Rad Physics Week 4 09 by mkVhHYy

VIEWS: 5 PAGES: 74

									Radiographic Physics

        Week 4
        RTEC A
Objectives
   General Science Review

   Atomic Structure

   Atomic Interactions in the Tube
• A few days after
  Roentgen's initial public
  announcement of his
  discoveries, a doctor in
  America took X-ray
  photographs of a
  person with gunshot
  wounds in his hands
• Why can you see the
  bullet fragments?
Atomic Models
       • The theory of how
         radiation is created is
         based on the BOHR
         model of the atom.

       • Electrons orbit around
         a nucleus
ATOM
    Structure of the Nucleus



•   central core of the atom
•   contains p+ & n0 (& other particles)
•   identifies the atom as a specific element
•   held together by fundamental force ("strong")
Binding Energy
Binding
Energy
K L M Shells
        Law of Electrostatics
• Repulsion/Attraction – like charges repel,
  unlike charges attract

• The inverse square law – the force
  between two charges is directly
  proportional to the product of their
  magnitudes - The more energy they have
  the more attracted/repelled they will be
    How “X-rays” are created
      SEE:   MAN MADE RADIATION (PG.93)



      TO PRODUCE X-RAYS
          YOU NEED:

• A SOUCE OF ELECTONS
• A FORCE TO MOVE THEM QUICKLY
• SOMETHING TO STOP THEM SUDDENLY
How Are X-rays Made?
             • X-rays are produced
               when electrons strike
               a metal target.
             • The electrons are
               liberated 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 and nuclei
               of the metal target.
The X-ray Photons that exit the tube are many
energies – “polyenergetic or heterogenous beam”
    How “X-rays” are created
• Power is sent to x-ray tube via cables
• mA (milliamperage) is sent to filament
  on cathode side.
• Filament heats up – electrons “boil
  off”
• Negative charge
     How “X-rays” are created
• Positive voltage (kVp) 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
           Kinetic energy
• Energy of motion

• The electrons KINETIC energy is
  converted to electromagnetic or PHOTON
  energy
kVp   & mAs
• they are
  electromagnetic
  waves of shorter
  wavelength and
  higher energy than
  normal light. But the
  debates over the
  nature of the rays –
  waves or particles? –
• Photons can be
  described both as
  waves and particles.
Electromagnetic Energy Spectrum
• continuous range of energy
   – spectrum indicates that the distribution of
     energies exist in an uninterrupted band
     rather than at specified levels
• released by accelerating charged
  particles
   – moves through space or matter as
     oscillating magnetic & electric fields
      • needs no carrier medium but can have one
      • can penetrate or interact with matter
         ELECTROMAGNETIC
            RADIATION
• maybe described as wavelike fluctuations of
  electric and magnetic fields.
• There are many kinds of electromagnetic
  radiation;
• EMS illustrates that visible light, microwaves,
  and radio waves are within the electromagnetic
  spectrum. All the electromagnetic radiations
  have the same velocity, ie, 186,000 miles per
  second; however, they differ greatly in
  wavelength.
 The Electromagnetic Spectrum
• X-rays have wavelengths much shorter than
  visible light, but longer than high energy gamma
  rays.
The Electromagnetic Spectrum
energy (eV)
           (EMS)
               f requency (Hz)                                        wavelength (m)
                25
      10 10    10                                                               -15
                                          
                                                                           10
                                 
                                         gamma
      10 5     1020          xray                                              -10
                                                                          10
                                                                  
                                                      ultraviolet
                                                                  
      10 0     1015                                                             -5     light
                                                                          10
                                                          infrared
                                                     
      10 -5    1010
                                         microwave
                                               
                                                                           10 0
                         TV/FM radio 
                                     
                                               
                                     radio     
      10 -10   105
                                                     long waves            10 5

      10 -15   100
                                                                           10 10

      10 -20   10
                    -5
                                                                           10 15
   General Characteristics of EMS
           X-ray photons:
• Have no mass or physical form
• travel at speed of light (c) in a vacuum (or air)
   – c = 3 x 108 m/s or 186,400 miles per sec
• travel in a linear path (until interaction occurs)
• dual nature: wave vs. particle
• unaffected by
   – electric or magnetic fields
   – gravity
Wavelength
The shorter the wavelength –
  the higher the frequency
• Wavelength refers to the distance between two
  consecutive wave crests
• Frequency refers to the number of cycles per
  second (cps); its unit of measure is the hertz
  (Hz), which is equal to 1 cps.
• Frequency and wavelength are closely
  associated with the relative energy of
  electromagnetic radiations. More energetic
  radiations have shorter wavelengths and higher
  frequency. The relationship among frequency,
  wavelength, and energy is illustrated in the
  electromagnetic spectrum (Fig 1).
X-rays can ionize atoms…
    What is Ionization?
Some radiations are called ionizing
 because they have the energetic
 potential to break apart
 electrically neutral atoms,
 resulting in the production of
 negative and positive ions.
When an electron is added or
removed from the atom- it is
          ionized
INTERACTIONS
X-ray production begins
at the atomic level
 Energy (photons) are released when
the electron collides with another electron,
or passes close to the nucleus of the atom –
the change in energy of the shells
 –produces photons
             X-ray PRODUCTION in the TUBE
   X-rays are produced by the conversion of e- radiation
    (photons)
   A large potential difference is applied across the two electrodes
    in an evacuated envelope
       Neg. charged electrode (cathode): source of e-
       Pos. charged electrode (anode): target of e-




