[PHYSICS LECTURE]
CHAPTER 1 RADIATION
-
November 8, 1895


Wilhelm Conrad Roentgen
discovered x-ray
Used glass envelop and barium
platinocyanide
cellulose nitrate – glass plate
substitute
Michael Pupin – 1st to use
intensifying screen
December 28, 1895
-
investigated properties of the rays
and made a manuscript
ELECTOMAGNETIC RADIATION



transport of energy through space
as a combination of electric and
magnetic fields
produced by a charge that is
accelerated
travels at 186,000 miles/sec or 3 x
10 8 m/sec
propagated through space by form
of waves
consist of wavelength (distance
between 2 crest) and frequency (#
of waves passing a particular point)
frequency (hertz) is inversely
proportional to wavelength
diagnostically, we need a short
wavelength and increase the
frequency
VELOCITY = F x W
Coulomb’s Forces – force between 2
charged objects
Electron volt (eV)
-
unit used to measure the energy of
photons
amount of energy that an electron
gains as it is accelerated by a
potential difference of 1V
CHAPTER 2 PRODUCTION OF X-RAY

x-rays are produced by energy
conversion when a fast-moving
stream of electron is suddenly
decelerated in the target anode
Glass enclosure
1
vacuum; both glass and wires are
heated to high temperature on
operation
Cathode
-
-
negative terminal / filament
source of electrons for the tube
2 element of cathode:
o connecting wires – supply
the voltage and amperage
o focusing cup
filament is made of tungsten 0.2
mm x 0.2 cm x 1 cm
Thermionic emission
-
emission of electron resulting from
the absorption of thermal energy
occurs when an acquired energy
allows electrons to move a small
distance from the filament
Edison Effect – electron cloud produced by
thermionic emission
Space Charge
-
a small cloud produced by electrons
emitted by the filament
collection of negatively charged
electrons
Space Charge Effect
-
tendency of the space charge to
limit the emission of more electrons
from the filament
Ampere (A)

unit of electric current
rate of flow when 1 coulomb of
electricity flows through a
conductor in 1 sec
coulomb is the equivalent of he
amount of electric charge carried
by 6.25 x 1018 electrons
Focusing Cup
-
made of Nickel
designed so that the electrical
forces cause the electron stream to
converge onto the target anode
Line Focus Principle
-
the size of the area bombarded by
the electron is larger that the size
Summary of Christensen – Karol Pauline Farinas, M.D.

that emits x-ray due to the slanted
character (see Figure 2-3 page 13)
the smaller the angle, the smaller
the apparent focal spot
for general radiography in 40 inch
distance, anode angle must not be <
15o
Anode
-
-
small plate of tungsten in a bed of
copper (W chosen due to high
melting point and high atomic
number 74)
copper facilitates heat dissipation
and increase the total thermal
capacity of the anode
Rotating Anode
-
large disc of tungsten at 3600 rpm
with a beveled edge from 6 to 29
degrees
purpose is to spread the heat
produced during an exposure over
a large area of the anode
remainder is composed of
Molybdenum and uses metallic
silver as lubricant
the shorter the stem of the rotator
the better!
HEEL EFFECT
-intensity of the beam is stronger at the
CATHODE than the anode
- always place the thick part in the cathode
side! =)
- less noticeable when larger focus-film
distance and smaller films are used
NATIONAL COUNCIL ON RADIATION
PROTECTION NO. 49
-
limit the load that can be safely
accepted by an x-ray tube
Kilowatt rating
-
abiliy of the tube to make asingle
exposure of a reasonable duration
(0.1 sec)
Ceramic insulators
positive electrodes
2 types: stationary and rotating
Stationary anode
-
-
the leakage radiation at the
distance of 1 meter from the source
shall not exceed 100 mR/hour
when the tube is operated at its
maximun continous rated current
ofr the maximun rated tube
potential
TUBE RATING CHARTS
-
insulate the high voltage parts of
the x-ray tube from the metal tube
envelope
most commonly used is ALUMINUM
OXIDE
Off-Focus Radiation
-
-
produced by an x-ray tube when
high speed electrons interact with
metal surfaces other than the focal
tract of the anode
main source is the “backscatter
from anode”
ATOMIC STRUCTURE
- Atomic # = # of protons
- Proton + neutron = Mass #
- IsotoPes – same # of Proton
- IsotoNes – same # of Neutron
BREMSSTRAHLUNG / GENERAL
RADIATION

braking electrons
reaction of the elecetron with the
nucleus of tungsten atoms
an electron passes near the nucleus
of a W atom, the + charge will act on
the – charge of the electron
the highest energy x-ray photon
leaving the x-ray tube depends on
the KVP used while the lowest
energy depends on the FILTER used
CHARACTERISTIC RADIATION

results when the electrons
bombarding the target eject INNER
orbit electrons
gives of a positive ion and an Auger
electron (additional electron)
Tungsten atom with an inner shell
vacancy is much more likely to
produce an x-ray than to expel an
electron
[PHYSICS LECTURE]


