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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 ?
