Normalized STEAM-based diffusion tensor imaging provides a robust assessment of muscle tears in football players: preliminary results of a new approach to evaluate muscle injuries

Objectives: To assess acute muscle tears in professional football players by diffusion tensor imaging (DTI) and evaluate the impact of normalization of data. Methods: Eight football players with acute lower limb muscle tears were examined. DTI metrics of the injured muscle and corresponding healthy contralateral muscle and of ROIs drawn in muscle tears (ROItear) in the corresponding healthy contralateral muscle (ROIhc_t) in a healthy area ipsilateral to the injury (ROIhi) and in a corresponding contralateral area (ROIhc_i) were compared. The same comparison was performed for ratios of the injured (ROItear/ROIhi) and contralateral sides (ROIhc_t/ROIhc_i). ANOVA, Bonferroni corrected post-hoc and Students t-tests were used. Results: Analyses of the entire muscle did not show any differences (p>0.05 each) except for axial diffusivity (AD; p=0.048). ROItear showed higher mean diffusivity (MD) and AD than ROIhc_t (p<0.05). Fractional anisotropy (FA) was lower in ROItear than in ROIhi and ROIhc_t (p<0.05). Radial diffusivity (RD) was higher in ROItear than in any other ROI (p<0.05). Ratios revealed higher MD and RD and lower FA and reduced number and length of fibre tracts on the injured side (p<0.05 each). Conclusions: DTI allowed a robust assessment of muscle tears in athletes especially after normalization to healthy muscle tissue. Key Points STEAM-based DTI allows the investigation of muscle tears affecting professional football players. Fractional anisotropy and mean diffusivity differ between injured and healthy muscle areas. Only normalized data show differences of fibre tracking metrics in muscle tears. The normalization of DTI-metrics enables a more robust characterization of muscle tears.(VLID)475075

In-vivo measurements of axon radius and density in the corpus callosum using anomalous diffusion from diffusion MRI

Axon radius and packing density provide information on the role and performance of white-matter pathways. MRI researchers have tried to measure axon radius, however, they assumed the radius of axons follows a gamma distribution or a single axon radius was considered. From diffusion-weighted data, we mapped axon radii and packing density in the corpus callosum by fitting the parameters of the space fractional Bloch-Torrey anomalous diffusion model. We were able to calculate axon radii and packing density without any assumptions of axon radius. We found the values of axon radii to be in good agreement with previous findings

Diffusion-weighted imaging with multiple diffusion time to assess water-exchange between restricted and hindered diffusion components in vivo

SynopsisWe performed multi-b and multi-diffusion-time DWI (MbMdt-DWI) on human brain to visualize the mixture of restricted and hindered diffusion components, and also the water exchange between them. The diffusion parameters including the exchange time were calculated. The observed signal patterns clearly indicated the existence of the inter-compartmental water exchange. The calculated exchange time was within the appropriate range assumed from a previous cell-based study in vitro. MbMdt-DWI may be useful for assessing micro-diffusion in human brain.\nPurposeTo assess the capability of multiple b-value with multiple diffusion-time (DT) diffusion-weighted imaging (MbMdt-DWI) to visualize the mixture of restricted and hindered diffusion-components and the water exchange between them in healthy human brain.\nMaterials and MethodsSeven healthy female volunteers were recruited for this study (20-33 years, mean 24). Their brain MbMdT-DWIs were acquired by 3T MRI (MAGNETOM Skyra, Siemens Healthcare, Erlangen, Germany) with a proto type sequence (Table 1). 11 b-values from 0 to 4000 sec/mm2 were selected, with two encoding directions, respectively. The separation times of the gradients (Δ) were set at 43.4, 63.4, and 83.4 msec, while the diffusion gradient duration (δ) was fixed at 25.0 msec. Regions-of-interest (ROI) were designated manually at the corticospinal tract of the left internal capsule (PLIC) and deep white matter of the left centrum semiovale (CS). A free-water phantom and a phantom of pure restricted-diffusion (Capillary Plate (CP), Hamamatsu Photonics, Japan) were scanned as well as references.1.DT dependency was assessed by plotting the intra-ROI signal intensity (the mean of the two encoding directions) of the subjects.2.A diffusion model based on the Karger model was assessed (Fig.1) [1-3]. The model consisted of restricted and hindered diffusion components (RDC and HDC: their fractions were fr and fh) with inter-compartment exchange. The measured signal at a certain DT was expressed as the sum of the signal from RDC (Cr(DT)) and HDC (Ch(DT)) (Fig.2 Eq.1). RDC was defined as the compartment of which the diffusion-coefficient (Dr) was inversely proportional to DT. A supplementary independent variable (A) was set to define this diffusion (A = Dr×DT) [3]. HDC was defined as the compartment with diffusion independent of DT. The diffusion-coefficient of HDC (Dh) was fixed at 0.0012 mm2/sec in this study. The inter-compartment exchange was defined by the exchange time from RDC to HDC (tr) and that from HDC to RDC (th) (Fig.2 Eq.2). The independent variables A, fr, and tr were calculated (Fig.2 Eq.3,4). The variables between PLIC and CS were statistically compared (Wilcoxon signed-rank test; P<0.05 was considered significant).\nResults1.Strong DT dependency nearly linear with the b-value was found in CP, while no DT dependency was found in free water (Fig.3, upper row). In PLIC and CS, DT dependency was found at high b-values. Signal-intensity was elevated or it was slightly decreased when DT was increased from Δ=43.4 to 63.4 msec, and was then decreased by increasing DT further from Δ=63.4 to 83.4 msec (Fig.3, lower row).2.The observed signal intensities were fit well by the signal-change-curve obtained from the calculated parameters of the proposed model (Fig. 2). The medians of fr and tr in PLIC were larger than those in CS, with significant differences. Statistical difference was not found in A (Table 2).\nDiscussion1.The model of mixed RDC and HDC was reasonable in the DTs applied in this study, because a DT relation was found in high b-values, but not in low b-values in vivo. Furthermore, the fact that the signal was first elevated (or slightly decreased) and then decreased as DT increased may prove the existence of inter-compartment water exchange, because if the compartments were independent, the difference between different DTs should have increased monotonically (as adding the signal of CP and free water).2.The significantly larger fr in PLIC than CS may suggest larger intra-axonal space, and the small difference in A may suggest a relatively consistent axon diameter by the analogy of the assessment of corticospinal tract by q-space imaging [4]. The significant difference found in tr (larger in PLIC) may possibly reflect myelin density. However, the results do not provide sufficient evidence to prove these hypotheses at this moment. Further study with larger numbers of MPG encoding directions, as well as longer diffusion time (requiring larger gradient strength to maintain TE) may support our results. On the other hand, another previous in vitro study that assessed water exchange in aquaporin-4-expressing and -non-expressing cells reported the exchange times from intra- to extra-cellular space as 43.1 msec and 100.7 msec, respectively [5]. The range included tr of PLIC and CS in this study, which may somewhat support the appropriateness of our results, as RDC may mostly belong to intracellular water.ISMRM 2016 2016 Annual Meetin

