Radiography Flashcards
Label the X-ray machine
A: + Spinning anode
B: Tungsten target
C: Glass envelope
D: Cathode
E: Focusing cup
F: - Hot cathode filament
G: Electrons
H: Anode heel effect
I: X-ray photon beam
J: Window
K: Anode
L: Bearings
M: Rotor
N: Stator
What is the main purpose of the cathode
Acceleration of the Electron
The cathode
a) How are electrons produced
b) How are electrons accelerated
c) How does filament size affect the X-ray
a) Current (mA) is applied through the wire, heating the wire. Electrons emmitted through thermionic emission, producing an electron cloud at the cathode
b) A large potential difference (kV) is applied across the X-ray tube to accelerate electrons produced at the cathode
c) Fine filament size: small electron beam, small focal spot, fine detail, dense heat concentration
Broad filament size: large electron beam, large focal spot, reduced detail, higher safe exposures
What is the main purpose of the anode
Deceleration of the electron
The anode
a) Properties od the anode (tungsten) target
b) How much kinetic energy of electrons is converted to x-rays. Where does this occur
c) Best angle for the focal spot
d) Large vs small focal spots
a) Efficiency due to high atomic number. High melting point and specific heat capacity. Thermoconductive
b) Less than 1%. Occurs at the focal spot
c) 6-20 degrees
d) Large actual focal spots: reduce overheating
Small effective focal spots: improve image detail, limit size of field of view
The anode
a) Why does overheating occur
b) Problems associated with overheating (3)
c) Tube ratings charts
a) Electon collisions at the anode causes the target to vaporise and condense on the glass envelope
b) 1. Hot anode may then emit electrons, reversing the tube current 2. Over time the anode will become pitted and cause reduced and uneven x-ray output 3. Filament may burn out from overheating, causing glass envelope cracks and implosion
c) Tube ratings are defined input parameters (kV, mA, exposure time) that can be safely used without causing damage (including overheating) to the x-ray tube itself
The anode
Stationary anode vs rotating anode
Stationary: Low power portable machines. Copper stem dissipates heat. Oil provides heat sink and electrical insulation.
Rotating: Continually changing the actual focal spot. Heat loss by radiation into oil bath (NOT by conduction)
Types of radiation
Continuous radiation
Produced by the electrons colliding with the nucleus. Bremsstrahlung (= braking) effect. Energy released at X-ray. Continuous spectrom of radiation produced
Characteristic Radiation
Produced by electron colliding with a shell electron. Shell electron is ejected, outer shell electorn drops into inner shell, emitting energy as X-ray. X-rays produced at characteristic peaks, dependent on atomic number and shell energies
Soft radiation
Does not contribute to image
Low energy, long wavelength X-rays which are absorbed in patient - health hazard
Legal requirement to filter them out (inherent tube filtration - 1mm aluminium. Extra filtration 2-2.5mm aluminium)
Changing parameters of the X-ray Beam
a) Effect of increasing kV
b) Effect of increasing mA & time
a) ↑ number of x-rays, ↑ electron velocity, ↑ x-ray energy and penetration, ↓ x-ray wavelength
b)↑ number of x-rays, ↑ filament heating, ↑ x-ray exposure
a) Define intensity of the X-ray beam and what it’s affected by
b) Define quality of the X-ray beam and what it’s affected by
a) Amount of energy in the beam ∝ number of photons ∝ photon energy
Affected by: kV, mA, filtration, rectification, target atomic number
b) Penetration power of the x-ray beam ∝ x-ray energy
Affected by: kV, filtration, rectification
Transformers
What is the puropse of transformers in an X-ray machine
Alter voltage from mains supply into the x-ray machine
Transformers
Describe:
a) Autotransformers
b) Step-down transformers (aka low tension)
c) Step-up transformers (aka high tension)
a) Function: provide steady voltage from the mains (240V) to the machine. Ensures even x-ray production
b) Function: heat the filament. Input = 240V, output = 10V and 3-5A
c) Function: supply a large potential difference for acceleration of electrons. Input = 240V, output = 40 to 100 kVs
Rectifiers
What is the purpose of rectifiers in an X-ray machine
Mains AC current is required for transformers to function, but X-ray production requires a DC current → rectification changes current from AC to DC (a one-way current)
a) Inverse square law
b) Photoelectric effect
a) Intensity of the X-ray beam is inversely proportional to the square of the distance from the source
intensity ∝ 1/distance^2
b) The predominant interaction up to 60keV
a) What is the Compton effect
b) Consequences of scatter radiation
c) How to recude scatter production
d) How to reduce scatter reaching the film
a) The predominant interaction in the diagnostic energy range
b) 1. Decrease in contrast (film fogging) 2. Radiation hazard 3. Increased radiation dose
c) 1. Collimation of the primary beam (reduce size of primary beam to only cover area of interest) 2. Lower kV (allow photoelectric effect to predominant, not Compton, although not always feasable, depends on area of interest and power of machine) 3. Compress area under investigation (less scatter, but more image distortion)
d) 1. Air gap technique (move patient away from detector so less patient generated scatter) 2. Lead backed cassettes and intensifying screens (inbuilt scatter reduction) 3. Grids (scatter absorbed by lead as lower energy and different direction, primary beam passes through radiolucent gaps)
Types of grids (4) + pros and cons
