RAD TECH A Week 5 Spring 2006
E. Turner Lecture
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General Science ReviewPhysics: X-ray Production & Radiation Interactions
X-ray Exposure Factors Radiographic Density & Contrast
ATOM
ATOM
•Smallest particle of matter that has the properties of an element.
•Contains a small, dense, positively charged center (nucleus).
•Nucleus surrounded by a negative cloud of electrons.
•Electrons revolve in fixed, well-defined orbits (energy levels).
3 Fundamental Particles of an Atom
•Electron
•Proton
•Neutron
•Electrons can only exist in certain shells that represent electron binding energies
•K, L, M shells (K is closest to the nucleus)
•The closer an electron is to the nucleus, the higher the binding energy (strength of
attachment to the nucleus).
Atoms
•In their normal state, atoms are electrically neutral
•If an atom has an extra electron or has had an electron removed, it has been ionized.
How X-rays are Created
To produce x-rays, you need 3 things:
•A source of electrons
•A force to move them rapidly
•Something to stop them rapidly
*All 3 conditions met in an x-ray tube
Early X-ray Tube
The X-Ray tube is the single most important component of the radiographic system. It is
the part that produces the X-rays
Glass Envelope
•MADE OF PYREX GLASS TO WITHSTAND HIGH HEAT LOAD
•IS GAS EVAUCUATED
•(so electrons won’t collide with the air molecules in the tube)
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How are X-rays Made?
•X-rays are produced when electrons strike a metal target.
•The electrons are ejected from the heated filament and accelerated by a high voltage
towards the metal target.
•The X-rays are produced when the electrons collide with the atoms of the metal target.
Cathode (-)
•The cathode (negative electrode) contains a tungsten wire (filament) wound in a coil.
•Mounted in a holder called a focusing cup.
•Filament is heated & acts as a source of electrons.
•The temperature of the filament controls the quantity of electrons emitted from it.
•Temp. is raised by increasing current (mA) milliamperage.
Thermionic Emission
•When the current (amps) in the filament is intense enough, the outer-shell electrons of
the filament are “boiled off” and ejected from the filament.
•Tungsten filaments provide higher thermionic emission than other metals.
•Negative charge
Anode (+)
•The anode (positive electrode) is usually a copper block with a plate of tungsten
(target).
•Tungsten has a high melting point, can withstand extreme heat, & is more efficient @
producing x-rays.
•Small area on target where electrons strike is called the focal spot – this is the origin of
the x-rays.
Rotating Anode
•Driven by an electromagnetic induction motor – 2 principle parts:
•Stator – outside the glass envelope, series of electromagnets
•Rotor – inside the glass envelope, shaft made of copper & iron.
X-ray Production
•Positive voltage is applied to ANODE
•Negative electrons = attracted across the tube to the positive ANODE.
•Electrons “slam into” anode – suddenly stopped.
•X-RAY PHOTONS ARE CREATED
X-ray Production
•When a high electrical potential (kilovolts) is applied across the cathode & anode, the
electrons will strike the target with tremendous energy.
•The higher the voltage, the greater the speed.
X-ray Production
•Electron beam is focused from the cathode to the anode target by the focusing cup
•Electrons interact with the electrons on the tungsten atoms of target material
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•PHOTONS sent through the window PORT – towards the patient
Heat Production
•Heat & x-rays are produced by the impact of the electrons.
•Only about 1% of the energy resulting from the impact become x-rays.
•Most of the energy will become heat.
X-ray Photons
Waves or Particles?
•electromagnetic waves of shorter wavelength and higher energy than normal light. But
the debate over the nature of the rays – are they waves or particles?
•Photons can be described both as waves and particles.
Electromagnetic Spectrum
•X-rays have wavelengths much shorter than visible light, but longer than high energy
gamma rays.
•Because of short wavelength & high freq., x-rays are able to penetrate materials that
absorb or reflect light.
X-ray Properties
•Are highly penetrating, invisible rays which are a form of electromagnetic radiation.
•Are electrically neutral and therefore not affected by either electric or magnetic fields
X-ray Properties
•Can be produced over a wide variety of energies and wavelengths (polyenergetic &
heterogeneous).
•Release very small amounts of heat upon passing through matter.
X-ray Properties
•Travel in straight lines.
•Travel at the speed of light, 3 X 108 meters per second in a vacuum.
•Can ionize matter.
X-ray Properties
•Cause fluorescence of certain crystals.
•Cannot be focused by a lens.
•Affects photographic film.
X-ray Properties
•Produce chemical and biological changes in matter through ionization and excitation.
•Produce secondary and scatter radiation.
Interactions
RAD TECH A Week 5 Spring 2006
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Kinetic Energy (KE)
•The energy of motion
•Stationary objects have no KE
•Objects in motion have KE proportional to their mass & to the square of their velocity
•Electrons traveling from cathode to anode are sometimes called projectile electrons.
•The electrons’ KE is converted into thermal energy (heat) & electromagnetic energy (xrays).
X-ray Exposure Factors
•TECHNIQUE SELECTION:
•Radiographer selects the kiovoltage peak (kVp), milliamperage (mA) & time (s).
