Abstract: Various embodiments of the present invention include devices for use in determining the concentration of a tissue energy absorber (e.g., hemoglobin) in an individual's blood. In particular embodiments, the device includes a photometric device (e.g., a pulse oximeter) and an imaging device (e.g., an ultrasound imaging device or other suitable imaging device), and the device is adapted for: (A) using the photometric device to measure a change in mass of the tissue energy absorber within a particular volume of the individual's blood between a first point in time and a second point in time, the particular volume of blood being blood within a particular portion of at least one vascular structure, the vascular structure comprising at least one of the individual's vessels (e.g.