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We believed that a totally noninvasive video urodynamic system could be invented based on the concept of Doppler ultrasonography. To develop this system we measured flow velocity in an experimental urethra using 4 materials, including natural urine.A cellulose tube was inserted into a standoff pad as an experimental urethra. Degassed water, distilled water, urine and liposome (multi-lamellar particles) solution comprised the 4 materials used to compare signal intensity and flow velocity at various flow rates. The flow rate from the tube was measured with a uroflowmeter. The Doppler image and digital uroflow signal data were processed by a computer.Doppler signals and flow velocity curves could be sufficiently obtained using distilled water, urine and liposome solution, while degassed water showed no Doppler signals at any flow rate. Minimum flow rate at which clear Doppler signals were continuously detected from the angle in the frontal plane was greater than 3.0 ml. per second for distilled water, greater than 1.5 for urine and greater than 0.3 for liposome solution. Maximum flow velocities were identical in these 3 materials at a flow rate of greater than 2.0 ml. per second. The functional cross-sectional area of the tube showed a constant value irrelevant to the initial flow rate.Flow velocity could be measured by Doppler ultrasound above a certain minimal flow rate. Dissolved gasses have an important role in creating Doppler signals. Flow velocity, velocity related parameters and functional cross-sectional area can diagnose and localize the area of bladder outlet obstruction. Liposome solution may be helpful in detecting velocity in low flow states in future systems.