Transcript RENTGENSKO ZRAČENJE - University of Zagreb Medical

X - RAYS IN DIAGNOSTICS
D. Krilov
22. 10. 2008.
HISTORY
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W.C.Röntgen (1845-1923) 8.11. 1895. discovered a new type of radiation in
experiments on gas discharges; he
demonstrated that this radiation:
induces the ionization in the air
penetrates through the matter
does not deflect in electric and magnetic field
foggs the photographic plate
induces the burns on skin
in January 1896. he produced the first
anatomical X-ray picture of the hand
Nature, Jan. 23 1896
Science, Feb.14 1896
The nature of X-rays
X - rays are electromagnetic waves (1 pm – 0,1 nm)
 natural sources of X-rays do not exist; the artificial source
is X-ray tube
X-rays are produced by two mechanisms:
1. by rapid decceleration of fast electrons in electric fields
generated by heavy nuclei
2. by relaxation of heavy atoms in tube target (anode)
 the medical application is based on specific interactions of
incident X-ray photons with atoms in tissues; the image is
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produced from the beams transmitted through the body
X-ray tube
mv2/2= eUa
Ie= 20-30 mA
cathode
anode
Ig = 3-5 A
tungsten disc
Ua= 30-150 kV
Generation of X-rays
Brehmsstrahlung (braking
radiation)
High speed electrons enter the crystal
lattice of target atoms and are deccelerated
in electric field of atomic nuclei.
Energy of emitted photon depends on
hn  Ein  Eout
energy loss of photon.
The photon with highest energy is
generated when electron is stopped
h n max  h
 min nm 
c
 min
 eU a
1,24
U a kV 
hn
Ein
Eout
transmitted beam is composed of photons with
different energies - continuous spectrum
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Collisions of incident
electrons with electrons in
target material
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Incident electron ejects one of
electrons from inner shell of target
atom.
The empty state is filled by an
electron from higher shell, the
energy difference is emitted as Xphoton
Only the photons with energies
equal to differences of particular
atomic levels are emitted - line
spectrum reflects the atomic
structure of target
The probability of such events is
low - the intensity of line spectrum
is low
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Linear spectrum consists of a number of
narrow lines
transitions into K shell: Ka, Kb
transitions into L shell: La, Lb
X-ray spectrum
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The spectrum is the plot of spectral
radiancy over wavelength
It is composed of continuous and linear
part
continuous spectrum has well defined
short-wavelength cutoff determined by
anode voltage
the highest radiancy is at wavelength
4
   min
3
linear spectrum is not important for
medical diagnostics
I(W/m3)
min
Influence of tube voltage on X-ray
spectrum
Beam power is determined by empiric relation:
P  k U a2 I e Z
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spectral radiancy
Ie is the current of electrons in tube which
depends on heating current of cathode; Z is
atomic number of target atoms
Intensity of beam I   I  d is the ratio of
power and the surface of the window on tube
I P
A
Increase of voltage enhances the beam intensity.
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The spectrum is shifted to shorter wavelengths –
the hardness of the beam is higher
Influence of heating current and window
filter on X-ray spectrum
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I
high heating
current
without
filter
with
filter
low heating current
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short wavelength cutoff and wavelength
of highest radiancy are not influenced
by heating current; only the intensity of
beam depends on the current
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short wavelength cutoff is the same but
wavelength of highest radiancy is shifted to
shorter wavelengths. The intensity is lower
but the hardness is higher.
