1. A medical imaging apparatus, comprising: an injection part configured to inject a contrast agent into a subject at a predetermined injection speed; an image acquiring part configured to image the subject with the contrast agent injected and acquire a plurality of image data with different imaging times; a pixel value calculator configured to obtain, for each of the image data, a pixel value within a region of interest set in each of the image data acquired by the image acquiring part; and an injection controller configured to control injection of the contrast agent by the injection part in accordance with the pixel value obtained for each of the image data.