Noninvasive photoacoustic microscopy of living cells in two and three dimensions through enhancement by a metabolite dye

Photoacoustic microscopy (PAM) is a powerful imaging modality that can generate high-resolution, three-dimensional (3D) images non-invasively. However, most types of cells cannot be imaged and quantified by PAM due to the lack of an absorption-based contrast mechanism. Here we demonstrate that this problem could be solved by employing 1-(4,5-dimethylthiazol-2-yl)-3,5-diphenylformazan (MTT formazan), a cellular metabolite of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), as the contrast agent. The as-formed MTT formazan crystals inside the cells have a strong absorption in the spectral region of 500-700 nm, and are generally nontoxic. It could serve as a superb contrast agent for PAM in imaging various types of cells cultured on two-dimensional (2D) substrates and in 3D porous scaffolds. This contrast mechanism is general, and can be applied to essentially all types of metabolically active cells including stem cells and tumor cells, which would be very useful for a number of biological and biomedical applications.