A method, performed by a computer, for volumetrically determining a mid-sagittal plane (MSP) from a plurality of magnetic resonance (MR) images of a brain is disclosed. The plurality of MR images is received and converted into a 3D volume image defined in a 3D coordinate space. Semantic segmentation of an anterior commissure (AC) region, a posterior commissure (PC) region, and a third ventricle region in the 3D volume image is performed based on a pre-trained segmentation model to generate a 3D mask image labeled with the AC region, the PC region, and the third ventricle region. Based on the labeled 3D mask image, a 3D volumetric image of the segmented third ventricle region is constructed. The MSP is determined based on the 3D volumetric image of the third ventricle region.