Deterministic Tractography Study Of The Long Association Fibers Around The Parietal Region In The Human Brain

Objective: The human parietal lobe is anatomically and evolutionarily unique among primates,and plays an important role in advanced cognitive functions,including learning,memory,spatial perception,language,and hearing.The execution of advanced functions of the human brain is not performed independently by a single region but requires the cooperation of several brain regions with different functions.White matter fiber bundles provide the structural basis for the cooperation among various brain regions,containing axons of nerve cells for transmission and integration of information from different brain regions.White matter fiber bundles are considered to constitute an information“highway” in the human brain.Axonal tracing techniques are considered a “gold standard” in white matter research,and can be used in living non-human primates to accurately determine the direction of fiber bundle transmission,and the termination area in the cortex.However,because of ethical issues,axonal tracing cannot be applied to the study of the human brain.Based on the results of studies in other higher primates,researchers have speculated about the anatomical features of the corresponding fiber bundles in the human brain by analogy.However,previous studies have indicated that the human brain has undergone major evolutionary changes compared with other higher primates,particularly the fiber bundles related to language and emotion.Therefore,drawing analogies based on results from the monkey brain may lead to incorrect conclusions.In recent years,diffusion tensor imaging(DTI)has made it possible to study human white matter fiber bundles in vivo.However,due to the limitations of data acquisition methods,resolution and tracking algorithms,DTI cannot solve the crossing problem or identify cortical termination areas of fiber bundles.DTI is also prone to generating false connections.Recently,high definition fiber tracking(HDFT)technology has been developed,combining diffusion spectrum imaging(DSI)data acquisition,generalized Q-sampling imaging(GQI)data reconstruction and deterministic tracking under specific parameters.HDFT is able to achieve resolution to the sub-millimeter level,overcoming some of the limitations of DTI technology.Previous studies have reported that the tracking results of HDFT are highlyconsistent with the known white matter fiber bundles in the human brain.At present,the superior fronto-occipital fasciculus(SFOF)and middle longitudinal fasciculus(MdLF)are the two most controversial fiber bundles among the known long association fibers around the parietal lobe in the human brain.Previous studies have confirmed the existence of the SFOF in the monkey brain,but the question of whether the SFOF exists in the human brain,as well as its specific trajectory and cortical connective patterns,remains controversial.The MdLF was discovered relatively recently,and its termination areas in parietal and occipital lobes,segmentation and potential functions remain controversial.In the current study,advanced HDFT technology and Klingler’s fiber microdissection were used to explore whether the SFOF exists in the human brain,as well as its trajectory and cortical connective patterns.HDFT was used to study the trajectory of the MdLF,as well as its cortical connective patterns and hemispheric laterality in the human brain.Moreover,the MdLF was segmented according to its anatomical and functional connective patterns.The current results provide an anatomical basis for further study of the functions of the aforementioned fiber bundles,and could be useful for informing preoperative surgical approach design,accurate resection of tumors and the protection of nerve bundles in clinical practice.Methods: Part 1: First,Klingler’s fiber dissection method was applied to dissect four cadaver brains layer by layer from the outside to the inside,and from the inside to the outside,to explore the anatomical location of the SFOF in the human brain.The HDFT tractography experiment included 852 healthy subjects,ten of which were healthy volunteers recruited for the current study.Data for the remaining 842 subjects were extracted from the human connectome project(HCP-842)for the template study.All subjects underwent 3.0 T magnetic resonance(MR)DSI sequence scanning and GQI reconstruction.According to the results of dissection study,a deterministic tracking method combining whole brain seed points with multiple ROIs was then used to track the SFOF.Part 2: The HDFT tractography experiment included 852 healthy subjects,10 of which were recruited as healthy volunteers for the current study of individual differences.Data for the remaining 842 subjects were extracted from the HCP-842 large sample template,which was used to study the ubiquitous pattern of fiber bundles in the population.All 852 subjects underwent 3.0 T MR DSI sequence scanning and GQIreconstruction.The human MdLF was reconstructed with a deterministic tracking method to analyze its trajectory.Using DSI-studio software,the Brodmann template and improved automatic anatomical labeling(AAL2)were registered in QA diffusion space for each subject through linear transformation to identify functional and anatomical connective patterns,and the results were used to segment the MdLF.SPSS 22.0 software was used to quantitatively analyze the volume and length of the MdLF and its sub-segments in the left and right hemispheres.Results: Part 1: The fiber dissection study revealed some fibers that were suspected to be part of the SFOF,which were associated with the superior thalamus peduncle and were located above the head of caudate nucleus and medial to the corona radiata.In addition,some fibers were suspected to be part of the SFOF above the caudal thalamus.However,due to the cross fibers around the dorsal thalamus,the target fibers could not be tracked completely.Based on the fiber dissection results,three ROIs were identified,which helped with tracking the SFOF in the HDFT tractography study.In the HDFT tractography study,the SFOF was successfully reconstructed in 20 hemispheres of ten healthy volunteers and in all subjects included in the large sample HCP-842 template.Combined with cortical segmentation technology,the cortical termination areas of SFOF were identified,revealing that the human SFOF mainly originated from the rostral and medial parts of the superior and middle frontal gyri and formed a compact fascicle at the level of the anterior horn of the lateral ventricle coursing above the head of the caudate nucleus and dorsal thalamus,under the corpus callosum and medial to the internal capsule,and terminated mainly at the superior parietal lobe and precuneus.This anatomical organization differs from that in the monkey brain.Part 2: The MdLF in 20 hemispheres of 10 healthy volunteers and HCP-842 large sample template were qualitatively and quantitatively examined using HDFT.The MdLF was found to mainly originate from the temporal pole,superior temporal gyrus and transverse temporal gyrus,and mainly terminated at the superior parietal lobe,precuneus,cuneus,and lateral occipital regions.According to the cortical connectivity results,the MdLF was divided into three subsegments: MdLF-I,MdLF II,and MdLF-III.MdLF-I originated from the middle of the superior temporal gyrus(BA 22)and terminated at the superior parietal lobe(BA 7)and lateral occipital regions(BA 19);MdLF-II originated from the transversetemporal gyrus and terminated at the superior parietal lobe;MdLF-III originated from the temporal pole and anterior superior temporal gyrus and terminated at the cuneus and lateral occipital regions.The volume and length of MdLF and its subsegments in the left and right hemispheres were quantitatively analyzed using an independent sample t-test.The results revealed that,in terms of volume,there was a significant lateralization of MdLF and MdLF-III(left side > right side)(P > 0.05).In contrast,there was no significant lateralization of MdLF-I and MdLF-II,(P > 0.05).In terms of length,MdLF showed significant lateralization(left side > right side)(P > 0.05).The three subsegments of MdLF showed no significant lateralization(P > 0.05).Conclusion: Part 1: 1.The current results indicate that the SFOF exists in the human brain.However,this finding should be further verified using other techniques.2.The human SFOF mainly connects the frontal and parietal lobes.3.The human SFOF may be involved in visual processing,spatial sensation and cognitive processing.Part 2: 1.The termination areas of the human MdLF in the parietal and occipital lobes were mainly in the superior parietal lobe,precuneus,cuneus,and lateral occipital cortex.2.According to the anatomical and functional connective patterns,the MdLF can be divided into three subsegments: MdLF-I,MsLF-II,and MdLF-III.3.The human MdLF may be involved in the regulation of visual-spatial,visual-auditory and language functions.