Lecture 3 _X-ray.pdf

Physics of Diagnostic X-ray Lecture 3 - 4 By Sarah Alsalhi  Atom Characteristics The particles of the atom are : the negative electrons (dark blue spheres in this figure), the positive protons (red spheres), and the uncharged neutrons (gray spheres). Protons and neutro...

Physics of Diagnostic X-ray Lecture 3 - 4 By Sarah Alsalhi  Atom Characteristics The particles of the atom are : the negative electrons (dark blue spheres in this figure), the positive protons (red spheres), and the uncharged neutrons (gray spheres). Protons and neutrons make up the tiny dense nucleus, which is surrounded by electrons. The Atomic Number (Z) of the atom is the number of protons in the nucleus. The Mass Number (A) of the atom is the total number of protons and neutrons in the nucleus. Atoms and Ions A neutral atom has the same number of protons as electrons. The electron “shells” are a schematic representation of the actual electron distribution, a diffuse cloud many times larger than the nucleus. A positive ion is an atom with one or more electrons removed. A negative ion is an atom with an excess of electrons. Radiation Radiation: Radiation is the energy that travels through space or matter. The radiation that we are exposed to can be ionizing or non-ionizing, depending on whether or not the radiation has enough energy to remove an electron from an atom with which it interacts. Ionizing radiation: is the type of radiation which causes the formation of ions (electrically- charged atoms or molecules) when interacting with matter. These ions can lead to biological damage in cells. X-rays, gamma rays, neutrons, cosmic rays are ionizing radiation. They contain enough energy per photon to eject electrons from the atoms with which they interact. Non-ionising radiation: is the type of radiation which does not have enough energy to eject electrons from the atoms with which they interact, hence can not cause ionization. Visible light, infrared waves, radiofrequency waves are non-ionizing radiation. What is the difference between X-rays & Gamma rays? X-rays and gamma rays are differed only by their origin in the nucleus: Gamma rays originate within (inside) the nucleus of the atom, whereas X-rays are generated outside the nucleus by the interaction of high speed electrons with the atom. Gamma rays emitted by a single radioactive atom and consist of one or several discrete energies. X-rays consist of a continuous spectrum of energies. X-rays and gamma rays are alike in their mode of interaction with matter, their biological effects and their photographic effects.  X-ray History X-rays were discovered by the German physicist Wilhelm Konrad Röntgen in 1895. He called the new kind of ray (X) for the unknown. With these new rays, he could make a photograph of his wife’s hand—showing the bones and her wedding ring. Soon afterwards, their usefulness to visualize the internal anatomy of humans was established. Today, imaging with X-rays is perhaps the most commonly used diagnostic tool with the medical profession, and the techniques from a simple chest radiography to computer tomography (CT) depend on the use of X-rays. X-ray Production Components of an X-ray Tube Cathode (-): The cathode is a heated filament that emits electrons through thermionic emission. Anode (+): The anode is usually made of a heavy metal like tungsten. It serves as the target for the accelerated electrons. Vacuum Chamber: The entire assembly is enclosed in a vacuum-sealed glass or metal chamber to prevent the electrons from colliding with air molecules. Control Mechanisms: The X-ray tube has mechanisms to control the voltage, current, and exposure time, allowing for precise control over the X-ray production. Components of an X-ray Tube X-ray Production Process 1. Electron Emission (Thermionic Emission): Electrons are emitted from the cathode filament when it is heated. The filament (Cathode) is the source of electrons. 2. Electron Acceleration: The high voltage applied across the cathode and anode accelerates the electrons towards the anode (the target). 3. X-ray Generation: When these high-energy electrons collide (hit) with the anode target, they produce X-rays through Bremsstrahlung and characteristic radiation. 4. X-ray Emission: The produced X-rays exit the tube through a window, which is usually made of a material like beryllium that allows X-rays to pass through with minimal absorption.  