Interaction of X-ray photons with atoms in
tissue
the type of interaction depends
on photon energy and atomic
composition of tissue
photoelectric effect
predominates for the photons with
energies lower than 80 keV; it is more
probable for heavy atoms in
tissue which are present in bones (Ca)
Compton scattering
predominates for the photons with
higher energies; it is more probable for
lighter atoms in soft tissue (O, N, C,
H)
Law of attenuation for X-rays
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Intensity decrease of monochromatic beam along its path through the
tissue:
I  ( x )  I  ( 0 )e m  x
m() is linear absorption coefficient which depends on tissue and
wavelength of radiation
 in medical diagnostics is commonly used mass absorption coefficient:
m
mm   t  s
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which depends on probabilities for photoelectric effect (t) i Compton
scattering (s)
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Half value layer
the thickness of absorber which attenuates half of the photons
I x  I 0e mm x
x  x1 / 2  I x1 / 2 
x1 2
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x1 / 2 
ln 2
mm
I0
2
parameter for determination of
hardness of polychromatic beam
x1 2
12
x
x
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higher x1/2 means harder beam
Plot for attenuation of real polychromatic beam
I
rapid decay due to absorption
of low energy photons
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The analytical expression for
the attenuation of
polychromatic beam does not
exist
The average energy of
polychromatic beam is chosen
as the energy of corresponding
monochromatic beam with
equal half value layer.
x
along the path through tissue the beam becomes
harder due to predominate influence of higher energy
photons – penetration power is increased
X-ray diagnostics
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In classical radiography we get the image obtained from transmitted beams; it
displays the shadows of tissue structures - the image is 2D projection of 3D object;
therefore the shadows of bones overlay the shadows of soft tissue
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Intensity of transmitted beam depends on absorption coefficient
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The images of bones are obtained with high contrast if we apply lower tube voltage low energy photons; then, the absorption coefficient for photoelectric effect in bones
is increased and the absorption in the soft tissue is very low
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the good images of soft tissue are obtained if we apply higher tube voltage - high
energy photons; in such conditions the absorption coefficient for Compton effect is
increased; we can see shadows of soft tissues but also the more pale shadows of the
bones due to lower absorption of high energy photons; however, the overall contrast
is worse than for low voltage
Computer Assisted Tomography (CT,CAT)
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Hounsfield and Cormack – 1972.
it is the combination of special way
of recording, accumulation of data
and mathematical processing for the
image reconstruction
The basic concept of the method
The narrow beam propagates
through the layer of the body, its
thickness is determined by the beam
width.
At the other end is a detector which
measures the intensity of transmitted
beam.
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The layer (perpendicular to the long
axis of the body) is divided in volume
elements - voxel (10 mm3).The size of
one voxel is determined by the cross
section of the beam ; the voxel size
determines the resolution of the image
To each voxel is attributed its
absorption cofficient. The beam
propagates throuh the row of voxels
and the intensity of transmitted beam
is:
I t  I 0 e  m1 m2 m3 ...mn  x
pixel – element of 2D image of the layer;
one pixel contains the information from one
voxel - the number of pixels depends on the
number of voxels
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By subsequent parallel translation of tube and detector across the layer,
the whole layer is recorded. The process is repeated after each rotation
of the pair tube-detector for a small angle. In that way we get enough
data for the processing of the complex mathematical algorithm for the
distribution of attenuation coefficients in the layer. The calculated data
are transformed into pixels and displayed in grey scale.
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The number of counted photons determines the precision of absorption
measurement along one row of voxels.
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The contrast depends on absorption differences in particluate tissues.
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The described procedure demanded time-consuming recording, so the
technological improvement was based on building the devices for
much faster recording which became possible after construction of
light, small and cheap detectors.
Novel equipments for CT
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They enable instantaneous recording for large number of directions in
the layer simuntaniously. In that way the interval of patient irradiation
is significantly shortened. Application of fan shaped beams and
automatic rotation of the tube, makes the recording time even shorter.
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CT equipment which is in use nowadays is constructed with immobile
detectors arranged in a circle perpendicular to the long axis of the
body, while the tube is rotating in that plane. By automatic shift of bed,
the new layers are recorded.
Spiral CT
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This novel method enables
additional shortening of
recording time, due to very fast
computers. The data are taken
and processed while the body is
continuously shifted.
The image of the heart can be
obtained in 0.1 s.
The computers enable
reconstruction of 3D image