of 48
7/29/2019 Production and Characteristics of X-Rays_2
1/48
7/29/2019 Production and Characteristics of X-Rays_2
2/48
X-rays are one of the main diagnostical tools in medicine
since its discovery by Wilhelm Roentgen in 1895.
Current estimates show that there are approximately 650
medical and dental X-ray examinations per 1000 patients per year.
X-rays are produced when high energetic electrons
interact with matter.
The kinetic energy of the electrons is converted into
electromagnetic energy by atomic interactions (see chapter 7.1.)
7/29/2019 Production and Characteristics of X-Rays_2
3/48
The X-ray tube provides an environment for X-ray production
via bremsstrahlimgand characteristic radiation mechanisms.
electron source
electron acceleration potential
target for X-ray production
The classical X-ray tube requires:
7/29/2019 Production and Characteristics of X-Rays_2
4/48
The intensity of the electron beam determines the intensity
of the X-ray radiation. The electron energy determines the shape of
the bremsstrahlungs spectrum, in particular the endpoint of the
spectrum. Low energy X-rays are absorbed in the tube material.
7/29/2019 Production and Characteristics of X-Rays_2
5/48
The X-ray energy determines also the emission of
characteristic lines from the target material.
7/29/2019 Production and Characteristics of X-Rays_2
6/48
The major components of the modern X-ray tube are:
cathode (electron source)
anode (acceleration potential)
rotor/stator(target device)
glass/metal envelope (vacuum tube)
7/29/2019 Production and Characteristics of X-Rays_2
7/48
The figure shows a modern X-ray tube and housing assembly.
Typical operation conditions are:
Acceleration Voltage: 20 to 150 kV
Electron Current: 1 to 5 mA (for continuous operation)
Electron Current: 0.1 to 1.0 A (for short exposures)
7/29/2019 Production and Characteristics of X-Rays_2
8/48
The cathodeconsists of:
a. a spiral of heated low resistance R tungsten wire (filament) for
electron emission. Wire is heated by filament current I = U / R.
( U 10 V, I 3-6 A )
Electrons are released by thermionic emission, the
electron current is determined by the temperature which depends
on the wire current. The electron current is approximately 5 to 10times less than the wire current.
b. a focusing cup with a negative bias voltage applied to focus the
electron distribution.
7/29/2019 Production and Characteristics of X-Rays_2
9/48
The anode is the target electrode and is maintained at a positive
potential difference Vawith respect to the cathode. Electrons are thereforeaccelerated towards the anode: E = wVa
Upon impact, energy loss of electrons takes place by scattering and
excitation processes, producing heat, electromagnetic radiation and X-rays.
0.5% of the electron energy is converted into X-rays.
7/29/2019 Production and Characteristics of X-Rays_2
10/48
Because of the relatively low X-ray production efficiency,
most of the released energy comes in form of heat:
heat generation is a major limitation for X-ray machines
high melting point material with high X-ray output
tungsten (high melting point) good overall radiative emission
7/29/2019 Production and Characteristics of X-Rays_2
11/48
molybdenum (high melting points) high emission of characteristic X-rays
7/29/2019 Production and Characteristics of X-Rays_2
12/48
The two majoranodeconfigurations are:
The stationary anode is the classical configuration,
tungsten target for X-ray production and copper block as heat sink
7/29/2019 Production and Characteristics of X-Rays_2
13/48
The rotating anode is a tungsten disc, large rotating surface
area warrants heat distribution, radiative heat loss (thermally
decoupled from motor to avoid overheating of the shaft)
7/29/2019 Production and Characteristics of X-Rays_2
14/48
The anode angle is defined as the angle of the target
surface to the central axis of the X-ray tube.
The focal spot size is the anode area that is hit by the
electrons.
effective focal length = focal length sinq
The angle qalso determines the X-ray field size coverage. For
small angles the X-ray field extension is limited due to absorption and
attenuation effects of X-ray photons parallel to the anode surface.
The anode angle qdetermines the effective focal spot size:
7/29/2019 Production and Characteristics of X-Rays_2
15/48
Typical angles are: q = Tto 20.
