For identification of macromolecular complexes in cryoelectron tomo- grams covering cytoplasmic regions, we used a combination of subtomogram averaging and classification. In a first step, the cross correlation function between two times binned tomograms and a 25 nm spherical template was computed. From the corresponding correlation peaks subvolumes (24 × 24 × 24 voxels) were extracted and classified into 12 classes via a missing-wedge corrected multi reference classification procedure, using the initial template with different representations of Gaussian noise added as starting references.
I found the answer myself :)
In transmission electron microscopy one obtains a 3D image by tilting the sample stage. Usually it's 180°-x and x is called "missing wedge".