Tracing the Thalamo-Limbic Connections of the Human Brain Arash Kamali MD, Roy Riascos-Castaneda MD Siva Jasti Prasad MD, Saeeded Mirbagheri MD, Khader M. Hasan PhD Department of Diagnostic Imaging of Johns Hopkins University Hospital, Division of Neuroradiology, Diagnostic Imaging of University of Texas in Houston

Purpose 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 parcellation of limbic pathways in relation to the gray matter nuclei using fiber tractography by deterministic approach and high spatial resolution DTI data on 3 T.

Introduction Limbic system include structures involved in papez circuit. The circuit consists of outflow of impulses from the hippocampus via the fornix to the mamillary bodies of the hypothalamus. From there via the mamillothalamic tract to the anterior thalamic nucleus and via the thalamocortical fiber system to the cingulate gyrus from which impulses returns to the hippocampus via the entorhinal area. Hippocampus and amygdala are the central components of the two major limbic subsystems. Hippocampus is a cortical structure that nborders the inferior horn of the lateral ventricles and closely associated with dentate gyrus (1).

Introduction 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 (2) and partial volume averaging upon using large voxel volumes(2, 3).

Introduction Cingulum has been partially traced using DTI tractography(5, 6). Tracing the temporal portion of cingulum and the whole fornix trajectories have been a challenge so far in DTI tractography studies. This work aimed to demonstrate for the first time the feasibility of in vivo visualization of human limbic pathways including cingulum and fornix in relation to the gray matter nuclei on 3T. We also show the ability to separate and quantify the tract volume and corresponding diffusion tensor metrics of limbic pathways.

Materials and Methods 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 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).

Result A representative tractogram of cingulum 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 whole trajectory of Cingulum and fornix for the first time.

http://homepage.ntlworld.com/teversal/myweb/CNS/Images/basalganglia-2.jpg

Results Figure 1. 3D superior View of limbic pathways on T1 W map. Cingulum (green), and fornix (blue)

Figure 2.

Results Figure 3. 11 year old normal male child Cingulum (red) Stria Terminalis (blue) Fornix (green) 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 .

Discussion 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 (4). 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 (2) and partial volume averaging due to using large voxel volumes (3).

Discussion Partial volume averaging effects caused by low spatial resolution negatively affects the DT-MRI results (2, 3). Since some of the limbic structures for example fornix is thinner than brain stuctures such as corpus collosum (see Table 1), 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 and quantified the cingulum and fornix as well as the anterior comissure pathway using high resolution DTI data on 3T to achieve optimal results. The choice of thinner slice thickness also provided higher and more detectable tensor anisotropy within gray matter structures such as amygdala(3, 7).

Discussion 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 (8). Challenges of 2D DTI studies also include partial volume contamination with adjacent pathways due to the selection of size and location of ROIs. Using high spatial resolution 3D tractography combined with using multiple clear anatomical landmarks increases the level of validity and helps resolve these problems (3,4).

Discussion Clinical applications of our study include the study of brain 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.

Acknowledgements This work is funded by the National Institute of Neurological Disorders and Stroke (NIH-NINDS) R01-NS052505-02 to KMH (Diffusion Tensor Imaging of Wallerian Degeneration in Multiple Sclerosis). The purchase of the 3.0 T MRI clinical scanner is partially funded by NIH grant S10 RR19186-01

References 1) Afifi AK, Bergman RA. Functional neuroanatomy. Text and atlas,USA: McGraw-Hill, 1998; pp 730 2) Alexander , et al. Analysis of partial volume effects in diffusion-tensor MRI. Magn Reson Med. 2001;45:770-780. 3) Hasan et al. Mapping the human brain white matter tracts relative to cortical and deep gray matter using diffusion tensor imaging at high spatial resolution. Magn Reson Imaging. 2009;27631-6. 4) Kamali, et al . Diffusion tensor tractography of the somatosensory system in the human brainstem: initial findings using high isotropic spatial resolution at 3.0 T. Eur Radiol. 2009;19:1480-8.

References 5) Fujiwara , et al. Anterior and posterior cingulum abnormalities and their association with psychopathology in schizophrenia: a diffusion tensor imaging study. Schizophr Res. 2007;95:215-22. 6) Concha, et al. Diffusion tensor tractography of the limbic system. AJNR Am J Neuroradiol. 2005;26:2267-74. 7) Kamali et al. Diffusion tensor tractography of the human brain cortico-ponto-cerebellar pathways: a quantitative preliminary study. J Magn Reson Imaging. 2010;32:809-17. 8) Mori et al. Fiber tracking: principles and strategies - a technical review. NMR Biomed. 2002;15:468-80.