Optimum b value for resolving crossing fibers: a study with standard clinical b value using 1.5-T MR

Nerve fibre organisation in the human optic nerve and chiasm: what do we really know?

A recent anatomical study of the human optic chiasm cast doubt on the widespread assumption that nerve fibres travelling in the human optic nerve and chiasm are arranged retinotopically. Accordingly, a scoping literature review was performed to determine what is known about the nerve fibre arrangement in these structures. Meta-analysis suggested that the average number of fibres in each optic nerve was 1.023 million with an inter-individual range of approximately 50% of the mean. Loss of nerve fibres with age (approximately 3,400 fibres/year) could not account for this variability. The review suggested that there might be a retinotopic arrangement of nerve fibres in the orbital portion of the optic nerve but that this arrangement is most likely to be lost posteriorly with a more random distribution of nerve fibres at the chiasm. Limited studies have looked at nerve fibre arrangement in the chiasm. In summary, the chiasm is more 'H-shaped' than 'X-shaped': nerve fibre crossings occur paracentrally with nerves in the centre of the chiasm travelling coronally and in parallel. There is interaction between crossed and uncrossed fibres which are widely distributed. The review supports the non-existence of Wilbrand's knee. Considerable further work is required to provide more precise anatomical information, but this review suggests that the assumed preservation of retinotopy in the human optic nerve and chiasm is probably not correct.

Methods of diffusion MRI tractography for localization of the anterior optic pathway: A systematic review of validated methods

The anterior optic pathway (AOP) is a system of three structures (optic nerves, optic chiasma, and optic tracts) that convey visual stimuli from the retina to the lateral geniculate nuclei. A successful reconstruction of the AOP using tractography could be helpful in several clinical scenarios, from presurgical planning and neuronavigation of sellar and parasellar surgery to monitoring the stage of fiber degeneration both in acute (e.g., traumatic optic neuropathy) or chronic conditions that affect AOP structures (e.g., amblyopia, glaucoma, demyelinating disorders or genetic optic nerve atrophies). However, its peculiar anatomy and course, as well as its surroundings, pose a serious challenge to obtaining successful tractographic reconstructions. Several AOP tractography strategies have been adopted but no standard procedure has been agreed upon. We performed a systematic review of the literature according to the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) 2020 guidelines in order to find the combinations of acquisition and reconstruction parameters that have been performed previously and have provided the highest rate of successful reconstruction of the AOP, in order to promote their routine implementation in clinical practice. For this purpose, we reviewed data regarding how the process of anatomical validation of the tractographies was performed. The Cochrane Handbook for Systematic Reviews of Interventions was used to assess the risk of bias and thus the study quality We identified thirty-nine studies that met our inclusion criteria, and only five were considered at low risk of bias and achieved over 80% of successful reconstructions. We found a high degree of heterogeneity in the acquisition and analysis parameters used to perform AOP tractography and different combinations of them can achieve satisfactory levels of anterior optic tractographic reconstruction both in real-life research and clinical scenarios. One thousand s/mm2 was the most frequently used b value, while both deterministic and probabilistic tractography algorithms performed morphological reconstruction of the tract satisfactorily, although probabilistic algorithms estimated a more realistic percentage of crossing fibers (45.6%) in healthy subjects. A wide heterogeneity was also found regarding the method used to assess the anatomical fidelity of the AOP reconstructions. Three main strategies can be found: direct visual direct visual assessment of the tractography superimposed to a conventional MR image, surgical evaluation, and computational methods. Because the latter is less dependent on a priori knowledge of the anatomy by the operator, computational methods of validation of the anatomy should be considered whenever possible.

Resolution and b value dependent Structural Connectome in ex vivo Mouse Brain

Diffusion magnetic resonance imaging has been widely used in both clinical and preclinical studies to characterize tissue microstructure and structural connectivity. The diffusion MRI protocol for the Human Connectome Project (HCP) has been developed and optimized to obtain high-quality, high-resolution diffusion MRI (dMRI) datasets. However, such efforts have not been fully explored in preclinical studies, especially for rodents. In this study, high quality dMRI datasets of mouse brains were acquired at 9.4T system from two vendors. In particular, we acquired a high-spatial resolution dMRI dataset (25 μm isotropic with 126 diffusion encoding directions), which we believe to be the highest spatial resolution yet obtained; and a high-angular resolution dMRI dataset (50 μm isotropic with 384 diffusion encoding directions), which we believe to be the highest angular resolution compared to the dMRI datasets at the microscopic resolution. We systematically investigated the effects of three important parameters that affect the final outcome of the connectome: b value (1000s/mm² to 8000 s/mm²), angular resolution (10 to 126), and spatial resolution (25 μm to 200 μm). The stability of tractography and connectome increase with the angular resolution, where more than 50 angles is necessary to achieve consistent results. The connectome and quantitative parameters derived from graph theory exhibit a linear relationship to the b value (R² > 0.99); a single-shell acquisition with b value of 3000 s/mm² shows comparable results to the multi-shell high angular resolution dataset. The dice coefficient decreases and both false positive rate and false negative rate gradually increase with coarser spatial resolution. Our study provides guidelines and foundations for exploration of tradeoffs among acquisition parameters for the structural connectome in ex vivo mouse brain.

