Quantitative Analysis of Magnetic Resonance Diffusion Tensor Imaging of the Human Brain – The effect of varying number of acquisitions and number of diffusion sensitizing directions

Magnetic resonance diffusion tensor imaging (DTI) is both a n advanced imaging technique and increasingly clinically important. DTI enables many optio ns and parameters such as number of slices, image matrix, slice thickness, diffusion weightin g (i.e. b-value), number of diffusion sensitizing directions, and number of measurements (NEX) to be kspace averaged in each direction. The aim of my master thesis project was to study which of sever al different combinations of DTI sequence parameters gives the best estimation of the dif fusion tensor (D) and a geometric measure of diffusion anisotropy the fractional anisotrop y (FA) index. The DTI parameters subject to experimental control were: (i) the number, K, of diffusion sensitizing gradient direction s, gk = (gkx,gky,gkz)(k = 1, . . . ,K;K ≥ 6), and (ii) the number of image excitations (NEX) used for averaging in e ach of the directions k = 1, . ,K. It is expected that both increasing K and NEX will increase th e quality of the images and the voxel-wise diffusion tensor derived from these image data. However, both factors are increasingly time consuming. The task was then to experimentally determi ne which combination of (i) and (ii) will give the best result for a fixed duration of measurement t ime (approximately 7 min). In this experiment we have used three different experimenta l setups (protocols) for each of the participating subjects, i.e. 1. K=6 and NEX =8; 2. K=13 an d NEX =4; 3. K=25 and NEX =2. Time consumption for each of these protocols was approxi mately 7 minutes. We compared results from 5 healthy volunteers using a GE Signa 1.5 Tesla E chospeed MR scanner. All subjects were males (age: 24-29 years). Other parameters that were co mmon for all subjects were: 24 axial slices covering the whole brain, 4mm slice thickness, 128x1 acquisition matrix (interpolated to 256x256), b=1000s/mm2. To analyze the goodness and difference between the three pro tocols we calculated standard deviation, mean and coefficient of variation of tissue speci fic (i.e. WM, GM, CSF using SPM2) fractional anisotropy values. We also plotted the standard eviation of FA, as a function of the principal direction of diffusion in three dimensional plot s (using spherical coordinates) and employed visual inspection of color-coded fractional anisot ropy images as well. All this was done to reveal differences in quality and direction dependent vari ation in FA between the three protocols. From our data and analysis we found that there was, in white ma ter, slightly higher quality for the protocol with K= 6 and NEX = 8 than for the other two prot ocols, and for CSF with K= 25 and NEX = 2 the quality was slightly higher than using K=6 an d NEX = 8. However the difference was too small to make any firm recommendations bet ween the three protocols. Since a large number of directions enables more sophisticated anal ysis (e.g. diffusion spectrum imaging, DSI) we recommend a protocol with a high number of directions (for a given measurement time). This is also in accordance with the latest recommendation fr om the vendor of the scanner where K=25 and NEX=1 is proposed in their standard DTI protocol.