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30-1-2021 White matter tract anatomy Meike Vernooij, MD PhD Professor of Population Imaging Radiology & Nuclear Medicine; Epidemiology Erasmus MC University Medical Center Rotterdam, The Netherlands Disclosures • no conflict of interest. 2 1 30-1-2021 Outline • Why study tract anatomy? • How study tract anatomy? • Major tracts and their function Why study WM tract anatomy? 2 30-1-2021 WhiteWhy matterstudy has tract functional anatomy? anatomy • GM dominates as the origin of consciousness and higher cortical functions • ~ 50% of the brain consist of white matter • WM thought of as “bulk substance” • many diseases have their origin in WM (with have cognitive and emotional effects) • critical organization and topology of the brain receives increasing interest • fundamental understanding of the brain’s working White matter tract anatomy – historical context • Johann Christian Reil & Friedrich Burdach (Germany) ~1800 • brain hodology: study of fiber anatomy • animal axonal tracings and in vivo dissections Johann Christian Reil 1812 3 30-1-2021 Meynert (1833-92) classification of WM 1. Projection fibres: ascending/descending pathways arising/terminating in cortex (CST) 2. Commisural fibres: connecting both hemispheres (corpus callosum) 3. Association fibres: connect cortical fibres within a hemisphere (e.g. arcuate fasciculus) https://neupsykey.com/ Wernicke (1848-1904): disconnection theory • brain is mosaic-like arrangement of areas with ‘psychic’ elements or motor/sensory function • only motor and sensory function occur in specialized areas. • higher functions not localized, but result from associative connections. • breakdown of these connections results in disease. 4 30-1-2021 Classical Wernicke theory: conduction aphasia (1874) 1 2 5 30-1-2021 How can we study tract anatomy in vivo? • 1994 Basser, Mattiello and LeBihan: seminal paper on diffusion tensor imaging • diffusion = random Brownian (thermal) motion of water molecules in brain tissue (3- dimensional process) • isotropic diffusion: How can we study tract anatomy in vivo? • free diffusion can be hindered by structures, such as white matter fibers, causing diffusion to take an elliptical shape (“rugby ball”) • anisotropic diffusion • information on direction of diffusion fibers 6 30-1-2021 How can we study tract anatomy in vivo? DWI b=0 DWI b=1000 ADC FA FA/direction 7 30-1-2021 Tractography How can we study tract anatomy in vivo? • earliest application DTI: 3D visualization of WM tracts • tract maps mirror classical postmortem descriptions • technical problems and limitations (e.g. crossing fibers, artifacts, ascending vs descending) • new insights: lateralization, layers within tracts 8 30-1-2021 Outline • Why study tract anatomy? • How study tract anatomy? • Major tracts and their function projection commissural association Projection tracts • connect cortex with other areas in the CNS (deep nuclei, brainstem, cerebellum, spine) • ascending/ descending (efferent (motor)/afferent (sensory)) • corticospinal tract (CST) Pyramidal tract • corticobulbar tract (CBT) • corticopontine tracts • brainstem tracts (medial lemniscus) • optic radiation 9 30-1-2021 Projection tracts Thalamic radiation Corticobulbar/pontine/reticular Corticospinal tract Oishi, Faria, Van Zijl, Mori; MRI atlas of human white matter; academic press. 21 Corticospinal/ corticobulbar tract Frontal lobe M1 (BA4) 60% Pre-motor & SMA Parietal lobe Post CG 30% Image credits: https://teachmeanatomy.info/ 10 30-1-2021 DTI of CST D. Fortin, The Canadian journal of K. Dalamagkas, Brain Imaging and neurological sciences 2012. Behavior doi: 10.1007/s11682-018-0006-y23 Optic radiation • geniculocalcarine tract: from LGN primary visual cortex. • transmits information from retina/optic nerve/optic tract to the cortex. • postoperative visual field defects in 50%–100% temporal lobe surgery (epilepsy!). • 4%-50% with permanent deficit. 11 30-1-2021 Optic radiation • 3 bundles: posterior, central, and the anterior bundle (Meyer’s loop: curves around temporal horn). • tractography challenging: curved course, low density, inter-subject variability ILF Hofer, Front. Neuroanat. 