High angular resolution diffusion imaging reveals intravoxel white matter fiber heterogeneity

“, 2002), or is chosen by a statistical criterion (Parker and Alexander, 2003; Tuch et al., 2002; Behrens et al., 2007).”

“2012) using DSI tractography (Tuch et al. 2002; Wedeen et al. 2008) in six subjects as well as the publicly available NTU-90 DSI atlas (Yeh and Tseng 2011) and complemented by fiber microdissection in four subjects, demonstrated the MdLF connection between TP, STG and SPL (BA 7), precuneus (BA 7) and cuneus (CN, BA 18…”

“For voxels that suffer from " partial voluming " (PV)—also known as " partial volume effect " , " powder averaging " , or " intra-voxel heterogeneity " —the observed diffusion signal is a combination of the signals associated with each tissue type (Frank 2001; Tuch et al. 2002).”

“To achieve a more complete map of the probability density function within each voxel, new methods such as high angular resolution diffusion imaging (HARDI) (Tuch et al., 2002) or diffusion spectrum imaging (DSI) (Wedeen et al., 2005) were developed to sample the three-dimensional probability distribution using scans f…”

“The DTI model cannot differentiate these components or even crossing fibers (Tuch et al., 2002; Jbabdi et al., 2010), which are estimated to occur in up to 90% of WM voxels at the typical dMRI resolution (Descoteaux et al., 2009; Jeurissen et al., 2013).”

“Diffusion magnetic resonance imaging (MRI) techniques (diffusion tensor imaging and high angular resolution diffusion imaging) utilizing the anisotropic diffusion of water molecules along the fiber tracts have been developed recently to noninvasively study brain connectivity (Basser et al., 1994; Conturo et al., 1999;…”

“The complex fibrous structure of white matter can be further resolved using methods of high angular resolution diffusion weighted imaging (HARDI) to determine the water displacement probability function (Tuch et al., 2002).”

“Beyond DTI-Based Tractography: HARDI Models High angular resolution diffusion imaging (HARDI) uses a larger number of diffusion gradient directions, often in combination with multiple b-values, to measure the diffusion signal (68).”

“However, DTI tractography is known to have limitations primarily resulting from partial volume averaging of complex fiber architecture (such as sharp curves and crossings) into single voxels (Alexander et al., 2001; Tuch et al., 2002, 2003).”

“However, DTI is limited tomodeling a single-fiber orientation per voxel and is therefore incapable of resolving complex intra-voxel geometry such as crossingfibers (Alexander et al., 2002; Frank, 2001; Tuch et al., 2002), which are thought to occur in at least one-third of voxels in white-matter (Behrens et al.”

“In DTI, the angular distribution of T r ^ D̄r ^ is often plotted in 3-dimensions, exhibiting a profile that can be ‘‘peanut’’ or ‘‘discoid’’ shaped in brain white matter [34,35].”

“, 2005), multi-tensor (Tuch et al., 2002) or ball-and-stick models (Behrens et al.”

“This is a case of high angular diffusion weighted imaging [21], [22] where the variance of the signal could give important information on the multimodal aspect of diffusion.”

“However, it is limited by a single tensor model, which does not take into account complex fiber configurations such as crossings and branchings (Tuch et al., 2002; Kubicki et al., 2011).”

“For HARDI, there is no specialized report on the effect of he number of gradient directions on the structural brain network easures.”

“[53]”

“To solve the problem of branching, kissing or crossing fibers algorithms have been developed, like the mixture of tensors (30), higher rank tensor (33,34) ball-and-stick (35), Q-ball imaging algorithm (36,37) and diffusion spectrum imaging (38) They try to estimate”

“Another possible limitation is that with basic deterministic tracking it is impossible to accurately reconstruct all the major pathways, especially where fibers branch and cross [Tuch et al., 2002].”

“Thereby, higher order mathematical tensor models, as proposed in different studies (Frank, 2001, 2002; Tuch, 2004; Tuch et al., 2002, 2003), could provide a possible basis.”

“However, it is well known that most of the white matter tissue in the brain has crossing fibers (Tuch et al., 2002), which must be taken into account for proper analysis of the estimated microstructural parameters.”

“In these regions, where the diffusion tensor model has been shown to be inadequate (Frank, 2002; Tuch et al., 2002), partial volume effects exist (Alexander et al., 2001; Vos et al., 2011) that can adversely affect the estimation of diffusivity metrics (Wheeler-Kingshott and Cercignani, 2009; Vos…”

“High angular resolution diffusion imaging (HARDI-DTI), which resolves multiple intravoxel fiber populations (Tuch et al., 2002; Behrens et al., 2003), can more accurately perform fiber mapping and allow for the evaluation of white matter abnormalities near lesions.”

