Fisica Medica

3 – Raggi X

Corso di laurea in FisicaA.A. 2007-2008

Electromagnetic spectrum

X-ray tube

Molybdenum Röntgen spectrum

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

Intreaction of photons with matter

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

Equipment

increases mean energy of photon beam

reduces the irradiated areareduces large angle scattering

X-ray detectors

•Screen-film detectors

•Image intensifiers

•Detectors for computed radiography

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

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

X-ray detectors

•Screen-film detectors

•Image intensifiers

•Detectors for computed radiography

Image intensifier

Image intensifierconverts 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 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 imagesSkeletal X rays

Chest imaging

Mammography

P. Suetens; “Fundamentals of Medical Imaging”; CUP (2002)

Radiographic room

Clinical use – Fluoroscopic images

Principal applications

• Intraoperative fluoroscopy

• Angiography

• Barium fluoroscopy

• Urography

Images are produced in real time

3D angiographic room

X-ray angio

3D angio