Production of X-rays PDF

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18 mock exams for the first FRCR examination Join 10k+ newsletter subscribers Please note: Your email address will never be shared with any 3rd parties. It will only be used for Radiology Cafe communications. Emails are sent less than once a month on average. Read our Privacy policy for more details. Production of X-rays Overview 1. A current is passed through the tungsten filament and heats it up. 2. As it is heated up the increased energy enables electrons to be released from the filament through thermionic emission. 3. The electrons are attracted 4. towards the positively charged anode and hit the tungsten target with a maximum energy determined by the tube potential (voltage). 5. 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%). 6. The x-ray photons are released in a beam with a range of energies (x-ray spectrum) out of the window of the tube and form the basis for x-ray image formation. Equipment Diagram of an x- ray tube Cathode Filament Made of thin (0.2 mm) tungsten wire because tungsten: o has a high atomic number (A 184, Z 74) o is a good thermionic emitter (good at emitting electrons) o can be manufactured into a thin wire o has a very high melting temperature (3422°c) The size of the filament relates to the size of the focal spot. Some cathodes have two filaments for broad and fine focusing Focusing cup Made of molybdenum as: o high melting point o poor thermionic emitter so electrons aren’t released to interfere with electron beam from filament Negatively charged to focus the electrons towards the anode and stop spatial spreading Anode Target made of tungsten for same reasons as for filament Rhenium added to tungsten to prevent cracking of anode at high temperatures and usage Set into an anode disk of molybdenum with stem Positively charged to attract electrons Set at angle to direct x-ray photon beam down towards patient. Usual angle is 5º – 15º Definitions  Target, focus, focal point, focal spot: where electrons hit the anode Actual focal spot: physical area of the focal track that is impacted Focal track: portion of the anode the electrons bombard. On a rotating anode this is a circular path Effective focal spot: the area of the focal spot that is projected out of a tube Anode angles and definitions Stationary anode: these are generally limited to dental radiology and radiotherapy systems. Consists of an anode fixed in position with the electron beam constantly streaming onto one small area. Rotating anode: used in most radiography, including mobile sets and fluoroscopy. Consists of a disc with a thin bevelled rim of tungsten around the circumference that rotates at 50 Hz. 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 Molybdenum stem, which is a poor conductor of heat to prevent heat transmission to the metal bearings Silver lubricated bearings between the stem and rotor that have no effect on heat transfer but allow very fast rotation at low resistances Blackened rotor to ease heat transfer Heating of the anode This is the major limitation of x-ray production. Heat (J) = kVe x mAs or Heat (J) = w x kVp x mAs key: kVe = effective kV w = waveform of the voltage through the x-ray tube. The more uniform the waveform the lower the heat production kVp = peak kV mAs = current exposure time product Heat is normally removed from the anode by radiation through the vacuum and into the conducting oil outside the glass envelope. The molybdenum stem conducts very little heat to prevent damage to the metal bearings. Heat capacity A higher heat capacity means the temperature of the material rises only a small amount with a large increase in heat input. Temperature rise = energy applied / heat capacity Tube rating Each machine has a different capacity for dissipating heat before damage is caused. The capacity for each focal spot on a machine is given in tube rating graphs provided by the manufacturer. These display the maximum power (kV and mA) that can be used for a given exposure time before the system overloads. The maximum allowable power decreases with: Lengthening exposure time Decreasing effective focal spot size (heat is spread over a smaller area) Larger target angles for a given effective focal spot size (for a given effective focal spot size the actual focal spot track is smaller with larger anode angles. This means the heat is spread over a smaller area and the rate of heat dissipation is reduced) Decreasing disk diameter (heat spread over smaller circumference and area) Decreasing speed of disk rotation Other factors to take into consideration are: By using a higher mA the maximum kV is reduced and vice versa. A very short examination may require a higher power to produce an adequate image. This must be taken into consideration as the tube may not be able to cope with that amount of heat production over such a short period of time. Anode cooling chart As well as withstanding high temperatures an anode must be able to release the heat quickly too. This ability is represented in the anode cooling chart. It shows how long it takes for the anode to cool down from its maximum level of heat and is used to prevent damage to the anode by giving sufficient time to cool between exposures. Anode heel effect The anode heel effect An x-ray beam gets attenuated on the way out by the target material itself causing a decrease in intensity gradually from the cathode to anode direction as there is more of the target material to travel through. Therefore, the cathode side should be placed over the area of greatest density as this is the side with the most penetrating beam. Decreasing the anode angle gives a smaller effective focal spot size, which is useful in imaging, but a larger anode heel effect. This results in a less uniform and more attenuated beam. ** smaller angle = smaller focal spot size but larger anode heel effect ** Others Window: made of beryllium with aluminium or copper to filter out the soft x-rays. Softer (lower energy) x-ray photons contribute to patient dose but not to the image production as they do not have enough energy to pass through the patient to the detector. To reduce this redundant radiation dose to the patient these x-ray photons are removed. Glass envelope: contains vacuum so that electrons do not collide with anything other than target. Insulating oil: carries heat produced by the anode away via conduction. Filter: Total filtration must be >2.5 mm aluminium equivalent (meaning that the material provides the same amount of filtration as a >2.5 mm thickness of aluminium) for a >110 kV generator Total filtration = inherent filtration + additional filtration (removable filter)