“f1ðd2; rÞ 1⁄4 jbð~h; d2; rÞ b̂j |fflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflffl} Residual between desired and achieved b-value [9]”

“4A), these insights may be attributable to the improved signal-to-noise ratio of the present HARDI dataset, related to the number of gradient directions and b values (65, 66).”

“A number of such models have been proposed, including multi-tensor models [23], spherical deconvolution [24], Q-ball imaging [25], persistent angular structure MRI [26], and diffusion spectrum imaging [27], among others (There is yet no general consensus as to which of these advanced models has the best performance, a…”

“In the past decade, several advanced approaches for characterizing the intra-voxel diffusion profile have been developed to overcome the limitations of the second-rank diffusion tensor model [5–11].”

“, tract-based spatial statistics [TBSS]) [120], as well as advanced approaches such as high angular resolution (HARDI) [121].”

“The tensor model is able to represent just one fiber tract orientation per voxel, an anatomical implausibility in much, if not most of the brain (Tuch et al., 2002).”

“Studies have used high angular resolution diffusion imaging (HARDI) (Tuch et al., 2002) and diffusion spectrum imaging (DSI) (Wedeen et al., 2005) tomodel the diffusion characteristics of axonal fibers.”

“STI tractography can be further extended to resolve fiber crossings (Basser et al., 2000; Frank, 2001; Lin et al., 2003; Tuch et al., 2002) by incorporatingmultiple tensors andhigher order tensors (Liu et al., 2004, 2010).”

“To resolve the uncertainty in these areas, q-space spectral imaging [16] and other high angular resolution diffusion imaging methods [17, 18] have been explored.”

“Techniques including diffusion tensor imaging (Basser et al., 1994), high angular resolution diffusion imaging (HARDI) (Tuch et al., 2002), and diffusion spectrum imaging (Wedeen et al., 2005), have been developed to infer tissue microstructural properties from the diffusion-driven NMR signal decay.”

“Two distinct fiber populations intersecting at an angle within a voxel will produce a major eigenvector at an angle between the actual fiber orientation vectors (34).”

“2004 [24] Human Field-strength = 3T Resolution = 3.”

“Datadriven approaches with high angular resolution diffusion (HARDI) measurements with two b-factors may use spherical harmonic decomposition (SHD) [58-63] which is based on the expansion of the measured apparent diffusion coefficient data in terms of a complete orthonormal set (e.”

“Frank (2002) and Tuch et al. (2002) have used such high angular resolution images to resolve multiple fibers in a single voxel.”

“…has prompted the development of several different measurement streams including diffusion spectrum imaging (DSI) (Wedeen et al., 2008) and a variety of high angular resolution diffusion imaging (HARDI) methods such as Q-ball imaging and the spherical deconvolutionmethod (Tuch et al., 2002).”

“, [4]–[13]), the principle is broadly common: it consists of integrating the local white matter orientation information provided by a dMRI data set across the brain [3].”

“We are in the process of extending FADTTS to the analysis of high angular resolution diffusion image (HARDI), which is important for resolving the issue of fiber crossing (Lenglet et al., 2009; Tuch et al., 2002).”

“Even in WM, the DTI model can fail if the tissue contains substantial crossing or diverging fibers [11, 12].”

“A tensor model cannot describe multiple fiber conditions, and therefore the estimation of the axonal direction is inaccurate in the crossing fiber regions.(22,23) As mentioned above, the diffusivity measured by diffusion MRI is an average measurement of the underlying diffusion environment.”

“…(single b-value), e.g., using a fixed diffusion time and gradient strength but with varying gradient directions, see (Campbell et al., 2005; CohenAdad et al., 2011; Descoteaux et al., 2008; Gur et al., 2012; Michailovich and Rathi, 2010; Tournier et al., 2004; Tuch et al., 2002; Tuch, 2004).”

“The complex fibrous structure of white matter can be further resolved using methods of high angular-resolution diffusion-weighted imaging (HARDI) to determine the water displacement probability function [59].”

“7,8 Common sampling approaches in DSI include sampling along high angular resolution diffusion imaging (HARDI) shells, 9 q‐ball imaging patterns, 10 or a cube on a Cartesian grid.”

“The former two imaging techniques (DSI and HARDI) were developed in order to better resolve crossing fibers at the scale of single MRI voxels (Tuch et al., 2002; Wedeen et al., 2005; Wedeen et al., 2012, Ozarslan et al., 2003).”

