If purely from the X-ray radiation dose point of view, CT is far greater than X-ray filming, and MRI does not have X-ray radiation. Hospitals use are not harmful to the human body.X-rays are a very short wavelength, a lot of energy electromagnetic waves, the wavelength of X-rays is shorter than the wavelength of visible light (about 0.001 to 100 nanometers, the medical application of X-rays wavelength of about 0.001 ~ 0.1 nanometers between), it is the photon energy of photons than the visible photon energy of photon energy of several tens of thousands to hundreds of thousands of times larger. ? X-rays because of its short wavelength, energy, shine on the material, only part of the material is absorbed, most of the through the atomic gap and through, showing a strong penetration ability. x-ray penetration of material and the ability of x-ray photon energy related to x-rays, x-rays, the shorter the wavelength, the greater the energy of the photon, the stronger the penetration. x-rays, the penetration of the density of the material is also related to the use of the difference between the absorption of this nature can be differentiated between substances of different densities. The use of differential absorption of this property can be differentiated from the density of different substances.