Lesions of the Corpus Callosum and Other Commissural Fibers: Diffusion Tensor Studies Christopher G. Filippi, MD, and Keith A. Cauley, MD, PhD The corpus callosum is the largest white matter tract in the brain, connecting the 2 hemispheres. The functions of the corpus callosum are many and varied, and lesions frequently cause only subtle clinical findings. The range of diseases that can affect the corpus callosum is vast and includes all potential white matter disease. The distribution of lesions in the corpus callosum is disease specific in only a few entities such as Susac syndrome and MarchiafavaBignami disease. Group studies have found significant differences of diffusivity metrics in the corpus callosum in preterm infants, patients suffering seizure activity, and patients with earlyonset Alzheimer’s disease. Given the challenges that multiple orientation of fibers within the callosum presents, advanced postprocessing methods may be required to reveal ultrastructural disease. Semin Ultrasound CT MRI 35:445-458 C 2014 Published by Elsevier Inc.

Introduction

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he corpus callosum is the largest commissural white matter bundle in the brain, consisting of 200-250 million axonal projections, and serves as the major communication pathway between left and right hemispheres. A series of nowfamous studies on patients with callosotomy led to the understanding that the left and right hemispheres of the human brain are specialized in different tasks.1 The corpus callosum is subject to a wide variety of pathologies, some with characteristic clinical presentations, and many with unique appearance at imaging. Diffusion tensor magnetic resonance imaging (DTI) is a relatively new tool for characterization of white matter tracts. The fundamentals of diffusion tensor imaging are the subject of many previous review articles.2-4 As the major white matter bundle of the brain, and a conspicuous structure on sagittal midline images of the brain (Fig. 1), fibers of the corpus callosum can be readily traced on diffusion tensor imaging studies (Figs. 1 and 2), and pathologies of the corpus are

Division of Neuroradiology, Columbia University College of Physicians and Surgeons, New York, NY. Address reprint requests to Christopher G. Filippi, MD, New York Presbeterian Hospital, 177 Fort Washington Ave, MHB 3-111, New York, NY 10032. E-mail: [email protected]

http://dx.doi.org/10.1053/j.sult.2014.06.004 0887-2171/& 2014 Published by Elsevier Inc.

readily characterized with routine clinical diffusion tensor imaging. Although the role of diffusion tensor imaging in clinical studies is a topic of ongoing research, DTI has been used in the clinical setting to characterize pathology with the goal of narrowing the differential diagnosis, offering information for surgical planning, clarifying prognosis, and understanding of clinical symptoms as they relate to the anatomy detailed by DTI. DTI offers 2 kinds of information: quantitative measure of the diffusion parameters and diffusion tensor tractography, which is typically qualitative. Quantitative diffusion measures can be used to assess regions of white matter that appear normal on routine clinical magnetic resonance imaging (MRI) (normal-appearing white matter [NAWM]), and diffusion tensor measures are reported to be more sensitive for the assessment of white matter disease than conventional MRI. The quantitative capabilities of DTI provide a powerful tool in the assessment of white matter but are less amenable to pictorial review; most of the images shown in this review are of qualitative diffusion tensor tractography. Anatomically, the corpus callosum comprises the genu, the body, the splenium, and the rostrum. The body of the corpus can be further divided into anterior body, posterior body, and isthmus, as has been described5 (Fig. 3A). These anatomical regions are most clearly seen on sagittal midline images. Routine deterministic DTI using regions of interest (ROIs) 445

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Figure 1 DTI of the corpus callosum. (A) Sagittal color-coded FA map shows the corpus callosum in red. The color convention for FA map illustrates transverse fibers in red, craniocaudad fibers in blue, and anterior-posterior fibers in green. The fibers of the corpus travel transversely as they cross between the hemispheres and are therefore homogeneously red on colorcoded FA maps. (B) Whole-brain tractography of fibers generated using a region of interest around the corpus callosum on midline sagittal images, as in (A). As in (A), the tricolor map reflects fiber direction. A large percentage of brain white matter fibers pass through the corpus callosum.