Some technical notes about X-ray generation High speed electron can convert some or all it's energy into an x-ray photons when it strikes an atom, and thus we need to speed up electrons to produce X-rays. The number of electrons accelerated towards the anode depends on: Temperature of the filaments. The advantage of the rotating of the anode is to avoid overheating of the anode, because the overheating produce a damage of the anode. Generation Methods of X-rays  Generation Methods of X-rays X-rays are produced in two ways: Production of Bremsstrahlung Radiation (Bremsstrahlung x-ray) Production of Characteristic Radiation (Characteristic x-ray)  Generation Methods of X-rays 1. Bremsstrahlung Radiation (Braking Radiation) This radiation is termed as bremsstrahlung, a German word meaning “braking radiation” or radiation produced from the braking of projectile electrons. Process: High-speed electrons are accelerated towards a metal target (anode) inside an X-ray tube. When these electrons pass close to the nuclei of the target atoms, they are decelerated due to the electrostatic attraction between the negatively charged electrons and the positively charged nucleus. The deceleration causes the electrons to lose kinetic energy, which is emitted as X-ray photons. The energy of the emitted X-rays can vary, resulting in a continuous spectrum of X-ray energies. Results: Produces a broad, continuous spectrum of X-rays. The intensity of the X-rays increases with decreasing wavelength (higher energy). This type also called "white radiation" because it's analogous to white light and has a range of wave length. Bremsstrahlung Radiation (Braking Radiation) Bremsstrahlung causes a spectrum of photon energies to be released and depends on the anode voltage. 80% of X-rays are emitted via Bremsstrahlung. Rarely, the electron is stopped completely and gives up all its energy as a photon  Generation Methods of X-rays 2. Characteristic X-ray Radiation Process: High-speed electrons collide with the inner-shell electrons of the target atoms, ejecting these electrons from their orbits. This creates vacancies in the inner electron shells. Electrons from higher energy levels fall into these lower energy orbits. The energy difference between the higher and lower energy levels is emitted as X-ray photons. These emitted X-rays have specific energies corresponding to the energy levels of the target atoms. Results: Produces X-rays with discrete energies (characteristic peaks) specific to the target material. These energies are determined by the differences between the energy levels of the target atoms. Characteristic X-ray Radiation  2. Characteristic X-ray Radiation The emitted photon is named according to the shell of the stroked electron. For example: K characteristic X-ray or L characteristic X-ray. An x-ray photon emitted when: 1. An electron falls from the L-level to the K-level is called Kα characteristic X- ray. 2. An electron falls from the M-level to the K-level is called Kβ characteristic X- ray Example: For tungsten: Ek – EL (Kα) = 59.3 keV Ek – EM (Kβ) = 67.6 keV 2. Characteristic X-ray Radiation It is called “characteristic” as energy of emitted electrons is dependent upon the anode material, not on the tube voltage. Energy is released in characteristic values corresponding to the binding energies of different shells. As a result of interaction between the electron and the target material, about 99% of the energy is converted into heat and about 1% is converted into X-ray energy.  X-ray Spectra The resulting spectrum of x-ray photon energies released is shown in the graph. At a specific photon energy there are peaks where more X-rays are released. These are at the characteristic radiation energies and are different for different materials. The rest of the graph is mainly Bremsstrahlung, in which photons with a range of energies are produced. Bremsstrahlung accounts for the majority of x-ray photon production. Intensity of X-rays Intensity of X-rays The intensity of X-rays refers to the amount of X-ray energy transmitted per unit area per unit time in a specific direction. It is essentially a measure of the strength or brightness of the X-ray beam. If the entire electron energy is converted to that of the X-ray photon, the energy of the X-ray photon is related to the excitation potential V by the relation: 𝑬 = 𝒉 𝒇𝒎𝒂𝒙 𝒉𝒄 𝑬= =𝒆𝑽 𝝀𝒎𝒊𝒏 Where e is the electron charge =1.6 x 10-19 C. Thus, x-ray wavelength λ ; 𝒉𝒄 𝝀𝒎𝒊𝒏 = 𝒆𝑽 Inserting the values of the constants h, c, and e, we have; The greater the kinetic energy of the 𝑪𝒐𝒏𝒔𝒕𝒂𝒏𝒕 Higher energy electrons electrons that strike the anode, the 𝝀 min 𝒏𝒎 = (𝒌𝑽) can convert their energy 𝑽 shorter the minimum wavelength of into higher energy the X-rays emitted by the anode. (λmin) which is named here, the cutoff wavelength. photons, which have a shorter wavelength. Technical Factors Affecting X-ray Tube Current (mA): The intensity of X-rays is directly proportional to the Intensity tube current. Increasing the current increases the number of electrons striking the target, thus increasing the X-ray output. Tube Voltage (kV): The intensity and energy of the X-rays increase with the tube voltage. Higher voltage accelerates the electrons more, resulting in higher energy X-rays. More is Better Exposure Time: Longer exposure times result in higher total X-ray intensity as more X-ray photons are produced over time. Target Material: Different materials produce X-rays with different efficiencies. Heavy elements like tungsten are commonly used due to their high atomic number (Z) and efficiency in producing X-rays. Technical Factors Affecting X-ray Intensity Technical Factors Affecting X-ray Intensity Problem 1: Problem 2: Problem 3: