properties of x-rays 1

Question 1

what is the production of x-rays?

Answer 1

-x-rays and electron can be generated by machines
-diagnostic x-ray units create images using a beam of x-rays transmitted through a patient
-therapy x-ray units treat cancer and other diseases using a beam of x-rays absorbed in the patient

Question 2

what is diagnostic radiology?

Answer 2

-a beam of x-rays is generate in an x-ray tube and directed at a patient
-the x-rays are attenuated by the patient’s tissues as the beam passes through the patient
-attenuate means to reduce intensity
-intensity is the number of x-rays per unit area per unit time (ex: number of x-rays per cm^2 per second)

Question 3

what shows up on x-rays of diagnostic radiology?

Answer 3

less coming out will look whiter (bone is very white, air is dark)
-fat, soft tissue, air and bone in the patient attenuate the x-rays differently
-this produces a variation of the transmitted x-rays or a pattern
-the pattern of transmitted x-rays is converted into a visible image
-the greater the number of x-rays attenuated, the white the image

Question 4

how are x-rays created?

Answer 4

-x-rays are created in an x-ray tube by accelerating electrons (to give them energy)
-the electrons interact with a target, producing x-rays

Question 5

what does an x-ray tube consist of?

Answer 5

-a cathode (negative electrode) and anode (positive electrode) in an evacuated glass envelope (to prevent electron collisions with gas molecules)
-the cathode (filament) is the source of electrons
-the anode is the target of the electrons

Question 6

how do we make the electrons at the cathode?

Answer 6

-as the cathode is heated by the filament current, electrons ‘boil’ off (thermionic emission)
-electrons are repelled by the negative cathode and attracted by the positive anode
-the electrons accelerate towards the anode because of the potential difference between the cathode and the anode

Question 7

what is potential difference?

Answer 7

-potential, or potential difference, is the electrical pressure that us used to accelerate the electrons (measured in volts, V)
-the voltage setting on the x-ray machine (‘tube voltage’) corresponds to the potential difference applied between the anode and cathode (in kilovolt range, kV)

Question 8

what happens as the electrons go down?

Answer 8

-as the electrons accelerate, they gain kinetic energy
-the energy acquired by the electrons is measured in electron volts eV
-1eV is the energy of an electron accelerated across a potential difference of 1V
-therefore, the energy of an electron accelerated across a potential difference of 100kV is 100eV
-the tube voltage (kV) applied therefore determines the maximum energy of the electrons hitting the target

Question 9

what is the tube current flow?

Answer 9

-the tube current is the flow of electrons through the tube (measured in ampere A)
-increasing the tube current will increase the number of electrons hitting the target

Question 10

as electrons interact with the target, their kinetic energy is ____________

Answer 10

converted into heat (99%) and x-rays (1%)
-the x-rays produced include characteristic x-rays and bremsstrahlung x-rays

Question 11

what are the two x-rays produced?

Answer 11

-characteristic x-rays are produced when an incoming electron knocks an orbital electron in a target atom out of it shell
-bremsstrahlung x-rays are produced when an incoming electron interacts with a nucleus in a target atom

Question 12

why do we use tungsten at the anode?

Answer 12

-in order to generate enough x-rays to acquire a radiograph, the anode will get very hot (anode rise in temperature for a 2 second exposure is 2900 C)
-tungsten is used for the target material because it has a melting point of 3370 celcius and is able to withstand the heat
-as well, the efficiency of x-ray production increases with increasing atomic number (Z) and Z=74 for tungsten

Question 13

what is tube voltage, tube current and exposure time?

Answer 13

Question 14

what is the graph of interactions of electrons with the target of characteristic x-rays?

Answer 14

Question 15

when are characteristic x-rays produced?

Answer 15

-produced when an incoming electron knocks an orbital electron out of its shell
-the incoming electron is the accelerated electron directed at the target
-the orbital electron is from an atom in the anode material

Question 16

what is the definition of characteristic x-rays?

Answer 16

-defined as the radiation produced when an electron shell vacancy is created and an outer shell electron fills the vacancy, emitting an x-ray with energy equal to the difference in shell binding energies
-possible characteristic x-ray energies found by taking energy differences between the K, L, M and N shells

Question 17

why are they called characteristic x-rays?

Answer 17

-each element has different shell binding energies (ex: K shell for tungsten is 69.5 keV and K shell for calcium is 4.0keV)
-x-rays are therefore emitted with discrete energies characteristic of atoms in the target and the energies between the shells where the transitions take place

Question 18

what is an example of a characteristic x-ray?

Answer 18

to hit electron out, it must have energy at least equal to the binding energy

Question 19

how much of diagnostic x-ray beam is composed of characteristic x-rays?

Answer 19

-a typical diagnostic beam is composed of less than 20% characteristic x-rays
-the remainder of the x-rays (over 80%) are bremsstrahlung x-rays

Question 20

what is the graph of interactions of electrons with the target of a bremsstrahlung x-ray?

Answer 20

Question 21

when are bremsstrahlung x-rays produced?

Answer 21

-produced when an incoming electron (-) interacts with a nucleus (+) through electrostatic (Coulomb) force
-the incoming electron is the accelerated electron directed at the target
-the nucleus is from an atom in the anode material

Question 22

what is the definition of bremsstrahlung x-rays?

Answer 22

-defined as the radiation produced when an electron gives up part or all of its energy during a ‘collision’ with a nucleus (the electron is slowed, or braked, by the nucleus)
-the electron loses kinetic energy which is converted into a photon (x-ray) to satisfy the law of conservation of energy (energy cant be created nor destroyed)

Question 23

what are bremsstrahlung x-rays referred to as?

Answer 23

-white radiation or braking radiation
-the x-ray is emitted with a spectrum of energies up to the initial electron energy

Question 24

what does the energy of bremsstrahlung x-rays depend on?

Answer 24

-depends on how much the electron is slowed, which depends on how close it passes to the positively charged nucleus
-if the electron directly impacts the nucleus, and stops, then the energy of the outgoing x-ray is equal to that of the incoming electron`

Question 25

what is the possibility of x-rays?

Answer 25

Question 26

what is the description of x-ray spectrum?

Answer 26

-a graph representing an x-ray beam generated by an x-ray tube
-the x-axis is x-ray energy
-the y-axis is the number of x-rays of each particular energy
-an x-ray spectrum includes both characteristic and brem x-rays

Question 27

what is the x-ray spectra?

Answer 27

Question 28

why is brem a continuous spectrum of x-ray energies?

Answer 28

brem x-rays energy depends on incoming electron energy and how close the electron passes to the nucleus
-the energy of the electrons accelerate in an x-ray tube is variable (a continuous spectrum of electron energies)
-distance of the electrons from the nucleus varies, and this determines the energy of x-ray produced

Question 29

what is the continuous spectrum?

Answer 29

Question 30

what does the x-ray spectra look like with brem x-rays?

Answer 30

Question 31

why are filters added to brem x-rays?

Answer 31

-filters (usually a metal) are inserted into the path of the beam, before it reaches the patient and preferentially remove low energy x-rays (resulting in curve b)
-there is also inherent filtration of low energy x-rays (ex: the wall of the tube)
-curve b represents the x-rays used to image the patient

Question 32

what is the maximum energy and average energy of x-rays?

Answer 32