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X-Ray Physics

X-ray Physics

Radiation: Radiation is a type of energy which travels from a source and behave like wave as well as photons.

Ionizing radiation: the radiation which dissociate matter into ions. For example: X-rays, gamma rays, and beta rays etc.

X-rays are the electromagnetic radiation having short wave length.

History of X-rays:

  • The x-ray was discovered by Sir Willihelum Cornad Roentgen on 8th November 1985.
  • First x-ray tube was used by Sir W.C Roentgen was the Crookes tube which was gas filled tube.
  • X-ray was getting in the diagnostic field in 1896.
  • First anode or target material used in the Crookes tube was platinum later it was replaced by aluminum.
  • In modern X-ray tube tungsten is used as anode or target and these tubes are called as Coolidge tube. This tube was introduced in diagnostic field in 1910.

Applications of X-rays:

  1. Diagnostic radiology: The X-ray used to produce images of internal anatomical structures of human body by using X-ray machine, computed tomography, and fluoroscopy.

Properties of X-ray

  • X-rays are electromagnetic radiation having short wave length.
  • X-rays are not visible to the human eyes.
  • X-rays exhibit dual nature behaving like waves as well as photos.
  • There is no charge carried by X-rays.
  • X-rays cannot be reflected of deflected by external magnetic and electrical fields.
  • X-rays cannot be converged or focused to form images.
  • X-rays travel in a straight line and may scattered or absorbed by the objects.
  • In vacuum X-ray travels with the speed of light.
  • X-ray has extremely high penetrating power, capable of penetrating through hard objects like card boards, bricks, human flesh etc.
  • X-rays causes ionization while passes though gaseous medium.
  • X-rays can affect photographic plates and causes phosphorescence and fluorescence.
  • X-rays are capable of causing biological and chemical effects.

X-ray physics

X-ray tube:

The X-ray tube is a vacuum tube that is used for converting electrical input into X-rays. X-rays are produced by accelerating electrons. These X-rays used in radiography.

There are two types of X-ray tube:

  1. Stationary anode X-ray tube
  2. Rotating anode X-ray tube

Stationary anode X-ray tube consists of

  1. Cathode: The cathode is the negative terminal of the X-ray tube it consist of filament, the filament and cathode are made up of the tungsten. The function of filament is to produce electron when it is heated by fixed amount of electric current. Cathode also contains a copper block for better heat conduction. Filament can be single or dual but at the same time only one filament is energized and other filament remains unfunctional. When the small filament is active then the electrons are bombarded on the small area of the target or anode therefore it is called fine focus.

If the electrons are bombarded on the large area of the target or anode then it is called as large or broad focus. Filament is polished with thorium oxide to increase the production of electrons from the filament. Filament requires a current of 10 volt 10 ampere for the productivity of electrons. At the cathode terminal of X-ray tube a focusing cup is also present to focus the electrons towards the anode, this focusing cup is made up of the molybdenum and it is coated with the nickel so it acts as an electrode.

The filament will experience a heating cycle of 25°C to 2600°C. Therefore the filament is heated at a temperature of about 800°C in the absence of oxygen to provide dimensional stability for expanding and contracting during the heating cycle.

  1. Anode: it is the target of the X-ray tube. In stationary anode X-ray tube the anode is made up of a piece of tungsten with a copper block fitted behind the anode piece, copper is used for better heat conductivity. Anode is placed in front of cathode or filament in such a way that the incident electron strike in the middle of the anode, the anode of the stationary x-ray tube is based on line focus principle which states that the apparent focus is much smaller than the actual area where the electron bombardment takes place on the anode.

The anode is slightly angulated so that the X-rays can pass the window of the X-ray tube and dispersed in all directions. The anode angle in stationary tubes varies from 6°-20°, an oil circulation is used around the copper block for the transfer of heat from the anode terminal.

  • Focal spot or focal point: it is the point of the target or anode where electron bombardment takes place.
  • Focal track or focal area: it is the area of the target where electron bombardment takes place.
  1. Glass envelope: The glass envelope is made up of the borosilicate or Pyrex glass. The cathode and anode assembly are placed in this glass envelope and it is evacuated to the highest extent. During the manufacturing all the components inside the glass envelope are baked to a high temperature for a long period of time to removing gases which are wrapped in the metal parts. If traces of gases are found inside the envelope the tube becomes gassy. During X-ray production this gas gets ionized and the production of X-ray ceases.
  2. Tube housing: It is made up of a metal which protects supports and insulates the X-ray tube from any physical damage, moisture and dust particles. A thin layer of lead is lined inside the tube housing to prevent or attenuates the X-rays to come out from X-ray tube. The metal covering is properly earthed to prevent any electrical shock. In this tube housing, a window which is made by the material beryllium is also present through which X-rays come out.

Rotating anode X-ray tube consists of:

  1. Cathode: The cathode is the negative terminal of the X-ray tube it consist of filament, the filament and cathode are made up of the tungsten. The function of filament is to produce electron when it is heated by fixed amount of electric current. Cathode also contains a copper block for better heat conduction. Filament can be single or dual but at the same time only one filament is energized and other filament remains unfunctional. When the small filament is active then the electrons are bombarded on the small area of the target or anode therefore it is called fine focus.

If the electrons are bombarded on the large area of the target or anode then it is called as large or broad focus. Filament is polished with thorium oxide to increase the production of electrons from the filament. Filament requires a current of 10 volt 10 ampere for the productivity of electrons. At the cathode terminal of X-ray tube a focusing cup is also present to focus the electrons towards the anode, this focusing cup is made up of the molybdenum and it is coated with the nickel so it acts as an electrode.

