Formation of X-rays

1. Current applied to the filament (heated)
1. 2. Electrons are produced from the tungsten wire via thermionic emission
  1. 3. Electrons form a cloud
    1. 4. Negative cathode repels the electron cloud and further focusses it by a negative focussing cup
      1. 5. The electons are attracted to the positive anode and so they accelerate across the tube
        6. Electrons strike the target of the anode and interact with tungsten atoms to produce x-rays
        7. Produced: 97% heat and 3% x-rays
        Electrons interact with anode in 2 ways:
        Characteristic x-rays: characteristic to tunsten. The particle interacts with an INNER shell to produce 70keV or an OUTER shell for 10keV. Depending on what shell s interacted with will determine the strength of x-rays produced. Ie: only energy at certain points ie: 10, 58 and 70keV.
        Bremsstrahlung radiation: particle interacts with the electromagnetic filed of the tungsten nucleus, slowing down and releasing that energy in the form of an x-ray. The greatest energy is given off when the photon is deviated to the maximum around the nucleus.
        If we set 130kVp, our maximum energy will be 130kVp and many others will be produced below this - BEAM IS NOT UNIFORM :)
        We use filters to manage this beam nonuniformity and turn the graph into a bell curve but getting rid of lower energy x-rays which only add to patient dose.
        Inherent Filters: built into the tube. Gets better with age as when you heat the filament and anode, they turn into gaseous states which can deposit on the vacuum tube, increasing the filtration.
        Additional Filters: added to the tube to meet CSP5 recommendations. Installed in the tube.
        Compensatory Filters: added by the MRT to further filter the beam
        Anode Heel Effect: a varying intensity across the x-ray field in the anode-cathode direction caused by attenuation of the x-rays in the heel of the anode. Intensity of the beam is stronger at the cathode end of the tube
        A potential difference is applied across the cathode and anode to accelerate electrons to collide with anode. This is the kVp (kilovoltage peak)
        kVp: is a measure of the maximum electrical potential across an x-ray tube
        Controls: the quality and wavelength of the x-rays. The penetration power of the beam and the radiographic contrast (second to the algorithm.)
        Affects: the penetration of the body part, scatter production (higher kVp = more forward scatter), radiographic contrast ( high kVp produces high energy photons that penetrate through body tissues decreasing the differential absorption and producing low contrast images.)
        Determined by: part thickness, atomic number, known pathology, desired contrast effect, radiation protection, IR properties.
        Critique: adequate penetration to show the cortical outlines of the thinnest and thickest bony structures of interest.
        Too much kVp: borderline is too dark, burn out of cortical outline
        Insuffient kVp: areas of ROI are underpenetrated and bright white
        To increase contrast, decrease kVp by 15% and increase mAs by 100% To decrease contrast, increase kVp by 15% and decrease mAs by 50%.
  2. The amount of electrons produced, is decided by how hot the filament is heated.
    1. So our mA refers to our current of electrons.
      1. But the time of exposure is also important - from the moment we hit the button until the exposure stops. This can be caused by a number of things:
        AEC: using ionisation chambers, the AEC measures the radiation reaching the IR and at a preset amount, will terminate the exposure.
        Backup Timer:
        Dead Man's Switch:
        The combination of electrons and time is called mAs because the cloud of electrons moving creates a current.
        mAs: a measurement of the total number of electrons (quantity) produced via thermionic emission. A measurement of the total number of x-rays in the beam.
        Controls: density (the quantity of the radiation that reaches the IR)
        Affects: patient dose, movement unsharpness, visibility of detail, quantum mottle
        Determined by: patient size, SID, OID, grid use, movement control, patient pathology
        Critique: check for whether the bony and soft tissue structures of interest are visualised with sufficient detail and no mottle or movement unsharpness.
        Insufficient mAs: density is light so some of the anatomic structures cannot be evaluated. Quantum mottle becomes more apparant.
        Too much mAs: excessive grey scale. Generally do not need repeating for
        Doubling mAs increases EI by 300. Halving mAs does the opposite.
2. The filament is found at the cathode and is used to produce electrons.
  1. There is a filament for broad focus and one for fine focus. They can be arranged in different orientations.
    1. On top of one another: anode will be biangular and filaments will interact with rotating anode at different points.
    2. If beside one another: the electrons just hit the same part of the track so you have a standard rotating anode.

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Formation of X-rays

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