Radiation imaging apparatus and radiation detection system

A radiation imaging apparatus comprises a sensor portion including a pixel array configured to acquire an image signal corresponding to radiation, and a plurality of detection elements arranged in the pixel array and configured to detect the radiation, and a readout circuit configured to read out the image signal from the sensor portion, wherein the readout circuit includes a signal processing circuit arranged to combine and process signals from the plurality of detection elements if determining the presence or absence of radiation irradiation, and to process a signal for each detection element or combine and process signals from a number of detection elements from among the plurality of detection elements, the number being less than the number of detection elements that include the plurality of detection elements, if determining a radiation dose.