outlining these anatomical regions on sagittal DT images can readily trace the course of principal callosal fibers (Fig. 2). Lesions of specific regions of the corpus can have specific clinical symptoms, though most lesions seen clinically and at imaging are manifestations of more systemic disease and do

not present with unique localizing signs or symptoms. ROIs can be chosen to map specific subsets of callosal fibers (Fig. 3B). Here, an ROI is drawn over the rostrum, and color fibers represent white matter fibers coursing between the inferior frontal lobes.

Figure 2 Diffusion tensor tractography of the corpus callosum: (A) coronal, (B) axial, (C) oblique coronal, and (D) oblique sagittal views.

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Figure 3 Segemental anatomy of the corpus callosum and tracts. (A) Anatomical segments of the corpus callosum, color coded. Yellow—rostrum, green—genu, blue—anterior body, red—posterior body, light blue—isthmus, purple—splenium. (Adapted with permission from Filippi et al.5) (B) Fibers generated through a region of interest (ROI) outlining the rostrum of the corpus callosum, with trajectories toward inferior frontal lobe centers.

Important details regarding the anatomical and functional organization of the corpus callosum remain unknown, and DTI delineation of white matter fibers permits anatomical and functional studies that had not been possible previously, promising to aid in the understanding of the function of the corpus in health and disease. One such study has elaborated a largely unknown callosal communication between motor cortices of the hemispheres.6 Virtually all white matter pathologies involving the corpus callosum are amenable to characterization by diffusion tensor methods. The only limiting factors are patient cooperation and available scan time. Clinical but relatively low-quality DTI can be performed in only a few minutes and can be obtained as part of a routine protocol. High-quality diffusion tensor data may require prospective planning, and the longer scan time may go unreimbursed, unless funded by a clinical trial or research study. Several previous articles have reviewed lesions of the corpus callosum,7-9 with inclusion of diffusion tensor studies.10,11 Here we review examples from broad classes of pathology involving the corpus callosum, which have been the subject of diffusion tensor methods. We focus on the value of diffusion tensor methods in diagnostic neuroimaging (Table).

Neoplastic Disease As a rule, primary brain tumors that involve the corpus callosum are glioblastoma multiforme (Fig. 4) and lymphoma. Metastasis can also involve the corpus. Previous characterization of neoplasms using diffusion tensor imaging has shown that more aggressive lesions invade and disrupt white matter fibers, whereas benign lesions tend to cause mass effect on white matter fibers, displacing

them without disruption.12 Diffusion tensor parameters can be used to quantitatively characterize the tumor itself as well as the areas immediately around the tumor, for mapping white matter fibers and assessment of tumor invasion.13

Table Callosal Lesions Covered in This Review Lesions of the Corpus Callosum Diffusion Tensor Studies Neoplastic disease (glioblastoma multiforme) Diseases of myelin Leukoencephalopathy ADEM/MS Susac syndrome Infection HIV Viral encephalitis Other viral-related disease Vascular (infarction alien hand) Traumatic or iatrogenic injury Traumatic brain injury (TBI) Postsurgery Hydrocephalus Developmental or congenital or genetic Collosal agenesis or dysgenesis Congenitally thickened corpus callosum NF-1 Neurodegenerative conditions Alzheimer’s disease Toxic or metabolic Metabolic derangement (DKA) Marchiafava-Bignami disease Psychiatric Postictal ADEM, acute disseminated encephalomyelitis; DKA, diabetic ketoacidosis.

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Figure 4 Glioblastoma in the corpus callosum: (A) axial, (B) coronal, and (C) sagittal tractography views of the corpus callosum in a patient with a left occipital glioblastmoa multiforme; the tumor displaces and disrupts the left splenial fibers.

