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Identification of the Corticobulbar Tracts of the Tongue and Face Using Deterministic and Probabilistic DTI Fiber Tracking in Pa

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Identification of the Corticobulbar Tracts of the Tongue and Face Using Deterministic and Probabilistic DTI Fiber Tracking in Pa Published August 6, 2015 as 10.3174/ajnr.A4430 ORIGINAL RESEARCH ADULT BRAIN Identification of the Corticobulbar Tracts of the Tongue and Face Using Deterministic and Probabilistic DTI Fiber Tracking in Patients with Brain Tumor M. Jenabi, X K.K. Peck, X R.J. Young, N. Brennan, and X A.I. Holodny ABSTRACT BACKGROUND AND PURPOSE: The corticobulbar tract of the face and tongue, a critical white matter tract connecting the primary motor cortex and the pons, is rarely detected by deterministic DTI fiber tractography. Detection becomes even more difficult in the presence of a tumor. The purpose of this study was to compare identification of the corticobulbar tract by using deterministic and probabilistic tractography in patients with brain tumor. MATERIALS AND METHODS: Fifty patients with brain tumor who underwent DTI were studied. Deterministic tractography was per- formed by using the fiber assignment by continuous tractography algorithm. Probabilistic tractography was performed by using a Monte Carlo simulation method. ROIs were drawn of the face and tongue motor homunculi and the pons in both hemispheres. RESULTS: In all subjects, fiber assignment by continuous tractography was ineffectual in visualizing the entire course of the corticobulbar tract between the face and tongue motor cortices and the pons on either side. However, probabilistic tractography successfully visualized the corticobulbar tract from the face and tongue motor cortices in all patients on both sides. No significant difference (P Ͻ .08) was found between both sides in terms of the number of voxels or degree of connectivity. The fractional anisotropy of both the face and tongue was significantly lower on the tumor side (P Ͻ .03). When stratified by tumor type, primary-versus-metastatic tumors, no differences were observed between tracts in terms of the fractional anisotropy and connectivity values (P Ͼ .5). CONCLUSIONS: Probabilistic tractography successfully reconstructs the face- and tongue-associated corticobulbar tracts from the lateral primary motor cortex to the pons in both hemispheres. ABBREVIATIONS: CBT ϭ corticobulbar tract; FA ϭ fractional anisotropy; FACT ϭ fiber assignment by continuous tractography recise localization of white matter fibers adjacent to brain tu- The DTI data can be reconstructed via tractography algo- Pmors, such as the corticospinal tracts, is important for presur- rithms to display the anatomic localization of the entire white gical planning to maximize resection of brain lesions and mini- matter.4,6-13 The algorithms have been classified into 2 types: de- mize iatrogenic complications.1,2 Diffusion tensor imaging is a terministic (streamline) and probabilistic. The fiber assignment rapidly growing technique to visualize the connectivity of white by continuous tractography (FACT) algorithm is a popular deter- matter networks insufficiently evaluated by other imaging modal- ministic method in which the propagation of tracking is primarily 2-4 ities. The DTI technique is based on the anisotropic diffusion of determined by the principal eigenvector of fractional anisotropy water that preferentially occurs along the longitudinal axis of ax- (FA) of each voxel. The measurement of FA is highly dependent ons, which allows the delineation of white matter from the diffu- on the distributions and intactness of axonal membranes and my- 4,5 sion patterns of water molecules. elin. However, FACT is unable to solve fiber-crossing problems. The FA decreases in axonal regions with complex architecture due Received December 5, 2014; accepted after revision March 26, 2015. to crossing fibers, intricate branching configurations, and modi- From the Departments of Radiology (M.J., K.K.P., R.J.Y., N.B., A.I.H.) and Medical fied tissue organization due to nearby tumors.1,3,12,14 One Physics (K.K.P.) and Brain Tumor Center (R.J.Y., A.I.H.), Memorial Sloan-Kettering Cancer Center, New York, New York; and Department of Radiology (R.J.Y., A.I.H.), method to overcome these problems is the probabilistic tracking Weill Medical College of Cornell University, New York, New York. algorithms.6,9 The probabilistic approach considers multiple fiber Please address correspondence to Andrei I. Holodny, MD, Department of Radiol- connectivity from large numbers of potential pathways to gener- ogy, Memorial Sloan-Kettering Cancer Center, 1275 York Ave, NY, NY 10021; e-mail: [email protected] ate a connectivity map with the most probable connected voxels Indicates article with supplemental on-line table. between seed points. http://dx.doi.org/10.3174/ajnr.A4430 Visualizing the corticobulbar tract (CBT) associated with the AJNR Am J Neuroradiol ●:●●2015 www.ajnr.org 1 Copyright 2015 by American Society of Neuroradiology. face and tongue by using FACT is a