Loneliness is linked to specific subregional alterations in hippocampus-default network covariation.

Social interaction complexity makes humans unique. But in times of social deprivation, this strength risks exposure of important vulnerabilities. Human social neuroscience studies have placed a premium on the default network (DN). In contrast, hippocampus (HC) subfields have been intensely studied in rodents and monkeys. To bridge these two literatures, we here quantified how DN subregions systematically covary with specific HC subfields in the context of subjective social isolation (i.e., loneliness). By codecomposition using structural brain scans of ∼40,000 UK Biobank participants, loneliness was specially linked to midline subregions in the uncovered DN patterns. These association cortex patterns coincided with concomitant HC patterns implicating especially CA1 and molecular layer. These patterns also showed a strong affiliation with the fornix white matter tract and the nucleus accumbens. In addition, separable signatures of structural HC-DN covariation had distinct associations with the genetic predisposition for loneliness at the population level.NEW & NOTEWORTHY The hippocampus and default network have been implicated in rich social interaction. Yet, these allocortical and neocortical neural systems have been interrogated in mostly separate literatures. Here, we conjointly investigate the hippocampus and default network at a subregion level, by capitalizing structural brain scans from ∼40,000 participants. We thus reveal unique insights on the nature of the "lonely brain" by estimating the regimes of covariation between the hippocampus and default network at population scale.

Groupwise track filtering via iterative message passing and pruning.

Tractography is an important tool for the in vivo analysis of brain connectivity based on diffusion MRI data, but it also has well-known limitations in false positives and negatives for the faithful reconstruction of neuroanatomy. These problems persist even in the presence of strong anatomical priors in the form of multiple region of interests (ROIs) to constrain the trajectories of fiber tractography. In this work, we propose a novel track filtering method by leveraging the groupwise consistency of fiber bundles that naturally exists across subjects. We first formalize our groupwise concept with a flexible definition that characterizes the consistency of a track with respect to other group members based on three important aspects: degree, affinity, and proximity. An iterative algorithm is then developed to dynamically update the localized consistency measure of all streamlines via message passing from a reference set, which then informs the pruning of outlier points from each streamline. In our experiments, we successfully applied our method to diffusion imaging data of varying resolutions from the Alzheimer's Disease Neuroimaging Initiative (ADNI) and Human Connectome Project (HCP) for the consistent reconstruction of three important fiber bundles in human brain: the fornix, locus coeruleus pathways, and corticospinal tract. Both qualitative evaluations and quantitative comparisons showed that our method achieved significant improvement in enhancing the anatomical fidelity of fiber bundles.

Anatomy and function of the fornix in the context of its potential as a therapeutic target.

The fornix is a white matter bundle located in the mesial aspect of the cerebral hemispheres, which connects various nodes of a limbic circuitry and is believed to play a key role in cognition and episodic memory recall. As the most prevalent cause of dementia, Alzheimer's disease (AD) dramatically impairs the quality of life of patients and imposes a significant societal burden on the healthcare system. As an established treatment for movement disorders, deep brain stimulation (DBS) is currently being investigated in preclinical and clinical studies for treatment of memory impairment in AD by modulating fornix activity. Optimal target and stimulation parameters to potentially rescue memory deficits have yet to be determined. The aim of this review is to consolidate the structural and functional aspects of the fornix in the context of neuromodulation for memory deficits. We first present an anatomical and functional overview of the fibres and structures interconnected by the fornix. Recent evidence from preclinical models suggests that the fornix is subdivided into two distinct functional axes: a septohippocampal pathway and a subiculothalamic pathway. Each pathway's target and origin structures are presented, followed by a discussion of their oscillatory dynamics and functional connectivity. Overall, neuromodulation of each pathway of the fornix is discussed in the context of evidence-based forniceal DBS strategies. It is not yet known whether driving fornix activity can enhance cognition-optimal target and stimulation parameters to rescue memory deficits have yet to be determined.

Uncovering a Role for the Dorsal Hippocampal Commissure in Recognition Memory.

