According to some embodiments there is provided a method for controlling a treatment effect on blood vessel tissue during an ultrasonic treatment, the method comprising positioning an ultrasonic transducer device in the blood vessel lumen； controlling a treatment effect by controlling fluid flow, wherein controlling comprises deploying a fluid restrictor at a location relative to the transducer effective to block at least a portion of fluid flowing upstream, downstream or adjacent the transducer. According to some embodiments there is provided an ultrasonic transducer device sized for placement in a body lumen, and comprising a fluid restrictor effective to block at least a portion of fluid flowing upstream, downstream or adjacent the transducer. In some embodiments, the fluid is blood.