                                           Cathode
              Anode



                                                                        35
INTERACTIONS IN THE TUBE
• BREMS (Bremsstrahlung)

• CHARACTERISTIC

• HEAT
            TUBE INTERACTIONS
     -
    e (electrons) are released from the cathode are
             accelerated towards the anode

• Heat = 99%
• X-ray = 1%
  – Bremsstrahlung
  (Brems) = 80%

  – Characteristic =
   20%
   The CATHODE is the            X-ray Tube Cathode
    e- source – a tungsten
    wire filament surrounded
    by a focusing cup
   Electricity sent though the
    wire at the Cathode
   Electricity heats the
    filament and releases e-
    via thermionic emission
   ELECTRONS ARE
    “BOILED OFF” &
    suspended in a space
    cloud.
                                                       38
When the ELECTRONS FROM THE CATHODE –
INTERACT WITH THE ELECTRONS IN THE
TUNGSTEN ANODE – X-RAY PHOTONS ARE
CREATED
e- (electrons) are released from the
cathode are accelerated towards the
anode
               X-ray Tube Anode
   Tungsten anode disk        Focal  track area
   Stator and rotor make up   (spreads heat out
    the induction motor        over larger area than
   Rotation speeds            stationary anode
      Low: 3,000 – 3,600
                               configuration)
       rpm
      High: 9,000 – 10,000
       rpm
   Molybdenum stem (poor
    heat conductor) connects
    rotor with anode to
    reduce heat transfer to
    rotor bearings
• HIGH VOLTAGE
  TO ANODE –
  ATTRACTS –
  ELECTRONS
  FROM CATHODE

• CURRENT TO
  STATOR CAUSES
  ROTATION OF
  ANODE
Rotating Anode
INTERACTIONS IN THE TUBE

• HEAT = 99%       X-RAYS = 1%

• BREMS (BRESTRAULUNG) (80%)

• CHARACTERISTIC (20%)
Start with the incoming line…
      Tube or Patient ?
                    Heat
• Most kinetic energy of projectile e- is
  converted into heat – 99%

• Projectile e- interact with the outer-shell e-
  of the target atoms but do not transfer
  enough energy to the outer-shell e- to
  ionize
Heat
                            INCOMING
HEAT                        ELECTRONS
                            FROM
                            CATHODE -
8 p+ + 8e- = neutral atom
                            pass by the
                            ELECTRONS
                            IN THE
  e
                            TUNGSTEN
                            TARGET – do
                            not interact
                            with e –
                            causes them
                            to VIBRATE –
                            RELEASING
  e
                            SMALL
                            AMOUNTS OF
                            HEAT
  Heat is an excitation
rather than an ionization
X-ray Photons – BREMS
                Creates a
              polychromatic
               spectrum –
                 xrays of
                different
                energies




                              50
 Energy (photons) are released when
the e passes close to the nucleus, then
          changes direction
BREMS RADIATION
           • Electron
           • Passes by
             nucleus
           • Changes
             direction
           • Energy
             released as a
             PHOTON
           • bremsstrahlung
             : “breaking”
             radiation
          Bremsstrahlung

is a german
word meaning
slowed-down
radiation
   Characteristic Radiation – 2
              steps
• Projectile e- with high enough energy to
  totally remove an inner-shell electron of
  the tungsten target

• Characteristic x-rays are produced when
  outer-shell e- fills an inner-shell void

• All tube interactions result in a loss of
  kinetic energy from the projectile e-
CHARACTERISTIC (in tube)
              • Electron hits inner
                shell e in orbit –
                knocked out &
                creates a hole
              • Other E’s want to
                jump in
              • Energy released
                as PHOTONS
• It is called
characteristic
because it is
characteristic of
the target element
in the energy of
the photon
produced
Characteristic
    Bremsstrahlung Radiation
• Heat & Characteristic produces EM energy
  by e- interacting with tungsten atoms e- of
  the target material

• Bremsstrahlung is produced by e- passing
  by closely with the nucleus of a target
  tungsten atom – the change in direction of
  the electon – releases a photon of energy
         REVIEW
  What type of Interaction?
   Where does it occur?
         X-RAY TUBE:
STRAIGHT LINE IN = E FROM TUBE
  WAVY LINE OUT = PHOTONS
Review Quiz
   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
Questions ?

								
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