Experimental A mouse brain tissue section was placed on an ITO conductive glass slide plate (Fig. 1). The matrix compound 9-aminoacridine was sprayed on the surface of the tissue and then the sample was introduced to the mass spectrometer. The imaging MS measurements were performed in negative-ion mode on the whole brain tissue section (6.3 mm × 9.24 mm ) with 60 μm spatial resolution. The images consisted of 16170 mass spectra equivalent to the accumulation of 500 laser shots for each mass spectrum.