X-rays are one of the main diagnostical tools in medicine since its discovery by
Neave, Charles, Contributing Editor has reference to this Academic Journal, PHwiki organized this Journal 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 in addition to 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.) The X-ray tube provides an environment as long as X-ray production via bremsstrahlimg in addition to characteristic radiation mechanisms. electron source electron acceleration potential target as long as X-ray production The classical X-ray tube requires:
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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:
The figure shows a modern X-ray tube in addition to housing assembly. Typical operation conditions are: Acceleration Voltage: 20 to 150 kV Electron Current: 1 to 5 mA ( as long as continuous operation) Electron Current: 0.1 to 1.0 A ( as long as short exposures) The cathode consists of: a. a spiral of heated low resistance R tungsten wire (filament) as long as 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. b. a focusing cup with a negative bias voltage applied to focus the electron distribution. The anode is the target electrode in addition to is maintained at a positive potential difference Va with respect to the cathode. Electrons are there as long as e accelerated towards the anode: E = wVa Upon impact, energy loss of electrons takes place by scattering in addition to excitation processes, producing heat, electromagnetic radiation in addition to X-rays. 0.5% of the electron energy is converted into X-rays.
Because of the relatively low X-ray production efficiency, most of the released energy comes in as long as m of heat: heat generation is a major limitation as long as X-ray machines high melting point material with high X-ray output tungsten (high melting point) good overall radiative emission molybdenum (high melting points) high emission of characteristic X-rays The two major anode configurations are: The stationary anode is the classical configuration, tungsten target as long as X-ray production in addition to copper block as heat sink
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 sinq The angle q also determines the X-ray field size coverage. For small angles the X-ray field extension is limited due to absorption in addition to attenuation effects of X-ray photons parallel to the anode surface. The anode angle q determines the effective focal spot size: Typical angles are: q = T to 20°. A small angle in close distance is recommended as long as small spot coverage, a large angle is necessary as long as 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 in as long as mation 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 there as long as e 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 as long as Va 100 kV The intensity drops exponentially with the thickness d: with eff as 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 as long as 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 as long as 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 in addition to m is the image magnification: The resulting geometric unsharpeness Ug is defined: Additional unsharpeness can be caused by the image receptor (grain size, resolution of the film, etc) in addition to by movement of the object (restless person).
The absorbed dose D as long as the patient is determined by the number of photons per area N, the mass energy absorption coefficient as long as tissue (), in addition to 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 in addition to contrast there is a minimum object size which can be visualized.
Neave, Charles Contributing Editor
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