physics: xray tube and production

what are the external components?

ceiling support system, protective housing, glass envelope

what is ceiling support?

Consists of 2 perpendicular sets of ceiling-mounted rails

what is the legal limit for leakage radiation in units?

1 mgy/hr

why is the glass enclosure vacuum in the tube?

to speed up the electrons 

which type of enclosure is considered superior to and why?

pyrex and metal more efficient x-ray production and longer tube life

what are the two internal components?

cathode and anode

what makes up the cathode?

filament, focusing cup, filament current

why is there two different sizes of filament?

to create two different focal spot sizes

what is the filament made of?

what is the principle for the focusing cup?

electrostatic repulsion and convergence

what is a focusing cup?

Is negatively charged so that it electrostatically confines the electron beam to a small area of
the anode

what is the filament current?

When emitted from the filament, electrons are in the vicinity of the filament before they are accelerated to the anode

what is protective housing?

x-ray tubes contains oil that serves as an insulator against electric shock and as a thermal
cushion to dissipate heat.

what are the anode functions?

Electrical conductor
– Provides mechanical support
– Must be a good thermal dissipater

what is an anode?

Positive side of the x- ray tube; it conducts electricity and radiates heat and contains the
target

what is stationary anode?

Used in dental machines or portables where high tube current and power are not needed

what is Rotating anode?

Most general purpose x-ray machines use a rotating anode because it must be capable of producing high-intensity x-ray beams in a short time

what is space charge?

electrons  repel one another and tend to form a cloud around the filament

what is the space charge effect?

Makes it difficult to subsequent electrons to be emitted by the filament because of electrostatic
repulsion

what is saturation current?

Where the tube current reaches a maximum level at a particular
filament current (all available electrons have been used)

what is thermionic emisson?

Emits electrons when heated

what is the target made of?

tungsten

why would you choose tungsten as a node material?

Atomic number, Thermal conductivity, High melting point

what is a target?

Area of anode struck by the electrons from the cathode

what is induction motor?

Anode is rotated inside an enclosure with no mechanical connection to the outside

what is another name for induction motor?

electromagnetic induction motor

what makes up the induction motor?

stator and rotor

what is the line focus principle?

The focal spot is the area of the target from which x-rays are emitted

what is the relationship between line focus principle?

effective focal spot size much less than the actual focal spot size

what is an effective focal spot?

what you see from a patient's perspective

what is an actual focal spot?

the physical area on the anode that is struck by electrons

what two things does line focus principle help?

improves spatial resolution and heat capacity

what is the anode heel effect?

greater radiation intensity is greater on cathode side 

why does anode heel effect occur?

The intensity of the x-rays that are emitted toward the anode side must traverse a greater
thickness of target material than the x-rays emitted toward the cathode

if you had a thicker part, which side of the tubes should it go under

under the cathode

how does an angle of the anode effect the heel effect?

The smaller the anode angle, the larger is the heel effect

what is off focus radiation?

when electron land somewhere other than the focal spot

why would a xray tube fail?

Excessive heat and skipping tube warm ups

what is radiation?

Transfer of heat by the emission of infrared radiation

what is an example of radiation?

anode to filament

what is conduction?

Transfer of energy from one area of an object to another

what is an example of conduction?

heat going from anode to oil

what is the convention?

Transfer of heat by the movement of a heated substance from one place to another

what is example of convention?

fan

what is the heat unit formula for single phase?

HU = kVp x mA x s

Radiographic examination of the lateral lumbar spine with a single-phase imaging system requires 98 kVp, 120 mAs. How many heat units are generated by this exposure?

11760 HU

what is the heat unit formula for three phase/high frequency phase?

HU = 1.4 x kVp x mA x s

6 sequential skull films are exposed with three-phase equipment operated at 82 kVp, 120 mAs. What is the total heat generated?

82656 hu

what is the power rating formula for a three phase?

(ma x kvp) /1000

what is the power rating formula for a single phase?

{(ma x kvp) /1000} x .7