https://www.cell.com/biophysj/pdf/S0006-3495(94)80775-1.pdf

MR diffusion tensor spectroscopy and imaging.

The success of diffusion magnetic resonance imaging (MRI) is deeply rooted in the powerful concept that during their random, diffusion-driven displacements molecules probe tissue structure at a microscopic scale well beyond the usual image resolution. As diffusion is truly a three-dimensional process, molecular mobility in tissues may be anisotropic, as in brain white matter. With diffusion tensor imaging (DTI), diffusion anisotropy effects can be fully extracted, characterized, and exploited, providing even more exquisite details on tissue microstructure. The most advanced application is certainly that of fiber tracking in the brain, which, in combination with functional MRI, might open a window on the important issue of connectivity. DTI has also been used to demonstrate subtle abnormalities in a variety of diseases (including stroke, multiple sclerosis, dyslexia, and schizophrenia) and is currently becoming part of many routine clinical protocols. The aim of this article is to review the concepts behind DTI and to present potential applications.

/pdf/diffusion-tensor-imaging-concepts-and-applications-40utq0zjun.pdf

Diffusion tensor imaging: Concepts and applications

Intravoxel incoherent motion (IVIM) imaging is a method the authors developed to visualize microscopic motions of water. In biologic tissues, these motions include molecular diffusion and microcirculation of blood in the capillary network. IVIM images are quantified by an apparent diffusion coefficient (ADC), which integrates the effects of both diffusion and perfusion. The aim of this work was to demonstrate how much perfusion contributes to the ADC and to present a method for obtaining separate images of diffusion and perfusion. Images were obtained at 0.5 T with high-resolution multisection sequences and without the use of contrast material. Results in a phantom made of resin microspheres demonstrated the ability of the method to separately evaluate diffusion and perfusion. The method was then applied in patients with brain and bone tumors and brain ischemia. Clinical results showed significant promise of the method for tissue characterization by perfusion patterns and for functional studies in the eva...

Separation of diffusion and perfusion in intravoxel incoherent motion MR imaging.

Indices of diffusion anisotropy calculated from diffusion coefficients acquired in two or three perpendicular directions are rotationally variant. In living monkey brain, these indices severely underestimate the degree of diffusion anisotropy. New indices calculated from the entire diffusion tensor are rotationally invariant (RI). They show that anisotropy is highly variable in different white matter regions depending on the degree of coherence of fiber tract directions. In structures with a regular, parallel fiber arrangement, water diffusivity in the direction parallel to the fibers (Dparallel approximately 1400-1800 x 10(-6) mm2/s) is almost 10 times higher than the average diffusivity in directions perpendicular to them (D + D)/2 [corrected] approximately 150-300 x 10(-6) mm2/s), and is almost three times higher than previously reported. In structures where the fiber pattern is less coherent (e.g., where fiber bundles merge), diffusion anisotropy is significantly reduced. However, RI anisotropy indices are still susceptible to noise contamination. Monte Carlo simulations show that these indices are statistically biased, particularly those requiring sorting of the eigenvalues of the diffusion tensor based on their magnitude. A new intervoxel anisotropy index is proposed that locally averages inner products between diffusion tensors in neighboring voxels. This "lattice" RI index has an acceptably low error variance and is less susceptible to bias than any other RI anisotropy index proposed to date.

Toward a quantitative assessment of diffusion anisotropy

https://www.sci.utah.edu/~wolters/LiteraturZurVorlesung/Literatur/F:_Anisotropy/AnderssonSkareAshburner_NeuroImage.pdf

How to correct susceptibility distortions in spin-echo echo-planar images: application to diffusion tensor imaging

Molecular diffusion and microcirculation in the capillary network result in a distribution of phases in a single voxel in the presence of magnetic field gradients. This distribution produces a spin-echo attenuation. The authors have developed a magnetic resonance (MR) method to image such intravoxel incoherent motions (IVIMs) by using appropriate gradient pulses. Images were generated at 0.5 T in a high-resolution, multisection mode. Diffusion coefficients measured on images of water and acetone phantoms were consistent with published values. Images obtained in the neurologic area from healthy subjects and patients were analyzed in terms of an apparent diffusion coefficient (ADC) incorporating the effect of all IVIMs. Differences were found between various normal and pathologic tissues. The ADC of in vivo water differed from the diffusion coefficient of pure water. Results were assessed in relation to water compartmentation in biologic tissues (restricted diffusion) and tissue perfusion. Nonuniform slow flow of cerebrospinal fluid appeared as a useful feature on IVIM images. Observation of these motions may significantly extend the diagnostic capabilities of MR imaging.

MR imaging of intravoxel incoherent motions: application to diffusion and perfusion in neurologic disorders.

BACKGROUND AND PURPOSE: Previous structural data obtained with diffusion tensor imaging axonal tracking have demonstrated possible in vivo connections between the human red nucleus (RN) and the sensorimotor and associative cortical areas. However, tractographic reconstructions can include false trajectories because of, for instance, the low spatial resolution of diffusion images or the inability to precisely detect fiber crossings. The rubral network was therefore reassessed by functional connectivity during the brain resting state. Because the RN is located very close to the substantia nigra (SN), the nigral network was also studied to ensure that these 2 circuits were correctly dissociated. MATERIALS AND METHODS: Data from 14 right-handed healthy volunteers were acquired at rest and analyzed by region-of-interest (ROI)–based functional connectivity. The blood oxygen level–dependent (BOLD) signal intensity fluctuations of separate ROIs located in the RN and SN were successively used to identify significant temporal correlations with BOLD signal intensity fluctuations of other brain regions. RESULTS: Low-frequency BOLD signal intensity of the RN correlated with signal intensity fluctuations in the cerebellum; mesencephalon; SN; hypothalamus; pallidum; thalamus; insula; claustrum; posterior hippocampus; precuneus; and occipital, prefrontal, and fronto-opercular cortices. Despite some cortical and subcortical overlaps with nigral connectivity, this rubral network was clearly distinct from the nigral network, which showed a strong correlation with the striatum; cerebellar vermis; and more widespread frontal, prefrontal, and orbitofrontal cortical areas. CONCLUSIONS: During the brain resting state, the human RN participates in cognitive circuits related to salience and executive control, and that may partly represent a subclass of its structural connectivity as revealed by tractography.

http://www.ajnr.org/content/ajnr/30/2/396.full.pdf

Functional connectivity of the human red nucleus in the brain resting state at 3T.

We used fMRI to study cerebellar activation during index finger-thumb opposition of the right hand and index finger-thumb opposition of both hands in in-phase and out-of-phase modes. The right hand movement activates the contralateral anterior lobe of the cerebellum. During bimanual in-phase movements, this activity pattern becomes bilateral. More interestingly, bilateral out-of-phase movements recruit the cerebellar posterior lobe VIII, which likely corresponds to the second homunculus. As out-of-phase movements differ from the in-phase movements only by their temporal complexity and their attentional awareness, this study demonstrates the preferential involvement of the cerebellar second homunculus in the control of complex movements.

E.A. Cabanis

Papers

MR imaging of intravoxel incoherent motions: application to diffusion and perfusion in neurologic disorders.

Functional connectivity of the human red nucleus in the brain resting state at 3T.

Specific neocerebellar activation during out-of-phase bimanual movements.

In vivo localized NMR proton spectroscopy of normal appearing white matter in patients with multiple sclerosis.

Magnetic resonance imaging using FLAIR pulse sequence in white matter diseases.