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RT 202 Chapter 7
Terms in this set (93)
Chapter 6 discusses the internal components of the x-ray tube:
the cathode and the anode within the evacuated glass or metal enclosure.
This chapter explains the interactions of the projectile electrons that are accelerated from the cathode to the x-ray tube target. These interactions produce two types of x-rays:
characteristic and bremsstrahlung
The x-ray imaging system description in chapter 6 emphasizes that it's primary function is to accelerate electrons from the:
cathode to anode in the x-ray tube.
The three principal parts of an x-ray imaging system:
the operating console, the high voltage generator, and the x-ray tube, are designed to provide a large number of electrons with high kinetic energy focused toward a small spot on the anode.
Kinetic energy is the:
energy of motion.
Stationary objects have no kinetic energy, objects in motion have:
kinetic energy proportional to their mass and to the square of their velocity.
Kinetic energy equation:
m=mass in kg
v=velocity in m per second
KE=kinetic energy in joules
Figure 7-1 a 1000 kilogram automobile has four times the kinetic energy of a 250 kilogram motorcycle traveling at the same speed. If the motorcycle were to double its velocity:
it would have the same kinetic energy as the automobile.
In determining the magnitude of kinetic energy of a projectile:
velocity is more important than mass.
In an x-ray tube the projectile is the:
All electrons have the same mass, therefore electron kinetic energy is increased by raising the:
As electron kinetic energy is increased, both the:
intensity (quantity) and the energy (quality) of the x-ray beam are increased.
At 100 milliamps how many electrons travel from the cathode to the anode of the x-ray tube every second:
In an x-ray imaging system operating at 70 keV, each electron arrives at the target with a maximum kinetic energy of:
70 keV, and because there are 1.6x10^16 J/keV
How much energy is in 70 keV:
(70)(1.6x10^-16 J/kg) = 1.12x10^-14J
When this energy is inserted into the expressions for kinetic energy and calculations are performed to determine the velocity of the electrons, the result is as follows:
v^2= (2)(1.2x10^-14J)/(9.1x10^31 kg)
= .25x10^17 m^2/s^2
v= 1.6x10^8 m/s
Math question: at what fraction of the velocity of light do 70 keV electrons travel?
v/c = (1.6x10^8 m/s)/(3.0x10^8 m/s)
The distance between the filament and the x-ray tube target is only approximately:
1 cm. Therefore the velocity of electrons goes from 0 to half the speed of liht in approximately one cm.
electrons traveling from cathode to anode constitute the x-ray tube current and are sometimes called projectile electrons.
When these projectile electrons hit the heavy metal atoms of the x-ray tube target, they transfer their:
kinetic energy to the target atoms.
The projectile electrons interact with the:
orbital electrons or the nuclear field of target atoms.
These interactions result in the conversion of electron kinetic energy into:
Thermal energy (heat) and electromagnetic energy in the form of infrared radiation (also heat) and x-rays.
Figure 7-1 kinetic energy is:
proportional to the product of mass and velocity squared.
Figure 7-2 most of the kinetic energy of projectile electrons is converted to:
heat by interactions with outer shell electrons of target atoms. These interactions are primarily excitations rather than ionizations.
Most of the kinetic energy of projectile electrons is converted into:
The projectile electrons interact with the outer shell electrons of the target atoms but do not:
transfer sufficient energy to these outer shell electrons to ionize them. Rather, the outer shell electrons are simply raised to an excited, or higher energy level.
The outer shell electrons immediately drop back to their:
normal energy level with the emission of infrared radiation.
The constant excitation and return of outer shell electrons are responsible for the most of the:
heat generated in the x-ray tube
Only approximately 1% of:
projectile electron kinetic energy is used for the production of x-radiation
The production of heat in the anode increases directly with increasing:
x-ray tube current.
Doubling the x-ray tube current doubles the:
Heat production also increases directly with increasing:
kVp at least in the diagnostic range.
Although the relationship between varying kVp and varying heat production is approximate, it is sufficiently exact to allow the computation of:
heat units for use with anode cooling charts.
The efficiency of x-ray production is independent of the:
tube current regardless of what mA is selected, the efficiency of x-ray production remains constant.
The efficiency of x-ray production increases with increasing:
At 60 kVp only:
.5% of the electron kinetic energy is converted to x-rays
At 100k kVp, approximately:
1% is converted to x-rays
at 20 mV
70% is converted
the projectile electron interacts with an inner shell of the target atom rather than with an outer shell electron, characteristic x-rays can be produced. Characteristic x-rays result when the interaction is sufficiently violent to ionize the target atom through total removal of an inner shell electron.
Figure 7-3 illustrates how characteristic x-rays are produced. When the projectile electron ionizes a target atom by removing a:
K shell electron, a temporary electron void is produced in the K shell. This is a highly unnatural state for the target atom, and it is corrected when an outer shell electron falls into the void in the K shell.
The transition of an orbital electron from an outer shell to an inner shell is accompanied by the:
emission of an x-ray. The x-ray has an energy equal to the difference in the binding energies of the orbital electrons involved.
Question: a K shell electron is removed from a tungsten atom and is replaced by an L shell electron. What is the energy of the characteristic x-rays that is emitted:
K shell electrons have binding energies of 69 keV, and L shell electrons are bound by 12 keV. Therefore, the characteristic x-rays emitted has energy of 69-12=57 keV
Only K characteristic x-rays of tungsten are useful for:
Although many characteristic x-rays can be produced, these can be produced only at specific energies, equal to the differences in:
electron binding energies for the various electron transitions.
