A photoacoustic flow cytometry (PAFC) device for the in vivo detection of cells circulating in blood or lymphatic vessels is described. Ultrasound transducers attached to the skin of an organism detect the photoacoustic ultrasound waves emitted by target objects in response to their illumination by at least one pulse of laser energy delivered using at least one wavelength. The wavelengths of the laser light pulse may be varied to optimize the absorption of the laser energy by the target object. Target objects detected by the device may be unlabelled biological cells or cell products, contrast agents, or biological cells labeled with one or more contrast agents.
1. A method of detecting individual fast moving target objects in vessels of a living organism, comprising: generating a focused elongated laser beam using a lens or an optical fiber as a series of consecutive laser pulses or modulated continuous radiation at different wavelengths in the x-ray spectra, visible spectra, the terahertz spectra, or the microwave spectra; simultaneously detecting photoacoustic, fluorescence, and scattering light signals from the same individual fast moving target object with a focused ultrasound transducer and a photodetector; and analyzing a combination of the photoacoustic, fluorescence, and scattering light signals to determine the presence of the object target and their characteristics.