Changes in living tissue elasticity or hardness are generally correlated with its pathological state or disease. This makes tissue elasticity information extremely useful for early detection of cancer, coronary heart disease, and liver cirrhosis. But almost all of medical ultrasound examinations are insensitive to hardness as an imaging parameter. Today, therefore, palpation is still widely used as a self-screening procedure for obtaining the tissue elasticity. In recent years, new methods using Doppler based techniques are proposed and have been examined. In their method the approach combines external mechanical stimulation of target tissues with Doppler ultrasound to map the relative tissue motion.
In this study, one dimensional distribution of the vibration amplitude inside the agar phantom is detected by Doppler ultrasound, then the relative hardness of tissues is evaluated by using above mentioned method. First, it was verified that the low frequency vibration amplitude propagated in the homogeneous phantom was almost uniform amplitude. Next, as for the phantom which has relatively “hard” inclusion to surrounding medium, it was shown that the low frequency vibration was shut out at the face of “hard” inclusion. And as for the phantom which has relatively “soft” inclusion to surrounding medium, it was shown that the low frequency vibration was propagated well up to the part of “soft” inclusion to surrounding medium, but it was propagated little in the area which is behind the “soft” inclusion. The phantom with a thickness of about 3cm was used in the experiments, in that case, the frequency of 150Hz was best to distinguish the inclusion from the surrounding medium. In the measurement system made for this study, the inclusion with a thickness of about 5mm was detectable.
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