1. A medical imaging apparatus, comprising: a processor configured to cause an X-ray source and detector to image a subject with a contrast agent injected, and acquire a plurality of image data of the subject respectively at different times; a pixel value calculator configured to obtain, for each image data of the plurality of image data, a pixel value for the image data; and an injection controller configured to control injection of the contrast agent by an injector configured to inject a contrast agent into a subject in accordance with each pixel value obtained by the pixel value calculator, wherein the injection controller is further configured to stop injection of the contrast agent by the injector when one of the pixel values obtained by the pixel value calculator becomes at least a preset stopping threshold.