What is X-Ray ?



X-Ray Production

      There are several kinds of X-Ray machines which used in medical applications for imaging purposes. It is generally used by doctors and dentists however airport luggage scanners, X-ray crystallography, and industrial inspections are other fields that it is used. Materials which have high density can be displayed by X-Ray due to its property to pass from inside of the materials. While it is passing from different materials, they absorb some of the X-Ray according to its attenuation coefficient and it creates a contrast on an X-Ray Film.
      The X-Ray machines produce the X-Ray with an X-Ray tube. An electron gun inside the tube shoots high energy electrons at a target made of heavy atoms, such as tungsten. X-rays come out because of atomic processes induced by the energetic electrons shot at the target. There are two different atomic processes that can produce x-ray photons. These are Bremsstrahlung and K-shell emission which both can occur in heavy atoms like Tungsten. Most elements emit x-rays when properly bombarded with electrons. Heavier elements are best because they emit a higher intensity through bremsstrahlung. All of the energy from the electrons' impact then goes into heating the tungsten. Tungsten is used because it can withstand this bombardment, as it has a high melting point and can conduct heat away very well.
      According to Bremsstrahlung, radiation is emitted when the velocity of the electron shot at the tungsten changes. This electron slows down after swinging around the nucleus of a tungsten atom and loses energy by radiating x-rays. According to quantum theory, a lot of photons of different wavelengths are produced after the bombardment, but only the atoms with high energy rate are used as an X-Ray. After emitting the spectrum of x-ray radiation the original electron is slowed down or stopped. The incoming electron is accelerated and strikes the tungsten at a high speed and has a lot of energy. As a result of contact with Tungsten the electron might be slowed a little or a lot. This rate of the negative acceleration determines the wavelength of the X-Ray.


X-ray Continuum Radiation


X-Ray Production

      The other way of producing X-Ray is K-shell emission. The atoms of target material have their electrons arranged in closed shells of different energies and the K-shell is the lowest energy state of an atom. Tungsten target atom can give incoming electron enough energy to push electrons located on the shell of tungsten out of its energy state. Then, a tungsten electron of higher energy from an outer shell can fall into the K-shell. The energy lost by the falling electron shows up in an emitted x-ray photon. After that, higher energy electrons fall into the vacated energy state in the outer shell. K-shell emission produces higher intensity x-rays than Bremsstrahlung and the x-ray photon comes out at a single wavelength.

      Peak kilovoltage(kVp), tube current (mA), exposure time (s) are significant factors for X-Ray production. Peak kilovoltage creates a potential difference between the cathod (source of the accelerated electrons) and the anod (Target Atom). Tube current determens the potential difference of the cathode`s terminals. Total energy delivered to tube is calculated according to the this formula;

Energy = kVp* mA* s = kVp* mAs

      And the instantaneous power delivered to tube is calculated with the multipication of kVp and mA.

Power = kVp* mA

      Anode characteristics also effect the efficiency and the kVp results of the X-Ray tube. Surface condition, composion, geometry are important factors. Aging of the anode surface can cause high probability of tungsten attanuetion to the patients. Also wrong chemical composions and the angle of the anode directly effects the final contrast of the X-Ray film.