Higher atomic number of target
atoms, increase in the efficiency of
the production of x-ray
Atomic #determines the energy or
quality of x-rays produced
KVP – quality
mAs – Quantity
3
RECTIFICATION
-
process of changing AC to DC
convert everything to useful rays =)
Half wave rectification
-1/2 of electrical wave is used to produce xrays
CHAPTER 3 X-RAY GENERATORS
Full-wave rectification
-
- utilizes the full potential of the electrical
supply
device that supplies electric power
to the tube
Transformer
-
device that either increases or
decreases the voltage in a circuit
Primary Coil and secondary coil
Current flows only through the
secondary circuit if the magnetic
field is changing
Step-up Transformer
-
more turns in secondary coil than
primary
increases the voltage and decreases
the current (ampere)
Step-down Transformer
-
more turns in the primary than
secondary coil
decreases the voltage and increases
the current
Self rectification
- x-ray tube itself serves as a rectifier
- 2 disadvantages: 1) 2x as long 2) repeated
and prolonged exposure heat the anode
enough to produce current during the
inverse half of the cycle.
Solid-state rectifiers
-
N-type semiconductor
-
-
consist of a single winding wound
on a laminated closed core
works on the principle of selfinduction
-
-


regulates current flow through the
filament of the xray tube
precise control of filament heating
is critical, a small variation is
filament current = large variation in
x-ray tube current
KVP meter can be placed in the
circuit between the
autotransformer and step-up
transformer
mA meter must be in the secondary
coil of the high voltage transformer
to record current flow accurately
3 valence electron, impurity is the
acceptor, made of “holes”
DIODE – device formed by a P-N junction
RIPPLE FACTOR
-
Filament Circuit
-
5 electrons, impurity is the donor
commonly arsenic and antimony
P-type Semiconductor
Autotransformer
-
more reliable and longer life
uses semiconductor made of
CRYSTALLINE SILICON
-
variation in the voltage across the
x-ray tube expressed as a % of the
maximum value
s single phase circuit has a ripple
factor of 100%
POWER STORAGE GENERATOR
1) Capacitor discharge generator
- electrical device for storing charge
or electrons
- provide high mA for very short
exposure times
- as a general rule, a drop of 1 kV for
1 mAs
2) Battery powered generator
Summary of Christensen – Karol Pauline Farinas, M.D.
-
a standard power supply is used to
charge large capacity nickelcadmium batteries
FALLING LOAD GENERATORS
-
purpose is to produce an x-ray
exposure in the shortest possible
exposure time by operating the
xray tube at its maximum kilowatt
rating during the entire exposure
CHAPTER 4 BASIC INTERACTIONS
BETWEEN X-RAYS AND MATTER
5 Basic x-ray interactions:
1) Coherent Scattering
- radiation undergoes change in
direction w/o change in wavelength
- 2 types: 1) Thomson (single
electron) 2) Rayleigh (all electrons)
- No ionization happens
2) Photoelectric Effect
- 3 products: characteristic radiation,
negative ion and positive ion
- incident photon must have
sufficient energy to overcome the
binding energy of electron
- most likely to occur when the
photon energy and electron binding
energy are nearly the same
- the tighter the electron is bound in
its orbit, the more likely it is
involved in the photoelectric
reaction! (high Atomic #)
- enhances natural tissue contrast
and does not produce scatter
radiation; more radiation for the
patient (bad)
- most common reaction in lowenergy photons
3) Compton Scattering
- contribute almost all the scatter
radiation in diagnostic radiology
incident photon strike a free
OUTER shell electron;
most common interaction between
xray and body tissues
5)
Photodisintegration
-
part of the nucleus is ejected by a
high energy photon; does not occur
in < 7MeV
CHAPTER 5 ATTENUATION
- reduction in the intensity of an x-ray beam
as it traverses matter by either the
absorption or deflection of photons
from the beam
- depends on both the quantity (mAs) and
quality (kVp); however, in monochromatic
radiation, quality is not affected!
ATTENUATION COEFFICIENTS
- measures of the quantity of radiation
attenuated by a given thickness of an
absorber
1) Linear Attenuation Coefficient
- most important coefficient for diagnostic
radiology!
- quantitative measurement of attenuation
per centimeter of absorber
- it is for monochromatic radiation
Half Value Layer – absorber thickness
required to reduce the intensity of the
original beam by ½
- inc. HVL; inc. beam penetration
2) Mass Attenuation Coefficient
- used to quantitate the attenuation of
materials independent of their physical
state
- unit is g/cm2
- MAC is same for water, ice and vapor
FACTORS AFFECTING ATTENUATION
* inc radiation energy = inc # of transmitted
photons = dec attenuation
4)
Pair Production
-
high energy photon interact with
the nucleus of an atom, the photon
disappears and its energy is
converted into matter to form 2
particles: positron and electron
* inc density, atomic # or electron/gram =
dec # of transmitted photons = inc
attenuation
cannot take place in < 1.02 MeV
- contains photons of various energies
-
POLYCHROMATIC RADIATION
[PHYSICS LECTURE]
5
- in general, the mean energy of
polychromatic radiation is between 1/3
and ½ of its peak energy
3 DIFFERENT LEVELS OF FILTRATION
- change in both the quality and quantity
- resulting from absorption of xrays as they
pass through the xray tube and housing
specifically the galss envelop, the window of
the tube and the insulating oil around the
tube
APPLICATION IN DIAGNOSTIC RADIOLOGY
*If all photons were transmitted, the film
would be black; if all were attenuated all
will be white
(e.g. bone will attenuate more photons than
lungs hence bones appear white)
* when Compton reactions predominate, the
differential attenuation entirely depends on
differences in density
* fat and water could only be demonstrated
effectively using low energy technique
(photoelectric)
SCATTER RADIATION
- make up 50 – 90% of total # of photons
from the patient
Factors affecting scatter radiation:
1) kilovoltage (kVp)
2) part thickness
3) field size
* increase in any of the 3 will increase
scatter radiation! However if we decrease
kVp we increase patient dose. =(
Field size is the most important factor in
production of scatter radiation but scatter
radiation has a saturation point too which
is 30 x 30 or 12 inch square wherein further
increase in field size doesn’t produce
increase in scatter radiation anymore.
CHAPTER 6 FILTERS
Filtration
- process of shaping the x-ray beam to
increase the ratio of photons useful for
imaging to those photons that increase
patient dose or decrease image contrast
* we must remember that the 1st few cms of
tissue receive much ore radiation than the
rest of the patient, filter is used to reduce
patient dose which is usually a sheet of
metal so low energy photons are absorbed
from the beam before it hits the patient
1) INHERENT FILTRATION
- measured in aluminum equivalent which
represents the thickness of aluminum that
would produce the same degree of
attenuation as the thickness of the material
in question (commonly 0.5 to 1.0 mm)
- special circumstance uses Beryllium
window tubes for essentially unfiltered
beam
2) ADDED FILTRATION
- resut from absorbers placed in the path of
the xray beam
*attenuation is most intense in photoelectric
reaction and diminishes in Compton
reaction
- usually copper (for high energy) and
aluminum (low energy and multipurpose)
* in compound aluminum and copper, the
aluminum faces the patient an the copper
faces the tube. Most filtration occurs in the
copper and the purpose of the aluminum is
to absorb the characteristic radiation from
the copper
Filter Thickness
Operating kVp
< 50 kVp
50 – 70 kVp
> 70 kVp
Total Filtration
0.5 mm aluminum
1.5 mm aluminum
2.5 mm aluminum
* major disadvantage of filter is reduction in
the intensity of x-ray beam, to comensate
we must increase the mAs
HEAVY METAL FILTERS (K – edge filters)
- to produce an x-ray beam that has high #
of photons in the specific energy range that
will be most useful in diagnostic imaging
- transmit a significantly narrower spectrum
of energies than aluminum w/ decreased #
of both low and high energy photon
Summary of Christensen – Karol Pauline Farinas, M.D.
CHAPTER 7 XRAY BEAM RESTRICTORS
1) APPERTURE DIAPHRAGMS
- simplest type of beam restrictor
- sheet of lead with a hole in the center
- diasadvantage: large penumbra at the
periphery of the beam but can be reduce the
farther away the aperture is to the patient
2) CONES AND CYLINDERS
- beam restriction with cylinder takes palce
at the far end of the barrel, so less
penumbra
GRID PATTERN
- orientation of the lead strips in their
longitudinal axis
1) Linear Grid
- lead strips are parallel to each other in its
longitudinal axis
- major advantage: allow us to angle the xray tube along the length of the grid without
loss of primary radiation from grid (“ grid
cut-off”)
2) Crossed grid
- diasadvantage: like aperture , severe
limitation in sizes available
- 2 superimposed linear grids with same
focusing distance; cannot be used with
oblique techniques
3) COLLIMATORS
- converge at convergence point
- best beam restrictor
3) Focused Grid
- provides infinite variety of rectangular
xray fields and a light beam shows the
center and exact configuration of the xray
field
- made up of lead strips that are angled
slightly so that they focus in space
Positive Beam Limiting devices –
automatic collimators wherein its shitters
are motor-driven; sensors in the tray
identify the size and alignment of the
cassette
- require to be no greater than 2% total
misalignment from the respective edges of
the xray field
CHAPTER 8 GRIDS
- converge at a line in space called
convergent line
- focusing range, which is the distance
within which the grid can be used without
significant loss in primary radiation, is wide
in low-grid ration and narrow for high grid
ratio
4) Parallel Grid
- lead strips are parallel when viewed in
cross section
- consists of a series of lead foil strips
separated by xray –transparent spacers
- focused at infinity so there are no
convergent lines
- invented by Dr. Gustave Bucky in 1913
- can only be used effectively with very
small xray fields or long target-grid
distances, commonly in fluoro machines
- most effective way of removing scatter
radiation from large radiographic fields
- primary radiation must be oriented the
same axis as the grids
GRID RATIO
- ratio between the height of the lead strips
and the distance between them
- usually 4:1 to 16:1
* the higher the ratio, the better the grid
function, better contrast
EVALUATION OF GRID PERFORMANCE
1) Primary Transmission
- measurement of the percentage of primary
radiation transmitted through the grid
2) Bucky Factor
- ratio of the incident radiation falling on
the grid to the transmitted radiation passing
through the grid
[PHYSICS LECTURE]
- indicates how much we must increase
exposure factors from a non-grid to grid
technique
- tells how much the patient’s exposure dose
is increased by the use of a grid
- measure of the total radiation (primary +
secondary) absorbed from the xray beam by
the grid
*the higher the bucky factor, the greater
the radiation dosage to the patient and
exposure factors
motion of the grid should be
synchronous with the pulses of the
x-ray generator
GRID SELECTION


