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The blood volume assessment provides important information on the circulatory system condition. Especially the intra-thoracic blood volume (ITBV) is related to the symmetry of the cardiac efficiency. The ITBV measurement, nowadays, is made by use of trans-pulmonary indicator dilution techniques, such as thermo- and dye-dilution. Since catheterization is required, these techniques are very invasive. The tracer is injected into a central vein and detected in the aorta. The detected indicator concentration-versus-time curve is referred to as indicator dilution curve (IDC). The mean transit time (MTT) of the IDC multiplied by the cardiac output (CO) gives the blood volume between the injection site (central vein) and the aorta, i.e., the ITBV plus the average volume of all the cardiac chambers. This paper presents a new non-invasive technique for the measurement of blood volumes in the circulatory system. The tracer is an ultrasound contrast agent (UCA) detected by an ultrasound transducer. The acoustic or video intensity analysis of the B-mode output of ultrasound scanners allows the measurement of UCA IDCs. Several cardiac echo-views permit the measurement of different IDCs from different sites. The blood volume between two different sites is given by the product of the blood flow (CO) times the MTT that the contrast takes to go from the first to the second site. For the ITBV assessment, two IDCs can be measured simultaneously in the right and left side of the heart. The system is validated by in-vitro experimentation. A Sonos 5500 ultrasound scanner is used to detect SonoVue® contrast agent IDCs. The results show very accurate volume measurements with a standard deviation smaller than 4% of the volume for a wide range of flows. Initial in-vivo application of the system in humans shows promising results.