Does the Shape of the Brain Depend on Gravity?

The estimation of cerebral atrophy in clinical routine is mostly a visual neuroradiological diagnosis depending on a subjective comparison of the cerebrospinal fluid (CSF) space relative to the brain tissue and size of the ventricles. With neuroimaging brain atrophy appears to be accentuated in the frontal lobe regions. This holds true for normal ageing and even more so in most neurodegenerative processes. This finding might be overestimated because of a brain shift caused by position-dependent gravitational influences in the scanner. MRI-based quantitative morphometric techniques like the Brain Image Analysis (BRIAN, MPI/Leipzig) are able to detect intraindividual changes by a three-dimensional matching of images. This pilot study tried to detect subtle intracranial brain movements relative to the skull and deformations of the brain depending on the position of the head in the scanner. A 1.5 T Siemens MRI scanner was used to obtain 3D T₁-weighted MRI images. Image analysis was carried out with the BRIAN tool. Consecutive MRI scans of 2 healthy female participants (age: 25, 62 years) were taken in supine, lateral and prone positions. The image quality did not differ with positioning. In a preliminary analysis, distances between defined intracerebral landmarks like the anterior commissure and defined landmarks of extracerebral structures were measured. In order to assess more complex deformations, a non-rigid image registration was applied. The linear analysis revealed no tissue movements relative to gravity. However non-rigid image registration showed subtle cortical deformation in the frontal and occipital regions. The deformations could not be described by a single main vector and, therefore, it is doubtful whether these changes can be attributed to the influence of gravity. This study demonstrates that acquisition of high quality MRI is possible irrespective of the position of the subject in the scanner. The present preliminarily findings suggest that the position of the brain in the skull and the shape of the brain do not großly depend on gravitational influences. This might result from mechanical obstacles posed by rigid structures like the sinuses, the cerebellar tentorium or the falx. A voxel-based analysis, however, revealed subtle deformations which have to be analyzed further. At present it is unclear whether more pronounced effects would be visible, e.g., with longer intervals between position changes.