Visible light is made up of many different wavelengths known as a spectrum. We observe these wavelengths as different colours, or all wavelengths together as white. The same principle can be used for X-rays.
X-rays are produced in a range of wavelengths called a spectrum. Although invisible to the naked eye, these different wavelengths can be detected with film and digital devices. Standard radiographs and CT measure how much of the X-rays are stopped/attenuated by the object they pass through. They cannot measure how each wavelength is being attenuated by the object – this is equivalent to measuring the brightness (density) but not the colour (composition). This is because all materials attenuate the various wavelengths differently. Spectral CT measures how specific wavelengths are attenuated, which enables visualisation of tissue composition. If iodinated contrast is administered to the patient, spectral CT can distinguish iodine from body tissues and determine the amount present.