The dorsal hippocampal commissure (DHC) is a white matter tract that provides interhemispheric connections between temporal lobe brain regions. Despite the importance of these regions for learning and memory, there is scant evidence of a role for the DHC in successful memory performance. We used diffusion-weighted magnetic resonance imaging (DW-MRI) and white matter tractography to reconstruct the DHC in both humans (in vivo) and nonhuman primates (ex vivo). Across species, our findings demonstrate a close consistency between the known anatomy and tract reconstructions of the DHC. Anterograde tract-tracer techniques also highlighted the parahippocampal origins of DHC fibers in nonhuman primates. Finally, we derived diffusion tensor MRI metrics from the DHC in a large sample of human subjects to investigate whether interindividual variation in DHC microstructure is predictive of memory performance. The mean diffusivity of the DHC correlated with performance in a standardized recognition memory task, an effect that was not reproduced in a comparison commissure tract-the anterior commissure. These findings highlight a potential role for the DHC in recognition memory, and our tract reconstruction approach has the potential to generate further novel insights into the role of this previously understudied white matter tract in both health and disease.

White-matter commissures: a clinically focused anatomical review.

PURPOSE: The objective of this review is to provide a structured approach to the main white matter commissures, their anatomic and radiological definition and disease implications. METHODS: The Pubmed database and The JAMA Network were used for the literature review and the following terms were searched using Sort by: Best Match and Sort by: Most Recent: telencephalic commissure, forebrain commissure anatomy, fornix anatomy, commissure of fornix, posterior commissure, corpus callosum, commissural agenesis, Probst bundle, corpus callosum disorders review, corpus callosum diseases review, Marchiafava-Bignami, Alzheimer's disease and Forel commissure; 36 papers were selected, one excluded due to the language barrier. RESULTS: The interhemispheric communication in the brain is achieved via the brain commissures, bundles of white matter linking the two cerebral hemispheres. Anterior white commissure (AWC)-related with olfactory and non-visual communication, hippocampal commissure-main efferent pathway of the hippocampus, connecting the hippocampal formation to structures beyond the temporal lobe, crucial in declarative memory formation and consolidation-and the corpus callosum (CC)-from the anterior commissure to the hippocampal commissure-are the main telencephalic commissures. Supramammilary commissure, posterior commissure, supraoptic commissure and habenular commissure are diencephalic commissures-unknown function, probably related to involuntary eye movements. Commissural agenesis (AWC is absent or impossible to recognize), Alzheimer's Disease (hippocampal commissure may contribute for disease dissemination) and agenesis of corpus callosum are some of the disturbances that involve the telenchephalic commissures. CONCLUSIONS: A comprehensive understanding of the clinic-anatomic correlation is pivotal to understand the pathology and therefore improve our diagnostic accuracy and treatment options, in the background of all patient management.

Schwalbe's Triangular Fossa: Normal and Pathologic Anatomy on Frozen Cadavers. Anatomo-Magnetic Resonance Imaging Comparison and Surgical Implications in Colloid Cyst Surgery.

BACKGROUND: The fornix is a region of greatest neurosurgical interest in regards to its complex anatomy and surgical approaches to this area. The objective of this study was to evaluate the morphology of the triangular recess (TR) and its role in the growth pattern of the colloid cysts (CC) within the third ventricle and in the choice of the surgical approach for their removal. Furthermore, to compare the results of the dissections with measurements performed on a magnetic resonance imaging scan. METHODS: In the anatomic study, 20 cadaveric specimens were dissected and analyzed. In the radiologic study, a magnetic resonance imaging scan was performed in 20 healthy volunteers. In the clinical study, a retrospective analysis of all the patients affected with CCs microsurgically removed at our institute between 2010 and 2018 was conducted. RESULTS: In the anatomic study, the width, height, and the area of the TR were respectively 0.31 cm, 0.33cm, and 0.051 cm2. In the radiologic study, 3 different typologies of TR were identified: open recess in ventriculomegaly (7 patients); open recess in physiologic ventricular system (3 patients); closed or blind recess (10 patients). Three different growth patterns of CCs were identified: type 1) CCs localized at the foramen of Monro growing behind the fornix and below the third ventricular roof; type 2) CCs growing rostrally between the column of fornix; and type 3) CCs growing above the plane of the third ventricular roof. CONCLUSIONS: The anatomy of the TR influences the growth pattern of CC within the ventricular cavity and determines the surgical strategy for their removal.

Anatomy of the Limbic White Matter Tracts as Revealed by Fiber Dissection and Tractography.

