We present a concept and early prototype for exploring volumetric medical data, e.g., from MRI or CT scans, in head-mounted Augmented Reality (AR) with a spatially tracked tablet. Our goal is to address the lack of immersion and intuitive input of conventional systems by providing spatial navigation to extract arbitrary slices from volumetric data directly in three-dimensional space. A 3D model of the medical data is displayed in the real environment, fixed to a particular location, using AR. The tablet is spatially moved through this virtual 3D model and shows the resulting slices as 2D images. We present several techniques that facilitate this overall concept, e.g., to place and explore the model, as well as to capture, annotate, and compare slices of the data. Furthermore, we implemented a proof-of-concept prototype that demonstrates the feasibility of our concepts. With our work we want to improve the current way of working with volumetric data slices in the medical domain and beyond.