Diseases of Myelin Leukoencephalopathy Any diffuse leukoencephalpathy may show secondary involvement of the corpus callosum (via Wallerian degeneration) or via direct disease predilection (i.e. mucopolysaccharidoses). The differing components of white matter damage may be revealed by different DTI metrics. In a study of “vanishing white matter disease,” DTI showed progressive declines in fractional anisotropy (FA) and increases in both axial and radial diffusivity reflecting a combination of effects of axonal damage and demyelination.14

Acute Disseminated Encephalomyelitis/ Multiple Sclerosis As perhaps the most common white matter disease, multiple sclerosis (MS) has been the subject for a large number of diffusion tensor research studies. DTI has been shown to be sensitive to white matter changes that are below the threshold for detection using conventional MRI,15,16 and may offer a quantitative biomarker for use in clinical trials. Since the first descriptions of MS, the corpus callosum has been described as the center of demyelinating lesions. Study of the corpus callosum shows diffuse declines in FA and mean diffusivity (MD) in MS.17 Lower FA in the splenium of the corpus callosum predicts a greater progression of disability in 5 years of follow-up.18 Reduced axial diffusivity of NAWM of the callosum was found in a comparison between children with MS and those with a monophasic demyelinating illness (acute disseminated encephalomyelitis); both groups showed reduced FA and increased radial diffusivity (indicative of myelin damage) compared with controls.19

Susac Syndrome Susac syndrome is a rare microangiopathy, thought to be due to autoimmunity to capillary endothelial cells, typically seen in young patients. Virtually all patients meeting the diagnostic criteria have callosal lesions, the majority of which are located centrally with the corpus callosum, typically the body of the corpus callosum (Fig. 5). MRI demonstrates classic “T1 holes,” which remain after the acute presentation has resolved.20,21 Symptoms can be varied and striking and are often not entirely explained by the routine clinical imaging findings. DTI enables not only tractography but also quantitative analysis of the diffusion tensor metrics, which can be sensitive to pathology even in NAWM,22 suggesting that DTI might prove helpful in strengthening the diagnosis of Susac syndrome in uncertain cases. Involvement of normal-appearing frontal white matter may account for the previously unexplained behavioral symptoms seen in the syndrome.

Infection Human Immunodeficiency Virus The corpus callosum is subject to the same (particularly white matter) infections seen elsewhere in the brain, and DTI has been used to investigate disease of NAWM in patients with human immunodeficiency virus (HIV),23 with a subsequent study documenting declines in FA and increases in MD in the corpus callosum,24 measures that appear to be more sensitive on DTI than conventional MRI for detecting pathologic changes of HIV.

Viral Encephalitis A case report using diffusion tensor tractography in a patient with rotavirus encephalopathy shows reversible

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Figure 5 Susac syndrome. (A) Sagittal T1-weighted image of a patient with Susac syndrome shows characteristic T1 hypointense lesions (arrows) within the corpus callosum. (Adapted with permission from Rennebohm et al.20) (B) Sagittal T2-weighted image of a patient with Susac syndrome (different patient than left) shows T2 hyperintense, cystic lesions located centrally within the corpus callosum. (Adapted with permission from Filippi et al.23)

changes in the forniceal crus.25 As diffusion tensor imaging has become part of many clinical protocols at university centers, cohort studies will enable determination of the reproducibility, sensitivity, and specificity of these types of observations.

alterations in diffusion tensor characteristics of the corpus callosum, although the described changes would be expected to extend to the corpus.

Vascular Other Viral-Related Disease Diffusion tensor imaging reveals changes to NAWM in other infectious disease conditions such as CreutzfeldtJakob disease26 and progressive multifocal leukencephalopathy.27 These studies did not specifically address

Lesions of the corpus callosum can have a spectrum of clinical manifestations, depending on the location and severity of the lesion. Anterior lesions may result in inability to move or speak (akinetic mutism) or inability to name objects that are felt but not seen (tactile anomia).

