An apparatus includes an image analysis unit arranged to extract, from the medical image, brightness intensities of a first body lumen region of the medical image and brightness intensities of a second body lumen region of the medical image; and a measuring unit arranged to calculate a first body lumen value and a second body lumen value, each of the first body lumen value and a second body lumen value being calculated as a predetermined linear combination of the brightness intensities of the corresponding first body lumen region and second body lumen region, and to compare the first body lumen value with the second body lumen value.