Despite its widespread use, diffusion-weighted MRI (DWI) provides crude stratification of heterogeneous ischemic tissue injury^1,2. Recent studies have shown that diffusion kurtosis imaging (DKI), measuring non-Gaussian diffusion, complements DWI for defining irreversible ischemic injury^3,4. However, the conventional DKI acquisition time is relatively long, limiting its use in the acute stroke setting. In addition, the complexity of cerebral structure/composition makes kurtosis map heterogeneous, limiting the specificity of kurtosis hyperintensity to acute ischemia. Herein we developed relaxation-normalized fast DKI for the improved characterization of ischemic tissue injury, aiding the translation of fast DKI to the acute stroke setting.

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 mm², 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/mm², 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 b₁=0, b₂=1000, and b₃=2500 s/mm². 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.

Fig. 1a shows parametric T₁, T₂, MD, FA, and MK maps from a representative normal Wistar rat brain. Fig. 1b compares the relationship between MK and MD, FA, relaxation rates of R₁ and R₂ in normal brain, and found strong correlation between MK and R₁ (P<0.001). Based on this correlation, we used the univariate linear regression coefficients determined from R1 and MK (MK_est= 1.36*R₁-0.22) to generate relaxation-normalized mean kurtosis (RNMK=MK/MK_est) maps. Fig. 2a shows that the RNMK approach could suppress intrinsic kurtosis heterogeneity in the intact brain. Importantly, it can also enhance ischemic kurtosis lesion segmentation over conventional MK map (Fig. 2b). Fig. 3 shows substantial MD and RNMK lesion mismatch in an acute stroke rat. We found significantly different RNMK and MD lesion volume, being 135±78 and 157±86 mm³, respectively (P<0.01, Paired-t test). Moreover, there was no significant difference in MD value from MD and RNMK lesions (0.62±0.04 µm²/ms vs. 0.61±0.03 µm²/ms, P>0.05, Paired-t test) while RNMK was significantly different between MD and RNMK lesions (1.52±0.15 vs. 1.70±0.13, P<0.01, Paired-t test). Our study demonstrates that relaxation-normalized fast DKI reasonably corrects for intrinsic regional variation in cerebral kurtosis, enabling semi-automatic segmentation of ischemic kurtosis lesion with reduced scan time. The relaxation-normalized kurtosis analysis represents a promising approach to aid elucidation of the diagnostic value of DKI in stroke prior to its translation to the acute stroke setting.