An image diagnostic system controls a probe to perform radial scanning within a body cavity produces data based on signals received by the probe to construct and display a tomographic image of the body cavity and surrounding biotissue. The system includes a generation unit, a selection unit and a conversion unit. The generation unit generates and outputs synchronization signals in synchronization with a timing of acquisition cycles of line units of the signals and generated at a higher frequency than output signals outputted corresponding to rotational angles of the probe upon performing the scanning. The selection unit successively receives the output signals, and selects and outputs first ones of the synchronization signals as received subsequent to the receptions of the output signals. Responsive to successive inputs of the synchronization signals selected by the selection unit, the conversion unit converts the reflected signals into digital signals and outputs the digital signals.