The Diffusion Magnetic Resonance Imaging Study Of Superior Longitudinal Fasciculus In Human Neonatal Brain

The superior longitudinal fasciculus(SLF)is a long association fiber tract which is thought to connect temporal and parietal lobe with the frontal lobe.Based on its extensive connections in the cerebral lateral hemisphere and involvement in various essential functions,such as language,attention and other related functions,the study of the superior longitudinal fasciculus has been ongoing over a hundred years for its important research value and also will continue in the futureIn the early time,the research of SLF were mainly limited to the anatomical study of adult specimen.With the development of medical imaging and image post-processing technology,the diffusion magnetic resonance imaging is increasingly applied in the study of human adult SLF.The advantage of this method is that it can quantitatively analyze the microstructure of human white matter fiber tracts noninvasively.Unlike adults,there are few reports about the SLF of neonates.On the one hand,it is very difficult to obtain high resolution images of diffusion MRI due to the scan limitations of neonates like scan time,magnetic field intensity and other conditions;For another,SLF is one of the slowest maturing fiber tracts which means a lower myelination degree,meanwhile there are many fibers intersecting with SLF in its running area,which are all limitations of DTI and make the tracking results of SLF not well during neonatal period.To observe the trajectory of each branch of SLF and the cortical regions connected by it as well as quantitatively evaluate the maturity status of each SLF segment,we chose the diffusion MRI data of 40 healthy term neonates which were imaged at 36.86-44.14 weeks post-menstrual age and conducted a qualitative and quantitative study of SLFIn the qualitative analysis,we used the new single shell 3-tissue constrained spherical deconvolution(SS3T-CSD)algorithm and obtained white matter fiber orientation distribution(WM-FOD)pictures.On the basis,we got the results of the whole brain fiber tracking.Then we segmented the SLF by manually drawing the region of interest(ROI)and successfully divided it into SLF Ⅰ,SLF Ⅱ and SLF Ⅲ.Combined with the Automated Anatomical Labeling(AAL)map which is registered to our picture,we obtain many meaningful results.We found that the fiber morphology and connectivity of SLF in neonatal brain are similar to adults.SLF Ⅰlocates laterally and superiorly compared with the cingulate fasciculus.SLF Ⅰ runs parallel to the cingulate fiber and exhibits an "S-shaped" configuration with two prominent bends,connecting the superior parietal lobule and precuneus as well as the superior frontal gyrus.SLF Ⅱ locates laterally and inferiorly compared with the SLF Ⅰ.The connectivity of SLF Ⅱ is from angular gyrus to middle frontal gyrus and precentral gyrus.SLF Ⅲ is indistinguishable from the anterior segment of the arcuate bundle(AF),which we consider to be the same fiber tract with SLF Ⅲ.The anterior segment of AF or SLF Ⅲ is located on the ventral side of SLF Ⅱ.At the sagittal plane where SLF Ⅱ and SLF Ⅲ both exist simultaneously,the fibers in a more dorsal direction are considered as SLF Ⅱ,and those in a ventral direction as SLF Ⅲ.SLF Ⅲconnects from supramarginal gyrus to precentral gyrus and inferior frontal gyrus.After obtaining fiber tracking results for the three branches of SLF,we measured the DTI(diffusion tensor imaging)and NODDI(neurite orientation dispersion and density imaging)parameter values of each branch of SLF,and quantitatively evaluated the maturity status of each SLF segment.Our results show that the FA and NDI values of SLF Ⅰ,SLF Ⅱ and SLF Ⅲ increase linearly,while the MD and RD values of them decrease linearly,as gestational age increases.Different from the above metrics,the ODI values of SLF Ⅰ and SLF Ⅲ change non-linearly with the increase of gestational age.Although the ODI values of SLF Ⅱ show a linear decrease,the P value of the slope has reached 0.047.The AD value of SLF Ⅱ changes non-linearly,but the decrease of AD in SLF Ⅰ and SLF Ⅲ is linear with the increase of gestational age.In addition,among the three branches of SLF,SLF Ⅲ has the highest FA value and the lowest ODI value.On the contrary,the FA value of SLF Ⅱ is the lowest,but the ODI value of it is the highest.These results are all statistically significant.To sum up,we came to conclusions:(1)The fiber morphology and connectivity of SLF in neonatal brain are similar to adults.(2)The three branches of SLF have different maturation degree:SLF Ⅲ with the highest FA value and the lowest ODI value is more mature compared to other two SLF subcomponents.While SLF Ⅱ with the lowest FA value and the highest ODI value is less mature than others,which may have more advanced brain functions.In our study,we reconstructed and visualized the trajectory of SLF and its branches,qualitatively analyzing the fiber morphology of SLF and the cortical areas its branches connect.These results can help clinicians choose reasonable surgical approach and provide the anatomical basis for reducing the damage of corresponding fiber tracts in surgery as much as possible.In addition,we used quantitative parameters to evaluate the maturation degree of each SLF branch in the neonatal period,providing a theoretical basis for early diagnosis and treatment of related diseases caused by abnormal development of SLF.