BACKGROUND: The limbic tracts are involved in crucial cerebral functions such as memory, emotion, and behavior. The complex architecture of the limbic circuit makes it harder to approach compared with other white matter networks. Our study aims to describe the 3-dimensional anatomy of the limbic white matter by the use of 2 complementary study methods, namely ex vivo fiber dissection and in vivo magnetic resonance imaging-based tractography. METHODS: Three fiber dissection protocols were performed using blunt wooden instruments and a surgical microscope on formalin-fixed brains prepared according to the Klingler method. Diffusion tensor imaging acquisitions were done with a 3-Tesla magnetic resonance scanner on patients with head and neck pathology that did not involve the brain. Fiber tracking was performed with manually selected regions of interest. RESULTS: Cingulum, fornix, the anterior thalamic peduncle, the accumbofrontal bundle, medial forebrain bundle, the uncinate fasciculus, the mammillothalamic tract, ansa peduncularis, and stria terminalis were dissected and fiber tracked. For each tract, location, configuration, segmentation, dimensions, dissection and tractography particularities, anatomical relations, and terminations are described. The limbic white matter tracts were systematized as 2 concentric rings around the thalamus. The inner ring is formed by fornix, mammillothalamic tract, ansa peduncularis, stria terminalis, accumbofrontal fasciculus, and medial forebrain bundle and anterior thalamic peduncle, and the outer ring is formed by the cingulum and uncinate fasciculus. CONCLUSIONS: This paper proposes a fiber-tracking protocol for the limbic tracts inspired and validated by fiber dissection findings that can be used routinely in the clinical practice.

The Surgical White Matter Chassis: A Practical 3-Dimensional Atlas for Planning Subcortical Surgical Trajectories.

BACKGROUND: The imperative role of white matter preservation in improving surgical functional outcomes is now recognized. Understanding the fundamental white matter framework is essential for translating the anatomic and functional literature into practical strategies for surgical planning and neuronavigation. OBJECTIVE: To present a 3-dimensional (3-D) atlas of the structural and functional scaffolding of human white matter-ie, a "Surgical White Matter Chassis (SWMC)"-that can be used as an organizational tool in designing precise and individualized trajectory-based neurosurgical corridors. METHODS: Preoperative diffusion tensor imaging magnetic resonance images were obtained prior to each of our last 100 awake subcortical resections, using a clinically available 3.0 Tesla system. Tractography was generated using a semiautomated deterministic global seeding algorithm. Tract data were conceptualized as a 3-D modular chassis based on the 3 major fiber types, organized along median and paramedian planes, with special attention to limbic and neocortical association tracts and their interconnections. RESULTS: We discuss practical implementation of the SWMC concept, and highlight its use in planning select illustrative cases. Emphasis has been given to developing practical understanding of the arcuate fasciculus, uncinate fasciculus, and vertical rami of the superior longitudinal fasciculus, which are often-neglected fibers in surgical planning. CONCLUSION: A working knowledge of white matter anatomy, as embodied in the SWMC, is of paramount importance to the planning of parafascicular surgical trajectories, and can serve as a basis for developing reliable safe corridors, or modules, toward the goal of "zero-footprint" transsulcal access to the subcortical space.

Manual segmentation of the fornix, fimbria, and alveus on high-resolution 3T MRI: Application via fully-automated mapping of the human memory circuit white and grey matter in healthy and pathological aging.

Recently, much attention has been focused on the definition and structure of the hippocampus and its subfields, while the projections from the hippocampus have been relatively understudied. Here, we derive a reliable protocol for manual segmentation of hippocampal white matter regions (alveus, fimbria, and fornix) using high-resolution magnetic resonance images that are complementary to our previous definitions of the hippocampal subfields, both of which are freely available at https://github.com/cobralab/atlases. Our segmentation methods demonstrated high inter- and intra-rater reliability, were validated as inputs in automated segmentation, and were used to analyze the trajectory of these regions in both healthy aging (OASIS), and Alzheimer's disease (AD) and mild cognitive impairment (MCI; using ADNI). We observed significant bilateral decreases in the fornix in healthy aging while the alveus and cornu ammonis (CA) 1 were well preserved (all p's<0.006). MCI and AD demonstrated significant decreases in fimbriae and fornices. Many hippocampal subfields exhibited decreased volume in both MCI and AD, yet no significant differences were found between MCI and AD cohorts themselves. Our results suggest a neuroprotective or compensatory role for the alveus and CA1 in healthy aging and suggest that an improved understanding of the volumetric trajectories of these structures is required.