Figure 6 Callosal infarct. DWIs show an acute infarct of the left genu and body of the corpus callosum in an 82-year-old woman with history of hypertension and type II diabetes, who presented with sudden-onset loss of control of left hand and tactile anomia. The left hand seemed to move of its own accord and often had to be restrained by her right hand. DTI of a very similar case showed reduced volume of corpus callosal fibers between motor cortices.29 DWI, diffusion-weighted imaging. (Case courtesy of Dr Adam G. Thomas.)

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Figure 7 DTI in diffuse axonal injury: (A) axial FLAIR scan in a 49-year-old patient with TBI taken 16 months after the initial trauma showing no abnormalities; (B) color-coded FA map showing reduced FA left frontal lobe; (C) axial T2-weighted image showing ROI for tractography; (D) superior oblique view of tractography including ROI (red) shows involvement of the forceps minor and frontotemporo-occipital fibers; and (E) removal of the red ROI shows discontinuity of fibers (arrow). FLAIR, fluid-attenuated inversion recovery. (Adapted with permission from Assaf et al.32)

Alien hand syndrome (AHS) is one of the more striking manifestations of damage to the corpus callosum and is 1 aspect of the split brain findings seen in subjects with callosotomy. In AHS, the subject fails to recognize ownership of his or her own limb in the absence of visual cues and experiences a feeling that 1 body part is foreign, with activity that is autonomous and outside of voluntary control.28 AHS can arise from damage to either frontal lobe or corpus callosum. Callosal AHS is seen as a consequence of an anterior callosal lesion and is thought to be experienced during activities requiring dominant hemisphere control.29 AHS is relatively uncommon and is probably seen most often as a consequence of infarction (Fig. 6). Lesions to the posterior corpus callosum (splenium) can present with an inability to read, while maintaining an ability to write (alexia without agraphia). Future studies might use diffusion tensor imaging to correlate clinical disconnection syndromes with specific DTI findings, to further our understanding of their clinical manifestations.

Traumatic or Iatrogenic Injury Traumatic Brain Injury Traumatic brain injury (TBI) has been the subject of recent attention, as a relatively minor traumatic injury, particularly if repeated, can have very serious long-term sequelae. Diffusion tensor imaging is currently under investigation as a tool for analysis of TBI, which may not be apparent at routine clinical imaging. The corpus callosum provides a logical target for analysis as TBI tends to involve this major white matter structure. A recent study finds mild TBI associated with significant decreases in the FA and MD of the genu o3 months after trauma.30 In more severe cases, both genu and splenium are affected.30 Diffusion tensor imaging may prove to be an important tool for the diagnosis and prognosis of TBI (Fig. 7).

After Surgery Diffusion tensor imaging was used to characterize a case of stereotactic subcaudate tractotomy performed for depression

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Figure 8 Callosal damage from psychosurgery. (A) Sagittal T1 and (B) tractography of the anterior callosum in a healthy patient. (C) Sagittal T1-weighted image in a 67-year-old male imaged for diplopia and dizziness with sequelae of a deep brain instrumentation showing absence of the rostrum of the corpus callosum (arrow). He had undergone subcaudate tractotomy for depression more than 30 years earlier. (D) Tractography reveals severe atrophy of rostral collosal fibers. (Adapted with permission from Cauley and Catatepe.33)

more than 30 years earlier31 (Fig. 8). Ablation aimed to disrupt the frontothalamic fibers, which are thought to be important in the control of mood. Diffusion tensor tractography revealed atrophy of the rostrum of the corpus

callosum as a consequence of the ablative procedure, which apparently had a very satisfactory outcome. The rostrum contains interhemispheric connections between prefrontal regions.

