Production and Characteristics of X-Rays_2

7/29/2019 Production and Characteristics of X-Rays_2

1/48


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.)


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:


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.


5/48

The X-ray energy determines also the emission of

characteristic lines from the target material.


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)


7/48

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)


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.


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.


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


11/48

molybdenum (high melting points) high emission of characteristic X-rays


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


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)


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:


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.


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.


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.


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:


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.


20/48


21/48


22/48


23/48


24/48


25/48


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!


27/48

The influence of different filter combinations for a 200 kV

X-ray spectrum is shown in the figure.


28/48

The X-ray beam size is limited by a collimator system, the

collimators are lead for complete absorption.


29/48

Collimator design allows to optimize the point exposure!


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).


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


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


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.)


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.


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.


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.


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


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:


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.


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


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.


42/48


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


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


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:


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.


47/48


48/48

Production and Characteristics of X-Rays_2

Documents

Transcript of Production and Characteristics of X-Rays_2