A small angle in close distance is recommended for
small spot coverage, a large angle is necessary for large
area coverage.
7/29/2019 Production and Characteristics of X-Rays_2
16/48
The X-rays pass through a tube window (with low X-ray
absorption) perpendicular to the electron beam.
Usually the low energy component of the X-ray spectrum
does not provide any information because it is completely absorbed in
the body tissue of the patient. It does however contribute significantly
to the absorbed dose of the patient which excess the acceptable doselimit.
These lower energies are therefore filtered out by aluminum
or copper absorbers of various thickness.
7/29/2019 Production and Characteristics of X-Rays_2
17/48
The minimum thickness d depends on the maximum
operating potential of the X-ray tube but is typically d 2.5 mm for
Va 100 kV
The intensity drops exponentially with the thickness d:
with eff
as material dependent absorption coefficient.
7/29/2019 Production and Characteristics of X-Rays_2
18/48
The absorption coefficient is determined in terms of
the Half-Value LayerHVLwhich is the thickness of a material
necessary to reduce the intensity to 50% of its original value.
The solution yields:
7/29/2019 Production and Characteristics of X-Rays_2
19/48
Graph showing how the intensity of an x-ray beam
is reduced by an absorber whose linear absorption
coefficient is = 0.10 cm1.
7/29/2019 Production and Characteristics of X-Rays_2
20/48
7/29/2019 Production and Characteristics of X-Rays_2
21/48
7/29/2019 Production and Characteristics of X-Rays_2
22/48
7/29/2019 Production and Characteristics of X-Rays_2
23/48
7/29/2019 Production and Characteristics of X-Rays_2
24/48
7/29/2019 Production and Characteristics of X-Rays_2
25/48
7/29/2019 Production and Characteristics of X-Rays_2
26/48
The spectral distribution of the X-rays can be defined by
the appropriate choice of filters.
The filter material depends on the energy range of the
original X-ray distribution!
7/29/2019 Production and Characteristics of X-Rays_2
27/48
The influence of different filter combinations for a 200 kV
X-ray spectrum is shown in the figure.
7/29/2019 Production and Characteristics of X-Rays_2
28/48
The X-ray beam size is limited by a collimator system, the
collimators are lead for complete absorption.
7/29/2019 Production and Characteristics of X-Rays_2
29/48
Collimator design allows to optimize the point exposure!
7/29/2019 Production and Characteristics of X-Rays_2
30/48
The size of the collimator (object size) determines the
geometric "unsharpness" (blurring) of the image.
The blurring B in the image is given by:
where a is the effective size of the collimator of the
X-ray tube and m is the image magnification:
The resulting geometric unsharpeness Ugis defined:
Additional unsharpeness can be caused by the image
receptor (grain size, resolution of the film, etc) and by movement of
the object (restless person).
7/29/2019 Production and Characteristics of X-Rays_2
31/48
For general radiography purposes the geometric
unsharpeness dominates the other components
Therefore the unsharpeness will increase with increasingmagnification. To keep magnification small (close to m=1) requires
the image receptor to be as close as possible to the patient and the
focus patient distance to be large.
Typical conditions are:
a 1mm
d1 1 m
d2 10 cm
110cm= =1.1100cm
m
1=1mm 1- =0.091mm
1.1g
U
7/29/2019 Production and Characteristics of X-Rays_2
32/48
For a close dental X-ray shot the conditions are:
a 1mm
d1 5 cm
d2 1 cm
6cm= =1.25cm
m
1=1mm 1- =0.167mm
1.2
gU
7/29/2019 Production and Characteristics of X-Rays_2
33/48
The radiographic image of the X-ray exposure is
determined by the interaction of the X-rays which are transmitted
through the patient with a photon detector (film, camera etc.)
7/29/2019 Production and Characteristics of X-Rays_2
34/48
Primary X-ray photons have passed through the patient
without interaction, they carry useful information.
They give a measure for the probability that a photon pass through
the patient without interaction which is a function of the body tissue
attenuation coefficients.
Secondary photons result from interaction inside the patient, they
are usually deflected from their original direction and carry therefore only
little information. They create background noise which degrades the
contrast of the image.
