Transcript Seminar iz klasičnih metoda rentgenske dijagnostike

X-ray radiography
Seminar
22.10. 2008.
Difficulties and problems of classical radiography
 The "geometrical" unsharpness of X-ray images is limited by
finite dimension of the source, i.e. the window on X-ray tube.
Therefore the optical shadow is always surrounded by halfshadow. By adjustment of distance between the tube, object
and film we try to minimalize the half-shadow.
 Secondary photons, emitted by relaxation of atoms after
Compton scattering, leave the body and uniformly increase the
exposure of the film reducing the contrast.
 The quality of radiographs can be improved by the use of
special filters, X-ray film coated with fluorescence layer and
image intensifier.
Contrast of the image
 The X-ray beams will be absorbed in different amount in
particulate tissues. The transmitted beams will have different
intensities - the basis for the contrast in the image.

I1  I 2
K
I1  I 2
I0
I1(bones)
I2 (soft tissues)
 The contrast is satisfactory if the intensity difference is at least
10%.
 Using the contrast agents we can improve the contrast.
 Soft tissues are absorbing Xradiation by the mechanism of
Compton effect. The probability
for absorption depends on
electron density which is almost
the same for the atoms of soft
tissue and the contrast is rather
low.
 It can be enhanced by
introduction of substance with
high density and high Z number
(contrast agent) in organ under
investigation. These agents are
absorbing X-radiation by
photoelectric effect. Using the
lower voltage, only the soft
tissues with contrast agent will
have strong absorption.
intestines with solution of barium salt
Influence of half-shadow on contrast
half-shadow is the consequence of finite dimension of the source
half-shadow
x
L
shadow
D
film
window aperture
x = D d/L
object
d
half-shadow is thinner if the distance from object
to film is small in comparison with distance from
object to source
Use of intensifying screen on X-ray film
 Photographic emulsion is not
enough sensitive for Xphotons, only 1% of incident
photons are absorbed in film it would require the long
irradiation of patient to get the
good image
 Intensifying screen (phosphor
in low Z matrix) enables the
transformation of X-photons
into light photons (1 X - 1000
VIS) which can be completely
absorbed in film
 The intensity of image can be
enhanced about 30 times
Image intensifier
- in fluorescent screen Xphotons are transformed in
photons of visible light
- these photons eject
electrons from photocathode
- in evacuated tube,
electrons are accelerated
toward anode; their motion
is controlled by electron
optics
convenient for surgery halls, teaching,
distance learning
- electrons hit the other
fluorescent screen and create
the image of high intensity
which can be monitored in
the dark
Image intensifier in surgery
Xerography
 Xerography uses photoconductivity photoelectric effect on a semiconductor
 the images have sharp boundary
lines - the borders between different
tissues
 The beam transmitted through the
body contacts with positively charged
selenium coated aluminum plate; at the
collision site, the photon energy causes
selenium to become conductive and the
charge is neutralized; the distribution of
positive charge density is inversely
proportional to the number of incident
photons (electrostatic image)
 by applying negatively charged dye
powder we can get visible image
X-beam
objects with
different density
edges are sharp due to changes
of electric field as a step function