An instrument configured to acquire co-registered orthogonal fluorescence and photoacoustic volumetric projections, the instrument comprising: an imaging tank (101, 111) filled with liquid, the liquid comprising a coupling medium selected from the group consisting of: a single chemical, mixture, solution, suspension, emulsion or gel; an object positioning mechanism configured to position an interrogated object, or part thereof, inside the coupling medium in the tank and to rotate the interrogated object about a rotation axis relative to the tank; an optical excitation unit fixed with respect to the tank and configured to induce both fluorescence and photoacoustic responses inside the interrogated object using the same optical excitation spectrum and the same irradiation pattern at the surface of the interrogated object, with light-emitting outputs arranged so as to illuminate two opposite sides of the interrogated object at each of a plurality of rotational positions of the interrogated object; an array of unfocused photoacoustic transducers fixed with respect to the tank, each element of the array configured to detect photoacoustic signals generated inside the interrogated object, the array being arranged such that a plane connecting the rotation axis and a central portion of the array cuts through the interrogated object separating the two sides irradiated by the optical excitation unit; at least one optical detector fixed with respect to the tank, configured to register planar projections of sources of fluorescence generated inside the object, and arranged opposite to the array of photoacoustic transducers with respect to the interrogated object; and, a data acquisition unit configured to synchronize acquisition of photoacoustic data, acquisition of fluorescence data, optical excitation, and rotational position of the interrogated obj ect; the instrument being further configured to amplify, filter, digitize, and transfer acquired data to a peripheral system for subsequent storage, processing, or visualization. an imaging tank (101, 111) filled with liquid, the liquid comprising a coupling medium selected from the group consisting of: a single chemical, mixture, solution, suspension, emulsion or gel; an object positioning mechanism configured to position an interrogated object, or part thereof, inside the coupling medium in the tank and to rotate the interrogated object about a rotation axis relative to the tank; an optical excitation unit fixed with respect to the tank and configured to induce both fluorescence and photoacoustic responses inside the interrogated object using the same optical excitation spectrum and the same irradiation pattern at the surface of the interrogated object, with light-emitting outputs arranged so as to illuminate two opposite sides of the interrogated object at each of a plurality of rotational positions of the interrogated object; an array of unfocused photoacoustic transducers fixed with respect to the tank, each element of the array configured to detect photoacoustic signals generated inside the interrogated object, the array being arranged such that a plane connecting the rotation axis and a central portion of the array cuts through the interrogated object separating the two sides irradiated by the optical excitation unit; at least one optical detector fixed with respect to the tank, configured to register planar projections of sources of fluorescence generated inside the object, and arranged opposite to the array of photoacoustic transducers with respect to the interrogated object; and, a data acquisition unit configured to synchronize acquisition of photoacoustic data, acquisition of fluorescence data, optical excitation, and rotational position of the interrogated obj ect; the instrument being further configured to amplify, filter, digitize, and transfer acquired data to a peripheral system for subsequent storage, processing, or visualization.