Fibre Dissection and Sectional Study of the Major Porcine Cerebral White Matter Tracts.

White matter anatomy is the basis for numerous applications in neurology, neurosurgery and fundamental neuroscience. Although the porcine brain is frequently used as experimental model in these fields of research, the description of its white matter is not as thorough as in the human brain or other species. Thus, the aim of this study is to describe the porcine white matter tracts in a complex manner. Two stepwise dissection protocols adapted from human anatomy were performed on six adult pig brain hemispheres prepared according to the Klingler method. Other four hemispheres were sectioned along section planes that were chosen similar to the Talairach coordinate system. As a result, three commissural tracts, seven association tracts and one projection tract were identified: corpus callosum, fornix, commissura rostralis, the short-association tracts, fasciculus longitudinalis superior, fasciculus uncinatus, fasciculus longitudinalis inferior, fasciculus occipitofrontalis inferior, cingulum, tractus mamillothalamicus and capsula interna. They were described and illustrated from multiple points of view, focusing on their trajectory, position, dimensions and anatomical relations. All in all, we achieved a three-dimensional understanding of the major tracts. The results are ready to be applied in future imagistic or experimental studies.

The differences of the precommissural and postcommissural fornix in the hippocampal location: a diffusion tensor tractography study.

PURPOSE: The precommissural fornix and postcommissural fornix have different connections to the basal forebrain and septal region, and mammillary body, respectively. However, little is known about the differences of the precommissural fornix and postcommissural fornix in the hippocampal location. In this study, using diffusion tensor tractography, we investigated the differences of the precommissural fornix and postcommissural fornix in the hippocampal location. METHODS: We recruited 25 healthy volunteers for this study. For reconstruction of the precommissural fornix and postcommissural fornix, we placed the seed region of interest on the septal nucleus, and the mammillary body, respectively. The target regions of interest (ROI) was given on the crus of the fornix on the coronal image. Evaluations of the anatomical location of the precommissural fornix and postcommissural fornix were performed using the highest probabilistic location in the hippocampal formation. RESULTS: The precommissural fornix and postcommissural fornix were located at an average of 83.9 and 87.5% between the lateral margin of the red nucleus and collateral sulcus on the axial plane, and 77.2 and 81.4% between the lateral margin of the midbrain and the inferior longitudinal fasciculus on the coronal plane. Significant differences of location in the medio-lateral direction were observed in the axial and coronal plane (p < 0.05). However, no significant differences of location in the antero-posterior direction were observed between precommissrual and postcommissural fornix (p > 0.05). CONCLUSIONS: The reconstructed precommissural fornix and postcommissural fornix were connected to the cornu ammonis 1(CA1) of the hippocampus, and the precommissural fornix was located more laterally to the postcommissural fornix in the CA1.

Topographic separation of fornical fibers associated with the anterior and posterior hippocampus in the human brain: An MRI-diffusion study.

BACKGROUND AND OBJECTIVE: Evidence from rat and nonhuman primate studies indicates that axons comprising the fornix have a characteristic topographical organization: projections from the temporal/anterior hippocampus mainly occupy the lateral fornix, whereas the more medial fornix contains fibers from the septal/posterior hippocampus. The aim of this study was to investigate whether the same topographical organization exists in the human brain. METHODS: Using high angular resolution diffusion MRI-based tractography at 3T, subdivisions of the fornix were reconstructed in 40 healthy adults by selecting fiber pathways from either the anterior or the posterior hippocampus. RESULTS: The tract reconstructions revealed that anterior hippocampal fibers predominantly comprise the lateral body of the fornix, whereas posterior fibers make up the medial body of the fornix. Quantitative analyses support this medial:lateral distinction in humans, which matches the topographical organization of the fornix in other primates. CONCLUSION: This novel tractography protocol enables the separation of fornix fibers from anterior and posterior hippocampal regions in the human brain and, hence, provides a means by which to compare functions associated with different sets of connections along the longitudinal axis of the hippocampus.

