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., at least one of the individual's arteries); (B) using an ultrasound imaging device to measure a change in interior volume, between the first point in time and the second point in time, of the particular portion of the vascular structure; and (C) using both the measured change in the mass of the tissue energy absorber, and the measured change in interior volume to determine at least an approximate blood total concentration of the tissue energy absorber within the individual's blood.