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Combined in vivo tractography and ex vivo dissections to investigate the topography of the acoustic radiation.

Maffei C., Jovicich J., De Benedictis A., Chioffi F., Sarubbo S.
  Martedì 12/09   09:00 - 13:00   Aula A209   V - Biofisica e fisica medica
The acoustic radiation (AR) is a primary sensory pathway, transferring auditory information from the thalamus to the auditory cortex (Heschl's gyrus (HG)). Very little is known about its topography and tractography reconstruction of the AR remains extremely challenging. In this scenario, where limited topographical knowledge is available, validation of tractography results is of great importance. This study aims at 1) characterizing the topography of the AR relative to other major bundles in the human brain, and 2) comparing $ex vivo$ dissections and diffusion-based reconstructions. Methods. Fiber dissection: 3 human cerebral hemispheres (2 right) were prepared according to a modified Klinger's technique. AR was approached posteriorly starting a layer-by-layer dissection from the posterior third of the superior temporal sulcus (STS). Tractography: data of 4 subjects from the Human Connectome Project (b-value = 1000-3000-5000-10000 $s/mm^{2}$ 1.25 isotropic resolution) were analyzed in MrTrix. A multi-tissue spherical deconvolution algorithm was fit to the data and anatomically constrained probabilistic tractography was performed (0.75 mm step-size, 45 angle threshold, 2000 seeds/voxel). Results: Ultra-high b-value tractography allowed the reconstruction of the AR profile. In accordance with blunt dissections, streamlines originate as a compact bundle and move in a lateral and anterior direction, fanning out in HG. Conclusions: To our knowledge this is the first study revealing the topography of the AR in the human brain combining $ex vivo$ dissections and tractography. Both methods showed the AR unique feature of a full transversal course from the midline to the posterior portion of the superior temporal lobe. Challenges in tractographic reconstruction are related to this unique anatomical profile. Dissections demonstrated, in fact, that AR is located in a region with a high density of crossing fibers: close to vertical thalamo-cortical fibers, and at the termination site of multi-layers associative pathways. Solving the fiber-crossing and defining the tract terminations represents an open challenge for tractography algorithms.