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Physics - Coggle Diagram

- - - - Beam-restricting devices decrease the x-ray beam field size
        
        Beam restriction serves two purposes:
        
        limiting patient exposure and
        
        reducing the amount of scatter radiation produced within the patient.
        
        can change the size AND shape of the x ray beam exiting the x ray tube (?)
        
        IR = image receptor
        
        The terms beam restriction and collimation are used interchangeably referring to a decrease in the size of the projected radiation field.
        
        Increasing collimation means decreasing field size, and decreasing collimation means increasing field size.
        
        MAKING UP FOR INCREASING COLLIMATION: Increasing collimation decreases the number of photons that strike the patient and decreases the amount of scatter radiation produced. Therefore exposure factors should be increased when increasing collimation.
        
        The kVp is typically not increased because it increases the proportion of scatter produced in the patient and results in decreased image contrast.
        
        To maintain exposure to the IR, the mAs (milliamperes/second) should be altered. [more energy into the x ray tube(?)
        
        collimation increases/field size decreases:
        
        patient dose decreases
        
        radiographic contrast increases (more clear image)
        
        film-screen: radiographic density decreases
        
        digital: quantum noise increases
        
        All beam-restricting devices are made of metal or a combination of metals that readily absorb x-rays.
        
        TYPES OF BEAM RESTRICTING DEVICES
        
        aperture diaphragms: consists of a flat piece of lead that has a hole in it (an aperture)
        
        you cannot actively change/adjust the size of the hole (ig while you are using it)
        
        because of the aperture’s proximity to the radiation source (focal spot), a large area of unsharpness surrounds the radiographic image (see image on one note)
        
        cones and cylinders: an aperture diaphragms with a cone or cylinder flange attached to it
        
        the explanations im not sure but cylinder is better than cones
        
        collimators: has two or three sets of lead shutters (like a camera shutter) that are stacked from the tube downwards with some space in between.
        
        The field shape produced by a collimator is always rectangular or square unless an aperture diaphragm, cone, or cylinder is placed below the collimator.
        
        Collimators are equipped with a white light source and a mirror to project a light field onto the patient. This light is intended to accurately indicate where the primary x-ray beam will be projected during exposure. [there is a light to predict where the x ray will scan]
        
        helps ensure that the radiographer does not open the collimator to produce a field that is larger than the IR.
        
        Another problem that may occur is the lack of accuracy of the light field. The mirror that reflects the light down toward the patient or the light bulb itself could be slightly out of position, projecting a light field that inaccurately indicates where the primary beam will be projected. There is a means of testing the accuracy of this light field and the location of the center of projected beam
        
        https://radiologykey.com/scatter-control/#:~:text=Located%20just%20below%20the%20x,of%20the%20projected%20radiation%20field.
  - - - Radiographic grids are used to improve radiographic image quality by absorbing scatter radiation that exits the patient, reducing the amount of scatter reaching the IR. It should be noted that grids do nothing to prevent scatter production; they merely reduce the amount of scatter reaching the IR.
  - - - Whereas the lead foil strips WILL absorb the x ray beams
    - - for the most common, focused grid pattern, the middle strips are parallels to each other but the ones on the outer edge are angle inwards slightly
        
        the photons that hit a lead strip are absorbed and therefore DO NOT reach the image receptor
    - - If the grid strips are 0.080 inch high, and the interspaces are 0.010 inch wide, the grid ratio is 8:1 (ALWAYS ends in ratio of one)
      - height/interspace width
        
        A high-ratio grid (16:1) will absorb more scattered radiation than a low-ratio grid (5:1) [the more closer together and longer the strips the less scatter) (this is because of angles) ->
- - - - speed is the speed of light c0
        
        ≈2.997 92 × 105 m s−1
      - λ=c0/fp [wavelength = speed over frequency]
      - λ = WAVELENGTH
      - ν or fp = frequency
  - - - ≈6.626 069 × 10−34 J s
    - - wavelength of most X-rays lies in the range of 0.01 nm up to 10 nm. This corresponds to an energy range of 100 keV down to 100 eV. (100,000 eV to 100eV)
- - - - The incident x-ray radiation is converted into an equivalent electric charge and then to a digital image through a detector sensor
    - - use a photo conductor like amorphous selenium (a-Se) or Cadmium telluride (Cd-Te) on a multi-micro electrode plate, providing the greatest sharpness and resolution. The information on both types of detectors is read by thin film transistors.
      - As there is no scintillator, lateral spread of light photons is absent here, ensuring a sharper image.
    - - has a scintillator layer which converts x-ray photons to photons of visible light and utilise a photo diode matrix of amorphous silicon to subsequently convert the light photons into an electrical charge. This charge is proportional to the number and energy of x-ray photons interacting with the detector pixel and therefore the amount and density of material that has absorbed the x-rays.
        
        in bold is the role of a x ray image intensifier (NOT intensifying screens) or Scintillators
        
        The energy of the incoming X-ray is completely absorbed by the material, exciting a molecule of the detector material. When the molecule de-excites, it emits a pulse of light, this time in the optical region of the electromagnetic spectrum. So, in this way, the incoming X-ray has been converted to optical light.
        
        SINTILLATORS AND PHOSPHOR DETECTORS (same role different application)
        We distinguish scintillators from phosphors, at least in X-ray astronomy, by defining bulk crystalline materials such as NaI and CsI as scintillators, and thin granular layers of rare earth oxysulphides as phosphors.
        