High-resolution diffusion kurtosis imaging at 3T enabled by advanced post-processing

Diffusion Kurtosis Imaging (DKI) is more sensitive to microstructural differences and can be related to more specific micro-scale metrics (e.g., intra-axonal volume fraction) than diffusion tensor imaging (DTI), offering exceptional potential for clinical diagnosis and research into the white and gray matter. Currently DKI is acquired only at low spatial resolution (2–3 mm isotropic), because of the lower signal-to-noise ratio (SNR) and higher artifact level associated with the technically more demanding DKI. Higher spatial resolution of about 1 mm is required for the characterization of fine white matter pathways or cortical microstructure. We used restricted-field-of-view (rFoV) imaging in combination with advanced post-processing methods to enable unprecedented high-quality, high-resolution DKI (1.2 mm isotropic) on a clinical 3T scanner. Post-processing was advanced by developing a novel method for Retrospective Eddy current and Motion ArtifacT Correction in High-resolution, multi-shell diffusion data (REMATCH). Furthermore, we applied a powerful edge preserving denoising method, denoted as multi-shell orientation-position-adaptive smoothing (msPOAS). We demonstrated the feasibility of high-quality, high-resolution DKI and its potential for delineating highly myelinated fiber pathways in the motor cortex. REMATCH performs robustly even at the low SNR level of high-resolution DKI, where standard EC and motion correction failed (i.e., produced incorrectly aligned images) and thus biased the diffusion model fit. We showed that the combination of REMATCH and msPOAS increased the contrast between gray and white matter in mean kurtosis (MK) maps by about 35% and at the same time preserves the original distribution of MK values, whereas standard Gaussian smoothing strongly biases the distribution

Intravoxel Incoherent Motion at 7 Tesla to quantify human spinal cord perfusion: Limitations and promises

International audiencePurposeTo develop a noninvasive technique to map human spinal cord (SC) perfusion in vivo. More specifically, to implement an intravoxel incoherent motion (IVIM) protocol at ultrahigh field for the human SC and assess parameters estimation errors.MethodsMonte‐Carlo simulations were conducted to assess estimation errors of 2 standard IVIM fitting approaches (two‐step versus one‐step fit) over the range of IVIM values reported for the human brain and for typical SC diffusivities. Required signal‐to‐noise ratio (SNR) was inferred for estimation of the parameters product, fIVIMD* (microvascular fraction times pseudo‐diffusion coefficient), within 10% error margins. In‐vivo IVIM imaging of the SC was performed at 7T in 6 volunteers. An image processing pipeline is proposed to generate IVIM maps and register them for an atlas‐based region‐wise analysis.ResultsRequired b = 0 SNRs for 10% error estimation on fIVIMD* with the one‐step fit were 159 and 185 for diffusion‐encoding perpendicular and parallel to the SC axis, respectively. Average in vivo b = 0 SNR within cord was 141 ± 79, corresponding to estimation errors of 12.7% and 14.7% according to numerical simulations. Slice‐ and group‐averaging reduced noise in IVIM maps, highlighting the difference in perfusion between gray and white matter. Mean ± standard deviation fIVIM and D* values across subjects within gray (respectively white) matter were 16.0 ± 1.7 (15.0 ± 1.6)% and 11.4 ± 2.9 (11.5 ± 2.4) × 10−3 mm2/s.ConclusionSingle‐subject data SNR at 7T was insufficient for reliable perfusion estimation. However, atlas‐averaged IVIM maps highlighted the higher microvascular fraction of gray matter compared to white matter, providing first results of healthy human SC perfusion mapping with MRI