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.
The X-ray energy determines also the emission of characteristic lines from the target material.
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)
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)
The cathode consists 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 10 times less than the wire current.
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)
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 • sin
The angle also 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 determines the effective focal spot size:
Typical angles are: = Tto 20°.
A small angle in close distance is recommended for small spot coverage, a large angle is necessary for large area coverage.
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 dose limit.
These lower energies are therefore filtered out by aluminum or copper absorbers of various thickness.
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 effas material dependent absorption coefficient.
The absorption coefficient is determined in terms of the Half-Value Layer HVL which is the thickness of a material necessary to reduce the intensity to 50% of its original value.
The solution yields:
Graph showing how the intensity of an x-ray beam is reduced by an absorber whose linear absorption coefficient is = 0.10 cm1.
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!
The influence of different filter combinations for a 200 kV X-ray spectrum is shown in the figure.
The X-ray beam size is limited by a collimator system, the collimators are lead for complete absorption.
Collimator design allows to optimize the point exposure!
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:
Additional unsharpeness can be caused by the image receptor (grain size, resolution of the film, etc) and by movement of the object (restless person).
For general radiography purposes the geometric unsharpeness dominates the other components
Therefore the unsharpeness will increase with increasing magnification. 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:
d1 1 m
d2 10 cm
For a close dental X-ray shot the conditions are:
d1 5 cm
d2 1 cm
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.)
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.
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 N of energy E, the energy absorption efficiency of the image receptor (E)(film) and the attenuation coefficients which have to be considered along the photon path in z-direction.
with S(E)as distribution of the scattered secondary X-ray photons.
The expression can be simplified to:
with R as the ratio of secondary to primary radiation.
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.
The quality of the image can be assessed by a few physical parameters:
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 t with a lower soft body tissue attenuation coefficient 1
The number of scattered photons depends on several parameters:
X-ray field size; an increase in field size increases R 3.5
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
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.
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
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?