Photoacoustic flow cytometry monitors cells circulating in vivo

Real-time detection of nanoparticles and cells, including cancer, is now possible in blood and lymph flow. Flow cytometry (FC) is a well-established technique that has rev-olutionized cell diagnostics in vitro.1 Extracted cells flow through anarea irradiatedbylaserbeams,anddetectors recordfluorescent and/orscatteredlightfromthesecells.Despite its longhistoryand manysuccessfulapplications,conventionalFCis limitedbythein-vasivenatureofcell extraction froma livingorganism,whichmay introduce artifacts. It also makes impossible long-term monitor-ing of the cells in their complexnatural environment for studiesof metastasis,cancerrecurrence,immunefunction,orcelldeath.Fur-thermore, thecytotoxicityof thefluorescent labelsusedinmostFC systemsmust be overcome before they can be used in humans.1–4 The main goal of our work has been to overcome these prob-lems by developing a photoacoustic (PA) technique using lase