X-ray Tubeyr1.pdf

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- o - o E E E E - -- - - e -- be e ↳ - - ↳ e - e cc * - - - - EE * - - E - - E ii e e ww - ss - By I I e e.. ↳ ↳ 11 ·st·sotroree ee : · ·.. # # EE S S So So e ↓ e ↓ * * N N -- - - - 8 8 # -- # - - - - - * sweweSo.-or * - ee araer.e eneene - BaBa So · · I I as * a * SS ·· e e ↳ · ee ↳ · ↑ e ↑ e - - S S EE I I ss * * Overview of X- ray tube X-ray tubes are evacuated glass tubes containing a negative filament (cathode) and a positive target made of tungsten W (anode). A current is passed through the tungsten filament and heated to a high temperature, which then emits electrons. As it is heated up the increased energy enables electrons to be released from the filament through thermionic emission. A high voltage (kV) from the generator is applied between the filament (cathode) and the target (anode). Flow of electrons from the filament to the target constitutes the tube current (mA). The electrons are attracted towards the positively charged anode and hit the tungsten target with a maximum energy determined by the tube potential (voltage). As the electrons bombard the target they interact via Bremsstrahlung and characteristic interactions which result in the conversion of energy into heat (99%) and x-ray photons (1%). The target region producing x-rays is the x-ray tube focal spot. Small focal spots/Fine focus or Large focal spots/Broad focus. X-ray tube: Effective Focal Spot To reduce the focal spot size the anode is at an angle producing a smaller ‘effective focal spot’ (Typical focal spot sizes: 0.6mm - 3mm). The heel effect produces an effective focal spot by having a smaller target (anode) Actual focal spot: where the electrons strike the anode, thus x-rays are produced and where heat is produced. Effective focal spot: the area of the focal spot that is projected out of a tube, the effective x-ray beam that is heading out towards the patient. 2  For example: The anode angle of A is the same as C, but there is a smaller effective focal spot on A than C. This happens due to a shorter filament being used (narrower electron beam) which means that the actual focal spot is decreased leading to sharper images. In B, the anode angle is smaller than C. Having this smaller anode angle produces a narrow effective focal spot which produces sharper images. In C, the anode angle is bigger than B which produces a larger anode angle. This results in a larger effective focal spot. Rotating anode: Consists of a disc with a thin bevelled rim of tungsten around the circumference that can rotate. Because it rotates it overcomes heating by having different areas exposed to the electron stream over time. It consists of Molybdenum disk with thin tungsten target around the circumference. The Heel effect The “Heel effect” relates to how the intensity of X-rays can vary across the X-ray beam because the X-rays from the thicker part of the anode get absorbed more. The conversion of the electron beam into x-rays doesn’t occur at the surface of the target, but it happens deep within the target material. 3  Because x-rays are produced deep within in the target material they must travel back out of the material, before they can get to the target field. The x-rays get blocked or absorbed more when they travel through the material close to the source of electrons (cathode). This means fewer X-rays reach the area in a straight line from the electron source. On the other hand, when X-rays travel closer to the anode, they have to pass through more material, which also absorbs some of the x-rays. As a result, the x-rays are more intense on the cathode side compared to the anode side (the field intensity towards the cathode is more than that towards the anode). Another explanation of the “heel effect”: The "anode," which is where the X-rays are produced. The anode is usually angled, and it has a thicker end and a thinner end. When X-rays are generated, they spread out in a cone-shaped beam from the anode. The "heel" of the anode is the thicker part, and the "toe" is the thinner part. X-rays that come from the heel of the anode (the thicker part) have to travel through more material, which can absorb some of them. So, they become weaker as they move away from the anode's heel. On the other hand, X-rays from the toe of the anode (the thinner part) have to pass through less material, so they are stronger. As a result, in X-ray images, the side of the image closer to the heel of the anode appears darker (weaker X- rays), and the side closer to the toe appears brighter (stronger X-rays). This is the "heel effect" in action. In summary, the "heel effect" is a phenomenon where the X-ray intensity is uneven across the X-ray beam because X-rays from the thicker part of the anode get absorbed more, making that side of the image appear darker, while X-rays from the thinner part are stronger, making that side appear brighter. It's an important consideration when taking X-ray images to ensure that the entire area of interest receives an appropriate amount of X-ray exposure. 4