Development, validation and application of a new fornix template for studies of aging and preclinical Alzheimer's disease.

We developed a merged younger-older adult template of the fornix and demonstrated its utility for studies of aging and preclinical Alzheimer's disease (AD). In Experiment 1, probabilistic tractography was used to reconstruct the fornix in younger and older adults and successful streamlines were then averaged to create a merged template in standard space. The new template includes the majority of the fornix from the hippocampal formation to the subcallosal region and the thalamus/hypothalamus. In Experiment 2, the merged template was validated as an appropriate measure for studies of aging, with comparisons against manual tracing measures indicating identical spatial coverage in younger and older adult groups. In Experiment 3, the merged template was found to outperform age-specific templates in measures of sensitivity and specificity computed on diffusion tensor imaging data of an independent participant cohort. In Experiment 4, relevance to preclinical AD was demonstrated via associations between fractional anisotropy within the new fornix template and cerebrospinal fluid markers of AD pathology (Aß42 and the t-tau/Aß42 ratio) in a third independent cohort of cognitively normal older adults. Our new template provides an appropriate measure for use in future studies seeking to characterize microstructural alterations in the fornix associated with aging and preclinical AD.

Comparative anatomical analysis of the transcallosal-transchoroidal and transcallosal-transforniceal-transchoroidal approaches to the third ventricle.

OBJECTIVE Access to the third ventricle is a veritable challenge to neurosurgeons. In this context, anatomical and morphometric studies are useful for establishing the limitations and advantages of a particular surgical approach. The transchoroidal approach is versatile and provides adequate exposure of the middle and posterior regions of the third ventricle. However, the fornix column limits the exposure of the anterior region of the third ventricle. There is evidence that the unilateral section of the fornix column has little effect on cognitive function. This study compared the anatomical exposure afforded by the transforniceal-transchoroidal approach with that of the transchoroidal approach. In addition, a morphometric evaluation of structures that are relevant to and common in the 2 approaches was performed. METHODS The anatomical exposure provided by the transcallosal-transchoroidal and transcallosal-transforniceal-transchoroidal approaches was compared in 8 fresh cadavers, using a neuronavigation system. The working area, microsurgical exposure area, and angular exposure on the longitudinal and transversal planes of 2 anatomical targets (tuber cinereum and cerebral aqueduct) were compared. Additionally, the thickness of the right frontal lobe parenchyma, thickness of the corpus callosum trunk, and longitudinal diameter of the interventricular foramen were measured. The values obtained were submitted to statistical analysis using the Wilcoxon test. RESULTS In the quantitative evaluation, compared with the transchoroidal approach, the transforniceal-transchoroidal approach provided a greater mean working area (transforniceal-transchoroidal 150 ± 11 mm2; transchoroidal 121 ± 8 mm2; p < 0.05), larger mean microsurgical exposure area (transforniceal-transchoroidal 101 ± 9 mm2; transchoroidal 80 ± 5 mm2; p < 0.05), larger mean angular exposure area on the longitudinal plane for the tuber cinereum (transforniceal-transchoroidal 71° ± 7°; transchoroidal 64° ± 6°; p < 0.05), and larger mean angular exposure area on the longitudinal plane for the cerebral aqueduct (transforniceal-transchoroidal 62° ± 6°; transchoroidal 55° ± 5°; p < 0.05). No differences were observed in angular exposure along the transverse axis for either anatomical target (tuber cinereum and cerebral aqueduct; p > 0.05). The mean thickness of the right frontal lobe parenchyma was 35 ± 3 mm, the mean thickness of the corpus callosum trunk was 10 ± 1 mm, and the mean longitudinal diameter of the interventricular foramen was 4.6 ± 0.4 mm. In the qualitative assessment, it was noted that the transforniceal-transchoroidal approach led to greater exposure of the third ventricle anterior region structures. There was no difference between approaches in the exposure of the structures of the middle and posterior region. CONCLUSIONS The transforniceal-transchoroidal approach provides greater surgical exposure of the third ventricle anterior region than that offered by the transchoroidal approach. In the population studied, morphometric analysis established mean values for anatomical structures common to both approaches.

The enigmatic psalterium: a review and anatomic study with relevance to callosotomy procedures.

BACKGROUND: Recalcitrant seizures after callosotomy procedures are challenging to treat. One commissure, the psalterium, has received little attention. OBJECTIVE: To review the literature on this structure, and with dissection, better elucidate its morphology and relationships. METHODS: Twenty adult cadaver brains underwent microsurgical dissection of the psalterium. Measurements included the size and distance from the splenium of the corpus callosum and posterior border of the hippocampus tail. Observations included the relationships between the psalterium and the vein of Galen and the fiber direction within this structure. RESULTS: The psalterium was identified in all specimens. It intimately contacted the undersurface of the splenium superiorly and the velum interpositum inferiorly. It was always in the midline. Just posterior to the psalterium, the internal cerebral veins were found leaving the velum interpositum to drain into the vein of Galen. In most specimens, the psalterium fibers traveled in the same plane as the commissural fibers of the splenium. Mean width of the psalterium was 11 mm; mean length was 13 mm. Average thickness in the midline was 1.4 mm, and at the lateral edges, it was generally thicker as it joined the fornix with an average of 2 mm. In 30%, efferents from the hippocampus were arranged in the form of a sheet bilaterally rather than the normal bundle-like formation forming the posterior crura. CONCLUSION: Better understanding of the commissures of the human brain, including the psalterium, might help the neurosurgeon during procedures near this structure.

A 3D endoscopic transtubular transcallosal approach to the third ventricle.

OBJECT: Surgical approaches to deep-seated brain pathologies, specifically lesions of the third ventricle, have always been a challenge for neurosurgeons. In certain cases, the transcallosal approach remains the most suitable option for targeting lesions of the third ventricle, although retraction of the fornices and wall of the third ventricle have been associated with neuropsychological and hypothalamic deficits. The authors investigated the feasibility of an interhemispheric 3D endoscopic transcallosal approach through a minimally invasive tubular retractor system for the management of third ventricular lesions. METHODS: Three-dimensional endoscopic transtubular transcallosal approaches were performed on 5 preserved cadaveric heads (10 sides). A parasagittal bur hole was placed using neuronavigation, and a tubular retractor was inserted under direct endoscopic visualization. Following observation of the vascular structures, fenestration of the corpus callosum was performed and the retractor was advanced through the opening. Transforaminal, interforniceal, and transchoroidal modifications were all performed and evaluated by 3 surgeons. RESULTS: This approach provided enhanced visualization of the third ventricle and more stable retraction of corpus callosum and fornices. Bayonetted instruments were used through the retractor without difficulty, and the retractor applied rigid, constant, and equally distributed pressure on the corpus callosum. CONCLUSIONS: A transtubular approach to the third ventricle is feasible and facilitates blunt dissection of the corpus callosum that may minimize retraction injury. This technique also provides an added degree of safety by limiting the free range of instrumental movement. The combination of 3D endoscopic visualization with a clear plastic retractor facilitates safe and direct monitoring of the surgical corridor.

Interindividual variation in fornix microstructure and macrostructure is related to visual discrimination accuracy for scenes but not faces.

Transection of the nonhuman primate fornix has been shown to impair learning of configurations of spatial features and object-in-scene memory. Although damage to the human fornix also results in memory impairment, it is not known whether there is a preferential involvement of this white-matter tract in spatial learning, as implied by animal studies. Diffusion-weighted MR images were obtained from healthy participants who had completed versions of a task in which they made rapid same/different discriminations to two categories of highly visually similar stimuli: (1) virtual reality scene pairs; and (2) face pairs. Diffusion-MRI measures of white-matter microstructure [fractional anisotropy (FA) and mean diffusivity (MD)] and macrostructure (tissue volume fraction, f) were then extracted from the fornix of each participant, which had been reconstructed using a deterministic tractography protocol. Fornix MD and f measures correlated with scene, but not face, discrimination accuracy in both discrimination tasks. A complementary voxelwise analysis using tract-based spatial statistics suggested the crus of the fornix as a focus for this relationship. These findings extend previous reports of spatial learning impairments after fornix transection in nonhuman primates, critically highlighting the fornix as a source of interindividual variation in scene discrimination in humans.

Symmetry of the fornix using diffusion tensor imaging.

