIs with weaker signals, however, considering that reducing of scanning time to lower possibility of movement-derived artifact and increasing of NDGD to improve precision of FA values, NDGD=20 is an optimal choice.