ELECTROMAGNETIC (EM) RADIATION

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15 Νοε 2013 (πριν από 3 χρόνια και 10 μήνες)

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1

Principles of Imaging Science I
(RAD119)

Electromagnetic Radiation

Energy


Definition of energy


Ability to do work


Physicist

s definition of work


Work = force x distance


Force acting upon object over
distance expends energy

Mechanical Energy


Action of physical movement


Two types:


Potential


Kinetic

2

Chemical Energy


Energy released from chemical reaction


Examples:


Body converts chemical energy from
food into mechanical energy or
movement


Battery converts chemical energy into
electrical energy

Heat Energy


Also known as thermal energy


Results from movement of molecules


Temperature measures thermal
energy


Example:


Toaster converts electrical energy
into heat energy

Electrical Energy


Electricity


Results from movement of electrons
in conductor


Example:


Light bulb converts electric energy
to light

3

Electromagnetic Energy


Exists independently of objects


Present ubiquitously and spans an
energy continuum



Endless ordered arrangement


Combination of electrical and
magnetic bundles called photons or
quantum

Electromagnetic Energy (EM)


All types of
electromagnetic
radiation are a
form of energy


EM energy is the
result of electric
and magnetic
disturbances
traveling through
space



Typically, only the electric wave
is depicted in illustrations

Electromagnetic Energy (EM)


Pure energy travels through
space at speed of light


Electric and magnetic waves
90 degrees to each other


Does not need a medium to
be transmitted unlike
mechanical waves in water
or sound waves in air


Can travel in a vacuum


Entire band of energies is
grouped in the EM spectrum


4

PHOTONS


Smallest quantity of any electromagnetic
energy


Have no mass, no form


Quantum refers to a small bundle of energy
that travels through space at the speed of
light


Speed of light = 186,400 miles/sec





= 1.864 x 10
5
miles/sec





= 3 x 10
8

m/sec


Velocity of all electromagnetic radiation

PHOTON PROPERTIES


Electric & magnetic fields
that continuously change


in a wavelike motion


Field: Interaction among
the electric and magnetic
energies



Sine Wave: Variation of the
interactions is represented
as a sine wave

SINE WAVE DEFINITION & TERMS


Disturbance in a medium



Amplitude


One half the range of
the wave that varies
from crest to valley


Height of the wave

5

Sine Wave Terms


Wavelength




Distance
between adjacent
crests or valleys



Measured in
metric meters



Represented by
lambda (
λ
)

SINE WAVE TERMS


Frequency


# of wavelengths
that pass a given
point per second


Cycles/sec


Oscillations/sec


Measured in Hertz
(Hz)

1 Cycle/second = 1
Hz

Wavelength & Frequency Relationship

6

SINE WAVE TERMS



Period: Time to complete
one cycle of a wave



TIME /#CYCLES


Wave with a
frequency of one cycle
per second


= 1.0 sec period



Wave with a
frequency of two
cycles per second
= 0.5 sec period

Two Sine Wave Comparison

1 sec/2cps =
0.5 Period

1 sec/4cps =
0.25 Period

WAVE EQUATION


Relationship between the sine wave parameters


Change in one parameter affects the value of one or
both parameters



Amplitude is not related to frequency or wavelength



Equation


Velocity = Frequency x Wavelength


As velocity decreases, frequency decreases to
maintain wavelength



As velocity is maintained, frequency and
wavelength are inversely proportional



frequency = Velocity/Wavelength



Wavelength = Velocity/frequency

7

PARTICLE MODEL


Applied to electromagnetic radiation


Planck’s Quantum Theory


Direct relationship
between photon energy and
frequency


E =
hf



E = Photon Energy


h = Planck’s constant 4.15 X 10
-
15
eVs



f = Frequency Velocity
(c) = frequency x
wavelength

EM SPECTRUM


Continuum of electromagnetic energies


The full range of all of the different types of
electromagnetic radiations arranged in order of
increasing energy:


Radio


Radar/microwaves


Infrared


Visible light


Ultraviolet


X
-
rays and gamma rays


Represents frequency, wavelength, and energy

EM SPECTRUM

8

EM SPECTRUM

EM SPECTRUM


Wavelength and frequency are inversely
proportional.


Wavelength and energy are inversely
proportional.


Energy and frequency are directly proportional.

Electromagnetic Interactions


EM interaction with matter is based upon
wavelength


EM energy interacts with objects that have
a size similar to the wavelength


Radio/TV (km)


antennae


Microwaves (cm)


food


X
-
ray, Gamma ray


atoms


Visible light acts more like a wave
when it interacts with matter

»
Has particle properties


X
-
rays behave more like particles due
to ionizing potential

9

Light Characteristics


Wave and particle
characteristics


Visible light refers to the
light we can see (wave)


Infrared light, ultraviolet
light


Warmth and sunburn are
the manifestations of UV
energy (particles)


The intensity of light is
related to how many
particles are emitted from
the source and distance


Light Characteristics


Transmission


Passing of light rays
through a substance


Air, clear glass, or the
near vacuum of space

Wave Model



Light photons are
transmitted,
attenuated, or
absorbed

10

Wave Model


X
-
ray photons that interact with the
body are attenuated or absorbed

RADIOGRAPHIC TERMINOLOGY


Radiopaque


Anatomical structures that absorb x
-
ray
photons


Demonstrate anatomical structures white in
the image


Bones


Radiolucent


Anatomical structures that partially absorb or
attenuate x
-
ray photons


Demonstrate structures grey in the image


Soft tissue, organs, muscle

Radiographic Terminology


Density (Brightness)


Degree of blackening
in the image


High Density (Dark)


Low Density (White)



Contrast (Grey Scale)


Long Scale Contrast


Many shades of
grey


Low contrast


CXR, Abdomen


Short Scale Contrast


Black and white


High contrast


Bony anatomy


11

Radiopaque or Radiolucent?

Density

Long Scale vs Short Scale Contrast

12

Scale of Contrast

Contrast

INVERSE SQUARE LAW


Demonstrates the similarity of
x
-
rays and light rays


The intensity of radiation
decreases with the square of
the distance from the source


Doubling the distance from
the source decreases the
intensity 4x.


Halving the distance from the
source increases the intensity
4x.

13

Inverse Square Law


Light also acts like particles


Even though these photons are
steadily emitted by the light
source, as you move farther away
from the source, fewer photons
reach you


They spread out as they travel in a
wider area away from the source

THE INVERSE SQUARE LAW

THE INVERSE SQUARE LAW


The intensity of the radiation decreases with
an increase of distance from the source (and
vice versa)


Intensity is
inversely proportional
to the
square

of the distance


Formula: I
2

= I
1

(d
1
/d
2
)
2



I
1

= Old intensity I
2

= New intensity


d
1

= Old distance d
2

= New distance


Formula may also be expressed as:
I
1
/I
2

= d
1
2
/d
2
2

14

INVERSE SQUARE LAW


I
1 =
D

2
2


___ _____


I
2
D

1
2




I
1 = Original Intensity

I
2 = New Intensity

D

1

= Original Distance

D

2

= New Distance


Examples