The effect of different number of diffusion gradients on SNR of diffusion tensor-derived measurement maps

th , 2008; revised Feb. 12 th , 2009; accepted Feb. 16 th , 2009 ABSTRACT Diffusion tensor imaging (DTI) is mainly applied to white matter fiber tracking in human brain, but there is still a debate on how many diffusion gradient directions should be used to get the best results. In this paper, the performance of 7 protocols corresponding to 6, 9, 12, 15, 20, 25, and 30 noncollinear number of diffusion gradi- ent directions (NDGD) were discussed by com- paring signal-noise ratio (SNR) of tensor- de- rived measurement maps and fractional ani- sotropy (FA) values. All DTI data (eight healthy volunteers) were downloaded from the website of Johns Hopkins Medical Institute Laboratory of Brain Anatomi- cal MRI with permission. FA, apparent diffusion constant mean (ADC-mean), the largest eigen- value (LEV), and eigenvector orientation (EVO) maps associated with LEV of all subjects were calculated derived from tensor in the 7 proto- cols via DTI Studio. A method to estimate the variance was presented to calculate SNR of these tensor-derived maps. Mean ± standard deviation of the SNR and FA values within re- gion of interest (ROI) selected in the white mat- ter were compared among the 7 protocols. The SNR were improved significantly with NDGD increasing from 6 to 20 (P 0.05). There were no significant variances in FA val- ues within ROI between any two protocols (P> 0.05). The SNR could be improved with NDGD in- creasing, but an optimum protocol is needed because of clinical limitations.