•Milliamperage + time = mAs
(milliamperage multiplied by a set time measured in seconds)
Kilovoltage Peak
•kVp
•One kilovolt = 1000 volts
•The amount of voltage selected for the x-ray tube.
•Range 30 to 150 kVp
•kVp controls contrast
Milliamperage
•mA
•One milliampere = one thousandth of an ampere.
•The amount of current supplied to the x-ray tube
•How many x-rays will be produced
•Range 10 to 1200 mA
Time
•In seconds
•How long x-rays will be produced
•0.001 to 6 seconds
Milliampere Seconds
•Technologists think in terms of mAs
•Calculated by mA x seconds
•Ex: 100mA X 0.2s = 20 mAs
•How many x-rays will be produced and for how long.
•Modern x-ray machines only allow control of mAs
Imagine this…
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•If one changes the mA station from 200 to 400 mA, twice as many electrons will flow
from the cathode to the anode.
•So mA controls how many electrons are coming at the target.
•mAs is a combination of how many and for how long (seconds)
Change in kVp
•kVp controls the energy level of the electrons and subsequently the energy of the x-ray
photons.
•A change from 72 kVp will produce
x-rays with a lower energy than at
82 kVp
•Difference between a ball traveling 72 mph and 82 mph (how much energy did it take to
throw the ball at the rates?)
Image Production
•Primary Radiation – The beam of photons, B4 it interacts with the pt’s body.
•Remnant Radiation – The resulting beam that is able to exit from the patient.
•Scatter Radiation – Radiation that interacts with matter & only continues in a different
direction – not useful for image production.
•Attenuation – Primary radiation that is changed (partially absorbed) as it travels through
the pt.
Path & Attenuation of X-ray Beam
Tube Interactions
•Heat = 99%
•X-ray = 1%
Bremsstrahlung
(Brems) = 80%
Characteristic=20%
Anode Heat
•Projectile e- interact with outer shell e- but do not have enough energy to remove them.
•Outer shell e- only becomes excited into a higher energy level & then drop back down to
normal energy
•Heat is generated
Characteristic Radiation
•Projectile electron removes K shell electron (hole is produced)
•Outer shell electron falls into hole in the K shell (ionization)
•Emission of x-ray photon
Bremsstrahlung Radiation
(Slowing down or Braking)
•Projectile e- completely passes by the orbital e•Comes very close to the nucleus
•As the proj. e- passes the nucleus, it slows down and changes direction
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•Slowing down reduces KE & x-ray is produced
Patient Interactions
•Photoelectric Effect
•Compton Scattering
•Classic Coherent Scatter
•Pair Production
•Photodisintegration
Photoelectric Effect
•X-ray photon absorption interaction
•Occurs when an x-ray is totally absorbed during the ionization of an inner-shell electron
•X-ray photon disappears and the k-shell e- (now called a photoelectron) is ejected from
the atom.
Photoelectric Effect
Compton Scatter
•X-ray photon interacts with an outer-shell e- & ejects it from the atom
•The x-ray continues in a different direction & with less energy
Compton Scatter
Compton Scatter
•Scattered x-rays provide NO useful information on the film
•Contribute to film fog – inferior radiograph, not as clear an image
•Can create radiation exposure hazard (esp. during fluoroscopy): Personnel in the room
can be exposed.
Classical Coherent Scatter
•Interaction between low energy x-rays & the atom, causing the atom to become excited
•Direction of 2nd photon is changed, no loss in energy, no e- being ejected.
Classical Coherent Scatter
Classical Coherent Scatter
Pair Production
•Occurs when an x-ray photon interacts with the nuclear force field and two electrons are
created:
1. Positron (positively charged electron)
2. Negatively charged electron
Pair Production
Photodisintegration
•High energy x-ray photons that escape interaction with electrons & nucleus.
•Instead, photon is absorbed by the nucleus, the nucleus becomes excited, and releases a
nuclear fragment.
Photodisintegration
Review!!!
Photoelectric Effect - Patient
Classical Coherent Scatter - Patient
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Compton Scatter - Patient
Pair Production - Patient
Photoelectric Effect - Patient
Characteristic Radiation - Tube
Bremsstrahlung Radiation - Tube
Why you see what you see…
•The films or images have different levels of density – different shades of gray
•X-rays show different features of the body in various shades of gray.
•The gray is darkest in those areas that do not absorb X-rays well – and allow it to pass
through
•The images are lighter in dense areas (like bones) that absorb more of the X-rays.
Images
•DENISITY = THE AMOUNT OF BLACKENING “DARKNESS” ON THE
RADIOGRAPH
•CONTRAST – THE DIFFERENCES BETWEEN THE BLACKS TO THE WHITES
Density
•mAs
•mA = AMOUNT of electrons sent across the tube combined with TIME (S) = mAs
•mAs controls DENSITY on radiograph primary function of mAs is DENSITY
Contrast
•Kilovolts to anode side – kVp
•Kilovolts controls how fast the electrons are sent across the tube
•kVp – controls CONTRAST on images
Radiolucent vs. Radiopaque
•Radiolucent materials allow x-ray photons to pass through easily (soft tissue).
•Radiopauqe materials are not easily penetrated by x-rays (bones)
Short Scale vs. Long Scale
Radiographic Density
RAD TECH A Week 5 Spring 2006
E. Turner Lecture
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