Slide 1: JP© 1 X - RAYS Slide 2: JP© 2 Cathode Ray Discharge Tube ROENTGEN 1895 X – RAYS !! Slide 3: JP© 3 Early cold cathode X-ray tube Slide 4: JP© 4 Heater current X-Ray Tube BASIC PRINCIPLE Thermionic Emission of Electrons X-Ray Photons Slide 5: 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 Slide 6: JP© 6 BREMSSTRAHLUNG RADIATION SPECTRUM ACCELERATING CHARGES RADIATE ELECTROMAGNETIC RADIATION When a fast-moving electron swings around a heavily charged nucleus, its acceleration changes rapidly. BREMSSTRAHLUNG radiation Photon Energy, E = hf Slide 7: 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. Maximum electron energy available = eV Slide 8: JP© 8 FINE [ LINE ] SPECTRUM DEPENDS UPON THE TARGET METAL IN THE ANODE Slide 9: JP© 9 LINE SPECTRUM Produced after an electron knocks out an inner electron from one of the target atoms e 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 Slide 10: JP© 10 X-RAY SPECTRUM AS A FUNCTION OF TUBE P.D. Intensity Voltage / kV 100 kV 150 kV Slide 11: JP© 11 ROTATING ANODE X-RAY TUBE 6.3 V A.C. hot filament focusing cathode 0 V 100 000V vacuum motor stator motor rotor ball race bearings rotating anode tungsten target electron beam X-RAYS THE ANODE ROTATES AT 3000 RPM Slide 12: 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. Slide 13: JP© 13 ATTENUATION OF X-RAYS IN A VACUUM, OR AIR, ATTENUATION OBEYS THE INVERSE SQUARE LAW IN A MATERIAL OF THICKNESS X metres Where I = the transmitted intensity in Wm-2 , I0 = the incident intensity and
µ = the linear attenuation coefficient. the half thickness Slide 14: JP© 14 the half thickness is the thickness of material that halves the X-Ray intensity The mass attenuation coefficient, µm, is the attenuation
per unit mass of material. Slide 15: 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. Narrow X-Ray Beam Slide 16: JP© 16 X-RAY IMAGING Lead GRID absorbs scattered X-Rays Slide 17: JP© 17 IMAGE INTENSIFIER TUBE X RAYS 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 Slide 18: 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 Slide 19: 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. CT SCANNER Slide 20: 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)