- most important test to a grid’s
performance since grid function to improve
contrast
GRID CUT-OFF
- loss of primary radiation that occurs when
the images of the lead strips are projected
wider than they would be with ordinary
magnification
- greatest on the side directly under the xray tube
- resultant images will be light in the area of
the cut-off
Four causes of grid cut-off:
-
- uniform loss of radiation over the entire
surface of the grid producing a uniformly
light radiograph
3) focus-grid distance decentering
- the target of the xray tube is correctly
centered to the grid
4) combined lateral and focus-grid distance
decentering
MOVING GRIDS
-
invented by Hollis E. Potter, 1920
grids are moved to blur out the
shadows cast by lead strips
grids must move fast to blur the
lead strips and the transverse
reduce scatter radiation but lesser
patient exposure than using grids
scatter radiation decrease not from
filtration but from the scattered
photons missing the film
the larger the air gap, the more
effective in removing scatter
radiation
FACTORS IN CHOOSING GAP WIDTH:
1) thicker patients, larger gap
2) 1st inch of air gap improves contrast
more than succeeding inches
3) image sharpness deteriorates with
increasing air gap
4) greater air gap, more exposure ,
greater magnification
CHAPTER 9 LUMINESCENT SCREENS
Fluorescence
–
–
1) focused grid used upside down
2) lateral decentering (grid angulation)
8:1 - < 90 kVp
12:1 - > 90 kVp
AIR GAP TECHNIQUE
3) Contrast Improvement Factor
- ratio of the contrast with a grid to the
contrast without a grid
7
for of luminescence produced when
light is emitted instantaneously
ability of crystals of certain
inorganic salts (phosphors) to emit
light when excited by xrays
Phosphorescence
–
if the emission of light is delayed
beyond 10-8 sec
Calcium tungstate – phosphor in
intensifying screen; produces light in the
blue region of the spectrum
Cadmium sulfide – phosphor in fluorescent
screen
INTENSIFYING SCREEN
-
decrease the x-ray dose to patient,
allow shorter exposure time
*x-ray film used with intensifying scrren
has photosensitive emulson on both sides
(unlike mammo which is a single emulsion
film) =)
Summary of Christensen – Karol Pauline Farinas, M.D.
4 LAYERS OF INTENSIFYING SCREEN:
1)
2)
3)
4)




