Transcutaneous surgical procedures performed with a high intensity focused ultrasound (HIFU) therapy system can be monitored in real-time with an ultrasound imaging system if the HIFU is gated appropriately. Without synchronization, gated or continuous HIFU saturates the imaging system and interference occludes the image. If a gating signal is synchronized with the imaging cycle from any commercial imager, the location of any interference can be controlled and the HIFU treatment region can he visualized in real-time. Synchronization typically requires that an imaging system be customized for HIFU therapy, which is expensive and time consuming. We have developed a low-cost prototype system that synchronizes a HIFU therapy system with an arbitrary unmodified imaging system by using the HIFU transducer as a focused receiver that can detect scattered acoustic signals transmitted by the imaging probe. The receive signal is processed into a trigger that is used to control the gating and phasing of the HIFU relative to the imaging cycle. The technique is tested using a B-mode imager to monitor the formation of a lesion in a transparent tissue phantom; exposure time is 60 seconds with 40 W (time-averaged) electrical power delivered to the transducer. Performance is evaluated by recording the position of interference on the B-mode images and by comparing the B-mode images with CCD images that provide an optical view of lesion formation.