Diffusion tensor tractography of normal facial and vestibulocochlear nerves

PURPOSE

Diffusion tensor tractography (DTT) is not adequately reliable for prediction of facial and vestibulocochlear (VII-VIII) nerve locations, especially relative to a vestibular schwannoma (VS). Furthermore, it is often not possible to visualize normal VII-VIII nerves by DTT (visualization rates were 12.5-63.6%). Therefore, DTT post-processing was optimized for normal VII-VIII nerve visualization with and without manual noise elimination.

METHODS

DTT examinations of ten patients were evaluated to assess the improvement in performance by modifying seed region of interest (ROI) and fractional anisotropy (FA) threshold. Seed ROI was placed at the porus of the internal auditory meatus, and FA threshold values were either fixed or variable for each patient. DTT visualization of cranial nerves VII-VIII was evaluated and the noise effect was measured.

RESULTS

Cranial nerves VII-VIII were visualized in 90% of patients without using manual noise elimination by modifying the seed ROI and FA threshold. The visualization rate with FA threshold of the upper limit in each patient (100%) was significantly higher than that with FA threshold of 0.1 (75%) (p = 0.02). The incidence rate of noise with FA threshold of the upper limit (10%) was not significantly different than the FA threshold of 0.1 (20%) (p = 0.66).

CONCLUSION

Seed ROI modification and FA thresholding can improve the visualization of cranial nerve VII-VIII locations in DTT. This technique is promising for its potential to determine the relationship of cranial nerves VII-VIII to VS.

Diffusion MR imaging: How to get the maximum from the experimental time

Diffusion-based MR imaging is the only non-invasive method for characterising the microstructural organization of brain tissue in vivo. Diffusion tensor MRI (DT-MRI) is currently routinely used in both research and clinical practice. However, other diffusion approaches are gaining more and more popularity and an increasing number of researchers express interest in using them concomitantly with DT-MRI. While non tensor-based methods hold great promises for increasing the specificity of diffusion MR imaging, including them in the experimental routine inevitably leads to longer experimental times. In most cases, this may preclude the translation of the full protocol to clinical practice, especially when these methods are to be used with subjects that are not compatible with long scanning sessions (e.g., with elderly and pediatric subjects who have difficulties in maintaining a fixed head position during a long imaging session).

The aim of this review is to guide the end-users on obtaining the maximum from the experimental time allocated to collecting diffusion MRI data. This is done by: (i) briefly reviewing non tensor-based approaches; (ii) reviewing the optimal protocols for both tensor and non tensor-based imaging; and (iii) drawing the conclusions for different experimental times.

Dentatorubrothalamic tract in human brain: diffusion tensor tractography study

INTRODUCTION

The dentatorubrothalamic tract (DRTT) originates from the dentate nucleus in the cerebellum and terminates in the contralateral ventrolateral nucleus (VL) of the thalamus after decussating to the contralateral red nucleus. Identification of the DRTT is difficult due to the fact that it is a long, multisynaptic, neural tract crossing to the opposite hemisphere. In the current study, we attempted to identify the DRTT in the human brain using a probabilistic tractography technique of diffusion tensor imaging.

METHODS

Diffusion tensor imaging was performed at 1.5-T using a synergy-L sensitivity encoding head coil. DRTTs were obtained by selection of fibers passing through three regions of interest (the dentate nucleus, the superior cerebellar peduncle, and the contralateral red nucleus) from 41 healthy volunteers. Probabilistic mapping was obtained from the highest probabilistic location at 2.3 mm above the anterior commissure-posterior commissure level.

RESULTS

DRTTs of all subjects, which originated from the dentate nucleus, ascended through the junction of the superior cerebellar peduncle and the contralateral red nucleus and then terminated at the VL nucleus of the thalamus. The highest probabilistic location for the DRTT at the thalamus was compatible with the location of the VL nucleus.

CONCLUSIONS

We identified the DRTT in the human brain using probabilistic tractography. Our results could be useful in research on movement control.

Reconstruction of scattered data in fetal diffusion MRI

In this paper we present a method for reconstructing diffusion-weighted MRI data on regular grids from scattered data. The proposed method has the advantage that no specific diffusion model needs to be assumed. Previous work assume the tensor model, but this is not suitable under certain conditions like intravoxel orientational heterogeneity (IVOH). Data reconstruction is particularly important when studying the fetal brain in utero, since registration methods applied for movement and distortion correction produce scattered data in spatial and diffusion domains. We propose the use of a groupwise registration method, and a dual spatio-angular interpolation by using radial basis functions (RBF). Leave-one-out experiments performed on adult data showed a high accuracy of the method. The application to fetal data showed an improvement in the quality of the sequences according to objective criteria based on fractional anisotropy (FA) maps, and differences in the tractography results.