We are currently developing a research to control a thin catheter by forming acoustic radiation force in human body, where it is necessary to constantly monitor the position of the catheter in blood vessel to perform effective control. In this study, we propose a method to detect the tip position of the catheter by analyzing time variation of brightness, which reflects the distribution of microbubbles scattered from the tip in ultrasonic volumes. We used the four-dimensional mode of the echography. The acquired volumes were transformed into two dimensions of the xy, yz, zx planes to estimate the position of the tip. As the result, the maximum error perpendicular to the axial direction was 1.5 mm, whereas that in the axial direction was 5.5 mm, because of the diffusion of microbubbles.Considering that the beam width of the acoustic radiation distribution is about 5 mm, the position detection result is useful