1999 25 Commissural tracts • Corpus callosum – largest fiber bundle (> 300 million axons) – homologous connections (mirror-images) but also asymmetric heterotopic connections – connects the entire cortex (not seen by DTI) • Anterior commissure • Posterior commissure • Hippocampal commissure 26 12 30-1-2021 Gross morphology CC • rostrum • genu • body • isthmus • splenium Aboitiz 2003 Doron Cortex 2008 Images kindly provided by prof. Tarek Yousry Jea Childs Nerv Syst 2008 28 dailyanatomy.com Hofer Neuroimage 2006 13 30-1-2021 Callosal disconnection: symptoms often Interhemispheric transfer deficits subclinical • Middle genu Simple bimanual tasks • Ant genu Complex bimanual tasks • Ant body Deficient tactile localisation • Anterior & body Dichotic listening & somatosensory tasks • Splenium Visual information Association tracts • connect cortical areas within a hemisphere • short versus long fibers: – short = connecting closely spaced regions (U- fibers) – long = connect different lobes • Major long association tracts: SLF, ILF, SFO, IFO, UF, limbic (cg, fx, st) (high resolution atlas: brain.labsolver.org, based on HCP data) 14 30-1-2021 SFO SLF SFO IFO UF ILF SLF ILF UF Oishi, Faria, Van Zijl, Mori; MRI atlas of human white matter; academic press. 15 30-1-2021 Function of association tracts • superior longitudinal fasciculus (SLF)/arcuate fasciculus (AF) – speech/ language – attention – memory – emotion – visuospatial function • inferior longitudinal fasciculus (ILF) – object recognition, prosopagnosia ILO • cingulum ILF – emotional processing, default mode network • inferior fronto-occipital fasciculus (IFO) UF – semantic language processing • uncinate fasciculus (UF) – semantic language processing – episodic memory – emotional processing • frontal aslant tract (FAT) – speech, verbal fluency image kindly provided by prof. Tarek Yousry Function of association tracts • superior longitudinal fasciculus (SLF)/arcuate fasciculus (AF) – speech/ language – attention – memory – emotion – visuospatial function • inferior longitudinal fasciculus (ILF) – object recognition, prosopagnosia ILO • inferior fronto-occipital fasciculus (IFO) ILF – semantic language processing • cingulum UF – emotional processing, default mode network • uncinate fasciculus (UF) – semantic language processing – episodic memory – emotional processing • frontal aslant tract (FAT) – speech, verbal fluency image kindly provided by prof. Tarek Yousry 16 30-1-2021 Language processing: classic model • Broca and Wernicke • Broca’s area: inferior frontal gyrus • Wernicke’s area: superior posterior temporal area • Motor and sensory • Connection: arcuate fasciculus 35 Language processing: classic model multiple speech production areas multiple comprehension areas images kindly provided by prof. Tarek Yousry 17 30-1-2021 Language processing: modern model • ventral and dorsal information stream • ventral: semantic: anterior/middle temporal lobe speech recognition • dorsal: phonological: spatial processing, sensorimotor integration 37 Tracts involved in language processing 38 18 30-1-2021 Frontal aslant tract • recognized only 10 years ago • left-lateralized • SMA IFG: speech and verbal fluency Dick The Neuroscientist 2014 Why study tract anatomy? implications for neurosurgery • challenge of surgical management of e.g. gliomas: maximal resection with minimal deficits. • direct electrical stimulation for brain mapping = gold standard • pre-operative DTI: planning of surgical procedure, prediction of risks (closeness of tracts to tumor, infiltration). • concordance between DTI and stimulation mapping reported sensitivity/specificity ranging from 85 to 100%. • in vivo validation difficult, accuracy spatially uncertain to ~ 1 cm • use of DTI may reduce number of stimulations needed. • technical factors affecting tractography are important limitations. Berman J Neurosurg 2007 Henderson et al; Neurosurg Focus 2020 19 30-1-2021 Jimenez-Pena; Clin Neuroradiol 2017 Take home points • White matter MATTERS! • topographic organization of WM subdivided by cortical projections. • categorized into projection, association, commissural tracts. • deficits often subclinical but still meaningful. • DTI has revolutionized non-invasive study of WM tracts in vivo. • implications for surgical planning but important inter- individual variability and caveats in tractography, operative mapping still gold standard. 20 30-1-2021 Thank you for your attention [email protected] 21.
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