        While large area single crystals of NaI and CsI are valued for their stopping power in hard X-ray and gamma-ray astronomy, thin layers of phosphors are used to do high resolution, soft X-ray imaging
- - - - A failing vacuum, resulting from leakage or degassing of the materials, causes increased ionization (=excited electrons hitting atoms making them ionized) of the gas molecules, which slows down the electrons. Further, a current of positive ions flowing back (how?) could impair or destroy the
        cathode filament.
      - The space between the housing and the envelope is filled with transformer oil, serving as electrical insulation and for heat removal from the envelope surface, which is heated by the infrared radiation from the anode??.
    - - The X ray beam exits the tube through a window in the envelope. To reduce absorption, the thickness of the glass is reduced in this area. If low energy X rays are used, as in mammography, the exit port is a beryllium window, which has less absorption than glass due to its low atomic number.
        
        lower atomic number = will take away less energy from the original electron
    - - by increasing the negative bias voltage at the focusing the cup (increasing the difference in potential bias voltage between the filament (tungsten) and the focusing electrode (???)) the focus size will decrease and finally the electron current will be pinched off.
        
        This effect is sometimes used to control the focus size electronically, or for a fast switching of the anode current (grid controlled tubes) when short radiation pulses are required, as in pulsed fluoroscopy.
        
        Some bias can simply be achieved by connecting the filament and cup with a high resistance grid leak resistor. Some electrons emitted from the filament surface will hit and charge the cup. The cup is discharged via the grid leak resistor, maintaining the cup at a negative potential difference
    - - To minimize the evaporation of tungsten from the hot surface, the filament temperature is kept at a lower level, except during exposure, when it is raised to operational levels
    - - Exception being mammography although in digital mammography the advantages of using elements with lower atomic numbers are less significant and you can use tungsten anodes essential that the thermal properties are such that the maximum useful temperature determined by melting point, vapour pressure, heat conduction, specific heat and density is also considered. Tungsten (Z = 74) is the optimum choice here.
    - - for weaker tubes (dental units, portable x ray units and portable fluoroscopy systems) a stationary anode is possible as you are not heating it up too much
        
        in these uses the anode is attached to a copper block the dissipate the heat efficiently to the surrounding cooling medium
        
        the maximum loading (maximum kinetic energy of the electrons hitting the anode?) (the maximum anode current)
      - However for most use cases a rotating anode is necessary to dissipate and spread the heat over a larger surface area, allowing (?) a larger number of more powerful (more excited) electrons to be released.
        
        otherwise the anode will melt or get destroyed
        
        spreads the area bombarded by electrons to the circumference of a focal track
  - - - if an electron comes close to an atomic nucleus, the attractive colulomb forces cause a change in the electron's trajectory. An accelerated electron, or an electron changing its direction, emits electromagnetic radiation (breaking radiation) and this energy of the emitted photon(photonelectron?) is subtracted from the kinetic energy of the electron. -- this energy depends on the distance of the electron from the nucleus and the nucleus?
        
        Original energy - emitted energy when breaking = New slower energy of the electron
      - the electron bombardment (aka the energy of the electrons that hit the target) is anything from zero up to the initial electron kinetic energy
        
        as the electron hits each layer of the thick target, the energy of it slowly decreases due to breaking radiation
- - - - EMULSION LAYER
        
        silver halide consists of most silver bromide and very little silver iodide
        
        the silver iodide ions are necessary because a perfect (pure) crystal does not have photographic sensitivity
        
        A sensitivity speck is introduced into the silver halide crystal by silver sulfide in order to trap electrons. When exposed to light or X-rays, a bromide ion releases an electron that is attracted to a sensitivity speck. In this process, the bromide ion becomes a bromine atom and diffuses into the gelatin. The electron gets attached to the sensitivity speck and attracts a silver ion. The silver ion attaches to the sensitivity speck and forms a silver atom. The silver atom continues to attract other silver ions to form more silver atoms. At least two to six silver atoms are required for a latent image to be processed into a visible image. During film processing, more silver atoms are produced, thus metallic silver (black) image is formed 1,2.
        
        When x-rays, gamma rays, or light strike the grains of the sensitive silver halide in the emulsion, some of the Br- ions are liberated and captured by the Ag+ ions. This change is of such a small nature that it cannot be detected by ordinary physical methods and is called a "latent (hidden) image." However, the exposed grains are now more sensitive to the reduction process when exposed to a chemical solution (developer), and the reaction results in the formation of black, metallic silver. It is this silver, suspended in the gelatin on both sides of the base, that creates an image.
        
        reduction like redox
        
        Silver halide crystals are inherently sensitive to blue and ultraviolet light. Only with the addition of dyes, then the crystals can become more sensitive to other spectrums of light such as green and red
        
        photodiode semiconductor layer - converts light energy into electrical energy in the indirect thin-film transistor array
        
        The photodiode (hydrogenated amorphous silicon) semiconductor layer is responsible for converting light energy to electrical energy.
        
        Scintillator layer - converts x-ray photons to light photons
        
        Emulsion layer - contains silver atoms used in screen film radiography
        
        Photomultiplier tube - converts light energy to electrical energy in computed radiography readout process.
        
        Amorphous selenium semiconductor layer - converts x-ray energy directly into electrical energy.
      - A few of the films used for radiography only have emulsion on one side which produces the greatest detail in the image. However double sided means super speed.
- - - - ?
    - - because there is a potential difference of some 20-30kV (the less the potential difference the faster the electrons go?) , the photoelectrons are accelerated and guided by electron optics to strike the exit fluorescent screen and produce further fluorescent photons.
        
        The diameter of the output screen is
        typically 2.5 cm and this demagnification of the image coupled with the electron acceleration means that the image intensifier has a very high photon gain
        
        The number of photons emitted from the output phosphor is about 50 times the
        number produced by the input phosphor. This is referred to as the flux gain.