Figure 9 Comparison of preterm and term neonatal white matter integrity. Tract-Based Spatial Statistics (TBSS) with comparison between preterm and full-term subgroups. Significantly lower FA values in the white matter of preterm neonates are shown, with prominent differences in the body and splenium of the corpus callosum, as well as the centrum semiovale, internal capsule, external capsule, crus of the fornix, cerebral peduncle, and inferior longitudinal fasciculus. Mean FA skeleton is shown in green. Areas with significant difference between groups (P o 0.05) shown in red. (Adapted with permission from Tovar-Moll et al.37)

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Figure 10 Regional development of the corpus callosum in preterm infants. Representative diffusion tensor image of the corpus callosum in preterm child. The isthmus in designated as region 4 and shows the greatest difference when compared with age-matched control subjects. (Adapted with permission from Wahl et al.38)

Hydrocephalus Diffusion tensor imaging has been used to characterize changes in the corpus callosum in hydrocephalus.32 There appears to be reversible reduced FA in the corona radiata but irreversible increased FA in the callosum, possible owing to differences in fiber density and orientation.32 Within the pyramidal tract, DTI white matter changes may identify reversible vs nonreversible effects of hydrocephalus,33 the ultimate goal being the identification of patients with potentially reversible white matter damage and thus suitable surgical candidates.

Developmental or Congenital or Genetic Myelin Maturation Although it is well established that brain myelination is incomplete at birth, and that incomplete myelination of prematurity can portend developmental delay, diffusion tensor imaging and Tract-Based Spatial Statistical (TBSS)34 methods are providing greater precision in analysis of both the time frame and extent of myelin maturation. Maturational changes are readily noted in the corpus callosum35 (Fig. 9). The corpus

Figure 11 Topographic arrangement of the Probst fibers in collosal dysgenesis (A-C) show the Probst bundles (in purple), segregated from the cingulated bundles (in yellow), shown in transverse sections. Note that Probst fibers leave the bundle toward the cortex in different regions along their longitudinal course (A), projecting mainly to mesial cortical areas (B and C). (D) A directional FA map at the coronal plane, with the Probst bundles and cingulate bundles coded in green. (Adapted with permission from Rollins.39)

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Figure 12 Comparison of DTI and QBI tractography. For a control subject (top) and a subject with partial agenesis of the corpus callosum (bottom), segmented tracts are shown by using DTI tractography performed on a DTI acquisition at b ¼ 1000 s/mm2 (A and D) and on a HARDI acquisition at b ¼ 3000 s/mm2 (B and E). QBI tractography is shown for the same HARDI acquisition at b ¼ 3000 s/mm2 (C and F). QBI tractography yields more extensive fibers, including more lateral frontal fibers and denser temporal fibers in the control subject and an additional heterotopic fiber in the subject with partial agenesis of the corpus callosum (red in F) not recovered by DTI tractography. HARDI, high-angular resolution diffusion tensor imaging; QBI, Q-ball imaging. (Adapted with permission from Di et al.40)

callosum maintains a lower FA value in children born preterm, and the isthmus of the corpus is the most vulnerable subdivision in these patients36 (Fig. 10).

Callosal Agenesis or Dysgenesis Agenesis of the corpus callosum occurs when the corpus callosum fails to develop normally, and the white matter fibers of the corpus are longitudinally oriented rather than running in a transverse direction (Fig. 11). These abnormal longitudinal white matter tracts are called “Probst bundles” and can be identified with diffusion tensor imaging.37 Other abnormalities of callosum formation include hypogenesis (partial formation), dysgenesis (malformation), and hypoplasia (underdevelopment). Malformations of corpus development can be associated with a number of congenital syndromes. Diffusion tensor imaging of callosal agenesis and partial agenesis reveals variable callosal connectivity among patients,

with implications for etiology as well as prognosis; such studies may reveal the plasticity of processes in the developmental biology of the brain. Advanced techniques in DTI have been applied to the corpus callosum, as this dense white matter structure poses challenges to simpler DTI methods owing to the large numbers of crossing fibers in each voxel. Approaches such as high-angular resolution diffusion tensor imaging or Q-ball imaging can demonstrate detail not previously evident on less sophisticated methods38 (Fig. 12) and resolve artifactual tracts generated by miscalculation of the trajectory of crossing fibers.

