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JP©1
X - RAYSX - RAYS
Wilhelm Conrad Röntgen
1895
JP©2
Induction Coil
+ve-ve
Cathode Ray Discharge Tube
ROENTGEN 1895
Wrapped Photographic Platewas EXPOSED
X – RAYS !!
JP©3
Early cold cathode X-ray tube
Air at low pressure
0 V 100 kV
cathode anode
JP©4
Heater current
-ve high voltage +ve
BASIC PRINCIPLE
Anode +ve
Thermionic Emission of Electrons
X-Ray Photons
JP©5
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
JP©6
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
JP©7
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
JP©8
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
JP©9
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
JP©10
X-RAY SPECTRUM AS A FUNCTION OF TUBE P.D.
Intensity
Voltage / kV
100 kV 150 kV
JP©11
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
JP©12
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.
JP©13
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
JP©14
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
JP©15
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
JP©16
X-RAY IMAGING
Lead aperture
Lead cone
anode
Lead GRID absorbs scattered X-Rays
FILM
JP©17
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
JP©18
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
JP©19
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.
JP©20
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)