References and Resources
Mind Map Summary
Describe and explain the concept
of the two electrical quantities, KV and MA, that are associated with x-ray production.
NOTE: In a later module you will learn how these quantities are produced and controlled.
Sketch a cross-section of a basic x-ray tube and label the components.
Describe the events that occur within the tube that lead to x-ray production.
Describe a focal spot.
Name the two materials used as x-ray tube anodes and state the reasons for using each.
State the atomic number (Z) of the two anode materials and describe the significance of Z with respect to x-ray production.
Describe the filament and focusing cup (i.e., the cathode assembly) of an x-ray tube, including the purpose of two filaments in some x-ray tubes and the purpose of the focusing cup.
Name and briefly describe the process by which electrons are liberated from a heated filament at the cathode of an x-ray tube.
List the factors that determine the kinetic energy attained by electrons as they strike the x-ray tube anode.
Discuss at least two functions of the x-ray tube housing.
Describe how the production of characteristic radiation and bremsstrahlung varies with the voltage across the tube (KV) and the atomic number (Z) of the above.
Sketch graphs comparing the energy spectra for photons from:
. an x-ray tube with a tungsten anode operated at 40 kVp
. the same tube operated at 90 kVp
. a radioactive nuclide emitting 90 keV gamma-ray photons
Name the factor(s) that determine(s) the maximum energy of x-ray photons emitted from an anode.
Discuss the sequence of events leading to the production of characteristic radiation.
Explain why K x-rays emitted by an atom have higher energies than L or M x-rays from the same atom.
Explain why a high Z element produces some characteristic x-rays with energy greater than that of x-rays from a lower Z element, even though both elements are bombarded by 100 keV electrons.
Define the expression "efficiency of an x-ray tube," identify the factors that affect efficiency , and estimate its value for a diagnostic x-ray tube.
State how the efficiency is related to the electron accelerating voltage (KV).
State the relationship between x-ray production efficiency and the atomic number of the anode material.
Sketch typical and describe the variation in output (i.e., exposure rate) of an x-ray tube as a function of: 1. tube voltage (KV) 2. tube current (MA)
Estimate the skin entrance exposure to a 23 cm thick patient for a PA chest radiograph performed at 72 inches using a technique of 120 kVp and 15 mAs.
Estimate the increase in x-ray tube output exposure when the tube potential is increased from 60 to 75 kVp.
References and Resources
1. Chapter 7. The Physical Principles of Medical Imaging, 2ndEd. Perry Sprawls. Medical Physics Publishing.
Resources on the Web