Congenitally Thickened Corpus Callosum Congenitally thickened callosum is a rare condition associated with seizures and developmental delay. Diffusion tensor tractography illustrates that the thickening may be due to heterotopic cingulum, enlarged indusium griseum, and aberrant callosal fibers (Fig. 13).39

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Figure 13 Thickened corpus callosum. A 2-year-old boy with pervasive developmental delay. (A) Midsagittal T1 image shows marked thickening of the midcallosal body (14.2 mm) with subjective shortening of the callosal length. (B) Midsagittal directionally encoded color map. The arrow indicates longitudinal supracallosal fibers, which account for the callosal thickening. Longitudinal fibers are not normally seen in the midline; like association fibers, the normal cingulum does not cross the midline. (C) Coronal color map. The large midline longitudinal fibers are separate from the paired cingulum (arrows). (Adapted with permission from Canu et al.41)

Neurofibromatosis Type 1 Neurofibromatosis type 1 (NF-1) is an autosomal dominant genetic defect in the NF-1 gene on chromosome 17. NF-1 demonstrates specific lesions such as gliomas and “neurofibromatosis bright objects” on conventional MRI,

but more recent findings suggest more global manifestations of the disease with abnormalities of myelination resulting in a significantly larger corpus callosum. Quantitative analysis of DTI metrics of the 6 segments of the corpus callosum of children with NF-1 reveal statistically

Figure 14 Callosal involvement in Alzheimer’s disease. Tract-Based Spatial Statistics (TBSS) results in patients with early-onset Alzheimer vs age-matched controls. Statistically significant voxelwise group differences are shown (fractional anisotropy, FA, in red in the upper series; mean diffusivity, MD, in blue in the lower series). Results are overlaid on white matter skeleton (light green) and displayed on sagittal and axial sections of the Montreal Neurological Institute (MNI) brain map in neurologic convention. These study show greater callosal damage in patients with early-onset AD. (Adapted with permission from Sair et al.43)

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Figure 15 Voxelization tractography of the fornix. Voxelized tractography of the fornix on the sagittal plane of the color-coded FA map is shown (A, D, and G). Morphologic features are evaluated by using the voxelized tractography of the fornix (B, E, and H). The diffusion tensor metrics are evaluated in the voxelized tractography of the fornix after shape processing (C, F, and I). The tractography for idiopathic normal pressure hydrocephalus (iNPH) (A, B, and C), AD (D, E, and F), and controls (G, H, and I) is shown. There is degeneration of the fornices in both AD and iNPH but stretching of the fornix in iNPH (A) is appreciated. (Adapted with permission from White et al.44)

significantly lower axial and radial diffusivity than agematched control subjects.5 Quantitative measures of white matter abnormalities in NF-1 may help to delineate the contribution of structural abnormalities to cognitive impairment in NF-1.

Neurodegenerative Conditions Alzheimer’s Disease As Alzheimer’s disease (AD) is relatively common and difficult to diagnose and treat, the disease has been the subject of considerable imaging research. Changes in DTI metrics of the corpus callosum have been detailed in AD and may be useful in distinguishing AD from other forms of cognitive impairment, and DTI metrics may serve at a biomarker for disease progression and response to treatment.40 Using TBSS, it has been demonstrated that early-onset AD shows more severe

damage of the corpus callosum than late-onset disease (Fig. 14).41 Diffusion tensor characterization of the forniceal crus has shown reduction in volume in AD as well as in idiopathic normal pressure hydrocephalus (iNPH). Both conditions show reduced forniceal FA and volume, but the length of the tract is increased in iNPH but not in AD, reflecting different etiologies (stretching in iNPH but degeneration in AD) (Fig. 15).42

Toxic or Metabolic Metabolic Derangement Restricted diffusion in the splenium of the corpus callosum is a nonspecific finding, which can be seen in infarction, infection such as viral encephalitis, or as toxic or metabolic sequelae (Fig. 16). Diffusion tensor analysis of callosal white matter fibers has yet to be performed in this setting.

