We have achieved protein imaging mass spectrometry capabilities at sub-cellular spatial resolution and at high acquisition speed by integrating a transmission geometry ion source with time of flight mass spectrometry. The transmission geometry principle allowed us to achieve a 1-μm laser spot diameter on target. A minimal raster step size of the instrument was 2.5 μm. Use of 2,5-dihydroxyacetophenone robotically sprayed on top of a tissue sample as a matrix together with additional sample preparation steps resulted in single pixel mass spectra from mouse cerebellum tissue sections having more than 20 peaks in a range 3-22 kDa. Mass spectrometry images were acquired in a standard step raster microprobe mode at 5 pixels/s and in a continuous raster mode at 40 pixels/s.