Except for K x-rays, all of the characteristic x-rays have:
very low energy
The L x-rays, with approximately:
12 keV of energy, penetrate only a few centimeters in soft tissue. Consequently, they are useless as diagnostic x-rays, as are all the other low energy characteristic x-rays.
The effective energy of characteristic x-rays increases with:
atomic number of the target element.
The production of heat and characteristic x-rays involves interactions between the:
projectile electrons and the electrons of the x-ray tube target atoms.
A third type of interaction in which the projectile electron can lose its kinetic energy is an interaction with the:
nuclear field of a target atom. In this type of interaction, the kinetic energy of the projectile electron is also converted into electromagnetic energy.
A projectile electron that completely avoids the orbital electrons as it passes through a target atom may come sufficiently close to the:
nucleus of the atom to come under the influence of its electric field.
Because the electron is negatively charged and the nucleus is:
positively charged, there is an electrostatic force of attraction between them.
The closer the projectile electron gets to the nucleus, the more it is:
influenced by the electric field of the nucleus.
As the projectile electron passes the nucleus, it is:
slowed down and changes its course, leaving with reduced kinetic energy in a different direction. This loss of kinetic energy reappears as an x-ray.
Figure 7-5 bremsstrahlung x-rays result from the interaction between a:
projectile electron and a target nucleus. The electron is slowed, and its direction is changed. This results in the formation of an x-ray.
Bremsstrahlung is a German word that means:
slowed down radiation. Bremsstrahlung x-rays can be considered radiation that results from the braking of projectile electrons by the nucleus.
Unlike the production of characteristic x-rays, which have very specific energies. Bremsstrahlung X-rays:
can have a wide range of energies,0 to maximum kVp.
In the diagnostic range, most x-rays are:
Bremsstrahlung x-rays can be produced at any projectile electron energy. K characteristic x-rays require an x-ray potential of at least:
At 65 kVp, no useful:
characteristic x-rays are produced. Therefore, the x-ray beam is all breamsstrahlung.
At 100 kVp approximately:
15% of the x-ray beam is characteristic, and the remaining is bremsstrahlung.
The discrete energies of characteristic x-rays are characteristic of the differences between:
electron binding energies in a particular element.
A characteristic x-ray from tungsten, for example, can have one of:
15 different energies and no others.
Characteristic x-rays have precisely fixed (discrete) energies and form a discrete:
The relative intensity of the K x-rays is greater than that of the:
lower energy characteristic x-rays because of the nature of the interaction process.
K x-rays are the only characteristic x-rays of tungsten with sufficient:
energy to be of value in diagnostic radiology.
Although there are five K x-rays, it is customary to represent them as one, as has been done in this figure (7-10) with a single verticle line, at:
If it were possible to measure the energy contained in each bremsstrahlung x-ray emitted from an x-ray tube, one would find that these energies range from the:
peak electron energy all the way down to zero
When an x-ray tube is operated at 90 kVp, bremsstrahlung x-rays with energies up to:
90 keV are emitted
Bremsstrahlung x-rays have a range of energies and form a:
continuous emission spectrum.
The general shape of the bremsstrahlung x-ray spectrum is the same for:
all x-ray imaging systems
The maximum energy (in keV) of a bremsstrahlung x-ray is numerically equal to the:
kVp of operation
The greatest number of x-rays is emitted with energy approximately:
one third of the maximum energy.
The minimum wavelength of x-ray emission correspond to the maximum x-ray
energy, and the maximum x-ray energy is numerically equal to the kVp
The total number of x-rays emitted from an x-ray tube could be determined by adding together the:
number of x-rays emitted at each energy over the entire spectrum, a process called integration
The general shape of an emission spectrum is always the same, but its relative position along the energy axis can change. The farther to the right a spectrum is, the:
higher the effective energy or quality of the x-ray beam
The larger the area under the curve, the higher is the:
x-ray intensity or quantity
If one changes the current from 200 to 400 mA while all other conditions remain constant:
twice as many electrons will flow from the cathode to the anode, and the mAs will be doubled
A change in mA or mAs result in a proportional change in the:
amplitude of the x-ray emission spectrum at all energies.
When kVp is increased, the relative distribution of emitted x-ray energy shifts to the:
right to a higher average x-ray energy.
The maximum energy of x-ray emission always remain:
numerically equal to the kVp
A change in kVp affects both the:
amplitutde and the position of the x-ray emission spectrum
A change in kVp has no effect on the position of the:
discrete x-ray emission spectrum.
A 15% increase in kVp is equivalent to:
doubling the mAs
To double the output intensity by increasing kVp, one would have to raise the kVp by as much as:
Radiographically, only a 15% increase in the kVp is necessary because with increased kVp, the:
penetrability of the x-ray beam is increased.
Adding filtration to the useful x-ray beam reduces x-ray
beam intensity while increasing the average energy.
Added filtration more effectively absorbs:
low-energy x-rays then high-energy x-rays, therefore, the bremsstrahlung x-ray emission spectrum is reduced further on the left then on the right.
The result of added filtration is an increase in the:
average energy of the x-ray beam with the accompanying in x-ray quantity.
Adding filtration is sometimes called:
hardening the x-ray beam because of the relative increase in average energy.
The atomic number of the target affect both the number (quantity) and the:
effective energy (quality) of x-rays
As the atomic number of the target material increases, the:
efficiency of the production of bremsstrahlung radiation increases, and high-energy x-rays increase in number to a greater extent than low-energy x-rays
After an increase in the atomic number of the target material, the characteristic spectrum is:
shifted to the right, representing the higher energy charactertistic radiation.
Increasing target atomic number enhances the efficiency of x-ray:
production and the energy of characteristic and bremsstrahlung x-rays.
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