Journal of the Neurological Sciences 339 (2014) 235–236
Contents lists available at ScienceDirect
Journal of the Neurological Sciences journal homepage: www.elsevier.com/locate/jns
Letter to the Editor The importance of using a proper technique and accurate seeding of regions-of-interest in diffusion tensor tractography
Dear Editor, We read with interest a recent study published in this journal “Distribution of the corticobulbar tract in the internal capsule” by Yim et al. [1]. Yim et al. used probabilistic tractography to map corticobulbar tract in patients with bulbar symptoms and concludes that the corticobulbar tracts do not run through the genu of the internal capsule as has been described in the atlas of anatomy [2]. Yim et al. [1] claim that in their experience the corticobulbar tract passes through the posterior half of the posterior limb of internal capsule. There are two significant points worth to be mentioned regarding this study. First, the corticobulbar tract is a long projection pathway connecting the brainstem nuclei with the face area of the motor cortex [2]. The challenges facing diffusion tensor tractography of the corticobulbar tract have been the crossing fibers at the white matter of the superior corona radiata at the centrum semiovale just lateral to the lateral ventricles. At the centrum semiovale, there are heavy load of fiber bundles directed in the anterior–posterior orientation such as the superior longitudinal fasciculus intersecting with the vertically oriented ascending and descending fiber bundles. These crossing fibers result in intra-voxel orientation heterogeneity which lowers the sensitivity and specificity of both probabilistic and deterministic tractography algorithms [3–6]. Complex fiber architecture within the voxel may result in abrupt abortion of the tractography algorithm (false negative result) or creation of incorrect fiber bundles as a result of switching to adjacent fiber tracts (false positive result) due to the crossing and kissing fiber phenomenon [3]. This phenomenon has been a major obstacle for tractography of the corticobulbar tracts in prior diffusion tensor imaging (DTI) studies using the single tensor model [3–6]. Since the corticobulbar tract projects to the lateral aspect of the motor cortex, the neurons have to bend laterally at the centrum semiovale; hence crossing fibers become the major issue for fiber tracking of the corticobulbar tract. The same issue has been a major obstacle in the way of study of the lateral projections of the corticospinal tract corresponding to the somatotopic distribution of the motor cortex related to the arms and face using single tensor tractography model [3–6]. Recent advances in tractography using high magnetic field MRI and high angular DTI methods such as diffusion spectrum imaging [7] resolved to some extent the crossing fiber problem. Given that this study uses single tensor model and low magnetic field (7 out of 26 subjects were scanned by 3 T scanner) the probability of tracing the corticobulbar tract is low. Second, the location of the regions of interest (ROIs) used by Yim et al. [1]. The authors selected the ROIs at the crus cerebri of the midbrain, posterior limb of internal capsule, centrum semiovale 0022-510X/$ – see front matter. Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jns.2014.01.016
and the motor cortex. Since the corticopontocerebellar pathways also pass through the crus cerebri, posterior limb of internal capsule and centrum semiovale, the contamination of the designated corticobulbar tract with corticopontocerebellar tract is inseparable. Specifically the frontal projection of the corticopontocerebellar pathways known as the frontopontocerebellar tracts have been shown by Kamali et al. [8] to originate from the motor cortex and pass through the centrum semiovale, posterior limb of internal capsule and crus cerebri. The corticopontocerebellar tracts unlike the corticobulbar tract cross at the level of the pons and continue to the contralateral cerebellum through the middle cerebellar peduncle. To avoid contamination with the frontopontocerebellar, one ROI should be seeded at the lowest level of the pons or at the pontomedullary junction where the major corticopontocerebellar pathways are already crossed to the contralateral cerebellum. The importance of this consideration also applies to tractography of the corticospinal tract. Many prior DTI studies seeded the ROIs for tractography of the corticospinal tract at the pons and higher levels including the midbrain, internal capsule or motor cortex [9–11]. These studies mixed the corticospinal and corticopontocerebellar tracts specifically the frontopontocerebellar tract which runs side by side with the corticospinal tract and inserts into the motor cortex. By adding an ROI below the level of the pons for example at the pontomedullary junction or medulla, the corticopontocerebellar pathways will be excluded as they have already crossed to the contralateral cerebellum at the level of the pons [8]. In vivo depiction of three-dimensional anatomy of the major white matter tracts by fiber tracking is becoming more commonly used in preoperative and intraoperative planning of lesions located close to these eloquent brain structures to avoid postoperative deficits. It is very important to realize that even small misplacement of the regions-ofinterest for the tracking algorithm may result in significantly different reconstructed fiber tracts. Better understanding of technical limitations and accurate placement of ROIs to distinguish the complex anatomical relationships between fiber tracts are essential to avoid confusing the neighboring fiber bundles with variant physiologic significance. Conflict of interest No conflict of interest. References [1] Yim SH, Kim JH, Han ZA, Jeon S, Cho JH, Kim GS, et al. Distribution of the corticobulbar tract in the internal capsule. J Neurol Sci 2013;334:63–8. [2] Hains DE. Neuroanatomy. An atlas of structures, sections and systems. 7th ed. New York: Lippincott Williams & Wilkins; 2007. [3] Barrick TR, Clark CA. Singularities in diffusion tensor fields and their relevance in white matter fiber tractography. Neuroimage 2004;22:481–91. [4] Lazar M, Alexander AL. Bootstrap white matter tractography (BOOT-TRAC). Neuroimage 2005;24:524–32. [5] Qazi AA, Radmanesh A, O'Donnell L, Kindlmann G, Peled S, Whalen S, et al. Resolving crossings in the corticospinal tract by two-tensor streamline tractography: method and clinical assessment using fMRI. Neuroimage 2009;47:98–106. [6] Behrens TE, Berg HJ, Jbabdi S, Rushworth MF, Woolrich MW. Probabilistic diffusion tractography with multiple fibre orientations: what can we gain? Neuroimage 2007;34:144–55.
