The Magnetic Resonance-guided Focused Ultrasound Surgery (MRgFUS) system was developed as a promising method to non-invasively treat uterine fibrosis. When comparing diagnostic ultrasound imaging systems and MRIs, MRIs are very expensive and do not have a high resolution. To overcome these problems, we have developed the ultrasound-guided FUS system. This system non-invasively measures 3-D temperature distributions with a high resolution of sub-pixel inside the human body. The method for measuring temperature distributions was based on the thermal dependence of the local change in speed of sound and thermal expansion. In this paper, we measured temperature distributions inside the heated agar phantom using the FUS system. First, an algorithm of measuring temperature distributions using ultrasound images was proposed. Second, the agar phantom was heated inside a hot water bath to show the relationship between temperature changes and the resulting ultrasound images. Third, using the FUS system, we measured temperature distributions inside the heated agar phantom with two transducers. Two transducers have different focal length. In the experiments, an ultrasound probe was fixed vertically to the agar phantom for measuring temperature distributions. The three-dimensional temperature distribution of the heated area inside the agar phantom was constructed by taking several planes of two dimensional temperature distributions, which were measured every 2.0 mm. A thermal image of the agar phantom was taken by an infrared thermal camera directly after 5 seconds of heating with the FUS system. When comparing the thermal image and temperature distribution results from ultrasound images using the proposed method, both resulted in an error of under 1°C. From these results, it was confirmed that the proposed method was useful for non-invasively measuring temperature distributions at a high resolution within 0.233 mm inside the heated object.

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