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Physics
Ch 6
| Question | Answer |
|---|---|
| What percentage of kinetic energy of electrons is converted to heat in the x-ray tube? | Over 99%. |
| What percentage of kinetic energy is converted to x-rays? | Less than 1%. |
| What determines the efficiency of x-ray production? | Tube voltage (kVp). |
| How does efficiency of x-ray production change with kVp? | It increases with higher kVp. |
| What is the approximate efficiency of x-ray production at 60 kVp? | About 0.5%. |
| What is the efficiency of x-ray production at 100 kVp? | About 1%. |
| What is the kinetic energy of projectile electrons determined by? | The kVp applied across the tube. |
| What causes most of the heat in the anode? | Excitation of target atoms by incident electrons. |
| What happens to the anode when it overheats? | It can pit, crack, or melt. |
| What is the most common cause of x-ray tube failure? | Excessive heat. |
| What are the three primary heat damage mechanisms? | Excessive exposure, prolonged exposure, and improper warm-up. |
| What is an anode rating chart used for? | To determine safe exposure factors that prevent tube damage. |
| What factors affect anode heat loading capacity? | Anode rotation speed, target material, focal spot size, and angle. |
| What is a tube rating chart? | A graph that shows safe combinations of kVp, mA, and exposure time. |
| What happens if an exposure exceeds the tube rating chart? | The anode or filament may be damaged. |
| What does a higher anode rotation speed allow? | Greater heat dissipation and higher tube loading capacity. |
| What happens when a small focal spot is used? | Better spatial resolution but lower heat capacity. |
| What happens when a large focal spot is used? | Higher heat capacity but less image sharpness. |
| What is the purpose of a cooling chart? | To determine required cooling time before additional exposures. |
| What are the two types of cooling charts? | Anode cooling chart and housing cooling chart. |
| What is the unit of heat storage capacity? | Heat units (HU). |
| What is the formula for single-phase heat units? | HU = kVp × mA × s. |
| What is the formula for three-phase 6-pulse heat units? | HU = 1.35 × kVp × mA × s. |
| What is the formula for three-phase 12-pulse or high-frequency units? | HU = 1.41 × kVp × mA × s. |
| What does the correction factor in heat unit formulas represent? | The efficiency of the generator. |
| Why is a higher correction factor used for three-phase systems? | Because of the higher and more constant voltage output. |
| What is the purpose of tube housing oil? | To insulate and dissipate heat from the x-ray tube. |
| What additional device helps remove heat from the anode? | A cooling fan. |
| What causes bearing failure in rotating anodes? | Excessive heat and wear over time. |
| What are common signs of bearing failure? | Unusual noise and decreased rotation speed. |
| What happens if the rotor fails to spin properly? | Localized overheating and anode damage. |
| What is a common precaution to protect the anode? | Warm-up procedure before high exposures. |
| Why is tube warm-up necessary? | To prevent thermal shock and cracking of the anode. |
| What is thermal shock? | Cracking of the anode surface due to rapid temperature change. |
| What happens to the focal track after repeated use? | It becomes roughened or pitted. |
| What effect does a roughened anode surface have? | Reduces x-ray output and image sharpness. |
| What is a common result of tungsten vaporization? | Deposits on the inside of the glass envelope. |
| What can tungsten deposition cause? | Arcing and increased inherent filtration. |
| What happens when the glass envelope becomes gassy? | Reduced vacuum efficiency and tube failure. |
| What is the advantage of metal envelopes over glass ones? | They prevent tungsten vaporization from affecting tube performance. |
| What is off-focus radiation caused by? | Electrons striking areas of the anode outside the focal track. |
| How can off-focus radiation be minimized? | Using tube shielding and proper collimation. |
| What happens to off-focus radiation as the tube ages? | It increases due to tungsten deposition. |
| What is the tube housing made of? | Lead-lined steel. |
| What is the purpose of the lead lining in the tube housing? | To absorb leakage radiation. |
| What is the maximum permissible leakage radiation? | Less than 1 mGy/hour at 1 meter. |
| What is the function of the glass envelope? | To maintain a vacuum and contain the electron stream. |
| What is the function of the anode stem? | To connect the anode to the rotor and provide support. |
| Why is the anode stem made of molybdenum? | It’s strong and poor at conducting heat, limiting heat transfer to bearings. |
| What are the main causes of rotor damage? | Bearing wear and overheating. |
| What can be done to extend x-ray tube life? | Avoid high exposures, follow warm-up procedures, and use correct mA/kVp settings. |
| How should a technologist reduce tube stress during operation? | Use lower mA and kVp settings whenever possible. |
| What is the recommended warm-up procedure? | Two or three exposures at low technique to gradually heat the anode. |
| What determines the heat storage capacity of the anode? | The size, composition, and rotation speed of the anode disc. |
| What is the function of the rotor’s induction motor? | To spin the anode without physical contact. |
| What is the stator? | The series of electromagnets that induce the rotor to spin. |
| What happens if stator failure occurs? | The anode won’t rotate, causing localized overheating. |
| What are typical signs of stator failure? | Vibration, noise, or reduced rotation speed. |
| What happens to the focal track with repeated thermal stress? | It becomes roughened, leading to reduced image quality. |
| What is the benefit of using a large anode disc? | Increased surface area for heat dissipation. |
| What happens when kVp is increased while mA remains constant? | X-ray quantity and quality both increase. |
| What happens when mA is increased while kVp remains constant? | X-ray quantity increases but quality remains the same. |
| What happens when exposure time is doubled? | The total quantity (mAs) of x-rays doubles. |
| What are typical causes of filament failure? | Evaporation of tungsten and thinning of the wire. |
| What causes open filament failure? | Breakage of the filament wire due to evaporation and stress. |
| What causes short filament failure? | Arcing or tungsten deposition that bridges the filament to the focusing cup. |
| What determines the actual focal spot size? | The size of the filament and the anode angle. |
| What is blooming in x-ray tubes? | An increase in focal spot size due to electrostatic repulsion at high mA. |
| How can blooming be minimized? | By increasing the anode voltage or reducing space charge. |
| What is the most common method of x-ray tube cooling? | Conduction to the oil bath and convection to the housing. |
| What role does radiation play in tube cooling? | Radiation transfers a small portion of heat from the anode to the glass envelope. |
| What is the purpose of the rotor delay or "boost" phase? | To allow the anode to reach full speed before exposure. |
| What happens if the exposure button is held down too long before exposure? | Excessive filament heating and reduced tube life. |
| What is the main cause of anode pitting? | Excessive single exposures at high technique. |
| What is the main purpose of tube rating and cooling charts in clinical use? | To prevent tube damage and prolong x-ray tube life through safe exposure planning. |
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