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Chap 6. Tube
Equipment. Chap 6. X-ray tube
Question | Answer |
---|---|
Modern x-ray tube? First tube? | coolidge. First was crookes |
filaments are usually made of _________. Why? | thoriated tungsten. High melting point |
x-ray tube current is adjusted by controlling the _______ _______. | filament current |
Space charge is | electrons forming a thermionic cloud around filament, they build up and their neg charge repels new electrons to be emitted. |
when is the small filament used? The larger? | 300 mA or less for small, 400 mA or more for larger |
Small focal spots range is from __ to ___. Large is from ____ to ____. | .1-1mm. .3-2mm. |
The anode serves 3 functions: | receives e-/target, conducts electricity, thermal dissipater. |
most rotating anodes revolve at 3400 rpm. High capacity rotates at | 10,000 rpms. |
Tungsten symbol, atomic number, K shell energy (keV), melting temp. | W, 74, 69, 3400 degrees C. |
What is the stem of the anode made of? | molybdenum |
What turns the anode? | electromagnetic induction motor, consists of anode and stator. |
Stator is _________, rotor is _______. | outside, inside. ***** (equipment book has this backwards.. check prev. tests and prev. books stator is OUT rotor is IN.) ***** |
best degree target angle? | 12 (normal is between 7-20) |
What is the focal spot? What is the effective focal spot? | actual x-ray source.. the area of the TARGET from which x-rays are emitted. Effective is area projected onto the patient and IR. |
As the size of the focal spot decreases, the heating of the target is... | concentrated onto a smaller spot... not good. |
What is the line focus principle? | By angling the target, one makes the effective focal of the target smaller than the actual focal spot. |
When the target angle is made smaller, the effective focal spot size..... | is also made smaller. |
What is the heel effect? | the intensity of x-rays that are emitted through "heel" of the target is reduced because they have a longer path through the target and therefore increased absorption. (Steeper the angle the larger the heel effect.. cathode more intense than anode side) |
The smaller the anode angle, the larger the ____ | heel effect |
In chest rad, the cathode should always be _____. In abdominal, the cathode should be _____. | inferior, superior. (cathode always over thicker portion) |
the effective focal spot is smaller on the ______ side of the x-ray field. | anode |
The heel effect results in smaller _________ focal spot and less radiation intensity on the ____ side of the beam. | effective, anode |
Transfer of energy from one area of an object to another is termed | conduction |
transfer of heat by movement of a heated substance from one place to another is termed | convection |
3 types of x-ray tube rating charts: | radiographic rating chart, anode cooling chart, and housing cooling chart. |
Which chart is the most important because it conveys which techniques are safe and which are unsafe for tube operation. | radiographic rating chart |
single phase heat unit formula? | HU= kVp x mA x s = 0.7J |
exam of lumbar spine with single phase imaging system requires 98kVp, 120 mAs. How many heat units are generated by this exposure? | HU= 98kVp x 120mAs = 11,760 HU |
Three phase/high frequency heat unit formula? | HU= 1.4 x kVp x mA x s = 1J |
One heat unit is = to ____ Joule | one |
6 skull films are exposed with a 3phase generator operated at 82 kVp, 120 mAs. What is the total heat generated? | HU = 1.4 x 82 kVp x 120 mAs =13,776 HU. =6 x 13,776 HU = 82,565 HU |
The thermal capacity of an anode and its heat dissipation characteristics are contained in a rating chart called | an anode cooling chart |
Protective housing of a tube has 3 purposes: | reduces leakage radiation, provides mechanical support, conducts heat away from tube target |
How many heat units are generated by an exposure of 70 kVp, 300 mA, and 0.1 second on a 1f rectified unit? | 2100HU |