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[7] Wedeen VJ, Wang RP, Schmahmann JD, Benner T, Tseng WY, Dai G, et al. Diffusion spectrum magnetic resonance imaging (DSI) tractography of crossing fibers. Neuroimage 2008;41:1267–77. [8] Kamali A, Kramer LA, Frye RE, Butler IJ, Hasan KM. Diffusion tensor tractography of the human brain cortico-ponto-cerebellar pathways: a quantitative preliminary study. J Magn Reson Imaging 2010;32:809–17. [9] Hua K, Zhang J, Wakana S, Jiang H, Li X, Reich DS, et al. Tract probability maps in stereotaxic spaces: analyses of white matter anatomy and tract-specific quantification. Neuroimage 2008;39:336–47. [10] Yamada K, Sakai K, Hoogenraad FG, Holthuizen R, Akazawa K, Ito H, et al. Multitensor tractography enables better depiction of motor pathways: initial clinical experience using diffusion-weighted MR imaging with standard b-value. AJNR Am J Neuroradiol 2007;28:1668–73. [11] Wakana S, Caprihan A, Panzenboeck MM, Fallon JH, Perry M, Gollub RL, et al. Reproducibility of quantitative tractography methods applied to cerebral white matter. Neuroimage 2007;36:630–44.
Arash Kamali Department of Diagnostic Radiology, University of Medicine and Dentistry of New Jersey Cooper, 1 Cooper Plaza, Camden, NJ 08103, USA Corresponding author. Tel.: +1 856 342 2383; fax: +1 856 365 0472. E-mail address: [email protected]. Khader M. Hasan University of Texas Medical School at Houston, Radiology Department, United States 28 November 2013
Contents lists available at ScienceDirect
Journal of the Neurological Sciences journal homepage: www.elsevier.com/locate/jns
Letter to the Editor The importance of using a proper technique and accurate seeding of regions-of-interest in diffusion tensor tractography
Dear Editor, We read with interest a recent study published in this journal “Distribution of the corticobulbar tract in the internal capsule” by Yim et al. [1]. Yim et al. used probabilistic tractography to map corticobulbar tract in patients with bulbar symptoms and concludes that the corticobulbar tracts do not run through the genu of the internal capsule as has been described in the atlas of anatomy [2]. Yim et al. [1] claim that in their experience the corticobulbar tract passes through the posterior half of the posterior limb of internal capsule. There are two significant points worth to be mentioned regarding this study. First, the corticobulbar tract is a long projection pathway connecting the brainstem nuclei with the face area of the motor cortex [2]. The challenges facing diffusion tensor tractography of the corticobulbar tract have been the crossing fibers at the white matter of the superior corona radiata at the centrum semiovale just lateral to the lateral ventricles. At the centrum semiovale, there are heavy load of fiber bundles directed in the anterior–posterior orientation such as the superior longitudinal fasciculus intersecting with the vertically oriented ascending and descending fiber bundles. These crossing fibers result in intra-voxel orientation heterogeneity which lowers the sensitivity and specificity of both probabilistic and deterministic tractography algorithms [3–6]. Complex fiber architecture within the voxel may result in abrupt abortion of the tractography algorithm (false negative result) or creation of incorrect fiber bundles as a result of switching to adjacent fiber tracts (false positive result) due to the crossing and kissing fiber phenomenon [3]. This phenomenon has been a major obstacle for tractography of the corticobulbar tracts in prior diffusion tensor imaging (DTI) studies using the single tensor model [3–6]. Since the corticobulbar tract projects to the lateral aspect of the motor cortex, the neurons have to bend laterally at the centrum semiovale; hence crossing fibers become the major issue for fiber tracking of the corticobulbar tract. The same issue has been a major obstacle in the way of study of the lateral projections of the corticospinal tract corresponding to the somatotopic distribution of the motor cortex related to the arms and face using single tensor tractography model [3–6]. Recent advances in tractography using high magnetic field MRI and high angular DTI methods such as diffusion spectrum imaging [7] resolved to some extent the crossing fiber problem. Given that this study uses single tensor model and low magnetic field (7 out of 26 subjects were scanned by 3 T scanner) the probability of tracing the corticobulbar tract is low. Second, the location of the regions of interest (ROIs) used by Yim et al. [1]. The authors selected the ROIs at the crus cerebri of the midbrain, posterior limb of internal capsule, centrum semiovale 0022-510X/$ – see front matter. Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jns.2014.01.016
