fisica medica 3 – raggi xrenzo.campanella/appunti/fismed-3-xrays.pdf · fisica medica 3 – raggi...
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Fisica Medica
3 – Raggi X
Corso di laurea in FisicaA.A. 2007-2008
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Electromagnetic spectrum
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X-ray tube
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Molybdenum Röntgen spectrum
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X-rays source parameters
• emitted current * timetyp. values 6-100 mAs
• energy of emitted photons (keV)it is expressed by the voltage betweencathode and anodetyp. values 50-125 kV
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Intreaction of photons with matter
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Interaction of photons with tissues
[ ]expout inI I dµ= −
exp ( )out
in
x
out inx
I I x dxµ⎡ ⎤
= −⎢ ⎥⎢ ⎥⎣ ⎦∫
0
( )inI E dEσ∞
= ∫
0
( ) exp ( , )out
in
x
outx
I E E x dx dEσ µ∞ ⎡ ⎤
= −⎢ ⎥⎢ ⎥⎣ ⎦
∫ ∫
homogeneous medium
nonhomogeneous medium
energy spectrum
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Equipment
increases mean energy of photon beam
reduces the irradiated areareduces large angle scattering
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X-ray detectors
•Screen-film detectors
•Image intensifiers
•Detectors for computed radiography
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Screen-film detectorsFilm• contain an emulsion with silver halides crystals (AgBr)• the crystals when irradiated transform into metallic silver: the area becomes dark• the remaining silver halide crystals are removed during development• very inefficient (QE = 0.02)
Two screens enclose the film. These screens:• contain phosphors with high QE• the phosphors convert X rays into visible light• increase QE up to 25%
Characteristics:• graininess• contrast• speed• resolution
sensitometric curve
Optical density D = log IinIout
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Characteristics of screen-film detectors• speed: inversely proportional to the amount of light needed to produce a given
amount of metallic silver on development (the larger the size the higher the speed the lesser the number of photons needed for the whole image)
• graininess: inversely related to the speed (if the grain size is large (fast film) the grain distribution can be observed underneath the image)
• contrast: is described by the plot of the optical density D = log (Iin/Iout)
• resolution: depends mainly on the grain size
sensitometric curve
high contrast
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X-ray detectors
•Screen-film detectors
•Image intensifiers
•Detectors for computed radiography
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Image intensifier
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Image intensifierconverts X rays into visible light
the photon releases electrons from the catode
electrons are converted to light photons
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Image intensifier
• lower spatial resolution
• noise increase
• geometric distortion
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X-ray detectors
•Screen-film detectors
•Image intensifiers
•Detectors for computed radiography
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Detectors for CR : storage phosphors
• impurities in the phosphors act like electron traps;
• electrons excited from the valence band are trapped and energy is stored;
• the latent image is preserved for a long time (in 8 hrs decrease by 25%);
• the stored energy is extracted by pixel-wise scanning with a laser beam: the
electron falls in the valence band and releases visible light;
• optic array and photomultiplier;
• ADC digital image
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Detectors for CR : direct radiography
similar to CCD but new materials overcome the problem of large areas
e.g.: a-Si:H, a-Se
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Clinical use – Radiographic imagesSkeletal X rays
Chest imaging
Mammography
P. Suetens; “Fundamentals of Medical Imaging”; CUP (2002)
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Radiographic room
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Clinical use – Fluoroscopic images
Principal applications
• Intraoperative fluoroscopy
• Angiography
• Barium fluoroscopy
• Urography
Images are produced in real time
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3D angiographic room
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X-ray angio
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3D angio