Supplement 1: Polygon mask for voxel basis reconstruction

Illustration of the polygonal masking used in the voxel-basis reconstruction. 1D projections of a 2D slice are shown from three different views in blue, orange and yellow. The left and right edges (at 5 \% the maximum intensity) of each projection are traced back through the slice, forming a 6-sided polygon enclosing the majority of emission. The reconstruction algorithm forces voxel intensities outside of this polygon to be zero.<p></p>