Double wave vector experiments were performed on seven healthy volunteers, with no known history of neurological disease. Written informed consent was given prior to the scan. Experiments were performed with a clinical whole body system operating at 3 T magnetic field strength (Ingenia, Philips, Amsterdam), using an 8-channel head coil array (receive only). An in-house implementation of a spin-echo echo planar imaging (EPI) sequence that incorporates two diffusion encoding periods was used (Figure 1). Both refocusing pulses were slice selective and crusher gradients were positioned immediately before and after the refocusing pulses.

Diffusion gradients were applied in the transversal plane (xy) perpendicular to the subjects’ body axes (z) such that the x and y components are equal. Thus, sixteen diffusion gradient directions were used as shown in Figure 3.

Figure 3 Scheme of diffusion gradient directions in the xy plane. Sixteen different diffusion gradient orientations were used, where **G**^{(1)} (bold line) is fixed and **G**^{(2)} (dashed line) is rotated about an angle *θ*.

Twenty transversal slices of 3 mm thickness were acquired using 3 x 3 mm^{2} nominal in-plane resolution, fat suppression using Spectral Presaturation with Inversion Recovery (SPIR), repetition time TR = 4.4 s for the first volunteer and TR = 6.5 s for the other six, echo time TE = 180 ms for the first six volunteers and TE = 200 ms on volunteer number 7 to allow variation of mixing time. Gradient duration = 10 ms, gradient amplitudes **G** = 44 mT m^{−1}, diffusion time *Δ* = 62 ms, gradient rise time *t*_{r} = 900 *µ*s, mixing time *τ*_{m} = *δ* + *t*_{r} for all seven volunteers, corresponding to a total diffusion weighting of *b* = 2 x 812 s mm^{−2} and 15 repetitions. An extra experiment was performed on volunteer number 7 using *τ*_{m} = *δ*_{r}+15 ms, so that a decrease of the modulation amplitude is expected.

In addition, T1-weighted and diffusion tensor images (b = 800 s mm^{−2} and 32 gradient directions) were acquired for radiology control and fibre direction estimation, respectively. The acquisition time for the DWV experiment measurement was approximately 30 min, total measurement was not longer than 2 hours.

The pore sizes were calculated using the mean squared radius of gyration 〈*R*^{2}〉

$${\u3008{R}^{2}\u3009}_{est}=\frac{3}{2}\frac{S\left(q,\pi \right)-S\left(q,0\right)}{{q}^{2}{S}_{0}},$$(4)which allows comparison of in vivo results with [2]. For a region-of-interest (ROI) analysis both left and right corticospinal tracts (CST) were delineated manually from a thresholded diffusion-weighted image.

The images were realigned to the first non-diffusion weighted image (b = 0 s mm^{−2}) to correct for subject motion, using the Diffusion toolbox extension (http://sourceforge.net/projects/spmtools) for SPM8 in Matlab R2012b (The MathWorks, Inc., Natick, Massachusetts), and then averaged over all repetitions and directions independently. First, the geometric mean is calculated over equivalent gradient orientations (*θ* = 0, *π*/2, *π* and 3*π*/2) separately. Then, the difference between parallel and perpendicular gradient orientation is given by

$$\begin{array}{l}S\left(q,0\right)-S\left(q,\pi /2\right)=\\ \text{\hspace{1em}}\text{\hspace{1em}}\text{\hspace{1em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\underset{\xaf}{\overline{\left(S\left(q,0\right)+S\left(q,\pi \right)\right)}-\overline{\left(S\left(q,\pi /2\right)+S\left(q,3\pi /2\right)\right)}}\\ \text{\hspace{1em}}\text{\hspace{1em}}\text{\hspace{1em}}\text{\hspace{1em}}\text{\hspace{1em}}\text{\hspace{1em}}\text{\hspace{1em}}\text{\hspace{1em}}\text{\hspace{1em}}\text{\hspace{1em}}\text{\hspace{1em}}\text{\hspace{1em}}\text{\hspace{0.17em}}2\end{array}$$(5)
## Comments (0)

General note:By using the comment function on degruyter.com you agree to our Privacy Statement. A respectful treatment of one another is important to us. Therefore we would like to draw your attention to our House Rules.