Shaped CNT cathodes. Control over the electron beam distribution, and subsequent symmetry of the X-ray beam, can be achieved by shaping the electron emitting areas on the cathode by conventional lithographic techniques. (a) A screen printed elliptical shaped electron emitter and subsequent X-ray image of a leaf  (Scale bars: 2 μm, 1 mm). Copyright Institute of Physics. (b) Scanning electron micrographs of an elliptical electron source, formed from tepee-like CNTs, demonstrated by Ryu et al. (Scale bar: 500 μm, 300 μm) Adapted from , Copyright 2012, Multidisciplinary Digital Publishing Institute.