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The assessment of blood volumes provides useful information on the circulatory system. Especially the intra-thoracic blood volume (ITBV), which is measured by invasive trans-pulmonary indicator dilution techniques, is related to the symmetry of the cardiac efficiency. This paper presents a new non-invasive indicator dilution technique for blood volume measurements. The indicator is an ultrasound contrast agent, which is detected by videodensitometric analysis of echocardiographic videos. Different indicator dilution curves (IDC) are measured simultaneously in different sites. The mean transit time (MTT) of the contrast between the measurement sites is multiplied by the blood flow to determine the blood volumes. The local density random walk model is used to fit and interpret the IDC. The MTT is easily derived from the fitted model. A specific fast and accurate least-square fitting algorithm is adopted. The in-vitro results show very accurate volume measurements with standard deviation smaller than 4.8% of the volume. The in-vivo application of the system in humans shows promising results too.