base made of poleyester plastic
reflecting layer (TiO2)
phosphor layer
plastic protective coat (cellulose)
total thickness is 15 to 16 mils
the efficiency with which the
phosphor converts xrays to light is
called intrinsic conversion
efficiency of phosphor
ability of light emitted by the
phosphor to escape from the screen
and expose the film is the screen
efficiency
intensification factor is the ration
of the xray exposure needed to
produce the same density on a film
with and without the screen
CHAPTER 10 PHYSICAL
CHARACTERISTICS OF X-RAY FILM AND
FILM PROCESSING
*Transfer of information from the xray
beam to the screen-film combination always
results in a loss of information
-
LATENT IMAGE



photographically active or
radiation-sensitive, emulsion that is
usually coated in both sides of a
transparent sheet of plastic (base)
FILM BASE
-
cellulose nitrate (then) ;
polyester (now)
provide support for the fragile
photographic emulsion
3 charachteristics: 1) must not
produce a visible pattern or absord
too much light when the radiograph
is viewed 2) flexibility, thickness
and strength must allow ease of
processing 3) dimensional stability
EMULSION
-
-
gelatin and silver halide
gelatin – keeps silver halide
dispersed, prevent clumping of
grains, processing solution can
penetrate easily
silver iodobromide – lightsensitive material in emulsion (90 –
site at which the developing
process will cause visible amounts
of metallic silver to be deposited
metallic silver is black. It is silver
that produces the dark areas seen
in the developing radiograph
the minimum number to produce
developability is between 3 and 6.
The more silver atoms that exist at
a latent image center, the greater
the probability that the grain will
be developed
DIRECT X-RAY EXPOSURE

FILM
-
99% silver bromide and 1-10%
silver iodide)
chemical sensitization of the
crystal takes several forms and is
produced by adding alllylthiourea
to produce silver sulfide which is
located in the surface of the crystal
called the sensitivity speck
photographic effect of direct
absorption of x-rays by the
emulsion is not caused by
electromagnetic radiation itself but
by electrons emitted when the
photon interacts with the silver
halide in the emulsion
FILM PROCESSING
1) Development
amplifies the latent image to form a
visible silver pattern
- reduction of the silver ion changes
it into black metallic silver
- time is a fundamental factor in the
developing process
- developing solutions:
hydroquinone and sodium sulfite
(makes the solution colorless and
act as preservative)
- FOG – development of unexposed
silver halide grains that do not
contain a latent image
- Temperature must be 90 to 95
degrees Fahrenheit
2) Replenishment
- maintain developing agent
concentration, preservative
concentration, bromide
concentration and pH
- in high volume (developing
reaction dependent), low volume
(oxidation reaction dependent =
-
[PHYSICS LECTURE]
raises pH and produces no
bromide)
3) Fixing
- removes the remaining silver halide
- uses sodium thiosulfate
- chromium or aluminum
compound is used as hardener
4) Washing
- remove the fixing-bath chemicals
- incomplete washing = film that
browns with age
CHAPTER 11 PHOTOGRAPHIC
CHARACTERISTICS OF X-RAY FILMS
1) mAs controls film density
2) kVp controls image contrast
 increase in film density of 0.3
decreases transmitted light to 50%
of its previous value
 higher density means a blacker
film and less light transmission
Film exposure – product of the intensity of
the exposure and the time of exposure

