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1、TheCommissuresWhatarethey?Thecommissuresarefibretractsconnectingthetwohemispheres.Thereisaposteriorcommissure,whichconnectsmidbrainanddiencephalonstructures,andliesjustinfrontofandabovethesuperiorcolliculi,belowthepinealgland.Theposteriorcommissureisnotalwayseas
2、ytoseeonahighdetailedstructuralscan,anditisveryunlikelyyouwillbeabletodiscernitonfunctionalimages.Theanteriorcommissure(AC)connectsthemiddleandinferiortemporalgyriofthetwohemispheres,andrunsacrossthemidlinejustinfrontofthefornix.Itisrelativelyeasytoseeonmoststru
3、cturalscans.Whythecommissures?Thecommissurehavebecomekeylandmarks,becauseTalairachandTournouxchosethemtodeterminethestandardalignmentofthebrainfortheirfamousatlas.Toalignascanto Talairachspace,abrainhastobesetsothattheanteriorandposteriorcommissuresareonahorizon
4、atalline.Wherearethey?Belowisalittleschematic(fromChrisRorden's MriCro tutorial)ofasagittalbrainview.Theanteriorcommissureisshowninred,andtheposteriorcommissureinyellow. TheimagebelowshowstheanteriorcommissureonaniconicbrainfromtheMNIseries(seethe MNIspacepage f
5、ordetails).TheACisatthevirtualintersectionoftheredlines.TheACisbestseenonslice91inX,131inYand67inZoftheiconicbrainimage.BelowisanimageshowingthePConthesamebrain,againatthe(truncated)intersectionoftheredlines.ThePCcanbestbeseenatslicesX91,Y103andZ70.Notethatthisb
6、rainisnotexactlyorientedaccordingtotheTalairachsystem,asthetwocommissuresarenotonthesameaxialplane.Itmaybeusefultoscrollthroughtheiconicbrainimageprovidedwith MRIcroorMRIcroGL,togetusedtothelandmarksaroundthecommissures.Hereisthepositionoftheanteriorcommissureon
7、theMNI152averagebrain;firstonthetheT1average:andnextontheT2average(whichismoresimilartoEPIimages).Specifyingthecommissuresonlowresolutionimages,suchasfunctionalimages,requiresalittleguesswork,combinedwithaknowledgeofwherethecommissuresareinrelationtoanatomythaty
8、oucansee,suchasthethalamus,brainstem,andcorpuscallosum.MatthewBrett,ChrisRorden17/12/99ReferencesJ.TalairachandP.Tournoux,"Co-planarStereotaxicAtlasoftheHumanBrain:3-
