Unraveling the Fornix and Stria Terminalis Connections of the Human Brain.

1. Arash Kamali, MD, Doris Lin, MD, SairSair, MD, Elena Motuzko, MD, Saeedeh Mirbagheri, MD, Khader M. Hasan, PhD, Roy Riascos-Castaneda, MD. Department of Diagnostic Imaging of Johns Hopkins University Hospital, Division of Neuroradiology, Diagnostic Imaging of University of Texas in Houston Unraveling the Fornix and Stria Terminalis Connections of the Human Brain.

2. The human limbic system is a complex network of largely bidirectional pathways linking cortical areas and subcortical structures and involved inemotion, motivation, social behavior, self-awareness as well as certain primitive instincts. • The limbic system was described by James Papez in 1937 and was named Papez circuit [Papez, 1937]. The circuit of Papez containsmost of the major limbic gray and white matterstructures. The major cortical areas involved in limbic system consist of the cingulate gyrus and the parahippocampal gyrus. Introduction

3. Two major long white matter connections are involved in the limbic system. The first white matter loop is the fornix and stria terminalis which projects from hippocampus to the septal region and mammillary bodies. The cingulum is the second loop which connects the entorhinal cortex and the cingulate gyrus. The modified version of the Papez circuit later included also the amygdala and septum per Paul Maclean in 1952 [Newman and Harris, 2009]. • However, the trajectories of these crucial limbic fiber tracts; for example, the stria terminalis and precommissural and postcommissural connections of the fornix have not been elucidated in part because of the complex connections and directionality as well as tight proximity with adjacent fiber tracts. introduction

4. Fornix has been partially traced using diffusion tensor imaging (DTI) tractography. Tracing the stria terminalis and fine details of the fornix trajectories have been a challenge so far in DTI tractography studies. • Diffusion tensor tractography of white matter connections between the cortex and deep gray matter structures including the small limbic white matter trajectories such as fornix is challenged by the signal-to-noise ratio (SNR) due to overestimation of anisotropy at low SNR and partial volume averaging upon using large voxel volumes. Introduction

5. Lack of adequate neuroimaging sensitivity and spatial resolution, so far, impeded depiction of limbic pathways within the human brain. The purpose of the current study is to demonstrate the feasibility of tractography of the fornix and stria terminalis pathways in relation to the gray matter nuclei using fiber tractography by deterministic approach and high spatial resolution DTI data on 3 T. Introduction

6. Study Subjects This work was approved by our institutional review board (IRB) and was health insurance portability and accountability act (HIPAA) compliant. Five right-handed healthy men (age range 24-37 years) were included in this study and written informed consent was obtained from all the subjects. • Conventional MRI Data Acquisition All MRI studies were performed on a 3T Philips Intera scanner with a dual quasar gradient system with a maximum gradient amplitude of 80 mT/m, maximum slew rate 200 mT/ms/m, and an eight channel SENSE-compatible head coil (Philips Medical Systems, Best, Netherlands). The conventional MRI (cMRI) protocol included axially prescribed 3D spoiled gradient (repetition time /echo time/ flip angle = TR/TE/a = 8 ms / 4ms / 6o ), 3-D proton density-weighted (TR/TE/a = 10,000 ms / 10 ms / 90o and 3-D T2-weighted (TR/TE/a = 10,000 ms / 60 ms / 90o), with a square field-of-view (FOV) = 256 mm x 256 mm and a matrix of 256x256 pixels. The slice thickness for the cMRI sequences was 1.0 mm with 120 contiguous axial slices covering the entire brain (foramen magnum to vertex). Materials and Methods

7. DTI Data Acquisition Diffusion-weighted image (DWI) data were acquired axially from the same graphically prescribed cMRI volumes using a single-shot multi-slice 2D spin-echo diffusion sensitized and fat-suppressed echo planar imaging (EPI) sequence, with the balanced Icosa21 tensor encoding scheme (2,3, 4). The b-factor = 500 sec mm-2, TR/TE = 14460/60 msec. The spatial coverage for DTI data matched the 3D cMRI spatial coverage (FOV = 256 mm x 256 mm and slice thickness / gap/ #slices = 1 mm / 0 mm / 120). The EPI phase encoding used a SENSE k-space undersampling factor of two, with an effective k-space matrix of 112x112 and an image matrix after zero-filling of 256x256. The acquisition spatial resolution for DTI data was ~ 2.29mm x 2.29mm x 1mm, and the nominal resolution after image construction was 1mm x 1mm x 1mm.The number of b-factor ~ 0 (b0) magnitude image averages was four. The total DTI acquisition time was ~ seven minutes for the diffusion-weighted acquisition. The DTI acquisition was repeated three times to enhance signal-to-noise ratio (SNR). The selection of the b-factor, parallel imaging, repetition and echo times enabled entire brain coverage using single-shot and interleaved EPI. The detailed description of the method is available in two recent accepted articles (3, 4). Materials and Methods

8. We delineated the detailed trajectories of the fornix and stria terminalis and their connections bilaterally in five normal adult human brains. • A representative tractogram of the stria terminalis and fornix in one of the subjects on the 1mm slice thickness acquisition on 3T are shown in Figure 1 and 2. • As it’s visible in Fig. 1&2, we were able to trace the precommissural and postcommissural fibers pf the forniceal columns and complete trajectory of stria terminalis for the first time. Result

9. Figure 1.3D superior View of limbic pathways on T1 W map. Fornix (red), and stria terminalis (black) Results

10. Cingulum (red) Stria Terminalis (blue) Fornix (green) Figure 2 Results Figure 2.3D superio-lateral views of the visual pathways including Optic tract (red) and optic radiations (yellow) on T1-W (left) and color-DTI (right) images.