special challenge because of mented in Matlab (MathWorks, Natick, Massachusetts), was used the relatively small size of the CBT and the presence of prominent to analyze DTI data. Routine anatomic images including axial 3D crossing fibers—the longitudinal fasciculus.4,6,8 The CBT forms T1-weighted images by using a spoiled gradient-recalled-echo se- part of the descending pyramidal tract, in conjunction with the quence (TR/TE ϭ 22/4 ms, matrix ϭ 256 ϫ 256, flip corticospinal tract. It originates from the primary motor cortex angle ϭ 30°, section thickness ϭ 1.5 mm, FOV ϭ 240 mm), T2- and descends through the corona radiata and internal capsule into weighted images (TR/TE ϭ 4000 ms/minimum full), and fluid- the cerebral peduncles. The CBT then innervates the motor cra- attenuated inversion recovery images (TR/TE ϭ 6000/140 ms, nial nerve nuclei in the brain stem to control the muscles of the matrix ϭ 256 ϫ 256, section thickness ϭ 1 mm, FOV ϭ 240 mm) face, head, and neck. Studies based on single tensor deterministic were obtained. tractography have failed to identify the CBT fibers associated with the face and tongue.12,15-18 There have been a few, relatively so- ROIs for CBT phisticated, high-angular-resolution diffusion imaging–based The ROIs (mean number of voxels in the ROI ϭ 28 Ϯ 9) were multitensor techniques with some success in extracting fibers manually drawn under the direct supervision of a board-certified from wider areas of the primary motor cortex, including the face radiologist holding a Certificate of Added Qualification in Neu- and tongue areas15,17,18; however, high-angular-resolution diffu- roradiology on the B0 images after inspection of the anatomic sion imaging–based techniques have not been applied to large images, FA maps, and color FA maps. First, the “hand knob” of cohorts of patients with brain tumors. Multitensor techniques the primary motor cortex was identified.19 Next, an ROI for the also require long acquisition times, which are a limiting factor in face was placed in the primary motor cortex lateral to the hand patients with brain tumors and neurologic deficits. knob in the axial and sagittal planes.16,19,20 The ROI for the The purpose of this study was to compare the detection of the tongue was then placed at the lowermost portion of the primary CBT fibers associated with the face and tongue by using both motor cortex just above the Sylvian fissure in the sagittal deterministic and probabilistic tractography in patients with plane.16,18 The ROI for the pons was placed at the ventral part of brain tumors. Given that the probabilistic approach allows the the midpons through which the CBT travels, as manifested by 2 visualization of a distribution of multiple fiber connectivity, we symmetric blue circles on the color FA maps to represent de- hypothesized that probabilistic tractography will outperform de- scending tracts.19 Anatomic images in the sagittal, coronal, and terministic tractography. axial planes were used to confirm the final ROIs. The same seed ROIs were used for both deterministic and probabilistic tractog- MATERIALS AND METHODS raphy (Fig 1). Patients This retrospective study was approved by the local institutional Deterministic Tractography review board. Fifty consecutive patients 7–80 years of age (20 Deterministic tractography was performed by using the FACT4 males; mean age, 56.2 years; 30 females, mean age, 57.1 years), algorithm implemented in DTI&FiberTools. Whole-brain white with single unilateral tumors, who underwent DTI as part of their matter masks were estimated and reconstructed by applying 2 standard of care during a 12-month period were studied. DTI was thresholds to control the tracking area and minimize unwanted performed whenever the margin of the enhancing tumor or peri- noise: a start and a stop mask. The start mask was set to FA Ͼ 0.25, tumoral FLAIR/T2 abnormality or both were within 2 cm of the and mean diffusivity Ͻ 1.6 ϫ 10Ϫ3 mm2/s, and the stop mask was face and/or tongue motor cortex and/or descending CBT on an- set to FA Ͼ 0.1 and mean diffusivity Ͻ 2 ϫ 10Ϫ3 mm2/s. The atomic images. In none of the cases did the tumor (defined as maximum curvature of the tract was set to 60°. The whole-brain abnormal enhancement or abnormal findings on FLAIR/T2) in- mask was then used to extract specific tracts between individual volve the contralateral side. The tumors consisted of glioblas- motor ROIs and the pons for each subject. tomas (n ϭ 17), metastases (n ϭ 19), astrocytomas (n ϭ 6), ana- ϭ ϭ plastic astrocytomas (n 3), oligoastrocytomas (n 3), Probabilistic Tractography oligodendroglioma (n ϭ 1), and hemangiopericytoma (n ϭ 1). Probabilistic tractography was performed by using DTI&Fiber- Among the 50 patients, only 6 received treatments before DTI. Tools in an extended Monte Carlo simulation method similar to Five had chemotherapy and radiation therapy, and 1 had chemo- the probabilistic index of connectivity method.6,7 Tracking areas therapy only. were defined as regions with mean diffusivity Ͻ 2 ϫ 10Ϫ3 mm2 MR Imaging Acquisition and Analysis and FA Ͼ 0.1.
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