Enhanced contrast in X-ray microtomographic images of the membranous labyrinth using different X-ray sources and scanning modes

The vestibular system, located in the inner ear, plays a crucial role in balance and gaze stabilisation by sensing head movements. The interconnected tubes with membranous walls of the vestibular system are located in the skull bone (the 'membranous labyrinth'). Unfortunately, these membranes are very hard to visualise using three-dimensional (3D) X-ray imaging techniques. This difficulty arises due to the embedment of the membranes in the dense skull bone, the thinness of the membranes, and the small difference in X-ray absorption between the membranes and the surrounding fluid. In this study, we compared the visualisation of very small specimens (lizard heads with vestibular systems smaller than 3 mm) by X-ray computed micro-tomography (mu CT) based on synchrotron radiation and conventional sources. A visualisation protocol using conventional X-ray mu CT would be very useful thanks to the ease of access and lower cost. Careful optimisation of the acquisition parameters enables detection of the membranes by using mu CT scanners based on conventional microfocus sources, but in some cases a low contrast-to-noise ratio (CNR) prevents fast and reliable segmentation of the membranes. Synchrotron radiation mu CT proved to be preferable for the visualisation of the small samples with very thin membranes, because of their high demands for spatial and contrast resolution. The best contrast was obtained by using synchrotron radiation mu CT working in phase-contrast mode, leading to up to twice as high CNRs than the best conventional mu CT results. The CNR of the synchrotron radiation mu CT scans was sufficiently high enough to enable the construction of a 3D model by the means of semi-automatic segmentation of the membranous labyrinth. Membrane thickness was found to range between 2.7 and 36.3 mu m. Hence, the minimal membrane thickness was found to be much smaller than described previously in the literature (between 10 and 50 mu m).