increase in the log relative
exposure of 0.3 always
represents doubling of the
relative exposure
4 FACTORS OF FILM CONTRAST:
1) characteristic curve of the film
- tells us how much change in film
density will occur as film exposure
changes
- the slope of a straight line joining
the 2 points of specified density on
the characteristic curve is average
gradient
2) film density
3) screen or direct x-ray exposure
4) film processing
- increasing the time or temperatur
of development will: 1) increase
average gradient (increase film
contrast) 2) increase film speed
(increase density for a given
exposure) 3) increase fog (decrease
film contrast)
LATITUDE
9
EMULSION ABSORPTION
- standard silver halide films absorb light in
the ultraviolet, violet and blue regions
- ortho film - green-sensitive film
- pan film – absorbs red
CROSS-OVER EXPOSURE
- “print-through exposure”
- occurs when a double emulsion xray film is
exposed in a cassette containing 2
intensifying screen
- decrease cross-over by: 1) matching
screen light emission to silver halide natural
sensitivity 2) changing the shape of the
silver halide grains
CHAPTER 12 FLUOROSCOPIC IMAGING
Copper-activated zinc cadmium sulfide –
fluorescent material in the screen that
emitted light in the yellow-green spectrum
IMAGE INTENSIFIER DESIGN
Vacuum tube contains:
1) Input Phosphor and Photocathode
- cesium iodide
- CsI because: 1) vertical orientation of the
crystals 2) greater packing density 3) more
favorable effective atomic number
- the photocathode is a photoemissive
metal (combination of antimony and cesium
compounds)
2) Electrostatic Focusing Lens
- inverts and reverses the image (point
inversion)
3) Accelerating Anode
- located at the neck of the tube
- accelerate electrons emitted from the
photocathode toward the output screen
- range of log relative exposure (mAs) that
will produce density within the accepted
rage for diagnostic radiology
4) Output Phosphor
* the latitude of film varies inversely with
film contrast
- crystal size and layer thickness are
reduced; good resolution even if small
- silver-activated zinc-cadmium sulfide
Summary of Christensen – Karol Pauline Farinas, M.D.
BRIGHTNESS GAIN
- tends to deteriorate as an image intensifier
ages (10% / year)
- comes from minificaion gain ( produced
by reduction in image size) and flux gain (
increases the brightness by factor of 50; for
each light photon from input screen = 50
photons in output screen)
- depends on thickness difference, density
difference, atomic number and kVp
* low kVP the greater the subject
contrast
2) Film Contrast
3) Fog and Scatter
IMAGING CHARACTERISTICS
- the effect of fog and scatter I to reduce
radiographic contrast
Contrast – brightness ratio of the periphery
to the center of the output screen
- scatter is produced by Compton
Scattering
Lag – persistence of luminescence after xray
stimulation has been terminated
- increased by improper film storage,
contaminated or exhausted developer
solution, excessive time or temperature of
development and use of high speed film
Distortion – result from unequal
magnification due to the flaring of the
peripheral electron from its course.
Vignetting – fall-off in brightness at the
periphery of the image
IMAGE QUALITY
-
CHAPTER 13 VIEWING AND RECORDING
THE FLUOROSCOPIC IMAGE
-
* vertical resolution depends on te number
of vertical lines and horizontal resolution is
determined by bandpass (frequency range
that the electronic components of a video
system must be designed to transmit)
-
- image is processed by a television camera
tube which can either be a standard vidicon,
plumbicon or a CCD
-
-
(for more details read pp 175-195)
CHAPTER 14 THE RADIOGRAPHIC IMAGE
R.E Wayren – suggested that the term
image clarity be used to describe the
visibility of diagnostically important detail
in a radiograph
ability of the film to record each
point in the object as a point on the
film
influenced by:
1) radiographic mottle
2) sharpness
ability of the xray film or film
screen system to define an edge
composed of geometric,
motion,absorption, screen and
parallax unsharpness (only in
double-emulsion film)
3) resolution
resolving power is he ability to
record separte images of small
objects that are placed very close
together
1) high contrast images are
SHARPNESS limited
2) low contrast images are NOISE
limited
RADIOGRAPHIC CONTRAST
the only factor determining noise in
the number of photons used by the
screen
- difference in density between areas in the
radiograph
MODULATION TRANSFER FUNCTION
- depends on:
-
1) Subject contrast
- difference in x-ray intensity transmitted
through one part of the subject as compared
to that transmitted through another part
-
concept that has been formulated to
provide an objective measurement
if the combined effects of sharpness
and resolution
amount of information seen is less
than the signal that had been
modulated
[PHYSICS LECTURE]
CHAPTER 15 GEOMETRY OF THE
RADIOGRAPHIC IMAGE
Magnification is decreased the closer
the object is to the film and keep the
focus-film distance as large as possible
DISTORTION
-
results from unequal magnification
of different parts of the same object
PENUMBRA
-
edge-gradient / geometric
unsharpness
region of partial illumination that
surrounds the umbra
width of the penumbra is less on
the anode side than on the cathode
side
INVERSE SQUARE LAW
-
states that the intensity of light
falling on a flat surface from a point
source is inversely proportional to
the square of the distance from the
point source
FOCAL SPOT BLOOMING
-
variation of focal spot size varies
with tube operating conditions
the focal spot size increases in
direct proportion to tube current
marked blooming seen in low kVp
and high mAs
focal spot size will decrease slightly
with increasing kVp
FACTORS THAT WILL DECREASE
UNSHARPNESS CAUSED BY:
MAGNIFICATION
1) small object-film distance
2) large focal spot-film distance
PENUMBRA
1) small focal-spot size
2) small object-film distance
3) large focal spot-film distance
MOTION
1) short exposure time
2) maximum possible limitation of
actual object motion
1
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CHAPTER 16 BODY SECTION
RADIOGRAPHY
- special x-ray technique that
blurs out the shadows of
superimposed structures to
shows more clearly the
principal structures being
examined
BLURRING
- distortion of definition of
objects outside the focal plane
Width of Blur
- refers to the distance over
which the image of an object is
spread out on the film and is
determined by:
1)amplitude of tube travel
- increase will increase width of
blur
2) distance from focal plane
- the farther from focal plane
the more blurred the object
3) Orientation of tube travel
- if longitudinal axis of the
object is oriented at the same
direction that the the tube
travels the object is not blurred
even if its outside the focal
plane
4) distance from the film
- farther from film, more
blurred the object
FULCRUM
- pivot point about which the
lever arm rotates, it determines
the plane that will be on focus.
FOCAL PLANE
- plane of maximal focus, and
represents the axis about
which the xray tube and film
rotate
FOCAL PLANE LEVEL
- height of the focal plane above
the table top
TOMOGRAPHIC ANGLE
- amplitude of the tube travel
expressed in degrees
EXPOSURE ANGLE
- angle through which the xrya
beam moves during the
exposure
Summary of Christensen – Karol Pauline Farinas, M.D.
WIDE ANGLE
1) tomographic arc >
10 (30-50)
2) less section
thickness
3) Considerable
unsharpness of focal
plane images
4) bone
5) maximum blurring
outside focal plane
6) can be done with
either circular or
linear motion
7) unlikely to cause
phantom image
8) long exposure time
NARROW ANGLE
1) tomographic arc <
10
2) greater section
thickness
3) very little
unsharpness
CHAPTER 18 XERORADIOGRAPHY
XERORADIOGRAPHY
-
4) lungs
5) minimum blurring
6) usually done with
circular motion
7) frequently cause
phantom image
8) short exposure time
LINEAR
1) inexpensive
2) section thickness is
dependent on
orientation of body
parts
3) blur margins are
tapered and indistinct
4) objects outside the
focal plane may be
incompletely blurred ;
“parasite streaks”
5) does not produce
phantom images
* in stereoscopy, the radiologist should know
which film should be viewed by each eye
CIRCULAR
1) expensive
2) uniform section
thickness
3) blur margins are
sharp and well-defined
4) objects outside focal
plane are uniformly
blurred; no “parasite
streaks”
5) likely to produce
phantom images
PANTOMOGRAPHY
- radiographic technique that produces a
panorami roentgenogram of a curved surface (e.g
teeth)
-
Valence band – band wih the highest energy
that also has electrons
Conduction band – next higher permissible
band

