Chapter 5 Review Questions

Question 1

What conditions must exist for x-rays to be produced?

Answer 1

a source of electrons, an appropriate target material, a high voltage, and a vacuum

Question 2

What are the basic parts of a cathode assembly?

Answer 2

filament or filaments, focusing cup, and associated wiring

Question 3

What is the purpose of the focusing cup?

Answer 3

to narrow the thermionic cloud as it is driven toward the anode

Question 4

Explain the space charge effect.

Answer 4

as more and more electrons build up in the area of the filament, their negative charges begin to oppose the emission of additional electrons (limits x-ray tubes to maximum mA ranges of 1,000-1,200)

Question 5

Why is tungsten the best metal for the x-ray source?

Answer 5

high atomic number, high melting point, and heat-conducting ability

Question 6

Explain the line-focus principle.

Answer 6

used to reduce the effective area of the focal spot; is controlled by the size of the actual focal-spot and the anode target angle; as the actual focal-spot size increases, the effective focal-spot size also increases

Question 7

How does the anode heel effect affect radiation intensity?

Answer 7

radiation intensity is greater on the cathode side; photons that are emitted toward the anode end are more likely to be absorbed by the target material itself than those that are emitted toward the cathode end; cathode end should be positioned toward the thicker part of the body

Question 8

What is the advantage of a high-speed rotor?

Answer 8

assists in dissipating heat

Question 9

Why is it necessary for a vacuum to exist within the envelope?

Answer 9

the removal of air permits electrons to flow from cathode to anode without encountering the gas atoms of air and greatly increases the efficiency of the tube’s operation

Question 10

Define leakage radiation.

Answer 10

any photons that escape from the housing except at the port

Question 11

How is off-focus radiation produced?

Answer 11

occurs when the high-voltage electrons striking the focal spot produce scattered electrons or photons; in some cases, these scattered electrons or photons have sufficient energy remaining that when they strike another object in the tube, they produce photons; these photons are produced away from the focal spot and are, therefore, considered off-focus

Question 12

What is the function of rating charts and cooling curves?

Answer 12

to help radiographers avoid thermal damage to x-ray tubes

Question 13

Define a heat unit.

Answer 13

specialized radiography equipment used to conduct cranial studies; is calculated as kVp x mA x time x rectification constant (1 radiographic heat unit = 0.78 joule)

Question 14

Name five things a radiographer can do to extend the life of an x-ray tube.

Answer 14

1. warm up the anode following the manufacturer’s recommendation
2. do not hold the rotor switch unnecessarily; double-press switches should be completely depressed in one motion; dual switches should have the exposure switch depressed first, followed by the rotor switch
3. use lower mA stations when possible
4. use a lower speed rotor when possible
5. do not make repeated exposures near tube loading limits
6. do not rotate the tube housing rapidly from one position to another
7. do not use a tube when you can hear loud rotor bearings (unless checked)