PURPOSE: To: 1) Present fornix tractography in its entirety for 20 healthy individuals to assess variability. 2) Provide individual and groupwise whole tract diffusion parameter symmetry assessments prior to clinical application. 3) Compare whole tract diffusion parameter assessments with tract-based spatial statistics (TBSS). MATERIALS AND METHODS: Diffusion tensor imaging (DTI) data were acquired on a 3T Siemens magnetic resonance imaging (MRI) system using a single-shot spin echo planar imaging (EPI) sequence. Individual fornix tractography was conducted and whole tract diffusion parameter symmetries assessed. Whole tract results were compared with asymmetry contrasts conducted with voxelwise statistical analysis of diffusion parameters using TBSS. RESULTS: The fornix tract could be visualized in its entirety including the columns, body, crura, and fimbria. Contrary to the crus and body, there were some tractography inconsistencies of the columns and fimbria across subjects. Although whole tract diffusion parameter asymmetries were nonsignificant, fractional anisotropy (FA) values bordered on statistical significance (P = 0.052). Using TBSS, significant FA asymmetries were identified (P ≤ 0.01, corrected). CONCLUSION: The findings demonstrate consistency of fornix tractography as well as some variability in the columns and fimbria. While parametric assessment demonstrates diffusion parameter symmetry, permutation-based TBSS analysis reveals significant FA asymmetries in the crura and fimbriae.

Removal of partial volume averaging with free water in MR diffusion tensor imaging using inversion recovery for b0 image.

PURPOSE: To remove the partial volume averaging effect of free water in MR diffusion imaging of neural tissues by use of the fluid attenuated inversion recovery (FLAIR) without the penalty of an extended scan time. MATERIALS AND METHODS: The magnetic resonance images were obtained from a normal volunteer in a coronal slice orientation at 3T with the 20-channel rf coil. In diffusion imaging only the b0 images were obtained with the FLAIR contrast while the diffusion weighted images were obtained without the FLAIR contrast. A composition of FLAIR b0 and non-FLAIR diffusion weighted images was used in calculating the diffusion tensor and fractional anisotropy after compensating the reduced signal amplitude due to the inversion recovery in the FLAIR b0 images. The fractional anisotropy of the non-FLAIR, FLAIR, and the composite methods were analyzed for the mean and histogram in the corpus callosum, cervical spine, and the fornix tracts. RESULTS: The partial volume averaging effect was observed in the corpus callosum, the cervical spine, and the fornix tracts in the non-FLAIR b0 and diffusion images. The partial volume averaging effect was removed in the FLAIR diffusion images which took more than twice the scan time than the non-FLAIR diffusion imaging. The proposed composite FLAIR diffusion imaging removed the partial volume averaging effect as in the FLAIR diffusion imaging. The distribution of the FA histogram was very different between the non-FLAIR and FLAIR diffusion images, while it was very similar between the FLAIR and the composite FLAIR after correcting the white matter signal in the FLAIR b0 images. CONCLUSIONS: The proposed composite FLAIR diffusion imaging method was equally effective in removing the partial volume averaging effect as the FLAIR diffusion imaging at a limited increase of the scan time since only a small number of b0 images needed to be obtained with the FLAIR contrast.

Surgical anatomy of the hippocampus.

BACKGROUND AND PURPOSE: Hippocampectomy is an efficient procedure for medial temporal lobe epilepsy. Nevertheless, hippocampus anatomy is complex, due to a deep location, and a complex structure. In this didactic paper, we propose a description of the hippocampus that should help neurosurgeons to feel at ease in this region. METHODS: Embryological data was obtained from the literature, whereas adult anatomy was described after dissecting 8 human hemispheres (with and without vascular injection) and slicing 3 additional ones. RESULTS: The hippocampus is C-shaped and made of 2 rolled-up laminae, the cornu Ammonis and the gyrus dentatus. Its ventricular aspect is covered by the choroid plexus of the inferior horn excepted at the head level. Its cisternal aspect faces the mesencephalon from which it is limited by the transverse fissure. Its rostral part (head) curves dorso-caudally to form the uncus, located at the medial aspect of the temporal lobe. Its caudal part (tail) splits into the fimbria and the gyrus fasciolaris that respectively run ventral and dorsal to the corpus callosum, to become the fornix and indusium griseum. CONCLUSION: Consequences of this complex anatomy are presented, and the authors stress the need for a subpial resection. Important landmarks are provided to avoid lesions of the surrounding structures.