1. A radiation diagnostic apparatus comprising: an X-ray tube; a plurality of radiation detecting elements configured to detect X-rays radiated from the X-ray tube; a plurality of signal processing substrates configured to perform processing including at least A/D conversion processing on outputted signals of the plurality of radiation detecting elements and to output processed signals as the outputted signals after being subjected to the processing including at least A/D conversion; and processing circuitry configured to identify a non-observing element or a non-observing substrate based on information on an imaging region included in imaging conditions of an object, the non-observing element being a radiation detecting element of the plurality of radiation detecting elements which corresponds to a region other than the imaging region, and the non-observing substrate being a signal processing substrate of the plurality of signal processing substrates which corresponds to the non-observing element, and control an operation of the non-observing element or an operation of the non-observing substrate in imaging under the imaging conditions.