Lecture 7, 8 & 9 Biophysics I - Radiation Physics PDF

Lecture 7,8 &9 biophysics I Radiation Physics Production of X–Rays  x-rays emitted from x-ray tube. Components of the X-ray tube  cathode : contain a heated metal filament to supply electrons by thermionic emission  The material of the filament is tungsten which is chosen becaus...

Lecture 7,8 &9 biophysics I Radiation Physics Production of X–Rays  x-rays emitted from x-ray tube. Components of the X-ray tube  cathode : contain a heated metal filament to supply electrons by thermionic emission  The material of the filament is tungsten which is chosen because of its high melting point Components of the X-ray tube  The cathode contain negatively charged focusing cup.  The function of the cathode cup is to direct the electrons toward the anode so that they strike the target in a well- defined area, the focal spot. Components of the X-ray tube  An evacuated chamber across which a potential difference can be applied  This high potential to accelerate the negative electrons from cathode to anode Components of the X-ray tube  Anode (the target): a metal with a high efficiency for conversion of electron energy into X-ray photons Components of the X-ray tube  The material chosen for the anode should satisfy a number of requirements  (1) High conversion efficiency for electrons energy to X-rays. High atomic numbers(Z) are favored since the X-ray intensity is directly proportional to Z.  lead (Z = 82) converts 1% of the energy into X- rays but aluminium (Z = 13) converts only 0.1%.  (2) A high melting point so that the large amount of heat released causes minimal damage to the anode Physics of X–ray production  the energy of most of the electrons striking the target is dissipated in the form of heat, the remaining produce useful X-rays  There are two mechanisms by which X-rays are produced. One gives rise to Bremsstrahlung or (braking radiation) and the other characteristic X- rays. Physics of X–ray production  Bremsstrahlung :  The process of bremsstrahlung (braking radiation) is the result of collision (interaction) between a high- speed electron and a nucleus.  The electrons while passing near a nucleus may be deflected from its path by the action of uncles attraction and lose energy as bremsstrahlung x-ray. (predicted by Maxwell)  Physics of X–ray production  As the electron passes in the vicinity of a nucleus it suffers a sudden deflection and deceleration  As a result, a part electron energy convert to X-ray photons. Physics of X–ray production  The amount of bremsstrahlung produced depends upon two factors:  (1) The atomic (Z) no. of the target (i.e. the more protons in the nucleus, the greater the deceleration of the electrons).  (2) Potential difference on the tube.  high potential lead to faster the electrons (the more likely they will penetrate into the region of the nucleus) and Higher amount of X-ray produced. Physics of X–ray production  Characteristic X-rays:  An electron strikes a K-electron in a target atom and knocks it out of its orbit.  The vacancy in the K shell is filled almost immediately when an electron from an outer shell of the atom falls into it. Physics of X–ray production  An X-ray photon emitted when an electron falls from the L level to the K-level is called a K characteristic X-ray, and that emitted when an electron falls from the M shell to the K shell is called a K X-ray  Video explain x-ray production https://www.youtube.com/watch?v=Bc0eOjWkxpU How X – Rays Are Absorbed  X-rays are not absorbed equally by all materials; if they were, they would not be very useful in diagnosis.  Heavy elements such as calcium (bones) are much better absorbers of X-rays than light elements such as carbon, oxygen, and hydrogen (soft tissues ), and as a result, structures containing heavy elements, like the bones, stand out clearly.  The soft tissues (fat, muscles, and tumors) all absorb X- ray equally and are thus it is difficult to distinguish from each other on an X-ray image.  Of course, air is poor absorber of X-rays. X-ray beam attenuation  The attenuation (absorption) of an X-ray beam is its reduction due to the absorption of some of the photons out of the beam.  To measure the attenuation of an X-ray beam:  consider a narrow beam of X-rays is produced and an X-ray detector measures the beam intensity after passing through a certain (material)absorber. X-ray beam attenuation  The un-attenuated (initial) beam intensity is (Io).  When sheets of absorber are introduced into the beam; the intensity of the beam decreases to (I) transmitted intensity. Initial intensity I0 Transmitted intensity I Thickness(X) X-ray beam attenuation  The intensity of X-ray beam (I) decreases exponentially with the thickness of the absorber material (X). X-ray beam attenuation  The exponential equation describing the attenuation curve for a mono-energetic X-ray beam is:  x I  Ioe  where e = 2.718  (x) is the thickness of the attenuator  () is the linear attenuation (absorption) coefficient of the absorber material. X-ray beam attenuation  The linear attenuation coefficient is depending on:  The energy of the X-ray photons. As the energy increase the absorption decrease for the same material.  Type of material. heavy material (bone) has more attenuation coefficient than light material(soft tissue). X-ray beam attenuation  Application of the absorption law:  Calculate the transmitted intensity of the X- ray beam after passing through tissue of thickness 12 mm, knowing that the initial beam intensity was 1000 photon/cm2 and the µ=0.69 cm-1. X-ray beam attenuation  The half-value layer (HVL) for an X-ray beam:  is the thickness of a given material that will reduce the beam intensity by one-half X-ray beam attenuation  The half-value layer for the X-ray beam in is 2.5 mm Al.  The half-value layer is related to the linear attenuation coefficient by: 0.693 HVL  μ