“However, for acquisitions where the b value is constant, such as HARDI (Tuch et al., 2002) or single diffusion tensor analysis, the user-defined parameters, d (mean diffusivity) and FA, can be adjusted to suit the b value used for the acquisition (Yoshiura et al.”

“Unlike conventional MRI and diffusion tensor imaging, high-angular-resolution diffusion imaging (HARDI) meets these methodologic requirements by measuring water diffusion along axonal pathways using a multidirectional model designed to detect crossing and branching axons.(12) HARDI thus provides an opportunity to mode…”

“For these reasons, HARDI is better at resolving crossing fibers (such as the corpus callosum and the long association fibers), a major issue for DTI (Tuch et al., 2002).”

“One of such techniques is high angular resolution diffusion imaging (HARDI), which measures the diffusion directly by sampling the diffusion signal over a spherical shell in diffusion wavevector space [9], [10], [13], [14].”

“Diffusion data were collected with a spin-echo echo-planar high-angular resolution diffusion imaging (Tuch et al., 2002) sequence with diffusion encoded along 60 isotropically distributed orientations according to an optimized gradient vector scheme (Jones, Horsfield, & Simmons, 1999) Pierpaoli, &…”

“These images feature better quality and, together with a high-order local diffusion model (Tuch et al., 2002; Descoteaux et al., 2007), enable the identification of fiber crossing within a voxel, thus improving tractography results.”

“, [7], [22], [28], [29], [33], [35], [39], [44], [45]); and the introduction of higher -value [24] diffusion-weighted imaging (DWI) sequences [48] covering an increased number of gradient directions.”

“This high angular resolution diffusion imaging ( Tuch et al., 2002 ) holds the promise to be able to determine the directions of multiple fibers within a single voxel, but also leads to increased acquisition times.”

“For strong diffusion weightings (i.e., high b-values), the angular dependence of the diffusionMRI (dMRI) signal intensity inwhitematter is sensitive to the complex geometries associatedwith intersecting axonal fiber bundles (Tuch et al., 2002).”

“Alternative methods of measuring diffusion anisotropy, such as high angular resolution diffusion imaging (Tuch et al., 2002), may be required to further improve our understanding of those rat hippocampal regions where multiple coherent fiber pathways exist at the length scales of particular image resolutions.”

“The diffusion tensor model that is typically applied to DW imaging data in this context is, however, inadequate for regions that contain complex architectures such as crossing fibres (Alexander et al. 2001; Jeurissen et al. 2013; Tuch et al. 2002).”

“Diffusion MRI tractography is a non-invasive technique that reconstructs three-dimensional models of white matter fiber pathways based on voxel-wise diffusion patterns (Basser et al., 1994; Pierpaoli and Basser, 1996; Tuch et al., 2002; Behrens et al., 2003a; Tournier et al., 2004; Wedeen et al., 2005; Jbabdi et al.,…”

“The present study investigated HD-related differences in WM microstructure by utilizing both high angular resolution diffusion imaging (HARDI) (Tuch et al., 2002) and quantitative magnetization transfer imaging (qMT) (Henkelman et al., 1993, 2001) which provides improved myelin specificity compared to diffusion MRI. qMT e…”

“It is well known that the DTI model, which accounts only for Gaussian processes, does not adequately describe diffusion in areas of kissing, branching, and crossing fibers (16), and FA alone does not explain all possible morphologic abnormalities.”

“…the number of tensor components (eg. to two components, Parker et al., 2003, Caan et al., 2010), incorporating physiological constraints (Tuch et al., 2002), reducing the complexity of the model by only allowing identical prolate tensors (Tabelow et al., 2012), stabilizing the problem…”

“There are more general models such as High-Angular-Resolution Diffusion-Image (HARDI) (Tuch et al., 2002), Q-ball (Tuch, 2004) and Diffusion Spectrum Imaging (DSI) (Wedeen et al., 2005) etc. Extracting connectivity information from DT-MRI, termed " tractography " (Basser et al., 2000; Conturo et…”

“In this model the diffusion process taking place inside each compartment of parallel fibers is described by a second-order self-diffusion tensor [6].”

“Severalmodels have been suggested to allowamore comprehensive model of diffusion in the tissue than the diffusion tensor (Alexander et al., 2010; Barazany et al., 2009; Tuch et al., 2002).”

“Keywords: Magnetic resonance imaging; Diffusion anisotropy; Baboon brains; Brain development”

“To overcome this difficulty, a multi-tensor (or multi-compartment) extension of the single diffusion tensor model has been proposed (Tuch et al., 2002; Frank, 2001; Behrens et al., 2007).”

“Tuch [28] thresholds the correlation of the measurements with their predictions from a single-component model to decide whether to use one or two components.”