Enhanced efficiency and sensitivity in detection of acute ischemic brain injury using fast diffusion kurtosis imaging

Yin Wu^{1,2}, Jinsuh Kim^{3}, Suk-Tak Chan^{1}, Iris Yuwen Zhou^{1}, Yingkun Guo^{1}, Takahiro Igarashi^{1}, Hairong Zheng^{2}, Gang Guo^{4}, and Phillip Zhe Sun^{1}

**MRI**: Eleven rats were induced
unilateral stroke with a standard intraluminal MCAO procedure and imaged with a
4.7 T MR scanner 60 minutes after the procedure. Five slices (slice
thickness/gap = 1.8/0.2 mm) were
acquired with a single-shot EPI sequence. Imaging parameters include: FOV = 20x20 mm^{2}, matrix size = 48x48, diffusion
duration/diffusion time = 6/20 ms, TR/TE = 2500/36.6 ms, one reference image of
b = 0 s/mm^{2}, NSA = 4. For the conventional DKI protocol, two b-values of 1000
and 2500 s/mm^{2} were applied in fifteen diffusion directions. The scan time was
5 min and 10 s. For the fast DKI protocol, three images of b = 1000 s/mm^{2} were applied along gradient directions of
(1,0,0), (0,1,0) and (0,0,1), and nine images of b = 2500 s/mm^{2} along diffusion
directions of $$$\widehat{n}^{(1)}=(1,0,0)^T$$$, $$$\widehat{n}^{(1+)}=(0,1,1)^T$$$ and $$$\widehat{n}^{(1-)}=(0,1,-1)^T$$$, and similarly for *i* =2 and 3. Note that the superscript *i*
in $$$\widehat{n}^{(i)}$$$ labels the position of
the “1”, while in $$$\widehat{n}^{(i+)}$$$ and $$$\widehat{n}^{(i-)}$$$ it labels the position
of the “0”. The fast
DKI scan time was 2 min and 10 s.

**Image analysis**: For conventional DKI, fractional anisotropy (FA), axial (D_{∥}) and radial (D_{⊥}) diffusivity, axial
(K_{∥}) and radial (K_{⊥}) kurtosis, and tensor-based MD_{tensor} and MK_{tensor}
were obtained using DKE. For the fast DKI, MD_{fast} was calculated as
the mean of MD_{x,y,z} as ^{3}: $$$MD\scriptsize x,y,z \normalsize = \frac{(b_{1}+b_{3})D_{x,y,z}^{(12)}-(b_{1}+b_{2})D_{x,y,z}^{(13)}}{b_{3}-b_{2}}$$$, where $$$D_{x,y,z}^{(ij)}=\frac{lnS(b_i)/S(0)-lnS(b_j)/S(0)}{b_j-b_i}$$$, *i* = 1, *j* = 2, 3, and b_{1} = 0, b_{2}
= 1000, and b_{3} = 2500 s/mm^{2}. Furthermore, MK_{fast} was
obtained from^{ 2}: $$
MK_{fast}=\frac{\frac{6}{15}[\sum_{i=1}^3ln\frac{S(b_3,\widehat{n}^{(i)})}{S(0)}+2\sum_{i=1}^3ln\frac{S(b_3,\widehat{n}^{{(i+)}})}{S(0)}+2\sum_{i=1}^3ln\frac{S(b_3,\widehat{n}^{(i-)})}{S(0)}]+6\cdot b_3\cdot MD_{fast}}{b_3^2\cdot MD_{fast}^2}.$$

**Image
segmentation**: The ischemic lesion was defined by
thresholding at two standard deviations (SD) below the baseline MD of the
contralateral normal brain. The reference ROI was designated by
mirroring the segmented lesion into the contralateral brain. The MK_{tensor},
MD_{fast} and MK_{fast} lesions were similarly defined.

**Data Analysis**: The CNR and CNR efficiency
were calculated as^{ 4}: $$$CNR=(S_{ischemia}-S_{contralateral})/{\sqrt{{(\sigma_{ischemia}^2+\sigma_{contralateral}^2)}/2}}$$$ and $$$CNR/\sqrt{scan time}$$$. Two-tailed Student’s t-test was performed
between paired measurements in ipsilateral ischemic and contralateral normal
regions. Measurement differences across the ROIs were tested using one-way
ANOVA with Bonferroni correction.

[1] Jensen JH, Helpern JA, Ramani A, Lu H, Kaczynski K. Diffusional kurtosis imaging: The quantification of non-gaussian water diffusion by means of magnetic resonance imaging. Magn Reson Med 2005;53(6):1432-1440.

[2] Hansen B, Lund TE, Sangill R, Jespersen SN. Experimentally and computationally fast method for estimation of a mean kurtosis. Magn Reson Med 2013;69(6):1754-1760.

[3] Jensen JH, Helpern JA. MRI quantification of non-Gaussian water diffusion by kurtosis analysis. NMR Biomed 2010;23(7):698-710.

[4] Sun PZ, Lu J, Wu Y, Xiao G, Wu R. Evaluation of the dependence of CEST-EPI measurement on repetition time, RF irradiation duty cycle and imaging flip angle for enhanced pH sensitivity. Phys Med Biol 2013;58:N229-N240.

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)

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