ANSWERS

1. Define radiography.
   The production of diagnostic films by exposure to x-rays.
2. Define radiology.
   Radiation physics, although commonly defined as the interpretation of the radiographic image.
3. Which 2 members of the electromagnetic spectrum possess the highest frequency and the shortest wavelength?
   • X-rays.
   • Gamma rays.
4. What are electromagnetic radiations made up of?
   Discrete energy packets known as photons or quanta.
5. What 4 properties are possessed by all the members of the electromagnetic spectrum?
   • They can pass through a vacuum.
   • They travel at a uniform speed through a vacuum (3 x 10 to the power of 8 m/s).
   • They are able to interact with matter by being absorbed or scattered.
   • A medium is not required for transmission.
6. What 3 special properties do x-rays possess?
   • The ability to penetrate substances opaque to visible light.
   • The ability to produce an image on photographic film.
   • The ability to cause certain substances to fluoresce.
7. What is an atom?
   The smallest particle of an element that retains all the properties of that element. It consists of a positively charged nucleus with negatively charged electrons moving around it in orbit.
8. What is meant by the "atomic number" of an element?
   The unique number of protons possessed by each element.
9. What is a compound?
   A combination of 2 or more elements. One usually possesses positive ions, whilst the other possesses negative ions.
10. X-rays are generated in the tube head of the x-ray machine. Name the 2 types of tube head available.
    • Stationary or fixed anode.
    • Rotating anode.
11. Why is a rotating anode superior to a stationary anode?
    • More efficient means of heat removal.
    • Allows longer exposure times.
12. A rotating anode is mounted upon a molybdenum rod which spins at speeds of up to 10,000 rpm. Why is molybdenum used?
    It is a poor conductor, and therefore prevents the heater from overheating.
13. At what angle is the target area set on a rotating anode?
    6-20 degrees.
14. What is the cathode?
    The negatively charged electrode of the tube head consisting of a coiled tungsten wire filament.
15. Why is tungsten the material of choice for the cathode?
    Its high atomic number means that it has a very high melting point and it also a good emitter of electrons.
16. What is the anode?
    The positively charged electrode of the tube head. This is made of tungsten or rhenium tungsten since its atoms are large and efficient at impeding the incident electrons.
17. How are x-ray photons produced within the tube head?
    Fast moving incident electrons are produced by the cathode (filament) which collide with the atoms of the anode (target). These collisions generate x-ray photons. The faster the incident electrons, the more energy they have to lose, and thus the greater the intensity of the primary beam.
18. What 2 factors increase the speed of the electrons travelling from the filament to the target?
    • A high electrical potential distance applied between the filament and target.
    • A glass envelope containing a vacuum enclosing the cathode, anode and part of the copper stem.
19. What is the glass envelope contained within and why?
    An oil bath, which acts as a heat sink and electrical insulator.
20. What is the term used to describe a small electrical current passed through a filament and the subsequent release of an electron cloud following heat generation?
    Thermionic emission.
21. What is the function of the step-down or filament transformer?
    This decreases the mains current since only a small current is required to heat the filament.
22. What is the function of the step-up or high-tension transformer?
    This increases the voltage of the mains supply.
23. What is the function of the focusing cup?
    This forms the electrons into a narrow beam.
24. What is the focusing cup made of?
    Nickel or molybdenum.
25. Why is the target set in a copper stem in a stationary anode tube?
    Copper is a good heat conductor and is therefore efficient at removing heat from the target.
26. What is meant by the terms "actual focal spot" and "effective focal spot"?
    • Actual focal spot - the area of the target upon which the electrons impinge; this is the area over which heat is produced.
    • Effective focal spot - this should ideally be a point source; the x-ray beam appears to originate from a much smaller area as a result of the angulation of the target.
27. How is focal spot size adjusted?
    Some x-ray machines have 2 different sized filaments at the cathode. The fine focus is produced by the smaller of the two; a narrower electron beam is generated and thus a smaller effective and actual focal spot size. The coarse (or broad) focus is produced by the larger filament, resulting in a larger effective and actual focal spot size.
28. What is the advantage of the fine focus?
    Very fine radiographic definition is produced, making it suitable for areas such as the limbs.
29. What is the advantage of the coarse (or broad) focus?
    Higher exposures may be selected, making it suitable for dense areas of the anatomy such as the chest and abdomen.
30. What is the main disadvantage of the fine focus?
    Exposure factors are limited due to the large amount of heat generated over a very small area.
31. What is the main disadvantage of the coarse (or broad) focus?
    Image definition is poorer than that produced with the fine focus.
32. Define the "penumbra effect".
    Blurring of the margins of an image related to the geometry of the primary beam.