Lecture 5 -7

• Photons have the striking property of penetrating matter that are opaque to visible light.
• When photons interact with matter they may be transmitted through the material unaffected, scattered in a different direction from that traveled by the incident photon, or absorbed by the material such that no photon emerges.
• Attenuation of the photon beam can be considered a combination of scattering and absorption.
• Attenuation is the reduction in the intensity when radiation beam pass through a matter.
• Factors affecting attenuation include thickness, atomic number, density, and energy of the radiation.
• The production of X rays involves the bombardment of a thick target with energetic electrons.
• As a photon makes its way through matter, there is no way to predict precisely either how far it will travel before engaging in an interaction or the type of interaction it will engage in.
• These electrons undergo a complex sequence of collisions and scattering processes during the slowing down process, which results in the production of bremsstrahlung and characteristic radiation.
• In clinical applications, we are generally not concerned with the fate of an individual photon but rather with the collective interaction of the large number of photons.
• The energy of the electrons is converted into electromagnetic energy by atomic interactions.
• We are interested in the overall rate at which photons interact as they make their way through a specific material.
• A certain percentage of the photons will interact, or be attenuated, in a 1 cm thickness of material.
• The quantity (exposure) and quality (spectrum) of the x-radiation produced can be controlled by adjusting the electrical quantities (KV, MA) and exposure time, S, applied to the tube.
• The X-ray circuit is a circulatory system for electrons, which pick up energy as they pass through the generator and transfer their energy to the X-ray tube anode.
• Oxidation is the process of adding oxygen to a compound.
• Current estimates of medical and dental X-ray examinations per year are unknown.
• The three principle electrical quantities that can be adjusted in an X-ray circuit are the KV (the voltage or electrical potential applied to the tube), MA (the electrical current that flows through the tube), and S (duration of the exposure or exposure time, generally a fraction of a second).
• An X-ray tube is an energy converter that receives electrical energy and converts it into two other forms: x-radiation and heat.
• The fraction that interacts in the 1 - cm thickness is 0.1, or 10%, and the value of the linear attenuation coefficient is 0.1 per cm.
• The heat is an undesirable by product.
• X-ray tubes are designed and constructed to maximize X-ray production and to dissipate heat as rapidly as possible.
• Linear attenuation coefficients can sometimes be found in the literature, but it is often easier to locate attenuation data in terms of the mass attenuation coefficient.
• Mass Attenuation Coefficient ( α ) is obtained by dividing the linear attenuation coefficient by the density ( 𝜇 / 𝜌 ).
• Heel effect is the reduction in X-ray beam intensity toward the anode side of an X-ray field.
• As the electrical current flows through the tube from cathode to anode, the electrons undergo an energy loss, which results in the generation of x-radiation and heat.
• The HVL depends on the energy of the photons, the atomic number of the material, the density of the material, and the thickness of the material.
• Of that 1%, approximately 0.5% interact to form the image.
• The average smoker receives a dose of radiation equivalent to about 300 chest x-rays due to radioactive isotopes in the smoke.
• Interaction in the body begins at the atomic level.
• Exponential attenuation coefficient: The attenuation of photon beam through a given material follows an ( - )exponential curve.
• Kilovoltage peak (kVp) is the peak voltage applied to the x-ray tube, determines the highest energy of x-ray photon, and is responsible for the acceleration of electrons from the cathode to the anode.
• The linear attenuation coefficient is represented as µ.
• Differential absorption is the difference between those X-rays absorbed and those transmitted to the image receptor (IR).
• The major distinction between linear and exponential functions is the rate of their growth or change.
• Above 80 kVp, Compton scatter begins to increase.
• Approximately 1% of photons that interact with the patient reach the IR.
• Half Value Layer (HVL): It is the most frequently used factor for describing the penetrating ability of radiation.
• When x is incremented by 1, the value of the linear function has 2 added to its last value, while for the exponential function, 2 is multiplied by its last value.
• Pilots and flight attendants are officially classified as "radiation workers" due to the high level of radiation exposure they receive in their profession.
• Exponential growth is far more significant than linear growth.