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X - RAYSX - RAYS

Wilhelm Conrad Röntgen

1895

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Induction Coil

+ve-ve

Cathode Ray Discharge Tube

ROENTGEN 1895

Wrapped Photographic Platewas EXPOSED

X – RAYS !!

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Early cold cathode X-ray tube

Air at low pressure

0 V 100 kV

cathode anode

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Heater current

-ve high voltage +ve

BASIC PRINCIPLE

Anode +ve

Thermionic Emission of Electrons

X-Ray Photons

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On Average 99% of each electron’s energy is converted into heat energy.

X-RAYS ARE PRODUCED IN TWO WAYS.

1. A BACKGROUND BREMSSTRAHLUNG SPECTRUM

2. A FINE LINE SPECTRUM

Wavelength range : 10-9 - 10-11 metres

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BREMSSTRAHLUNG RADIATION SPECTRUM

ACCELERATING CHARGES RADIATE ELECTROMAGNETIC RADIATION

+

NUCLEUS

When a fast-moving electron swings around a heavily charged nucleus, its acceleration changes rapidly.

BREMSSTRAHLUNG radiation

Eenergyphoton

Photon Energy, E = hf

E

hc

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BREMSSTRAHLUNG BACKGROUND SPECTRUM

Many X-Ray wavelengths [ down to a certain minimum ] may be produced by a particular X-Ray tube, depending upon how much of the electron’s energy is converted in this way.

0 2 4 6 8 10λ x 10-11 m

Intensity

50 kV

20 kV

Maximum electron energy available = eV

minhc

hfeV

eV

hcminλmin

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FINE [ LINE ] SPECTRUM

DEPENDS UPON THE TARGET METAL IN

THE ANODE

50 kV

0 2 4 6 8 10λ x 10-11 m

Intensity

20 kV

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LINE SPECTRUM Produced after an electron knocks out an inner electron from one of the target atoms

NUCLEUS

K

L

M

e

Electrons from a higher energy level can then fall in to a vacant energy level.

Excess energy is lost as an X-Ray photon

e

e

LM EE

hc

KL EE

hc

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X-RAY SPECTRUM AS A FUNCTION OF TUBE P.D.

Intensity

Voltage / kV

100 kV 150 kV

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ROTATING ANODE X-RAY TUBE

6.3 V A.C.

hot filament

focusing cathode

0 V

100 000Vvacuum

motor stator

motor rotor

ball race bearings

rotating anode

tungsten target

electron beam

X-RAYS

THE ANODE ROTATES AT 3000 RPM

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THE ANODE IS ROTATED SO THAT IT DOES NOT MELT.

ROTATING ANODE X-RAY TUBE

THE TUBE IS IMMERSED IN OIL TO ASSIST COOLING.

X-Ray QUALITY [= penetrating power] is increased by increasing the p.d. across the tube.

X-Ray INTENSITY [ number of photons per second] is increased by increasing the filament current.

This is the way in which the exposure time to produce a photographic plate is controlled.

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ATTENUATION OF X-RAYS

IN A VACUUM, OR AIR, ATTENUATION OBEYS THE INVERSE SQUARE LAW

IN A MATERIAL OF THICKNESS X metres xeII 0

Where I = the transmitted intensity in Wm-2 , I0 = the incident intensity and

μ = the linear attenuation coefficient.

% t

ran

smis

sio

n 100

50

x

2

1x

xeII 0

the half thickness

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x

the half thickness is the thickness of material that halves the X-Ray intensity

2ln

2

1 x

The mass attenuation coefficient, μm, is the attenuation

per unit mass of material. 

m where ρ is the density of the material

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X-RAY IMAGING X-rays cannot be focused.

They only make shadow images.

We therefore need to produce a point source.

The geometry of the anode restricts the angular beam width to about 17o.

The width of the beam is further limited with strips of lead.

Lead aperture

Lead cone

anode

Narrow X-Ray Beam

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X-RAY IMAGING

Lead aperture

Lead cone

anode

Lead GRID absorbs scattered X-Rays

FILM

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IMAGE INTENSIFIER TUBE

fluorescent screen A

X R

AY

Sphotocathode anodes fluorescent

screen B

evacuated glass envelope

electrons

to TV camera

Screen A converts X – Rays into light

Light releases photoelectrons from the photocathode

Electrons accelerated by anodes

The energy gained by the electrons increases the intensity produces in screen B by a factor of 100

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Barium sulphate is used for X-rays of the digestive system. It is given as a white liquid drink (barium meal) or into the back passage (barium enema). X-rays cannot go through it, so when the X-ray pictures are taken, the outline of the stomach or bowel shows up on the X-ray.

Contrast Media

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X – Ray source moves around the circular tube sending X - Rays through the patient in a fan shaped beam.

X –Ray detector moves around in time with the source and measures the X – Ray strength in each position.

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Each time the x-ray tube and detector make a 360 degree rotation and the x-ray passes through the patient's body, the image of a thin section is acquired.

During each rotation, the detector records about 1,000 images (profiles) of the expanded x-ray beam.

Each profile is then reconstructed by a computer into a two-dimensional image of the section that was scanned.

Computed Tomography Imaging (CT Scan, CAT Scan)