A flat-panel detector includes: a ray-conversion layer configured to convert rays into a light having a first wavelength; and a plurality of imaging units. At least one of the plurality of imaging units includes: a photo sensor configured for receiving the light and converting the light to an electrical signal; and a light guider located a side of the photo sensor adjacent to the ray-conversion layer, the light guider having a light entry surface adjacent to the ray-conversion layer and a light exit surface adjacent to the photo sensor, the light entry surface being configured to receive the light from the ray-conversion layer and having an area greater than an area of the light exit surface, and an orthogonal projection of the light exit surface in a direction perpendicular to the ray-conversion layer at least partially overlapping that of the photo sensor.