Diffusion-weighted Imaging Contrast Research Articles

P11. Connectivity-based predictions of hand motor outcome at the sub-acute stage after stroke

Predicting motor recovery after stroke is a recognized aim in stroke rehabilitation. Here, we combine two different neuroimaging methods to test their prognostic power at the sub-acute stage after stroke. We measured resting state fMRI (rs-fMRI) and diffusion weighted imaging (DWI) in 19 stroke patients within the first days (5–9 days) after stroke and investigated short term (3 months) and long term (6 months) motor outcome. Resting-state functional connectivity (rs-FC) showed no relevant predictive value for motor outcome. We found trends for negative associations between the rs-FC of primary motor cortex (M1) of the lesioned hemisphere and dorsal premotor cortices (dPMC) of the contralesional side with grip force of the affected upper limb at 3 months. In addition, a poorer short-term outcome for pinch grip performance was associated with rs-FC of M1 and the supplementary motor area (SMA) in the lesioned hemisphere. In contrast DWI showed a predictive value for both short-term and long-term motor outcome. Integrity along the M1-pons tract showed good predictive value for a positive outcome at 6 months. Overall, structural connectivity measures derived from probabilistic DWI may offer better prognostic information than rs-fMRI at the early phase after stroke.

MR Microscopy for 3D Identification of Cortical Tubers, White Matter “Microtubers” and Radial Migration Lines in Ex Vivo Pediatric TSC with Epilepsy

We compared 7T MRI to optical microscopy for the detection of abnormalities in two pediatric TSC + AED-refractory resection specimens. 3D reconstruction of microscopy slides is challenging because there are non-uniform shrinkage artifacts and deformations. Nevertheless, 2D optical microscopy of stained blocks enables visualization of sub-cellular structures (<10 um). At 100 um resolution, 3D MRI enables visualization of cellular layers. Thus the 2D stained blocks provide a 'ground truth' for validating the features detected via 7T protocols designed to produce optimal T1w-, T2w-, and diffusion-weighted imaging contrast. After MR acquisition, the resection en bloc specimens were sectioned and stained with/for H&E, LFB, NFP, Smi31, and GFAP. The 7T images were reconstructed into a 3D volume and manually matched to the 2D histology slides. We show that (1) both 7T MRI and optical microscopy are sufficient to visualize cortical tubers and aberrant features in the white matter, (2) a “microtuber” in 2D stained slices is coincident with radial migration lines (RMLs) in 7T imaging and (3) diffuse abnormalities persist beyond the tuber in TSC. This preliminary study demonstrates the feasibility of MR microscopy for 3D characterization of abnormalities in two pediatric TSC cases. Hence, MRI may help identify treatment response biomarkers in TSC with epilepsy. Additionally, via histology we discovered a “microtuber” in the white matter of the optic radiation, which is coincident with RMLs in MRI. Next, we will pursue 3D reconstruction of microscopy slides and higher spatial resolution MRI via limited field-of-view or super-resolution reconstruction.

Intravoxel Incoherent Motion and Diffusion-Tensor Imaging in Renal Tissue under Hydration and Furosemide Flow Challenges

To assess the reproducibility and the distribution of intravoxel incoherent motion (IVIM) and diffusion-tensor (DT) imaging parameters in healthy renal cortex and medulla at baseline and after hydration or furosemide challenges. Using an institutional review board-approved HIPAA-compliant protocol with written informed consent, IVIM and DT imaging were performed at 3 T in 10 volunteers before and after water loading or furosemide administration. IVIM (apparent diffusion coefficient [ADC], tissue diffusivity [D(t)], perfusion fraction [f(p)], pseudodiffusivity [D(p)]) and DT (mean diffusivity [MD], fractional anisotropy [FA], eigenvalues [λ(i)]) imaging parameters and urine output from serial bladder volumes were calculated. (a)Reproducibility was quantified with coefficient of variation, intraclass correlation coefficient, and Bland-Altman limits of agreement; (b) contrast and challenge response were quantified with analysis of variance; and (c) Pearson correlations were quantified with urine output. Good reproducibility was found for ADC, D(t), MD, FA, and λ(i) (average coefficient of variation, 3.7% [cortex] and 5.0% [medulla]), and moderate reproducibility was found for D(p), f(p), and f(p) · D(p) (average coefficient of variation, 18.7% [cortex] and 25.9% [medulla]). Baseline cortical diffusivities significantly exceeded medullary values except D(p), for which medullary values significantly exceeded cortical values, and λ(1,) which showed no contrast. ADC, D(t), MD, and λ(i) increased significantly for both challenges. Medullary diffusivity increases were dominated by transverse diffusion (1.72 ± 0.09 [baseline] to 1.79 ± 0.10 [hydration] μm(2)/msec, P = .0059; or 1.86 ± 0.07 [furosemide] μm(2)/msec, P = .0094). Urine output correlated with cortical ADC with furosemide (r = 0.7, P = .034) and with medullary λ(1) (r = 0.83, P = .0418), λ(2) (r = 0.85, P = .0301), and MD (r = 0.82, P = .045) with hydration. Diffusion MR metrics are sensitive to flow changes in kidney induced by diuretic challenges. The results of this study suggest that vascular flow, tubular dilation, water reabsorption, and intratubular flow all play important roles in diffusion-weighted imaging contrast.

“Squashing peanuts and smashing pumpkins”: How noise distorts diffusion‐weighted MR data

New diffusion-weighted imaging (DWI) methods, including high-b, q-space, and high angular resolution MRI methods, attempt to extract information about non-Gaussian diffusion in tissue that is not provided by low-b-value (b approximately 1000 s mm(-2)) diffusion or diffusion tensor magnetic resonance imaging (DT-MRI). Additionally, DWI data with higher spatial resolution are being acquired to resolve fine anatomic structures, such as white matter fasciculi. Increasing diffusion-weighting or decreasing voxel size can reduce the signal-to-noise ratio so that some DWI signals are close to the background noise level. Here we report several new artifacts that can be explained by considering how background noise affects the peanut-shaped angular apparent diffusion coefficient (ADC) profile. These include an orientationally dependent deviation from Gaussian behavior of the ADC profile, an underestimation of indices of diffusion anisotropy, and a correlation between estimates of mean diffusivity and diffusion anisotropy. We also discuss how noise can cause increased gray/white matter DWI contrast at higher b values and an apparent elevation of diffusion anisotropy in acute ischemia. Importantly, all of these artifacts are negligible in the b-value range typically used in DT-MRI of brain (b approximately 1000 s mm(-2)). Finally, we demonstrate a strategy for ameliorating the rectified noise artifact in data collected at higher b values.