In-vivo ultra-high resolution structural imaging of the human superior colliculus at 9.4T: validation with ex vivo measurements at 9.4T and 14T

Purpose/Introduction: The Superior Colliculus is a paired layered structure that regulates eye and head movements and is involved in multisensory control. It is divided in 7 alternating fibrous and cellular layers and our goal is to find methods capable of improving the contrast in the SC and separating its layers in vivo. In this study the visibility of the SC in vivo at 9.4T was investigated and validated with ex vivo measurements at 9.4T and 14T. Subjects and Methods: For the in vivo measurements 10 subjects were measured at the 9.4T Siemens, Erlangen. For each participant three different sequences were used: acquisition-weighted (AW) [1] (nominal resolution of 0.175 9 0.132 9 0.6; TE = 18 ms), multiecho (ME) 3D GRE (0.4 mm isotropic resolution, TE = 6:6:36 ms), and whole brain MP2RAGE (0.8 isotropic resolution). The AW images were k-space filtered with a Hanning function in the phase encoding dimension [2] in order to increase the SNR. R2* maps were calculated from the ME images [3]. VOIs of the SC were drawn based on the AW images and the R2* maps in the coordinate space of the AW images for each subject and both hemispheres. The contrast-tonoise-ratio (CNR) between the SC and a WM VOI was calculated. The post-mortem brainstem sample was fixed in formalin (4 ), measured in the 9.4T using the in vivo protocol, and then cut to fit the 14T volume coil. At 14T, 3D GRE (50 lm isotropic resolution) and DTI EPI (6dir, b = 10,000, 300 lm isotropic resolution). Results: In-vivo the AW sequence yielded the highest CNR values in the SC. The quantitative R2*map did not improve the CNR and higher values were obtained for the 5thecho of the ME sequence (36 ms). CNR was very low for the MP2RAGE sequence (Fig. 1). The GRE ex vivo images were comparable to the in vivo images. The first eigenvector of the DTI images show the superficial layer in accordance to its anatomy and also a layering structure in the middle part of the SC, which may represent one of the intermediate layers of the SC. Structural details that are not visible in vivo were revealed with the ex vivo scans (Fig. 2). Discussion/Conclusion: This experiment emphasizes the potential of UHF MRI in measuring deep-bain structures. The use of 9.4T in vivo and the comparison of this data with ex vivo samples may enable us to create more robust and accurate atlases of the SC and possibly reveal the layering pattern of the SC