Scattered photons are often absorbed in grids between the patient
and the image receptor.
7/29/2019 Production and Characteristics of X-Rays_2
35/48
The two dimensional image I(x, y)of the three dimensional
distribution of the X-ray attenuating body tissue of the patient can be
described as a function of the initial photon intensity Nof energy E,
the energy absorption efficiency of the image receptor(E)(film) and
the attenuation coefficients which have to be considered along thephoton path in z-direction.
with S(E)as distribution of the scattered secondary X-ray photons.
The expression can be simplified to:
with Ras the ratio of secondary to primary radiation.
7/29/2019 Production and Characteristics of X-Rays_2
36/48
As higher the attenuation coefficient, as larger absorption,
as lower the final intensity of the image.
For bone tissue the attenuation coefficient is considerably larger
than for soft body tissue, therefore increased absorption.
7/29/2019 Production and Characteristics of X-Rays_2
37/48
The quality of the image can be assessed by a few physical parameters:
radiographic contrast
noise and dose
CONTRAST OF THE IMAGE
Consider that you want to image clearly a target tissue of thickness x
with an attenuation coefficient 2 inside the body of thickness twith a lower softbody tissue attenuation coefficient 1
7/29/2019 Production and Characteristics of X-Rays_2
38/48
The contrast Cof the target tissue volume is defined
in terms of the image distribution function I1and I
2:
I1gives the energy absorbed outside the target tissue
I2
gives the energy absorbed inside the target volume.
Approximating for an X-ray energy E:
7/29/2019 Production and Characteristics of X-Rays_2
39/48
The expression can be simplified to:
The contrast depends mainly on the difference of attenuation coefficients1 and 2as well as on the ratio of scattered to primary X-ray photons.
As higher the ratio R (the number of scattered photons), as lower the contrast.
Therefore it is important to understand and to reduce secondary
scattered photon intensity to minimize R.
7/29/2019 Production and Characteristics of X-Rays_2
40/48
The number of scattered photons depends on several parameters:
X-ray field size; an increase in field size increases R3.5
Thickness of radiated volume (increase is roughly proportional
with thickness due to increase in scattering events)
X-ray energy dependence - decrease of scatter with increasing energy
7/29/2019 Production and Characteristics of X-Rays_2
41/48
To reduce the number of secondary scattered photons a
led grid is typically between object and image receptor. Because
scattered photons will not meet the grid at normal incidence, they
will be absorbed by the grid stripes.
7/29/2019 Production and Characteristics of X-Rays_2
42/48
7/29/2019 Production and Characteristics of X-Rays_2
43/48
NOISE AND DOSE
Even if the imaging system may have high contrast the noise
level may prevent identification of the object.
Two major noise components are:
statistical fluctuations in the number of X-ray photons
fluctuations in the receptor and display system
Th fi t t f th i i ll d t i ll
7/29/2019 Production and Characteristics of X-Rays_2
44/48
The first component of the noise is called quantum noise can usually
be reduced by increasing the number of photons used to form an image.
What is the minimum surface dose required on a body of
thickness t to see a contrast C for an object of size x over an area A
against a background of pure quantum noise?
The signal to be detect is:
The image noise in an areaA results from a statistical Poisson processand can be derived as:
This however will increase the dose absorbed by the patient which
should be minimized.
Thi i ld f th i l t i ti SNR
7/29/2019 Production and Characteristics of X-Rays_2
45/48
This yields for the signal to noise ratio SNR:
An object becomes detectable if the SNRexceeds a threshold value of:
At these conditions the number of incident photons Nfor the patient can
be calculated to:
7/29/2019 Production and Characteristics of X-Rays_2
46/48
The absorbed dose D for the patient is determined by the
number of photons per area N, the mass energy absorption coefficient
for tissue (), and the photon energy E:
The minimum dose required to visualize a fixed object increases with
the fourth power of the object size.
For a fixed dose and contrast there is a minimum object size which
can be visualized.
7/29/2019 Production and Characteristics of X-Rays_2
47/48
7/29/2019 Production and Characteristics of X-Rays_2
48/48