introduced by J. MacKenzie Davidson at
1898
MONOCULAR DEPTH PERCEPTION
Insulator: large forbidden gap that
electrons absorb enough energy to bridge
the gap
Semiconductor: small forbidden gap
XERORADIOGRAPHIC PLATE
1)
2)
-
3)
4)
-
Size: near objects are larger than far objects
Overlapping contours: near objects overlap
distant objects
Stereopsis - binocular depth perception;
dependent on the brain’s ability to see discrepant
image and fuse it into a single object with depth
Parallax - apparent displacement of an object
when viewed from 2 different vantage points
MAGNITUDE OF TUBE SHIFT
-
equal to 10% of the target-film
distance; equal to an angle of 6 degrees
the energy difference across the
forbidden energy gap determines
whether a solid acts as a conductor,
insulator or a semi-conductor
Conductor: no forbidden region between
the valence and the conduction band
CHAPTER 17 STEREOSCOPY
-
production of a visible image utilizing
the charged surface of a
photoconductor (amorphous selenium)
as the detecting medium, partially
dissipating the charge by exposure to
xrays to form a latent image
invented by Chester F. Carlson in 1937
ALUMINUM SUBSTRATE
- made of cleaned aluminum with
smooth surface
INTERFACE LAYER
thin layer of aluminum oxide
prevent negative charges induced in the
aluminum from migrating into the
selenium and dissipating the positive
charge induced in the selenium surface
SELENIUM COATING
PROECTIVE OUTER COATING
cellulose acetate
resistance is high enough to prevent
lateral conduction of charges which
would degrade the electrostatic latent
image
Dark Decay – reduction of plate voltage
while the plate remains in darkness
-
for xeroradiography, the rate should
not exceed 5% / min
IMAGING CHARACTERSISTICS OF
XERORADIOGRAPHY:
1)
2)
3)
Edge enhancement
Deletion
Subdued broad area response
1
3
[PHYSICS LECTURE]
4)
Broad exposure latitude
- this system is useful for imaging lowcontrast objects defined by sharp edges
CHAPTER 19 COMPUTED TOMOGRAPHY
-
introduced by G.N. Hounsfield in April
1972
ALGORITHMS FOR IMAGE RECONSTRUCTION
1)
2)
-
BASIC PRINCIPLE
-
the internal structure of an object can
be reconstructed from multiple
projections of the object
SCANNING MOTIONS
First Generation (translate – rotate, 1
detector)
- pencil-like xray beam; linear and rotatory
Second Generation (translate-rotate, multiple
detectors)
- fan-shaped beam and multiple detector
3)



Third Generation (rotate-rotate)
-
translation motion was completely
eliminated
multiple detectors are aligned along the
arc whose center is the xray tibe focal
spot
Fourth Generation (Rotate-fixed)

