Relaxation-normalized fast diffusion kurtosis imaging for semi-automatic segmentation of acute stroke lesion

Iris Yuwen Zhou^{1}, Yingkun Guo^{1,2}, Yu Wang^{3}, Emiri Mandeville^{4}, Suk-Tak Chan^{1}, Mark Vangel^{1}, Eng H Lo^{4}, Xunming Ji^{3}, and Phillip Zhe Sun^{1}

Adult
male Wistar rats were anesthetized throughout
the experiments with 1.5-2.0% isoflurane. Multiparametric
MRI was performed on two animal groups:
normal rats (N=9) and stroke rats within 2 hrs after standard middle cerebral
artery occlusion (MCAO, N=11) using a 4.7T
Bruker scanner (Bruker Biospec, Billerica, MA). Multi-slice
MRI (five 1.8-mm slices, FOV = 20x20 mm^{2}, matrix = 48x48) was
acquired with single-shot EPI. Fast DKI was acquired using three b-values: 0,
1000 (three directions), and 2500 (nine directions) s/mm^{2}, gradient
pulse duration/diffusion time (δ/Δ) = 6/20 ms, TR/TE = 2500/ 36.6 ms, 4
averages, scan time = 2 min 10 s^{5,6}. T1-weighted
images were acquired using an inversion recovery sequence, with seven inversion
delays ranging from 250 ms to 3000 ms (TR/TE = 6500/14.8 ms). T2-weigthed SE
images were obtained with two TEs of 30 and 100 ms (TR = 3250 ms). Images were
analyzed in MATLAB (MathWorks, Natick, MA). We calculated mean diffusivity (MD) as
described by Jensen *et al*.^{7}.$$MD_{x,y,z}=\frac{(b_{1}+b_{3})D_{x,y,z}^{(12)}-(b_{1}+b_{2})D_{x,y,z}^{(13)}}{b_{3}-b_{2}}$$ 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}. We have $$$MD_{fast}=\frac{MD_{x}+MD_{y}+MD_{z}}{3}$$$. Mean kurtosis (MK) was
obtained using the method described by Hansen *et al*.^{5} $$MK_{fast}=\frac{\frac{6}{15}(\sum_{i=1} ^3ln\frac{S(b_{3},\hat{n}^{(i)})}{S(0)}+2\sum_ {i=1}^3ln\frac{S(b_{3},\hat{n}^{(i+)})}{S(0)}+2\sum_ {i=1}^3ln\frac{S(b_{3},\hat{n}^{(i-)})}{S(0)})+6 \cdot b_{3} \cdot MD_{fast}}{b_3^2 MD_{fast}^{2}}$$ where $$$\hat{n}^{(1)}=(1,0,0)^{T}$$$, $$$\hat{n}^{(1+)}=(0,1,1)^{T}$$$ and $$$\hat{n}^{(1-)}=(0,1,-1)^{T}$$$, and similarly for i =2 and 3. Fractional anisotropy (FA) was estimated based
on standard diffusion tensor model.

[1] Sobesky J J. Cereb. Blood Flow Metab. 2012;32:1416-25.

[2] Yamada R, et al. Case Rep. Neurol. 2012;4:177-80.

[3] Jensen JH, et al. NMR Biomed. 2011;24:452-7.

[4] Cheung JS, et al. Stroke 2012;43:2252-4.

[5] Hansen B, et al. Magn. Reson. Med. 2013;69:1754-60.

[6] Sun PZ, et al. NMR Biomed. 2014;27:1413-8.

[7] Jensen JH, et al. NMR Biomed. 2010;23:698-710.

Fig. 1 (a) Comparison
of multi-parametric R_{1}, R_{2}, mean diffusivity (MD), fractional anisotropy (FA)
and mean kurtosis (MK) maps of a representative normal rat. (b)
Linear regression analysis between
kurtosis and multiple MRI indices of a representative normal rat.

Fig. 2 Comparison of R_{1},
conventional MK map, MK
map estimated from R_{1} map and relaxation-normalized MK (RNMK) map in (a) a representative normal rat and (b) a representative acute ischemic
stroke rat.

Fig. 3 Ischemic
tissue segmentation on the MD and RNMK maps. The size of
ischemic lesions in MD or RNMK maps were semi-automatically defined using a
threshold-based algorithm which counted the pixels with indices two standard deviations
above the mean values of contralateral normal hemisphere.

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

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