11. Human DTI studies of the limbic pathways have been limited by inadequate spatial resolution (in-plane and slice thickness), partial volume effect and poor SNR. Improving the DTI acquisition methodology by sequence optimization with respect to shorter echo time and decreased partial volume averaging effects, will be advantageous to study these thin fibers. • Diffusion tensor tractography of white matter connections between the cortex and deep gray matter structures is affected by the overestimation of anisotropy at low SNR and partial volume averaging due to using large voxel volumes. Discussion

12. Partial volume averaging effects caused by low spatial resolution negatively affects the DT-MRI results. Since some of the limbic pathways for example the stria terminalis are very delicate structures, higher spatial resolution is needed to minimize partial volume averaging effects resolving the crossing fibers and reducing the incoherency within the voxel enabling the algorithm to trace the limbic pathways. • In this work for the first time we mapped the stria terminalis and fine details of the fornix such as the pre commissural and post commissural of the forniceal columns using high resolution DTI. The choice of thinner slice thickness also provided higher and more detectable tensor anisotropy within gray matter structures such as amygdala. Discussion

13. The advantage of high resolution 3D fiber tract reconstruction over 2D ROI placement in DTI studies is the ability to show better the integrity of the fiber tract trajectory or interruption by lesions. • Challenges of 2D DTI studies also include partial volume contamination with adjacent pathways due to the selection of size and location of ROIs. 2D ROI is also more operator dependent. Using high spatial resolution 3D tractography combined with using multiple clear anatomical landmarks increases the level of validity and helps resolve these problems. Discussion

14. Clinical applications of our study includes many pathologies including psychiatric disorders, developmental disorders, traumatic brain injury, infarcts , tumors and demyelinating diseases involving the limbic structures as well as in study of patients with temporal lobe epilepsy, alzheimer ‘s disease, schizopherenia, wenicke-korsakoff syndrome , kluver-bucy syndrome and many other pathologies involving the limbic pathways. Discussion

15. The complexity of the fibers within a voxel remains to be a limitation to diffusion tensor tractography model [Barrick et al, 2004; Jones et al, 2008]. Future studies using high magnetic field MRI and higher angular diffusion technique such as diffusion spectrum imaging[Wedeen et al, 2008] with focusing on the limbic system may further validate our results and overcome some of the challenges due to crossing fibers and multidirectional fiber orientation within a voxel. Discussion

16. 1. AggletonJP, O'Mara SM, Vann SD, Wright NF, Tsanov M, Erichsen JT (2010) Hippocampal-anterior thalamic pathways for memory: uncovering a network of direct and indirect actions. Eur J Neurosci. 31:2292-307. • 2.Hains DE (2007) Neuroanatomy. An atlas of structures, sections and systems 7th (edn) Lippincott Williams and Wilkins, New York • 3. Hasan KM, Kamali A, Kramer LA (2009) Mapping the human brain white matter tracts relative to cortical and deep gray matter using diffusion tensor imaging at high spatial resolution. MagnReson Imaging. 27:631-636. • 4. Kamali A, Kramer LA, Hasan KM (2010) Feasibility of prefronto-caudate pathway tractography using high resolution diffusion tensor tractography data at 3T. J Neurosci Methods. 191:249-254. References

17. 5. Kwon HG, Byun WM, Ahn SH, Son SM, Jang SH (2011) The anatomical characteristics of the stria terminalis in the human brain: a diffusion tensor tractography study. Neurosci Lett. 500:99-102. • 6.Schmahmann JD, Smith EE, Eichler FS, Filley CM (2008) Cerebral white matter: neuroanatomy, clinical neurology, and neurobehavioral correlates. Ann N Y Acad Sci. 1142:266-309. • 7. Stretton J, Winston GP, Sidhu M, Bonelli S, Centeno M, Vollmar C, Cleary RA, Williams E, Symms MR, Koepp MJ, Thompson PJ, Duncan JS (2013) Disrupted segregation of working memory networks in temporal lobe epilepsy. NeuroimageClin. 2:273-281. • 8. Zeineh MM, Holdsworth S, Skare S, Atlas SW, Bammer R (2012) Ultra-high resolution diffusion tensor imaging of the microscopic pathways of the medial temporal lobe.Neuroimage. 62:2065-6582. References