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Figure 16 Reversible splenial lesions. DWI shows restricted diffusion in the splenium, as well as white matter of the corona radiata and centrum semiovale in a 30-year-old male patient with a 1-week history of diabetic ketoacidosis. ADC maps showed corresponding hypointensity. Reversible splenial signal change has many causes including viral infection, seizure activity, medication toxicity, and other metabolic disorders. ADC, apparent diffusion coefficient map; DWI, diffusion-weighted imaging.

Marchiafava-Bignami Disease Marchiafava-Bignami disease is typically seen as sequelae of alcohol toxicity and causes demyelination and necrosis of the corpus callosum. The disease presentation is variable but may

show an interhemispheric disconnection syndrome similar to that described previously in callosal infarction. Diffusion tensor metrics and diffusion tensor tractography findings are abnormal throughout the callosum (Fig. 17).43 DTI may be useful in assessing disease severity.

Figure 17 Marchiafava-Bignami disease. FA maps show diminished intensity in the genu and splenium of the corpus callosum in a patient with MBD compared with normal. An area of cystic necrosis is seen in the genu of the patient with MBD. Sagittal white matter fiber-tracking images demonstrate significantly diminished fiber density throughout the corpus callosum, with relative decrease in white matter volume within the body of the patient with MBD. The oblique white matter fiber-tracking images demonstrate similar findings with additional loss of lateral projecting white matter tracts from the splenium. MBD, Marchiafava-Bignami disease. (Adapted with permission from Concha et al.46)

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Figure 18 Spatial distribution of diffusion changes in postictal patients vs controls. Areas of significantly but transiently increased mean diffusivity compared with the control group are overlaid in color on the patient’s normalized b0 image (A and B). The difference analysis shows an area of decreased mean diffusivity postictally in the posterior temporal lobe than seen on the interictal image (C). (Adapted with permission.45)

Psychiatric As the reliability of diffusion tensor methods increases, and the database of normal subjects increases, diffusion tensor methods are finding application to diseases that have been refractory to traditional imaging analysis. Postprocessing methods that establish voxel-wise averaging and comparison of cohorts such as TBSS are promising in the evaluation of psychiatric disease such as schizophrenia.44

Postictal Diffusion characteristics of the corpus callosum undergo dynamic changes over time following seizure activity (Fig. 18).45 MD values appear to undergo greater changes than anisotropy measures. The spatial distribution of subtle white matter changes, only revealed by DTI, may have a role in surgery planning for epilepsy. 46

Conclusion The corpus callosum is the largest white matter bundle of the brain, argued to “enable the human condition,”1 yet much remains unknown regarding its anatomical and functional organization. Diffusion tensor imaging offers a tool for the analysis of white matter in vivo, both locally and through tractographic mapping of fiber direction and connectivity. Clinically, diffusion tensor tractography can offer immediate utility for delineating white matter anatomy, as in congenital malformations or in surgical planning. Diffusion tensor imaging can reveal abnormalities of the corpus callosum that are not apparent at conventional MRI, with implication for diagnosis, prognosis, and treatment planning. Increased diffusivity and reductions in FA are nonspecific findings of white matter pathology, and anatomical localization and monitoring rates of change are necessary to gain specific information about disease

involving callosal white matter, with the potential to serve as a quantitative biomarker in clinical trials, and in monitoring treatment in the clinical setting.

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Lesions of the corpus callosum and other commissural fibers: diffusion tensor studies.

The corpus callosum is the largest white matter tract in the brain, connecting the 2 hemispheres. The functions of the corpus callosum are many and va...
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