X-ray Tube Lecture 8 PDF

The X-ray Tube Course: FRD1011 Radiologic Physics & Radiation Slide 1 Protection Lecture 8: The X-ray Tube fchs.ac.ae Learning Outcomes At the conclusion of this lecture, associated tutorial and practical session (if relevant), you will be able to: 1. Describe the general d...

The X-ray Tube Course: FRD1011 Radiologic Physics & Radiation Slide 1 Protection Lecture 8: The X-ray Tube fchs.ac.ae Learning Outcomes At the conclusion of this lecture, associated tutorial and practical session (if relevant), you will be able to: 1. Describe the general design of an x-ray tube 2. List the external components that house and protect the x-ray tube 3. Explain the purpose of the glass or metal enclosure 4. Discuss the cathode and filament currents 5. Describe the parts of the anode and the induction motor 6. Define the line-focus principle and the heel effect 7. Identify the three causes of x-ray tube failure 8. Explain and interpret x-ray tube rating charts, relating current, exposure time and voltage Slide 2 fchs.ac.ae Prescribed Text Bushong, S.C., Radiologic Science for Technologists, 10th edition, Mosby/Elsevier; St Louis, 2012, pages 104-122. Notes: 1. Each lecture in this course will relate very closely to a specific set of pages in the above text. It is strongly recommended that students read the pages indicated prior to coming to the lecture. 2. The students outcomes listed at the commencement of each lecture are essentially those found in the prescribed text for the relevant chapter. Slide 3 fchs.ac.ae The X-ray Tube Exposed The main parts of a medical diagnostic x-ray tube are shown below Bushong, Figure 6-1, page 105 Slide 4 fchs.ac.ae External Components x-ray sets are heavy; usually supported at the ceiling; this provides maximum flexibility some systems are floor- or wall-mounted Of importance is the source to image (receptor) distance (SID) The tube position should be locked prior to exposure Slide 5 fchs.ac.ae Possible Configurations A. Ceiling-mounted B. Floor-mounted with vertical wall- mounted image receptor C. C-arm with separate x-ray system and patient couch Bushong, Figure 6-2, page 106 Slide 6 fchs.ac.ae Protective Housing x-rays from an x-ray tube target material, the anode, are emitted isotropically – of equal intensity in all directions Most are absorbed in the target material itself, or the tube housing The fraction used – the useful beam – exits the tube through a window, in the desired direction Bushong, Figure 6-3, page 107 Slide 7 fchs.ac.ae Protective Housing The tube housing allows some leakage of radiation in undesired directions, not part of the final useful beam Patient and radiographer (radiologic technologist in the US) must be protected from this leakage radiation The housing also protects the tube from mechanical shock; The housing may also contain oil that acts as a thermal cushion (protects the tube from thermal shock) Protective housing should reduce the intensity of leakage radiation to less than 1 mGy/hr at 1 m (mGy = milli-gray) The housing may be metal (lead) or glass Slide 8 fchs.ac.ae Internal Components – Cathode The cathode is on the electric negative side of the tube It contains the filament that produces electrons by thermionic emission (usually a tungsten-alloy wire) The emitted electrons are attracted towards the positive anode and are ‘shaped’ by electrically negative ‘cups’ Bushong, Figure 6-5, page 108 Slide 9 fchs.ac.ae Internal Components – Cathode The effect of the cups is shown opposite The aim is to have the electrons strike a small area on the anode target material This area is typically of the order of about 5 mm2 or less; it is somewhat dependent on kVP and mA Bushong, Figure 6-6, page 109 Slide 10 fchs.ac.ae Internal Components – Cathode The cathode filament is usually warmed by a low current in preparation for the exposure pulse The exposure current value and time are set by the mA and s, or mAs, settings Bushong, Figure 6-4, page 107 Slide 11 fchs.ac.ae Internal Components – Anode The anode is the electric positive side of the x-ray tube The anode may be stationary (as in image A) or rotating (B) The energetic electrons that strike the anode target produce far more heat energy (about 99%) than x-ray energy (about 1%), so the anode is designed to conduct heat away from the tube Anode base material is usually Cu, Mo or C (as graphite) Bushong Figure 6-10, page 110 Slide 12 fchs.ac.ae Internal Components – Anode Anode target area is W (tungsten) or Mo (molybdenum) The target material is the surface that the energetic electrons strike, and must conduct heat Bushong, Figures 6-11, 6-12, pages 111 & 112 Slide 13 fchs.ac.ae Internal Components – Anode For a stationary tube, the target will be about 1 mm  4 mm Rotating anodes are more common than stationary anodes: Target material is a strip about 7 mm wide around the anode Target area of 1500 - 2000 mm2 for a 15 cm diameter anode Tubes can rotate at speeds of 10,000 rpm (revs per minute) Bushong, Table 6-1, page 111 Slide 14 fchs.ac.ae Anode Focal Spot The focal spot is the target area, and the source of x-rays The aim is to have this as small as possible – a point source This is limited by heat dissipation capabilities The effective focal spot is the x-ray emission area as viewed from the patient/image receptor, along the centre of the beam The target angle, typically between 5 and 20, is shown opposite Bushong, Figure 6-17, page 114 Slide 15 fchs.ac.ae  Focal Spot Portion of anode struck by electron stream Focal spot size affects & limits resolution + fchs.ac.ae  Focal Spots Most tubes have 2 filaments & thus 2 focal spots only one used at a time small focus improves resolution large focus improves heat ratings Electron beam strikes larger portion of target fchs.ac.ae  Focal Spot Size & Resolution The larger the focal spot the more it will blur a tiny place on the patient. fchs.ac.ae Larger Focal Spot = Better Heat Ratings Electron beam applies huge amount of heat to target fchs.ac.ae Larger Focal Spot = Better Heat Ratings The larger the area the electron beam hits, the more intense the beam can be without melting the target fchs.ac.ae Focal Spot “Heel Effect” The effective focal spot design means that the x-ray beam is more intense on the cathode side (red on the diagram) than the anode side (yellow) This heel effect is diminished as the anode angle is increased The focal spot ‘intensity’ is also not ‘uniform’ and this can be used to advantage in some examinations Bushong, Figure 6-20, page 115 Slide 21 fchs.ac.ae X-ray Tube Failure x-ray tubes last many months, even years, if used carefully. They fail from: Thermal shock – usually from incorrect starting procedures Prolonged ON times – excessive thermal load Filament damage – evaporated tungsten may cause internal electric discharge Lack of use – the vacuum fails Tube life is usually expressed in the number of exposures Tubes are often guaranteed, if used according to specifications to 50,000 or more exposures Slide 22 fchs.ac.ae Rating Charts These specify the maximum extreme conditions under which tubes should be operated You will use these in some tutorial problems Bushong, Figure 6-26, page 120 Slide 23 fchs.ac.ae Rating Charts – Example Calculation OK or not? 95 kVp, 150 mA,1 s 3400 rpm, 0.6 mm focal spot? Ans: 75 kVp, 700 mA, 0.3 s 10,000 rpm, 01.0 mm focal spot? Ans: Bushong, Figure 6-26, page 120 Slide 24 fchs.ac.ae Q. With reference to Figure 6-26, which of the following conditions of exposure are safe, and which are unsafe? a. 95 kVp, 150 mA, 1 s; 3400 rpm; 0.6-mm focal spot b. 85 kVp, 400 mA, 0.5 s; 3400 rpm; 1-mm focal spot c. 125 kVp, 500 mA, 0.1 s; 10,000 rpm; 1-mm focal spot. d. 75 kVp, 700 mA, 0.3 s; 10,000 rpm; 1-mm focal spot. e. 88 kVp, 400 mA, 0.1 s; 10,000 rpm;0.6-mm focal spot Answer: Slide 25 fchs.ac.ae Anode Cooling Chart The anode has a limited cooling capacity The chart shown expresses this in “heat units”, HUs 1 HU = 1 kVP  1 mA  1 s Heat units are, effectively, energy in joules, J Bushong, Figure 6-27, page 121 Slide 26 fchs.ac.ae Anode Cooling Chart – Example Calculation How much heat energy (in joules, J) is produced during a single high- frequency mammographic exposure of 25 kVp and 200 mAs? Use: HU = kVP  mAs Ans: 25 kVp × 200 mAs = 5000 HU 5000 HU × 1.4 J/HU = 7000 J = 7 kJ Bushong, Figure 6-27, page 121 Slide 27 fchs.ac.ae Summary Check that you can satisfy the learning outcomes for this lecture Go over calculations/exercises undertaken during the lecture Make sure you can define/describe the following terms: tube filament cathode and anode target focusing cups rotating anode SID and heel effect Check that you can list the common causes for tube failure. Slide 28 fchs.ac.ae Types of x ray tubes Slide 29 fchs.ac.ae  The X-Ray Tube Development This is a modern rotating anode x-ray tube. It is encased completely in a metal protective housing. The housing provide electrical and radiation safety fchs.ac.ae Slide 31 fchs.ac.ae