detectors from a ring that completely
surrounds the patient
each detector has a collimator at 2
points, 1 close to the xray tube and the
other at the detector
DETECTORS
-
2 TYPES: 1) scintillation crystals 2)
xenon gas ionization chamber
photomultiplier tubes are replaced by
silicon photodiodes (converts a light
signal into electron flow output that is
proportional to the intensity of the light
signal)
Components of Detectors
1)
2)
3)
4)
5)
an anode and cathode
a counting gas (inert gas)
a voltage between the anode and
cathode
walls that separate the detector from
the rest of the world
a window for the radiation photon to
enter the detector
BACK PROJECTION
summation method; oldest means of
image recontruction
none of the commercial scanners use
this method
ITERATIVE METHOD
starts with assumption and compares
this with measured values and averages
it
can either be simultaneous
reconstruction, ray-by-ray correction
and point-by-point correction
ANALYTICAL METHOD
used in almost all CT scanners today
exact formulas are used
can either be 2-dimensional fourier
analysis or filtered back-projection
a lack of precision, or the presence of
mottle, is the limiting factor in CT
performance at the present time
Quantum mottle is a variation in the
number of xray photon absorbed by the
detector
Most of the noise I current Ct images is
a result of statistical fluctuations and is
not related to mathematical
reconstruction
Voxel – volume element
Pixel – picture element; a flat surface without
thickness
RESOLUTION
Spatial Resolution
-
ability of the scanner to display
separate images of 2 objects placed
close together
Contrast Resolution
-
ability of an imaging system to display
an image of a relatively large object that
is only slightly different in density from
the surrounding
ARTIFACTS
Motion Artifacts
Streak Artifacts
-
when a high density material severely
reduces the transmission, streak will
appear as an image
Beam Hardening Artifacts
-
as a heterogeneous xray beam passes
through the patient the low energy
Summary of Christensen – Karol Pauline Farinas, M.D.
photons are rapidly absorb causing
artifact
Ring Artifact
- result of miscalibration of 1 detector in a rotaterotate geometry scanner
CHAPTER 20 ULTRASOUND
- velocity of sound depends on the velocity of the
medium
- oscillate in longer distances: gas > liquid > solid
Longitudinal waves
- ultrasonic pulses are transmitted through
liquids as longitudinal waves (meaning that the
motion of the particles in the medium is parallel
to the direction of wave propagation)
- length of the wave is the distance between 2
bands of compression, or rarefraction
- by definition, ultrasound has a frequency of
greater than 20,000 cycles/sec
convert ultrasonic energy eflected back from the
tissues into electric signal
- most important component is piezoelectric
crystal located near the face of the transducer
-“strain” refers to the deformity of the crystal
caused when a voltage is applied to the crystal
Characteristic of the Piezoelectric Crystal
- madeup of innumerable dipoles arranged in
geometric pattern
- ferroelectrics is a group of artificial
piezoelectric materials
Curie Temperature
- ceramic crystals are made-up of tiny dipoles
but to be able to have a piezoelectric
characteristic, the dipole must be in perfect
geometric configuration – to produce this
pattern, it will be heated at high temperature in
high electric field because at high temperature
the dipoles are free to move.
- diagnostic imaging have frequencies from
1,000,000 – 20,000,000 cycles/sec (hertz) or 120 MHz
- curie temperature is the temperature at which
this polarization is lost. Heating a piezoelectric
crystal above the curie temperature reduces it to
a useless piece of ceramic =(
Velocity of Sound
Transducer Q Factor
- independent of frequency and depends
primarily on the physical makeup of the material
through which the sound is being transmitted
- refers to 2 characteristics of piezoelectric
crystals: the purity of their sound and the length
of time that the sound persists
- sound travels slowest in gas then liquids and
fastest in solid
- high Q transducer (produces a nearly pure
sound with narrow range of frequency) and a
low Q (produces whole spectrum of sound with
wide range of frequency)
- inversely related to the compressibility of the
conducting material (more compressible, the
more rapidly it transmit sound)
- the more dense the transmitting material, the
lesser the velocity of sound
* in the ultrasonic frequency range, the velocity
of sound is constant in any particular medium. If
the frequency is increased, the wavelength must
decrease
Relative Sound Intensity
- measured in decibels
- positive decibels indicate a gain in power,
whereas negative decibels express loss of power
TRANSDUCERS
- device that converts one for of energy to
another
- convert electric signal into ultrasonic energy
that can be transmitted into tissues, and to
Spatial Pulse Length
- length of the sonic pulse
- it is the number of waves x wavelength
* in a transducer, a backing block is incorporated
to quench the vibrations and to shorten the sonic
pulse (to ready for the coming of the signals from
the tissues =))
Fresnel zone – parallel component
Fraunhofer zone – diverging portion of the
beam
________________________
__Fresnel zone_______Fraunhofer zone
* the length of the Fresnel zone is longest with a
large transducer and high frequency sound,
[PHYSICS LECTURE]
shortest with small transducer and lowfrequency sound
* tissue absorption increases with increasing
frequency
INTERACTIONS BETWEEN ULTRASOUND AND
MATTER
1) REFLECTION
- image is produced by the reflected portion of
the beam
- transmitted sound contributes nothing to image
formation
- the percentage of the beam reflected at tissue
interface depend on 1) acoustic impedance
(density x velocity, the velocity of sound in tissue
is fairly constant over a wide range of
frequencies, so a substance’s acoustic impedance
is constant) 2) beam’s angle of incidence (the
greater the angle of incidence, the less the
amount of reflected sound
2) REFRACTION
- bending of waves as they pass from 1 medium
to another
- can cause spatial distortion (real structures are
imaged in wrong location) and loss of resolution
of image
3) ABSORPTION