and the motor cortex. Since the corticopontocerebellar pathways also pass through the crus cerebri, posterior limb of internal capsule and centrum semiovale, the contamination of the designated corticobulbar tract with corticopontocerebellar tract is inseparable. Specifically the frontal projection of the corticopontocerebellar pathways known as the frontopontocerebellar tracts have been shown by Kamali et al. [8] to originate from the motor cortex and pass through the centrum semiovale, posterior limb of internal capsule and crus cerebri. The corticopontocerebellar tracts unlike the corticobulbar tract cross at the level of the pons and continue to the contralateral cerebellum through the middle cerebellar peduncle. To avoid contamination with the frontopontocerebellar, one ROI should be seeded at the lowest level of the pons or at the pontomedullary junction where the major corticopontocerebellar pathways are already crossed to the contralateral cerebellum. The importance of this consideration also applies to tractography of the corticospinal tract. Many prior DTI studies seeded the ROIs for tractography of the corticospinal tract at the pons and higher levels including the midbrain, internal capsule or motor cortex [9–11]. These studies mixed the corticospinal and corticopontocerebellar tracts specifically the frontopontocerebellar tract which runs side by side with the corticospinal tract and inserts into the motor cortex. By adding an ROI below the level of the pons for example at the pontomedullary junction or medulla, the corticopontocerebellar pathways will be excluded as they have already crossed to the contralateral cerebellum at the level of the pons [8]. In vivo depiction of three-dimensional anatomy of the major white matter tracts by fiber tracking is becoming more commonly used in preoperative and intraoperative planning of lesions located close to these eloquent brain structures to avoid postoperative deficits. It is very important to realize that even small misplacement of the regions-ofinterest for the tracking algorithm may result in significantly different reconstructed fiber tracts. Better understanding of technical limitations and accurate placement of ROIs to distinguish the complex anatomical relationships between fiber tracts are essential to avoid confusing the neighboring fiber bundles with variant physiologic significance. Conflict of interest No conflict of interest. References [1] Yim SH, Kim JH, Han ZA, Jeon S, Cho JH, Kim GS, et al. Distribution of the corticobulbar tract in the internal capsule. J Neurol Sci 2013;334:63–8. [2] Hains DE. Neuroanatomy. An atlas of structures, sections and systems. 7th ed. New York: Lippincott Williams & Wilkins; 2007. [3] Barrick TR, Clark CA. Singularities in diffusion tensor fields and their relevance in white matter fiber tractography. Neuroimage 2004;22:481–91. [4] Lazar M, Alexander AL. Bootstrap white matter tractography (BOOT-TRAC). Neuroimage 2005;24:524–32. [5] Qazi AA, Radmanesh A, O'Donnell L, Kindlmann G, Peled S, Whalen S, et al. Resolving crossings in the corticospinal tract by two-tensor streamline tractography: method and clinical assessment using fMRI. Neuroimage 2009;47:98–106. [6] Behrens TE, Berg HJ, Jbabdi S, Rushworth MF, Woolrich MW. Probabilistic diffusion tractography with multiple fibre orientations: what can we gain? Neuroimage 2007;34:144–55.
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Letter to the Editor
[7] Wedeen VJ, Wang RP, Schmahmann JD, Benner T, Tseng WY, Dai G, et al. Diffusion spectrum magnetic resonance imaging (DSI) tractography of crossing fibers. Neuroimage 2008;41:1267–77. [8] Kamali A, Kramer LA, Frye RE, Butler IJ, Hasan KM. Diffusion tensor tractography of the human brain cortico-ponto-cerebellar pathways: a quantitative preliminary study. J Magn Reson Imaging 2010;32:809–17. [9] Hua K, Zhang J, Wakana S, Jiang H, Li X, Reich DS, et al. Tract probability maps in stereotaxic spaces: analyses of white matter anatomy and tract-specific quantification. Neuroimage 2008;39:336–47. [10] Yamada K, Sakai K, Hoogenraad FG, Holthuizen R, Akazawa K, Ito H, et al. Multitensor tractography enables better depiction of motor pathways: initial clinical experience using diffusion-weighted MR imaging with standard b-value. AJNR Am J Neuroradiol 2007;28:1668–73. [11] Wakana S, Caprihan A, Panzenboeck MM, Fallon JH, Perry M, Gollub RL, et al. Reproducibility of quantitative tractography methods applied to cerebral white matter. Neuroimage 2007;36:630–44.
Arash Kamali Department of Diagnostic Radiology, University of Medicine and Dentistry of New Jersey Cooper, 1 Cooper Plaza, Camden, NJ 08103, USA Corresponding author. Tel.: +1 856 342 2383; fax: +1 856 365 0472. E-mail address: [email protected]. Khader M. Hasan University of Texas Medical School at Houston, Radiology Department, United States 28 November 2013
