Fisica Medica 3 – Raggi X
Corso di laurea in Fisica
A.A. 2007-2008
Electromagnetic spectrum
X-ray tube
Molybdenum Röntgen spectrum
X-rays source parameters
• emitted current * time
typ. values 6-100 mAs
• energy of emitted photons (keV)
it is expressed by the voltage between cathode and anode
typ. values 50-125 kV
Intreaction of photons with matter
Interaction of photons with tissues
[ ]
out in exp
I = I − µ d
exp ( )
out
in
x
out in
x
I = I ⎡ ⎢ − µ x dx ⎤ ⎥
⎢ ⎥
⎣ ∫ ⎦
0 in ( )
I = ∞ ∫ σ E dE
0
( ) exp ( , )
out
in
x out
x
I = ∞ σ E ⎡ ⎢ − µ E x dx dE ⎤ ⎥
⎢ ⎥
⎣ ⎦
∫ ∫
homogeneous medium
nonhomogeneous medium
energy spectrum
Equipment
increases mean energy of photon beam
reduces the irradiated area
reduces large angle scattering
X-ray detectors
•Screen-film detectors
•Image intensifiers
•Detectors for computed radiography
Screen-film detectors
Film
• 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 Iin
Iout
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 Æ the higher the speed Æ 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) Æ 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
X-ray detectors
•Screen-film detectors
•Image intensifiers
•Detectors for computed radiography
Image intensifier
Image intensifier
converts X rays into visible light
the photon releases electrons from the catode
electrons are converted to light photons
Image intensifier
• lower spatial resolution
• noise increase
• geometric distortion
X-ray detectors
•Screen-film detectors
•Image intensifiers
•Detectors for computed radiography
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 Æ digital image
Detectors for CR : direct radiography
similar to CCD but new materials overcome the problem of large areas e.g.: a-Si:H, a-Se
Clinical use – Radiographic images
Skeletal X rays
Chest imaging Mammography
P. Suetens; “Fundamentals of Medical Imaging”; CUP (2002)