- there are 3 factor that affects absorption 1)
frequency of sound 2) viscosity of conducting
medium and 3) “relaxation time” of the medium
- bone has the highest absorption
- the relaxation time is the time that it takes for a
molecule to return to its original position after it
has been displaced
ULTRASONIC DISPLAY
- and electronic representation of data generated
from returning echoes and and displayed on a
TV monitor
A Mode
- echoes are displayed as spikes projecting from
a baseline
- the display on the cathode ray tube contains
information about the depth of structures and
the amplitude of the returning echo
TM Mode
- spikes are converted into dots
1
5
- used in echocardiography
B Mode
- produces a picture of a slice of tissue
- echoes are displayed as dots similar to TM
mode but in contrast, the transducer is moved so
that the sound beam traverses a plane of the
body
Pulse Rate
- refers to the nuber of separate little packets of
sound that are sent out each second
IMAGING PRINCIPLES
RESOLUTION
- depth resolution is the ability of the beam to
separate 2 objects lying in tandem along the axis
of the beam
- lateral resolution is the ability to separate 2
adjacent objects
- to narrow the beam and to improve resolution
we have to use a smaller transducer
Reverberation Echoes
- image appears with no physical existence
- transducer itself may act as a reflecting surface
and produce a reverberation artifact
* a close approximation of the focal length is
the diameter of curvature of the lens
DOPPLER TECHNIQUES
- the Doppler effect is a change in the perceived
frequency of sound emitted by a moving source
- the sound usually moves faster than the source,
so the source never catches the crest
- source motion does change the gap between
the crest
Scattering of Ultrasound by Blood
- sound reflection from solid structures is usually
referred to as specular reflection
- ultrasound encountering blood is not reflected ;
it is scattered in all directions. This scattering is
called Rayleigh-Tyndall scattering and is
caused by the RBC
REAL TIME ULTRASOUND
Summary of Christensen – Karol Pauline Farinas, M.D.
- real-time imaging systems are those that have
frame rates fast enough to allow movement to be
followed
Rem – unit of absorbed dose equivalent; unit
used only in radiation protection; Si unit is
sievert; 1 sievert = 100 rems
- line density refers to the number of vertical
lines per field of view
Linear energy transfer (LET) – amount of
energy deposited per unit length of travel,
expressed in keV/micron
Types of Real time Instruments
3 Types of Mechanical Scanning
1) Oscillating Transducer: Unenclose crystal
- a single transducer crystal is caused to osillate
through an angle
Relative biologic effectiveness (RBE) – another
expression used to compare the effectiveness of
several types of radiation
Natural Radiation
- produces a sector image
- from external ( cosmic and terrestrial gamma
radiation) and internal sources (radionuclides
w/in the body)
2) Oscillating Transducer” Enclosed crystal
External Sources
- with this system, the patient does not feel any
vibration because the moving transducer does
not touch the screen
- average cosmic ray annual dose equivalent is
0.26 mSv at sea level
- produces a trapezoidal image
- average annual gamma ray effective dose
equivalent is 0.28 mSv (variesfrom 0.16 – 0.63
mSv)
3) Rotating wheel Transducer
CHAPTER 21 PROTECTION
1928, the 2nd International Congress Of
Radiology appointed a committee to define the
roentgen (R) as a unit of exposure
1931, the 1st dose limiting recommendation was
made by a group of American scientists, the
Advisory Committee on X-ray and Radium
Protection (recommendation is 0.2 R/day or
50R/year) Now, MPD is brought down to 1/10th
of its original levels
Radiological Effects of Radiation
- can be somatic or genetic
- the most important somatic effect is
carcinogenesis; leukemia is the most common
neoplasia
- genetically significant dose – is the dose that
if received by every member of the population,
would expect to produce the same total genetic
injury as the actual doses received by the various
individual
Internal Sources
- from ingested food and inhaled particles
- annual effective dose equivalent from these
nuclides is 0.39 mSv
Medical Radiation
- estimated annual effective dose equivalent
from nuclear medicine studies is 0.14 mSv
- estimated annual effective dose equivalent from
diagnostic xray examinations is 0.39 mSv
EXAMINATION
Chest
Skull
Lumbar Spine
Upper GI
Abdomen
Barium Enema
Pelvis
IVP
Extremities
EDE / EXAMINATION
(mSv)
0.06
0.20
1.30
2.45
0.55
4.05
0.65
1.60
0.01
Radiation Units
Roentgen (R)- unit of radiation exposure
Stochastic Effects
Rad – unit of absorbed dose; 1 rad is equal to the
radiation necessary to deposit energy of 100 ergs
in 1 gram of irradiated material
- effect in which the probability of occurrence
increases with increasing absorbed dose
- 1 gray = 100 rads
- severity of the effects does not depend on the
magnitude of absorbed dose
(eg, cancers and genetic effects)
[PHYSICS LECTURE]
Non- stochastic effects
- somatic effect that increases in severity with
increasing absorbed dose
(eg, lens opacification and blood changes)
PLEASE SEE P 378 FOR THE DOSES OF
RADIATION =)
Protective Barriers
- distance is one of the most effective method of
radiation protection because exposures change
inversely with the square of the distance
* as a general rule, no secondary barrier is
required for areas protected by a primary barrier
Half Value Layer
- thickness of a specific substance that when
introduced into the path of a beam of radiation,
reduces the exposure rate by 1/2
Scatter Radiation
- the energy of scatter radiation is assumed to be
equal to that of the primary radiation
- the intensity of 90 degrees scatter radiation,
relative to the primary beam is reduced by a
factor of 1000 at a distance of 1 m for a field size
of 400 cm2
* maximum permissible leakage exposure 1
meter from a diagnostic xray tube is 0.1 R/h
CHAPTER 22 DIGITAL RADIOGRAPHY
CHAPTER 23 NUCLEAR MAGNETIC
RESONANCE
Precession – change in the direction of the axis of
rotation
Angular momentum - describes the rotational
motion of a body; has direction as well as
magnitude
1
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