Abstract: The elastic properties and velocity of acoustic sound propagation in soft tissues, permits applying the liquid model in research. In regard to the low level of ultrasound power admissible in research in vivo, a new method, implementing quasi-continuous wave, was proposed. The base for determination of the medium parameters, is utilising the conversion of receiver signal to digital form and counting the phase of signal numerically. Presented method was implemented in monitoring the condition of brain, and it gives the possibility to detect changes of density on level 50 ppm, and changes of temperature 0,02^oC with medium power over JOD times less then the admissible. That method, in regard to the measurement set-up's simplicity and high accuracy, can find implementation in other technical solutions and scientific research.