The filament will experience a heating cycle of 25°C to 2600°C. Therefore the filament is heated at a temperature of about 800°C in the absence of oxygen to provide dimensional stability for expanding and contracting during the heating cycle.

  1. Rotating anode: The anode in rotating anode X-ray tube is made up of tungsten and is present in form of a disc which has 55mm-100mm diameter and 7mm in thickness, the disc will experience a rotation speed of 3000-10000 rotation per minute. During the construction of rotating anode a small quantity of rhenium oxide is added to prevent the cracking or pitting of the anode. The melting point of the tungsten disc is 3370°C. The anode angle in these tubes ranges from 15°-22°. The anode of these tubes is based on line focus principle. The anode disc is attached with the copper rod which helps in the transmission of heat from the anode. An anode stem made up of molybdenum is also used to place the anode disc on the copper rod is present inside the glass envelope and rest is outside. An oil circulation is used for the transfer of heat from the copper rod to the environment. The anode disc has depressed boundaries so that X-rays are less emitted in all directions.
  2. Rotor and stator assembly: The copper rod of the anode disc is connected with the rotor which is made up of metal and it rotates when an electric current is supplied to the X-ray tube with the formation of magnetic field. A ball bearing system is mounted on which rotor will move at a very high speed. This ball bearing system will experience a temperature range of 500°C so to withstand this large amount of heat, silver powder are filled around the balls of bearing this powder also provides lubrication during the movement.

The stator is made up of metallic plates which are fitted together and it is in the form of a ring. Small depressions or cuts are present in which copper wires are wrapped to form the coils. These coils forms magnetic field, when electric current is supplied to them. The rotor and stator assembly is responsible for the movement of anode disc.

  1. Glass envelope: The glass envelope is made up of the borosilicate or Pyrex glass. The cathode and anode assembly are placed in this glass envelope and it is evacuated to the highest extent. During the manufacturing all the components inside the glass envelope are baked to a high temperature for a long period of time to removing gases which are wrapped in the metal parts. If traces of gases are found inside the envelope the tube becomes gassy. During X-ray production this gas gets ionized and the production of X-ray ceases.
  2. Tube housing: It is made up of a metal which protects supports and insulates the X-ray tube from any physical damage, moisture and dust particles. A thin layer of lead is lined inside the tube housing to prevent or attenuates the X-rays to come out from X-ray tube. The metal covering is properly earthed to prevent any electrical shock. In this tube housing, a window which is made by the material beryllium is also present through which X-rays come out.

Production of X-rays:

When the electric current is applied on the cathode terminal of the X-ray tube it produces or release electrons after heating. This current has fixed voltage which is of the order of 10 volt 10 ampere. A high voltage or potential difference is applied between the cathode and anode to provide acceleration to the electrons. This energy provides kinetic energy to the electrons and they strikes with the anode or target of the X-ray tube all the energy of the incident electrons are absorbed by the electrons or nucleus of the target material. After gaining this extra amount of energy the electrons of the target or anode reaches at the excited state and they releases extra amount of energy in the form of X-rays and heat. About 99% heats is formed at the target and only 1% X-rays are formed.

Types of radiation produced from the X-ray tube:

  1. Bremsstrahlung’s radiation: The radiation produced due to the interaction of incident electrons with nucleus of the target or anode. These type of radiation produces only heat and not useful for diagnostic purpose.
  2. Characteristic radiation: These radiations are produced due to interaction of incident electrons with the inner shell electrons of the target or anode. These radiations are useful in diagnostic purposes.

Exposure factors:

Each time a radiograph is to be produced a set of exposure factors has to be chosen to give the type of image required. To choice of these factors will depend on the region being examined, including its thickness, density, pathology etc.

Exposure factors to be selected:

Exposure factors chosen will different for different types of image acquisition, device and depend on whether a grid is being used.

Mili ampere (mAs):

This indicates the intensity or, put simply the amount of radiation being used. If the radiation has enough energy to penetrate the body, then it will be detected by image density or, again put the image blackening.

mAs = produce of the X-ray tube current and exposure time in seconds

mAs= mA x s mA is mili ampere & s is time

mA should be as high as possible with a short time to reduce the risk of movement un-sharpness. Highest mA and lowest time select automatically by X-ray generator. If insufficient mA is used, than a photographic film will be underexposed and lack photographic density. If mAs is too high it will result in an overexposed film excessive density and lack of contrast. mAs only effect quality of electrons and X-ray production.

Kilo voltage:

This indicates how X-ray beam will penetrate the body. Range of Kvp is used in diagnostic radiography is normally between 50-120 Kvp. For soft tissues examined Kvp can be as low as 25 Kvp such as mammography. High Kvp such as those used in chest radiograph (120 Kvp). If Kvp increases X-rays produced have a high energy and more able to penetrate the body and it is detected by image acquisition device. Kvp is the most important factor in control of the contrast of radiographic image, therefore should be chosen carefully. Kvp should be such that the radiation has enough energy to penetrate the body part and reach the image acquisition device. Maximum contrast will achieve if the lowest Kvp is used. Kvp increases more radiation will be able to penetrate the dense body part compared with less dense part.

Focus to film distance (FFD):

Greater the FFD lower the intensity of the radiation reaching the film for given Kvp & mAs. If FFD is increased mAs must be increased. While choosing the FFD the following factors are taken into consideration. X-ray tube must not too close to patient’s skin otherwise radiation damage could be caused. Short FFD could give unacceptable geometric un-sharpness. The FFD must not be excessive, otherwise the large increase in mAs required would wear high tube loading.

Grid:

It is a device consists of a series of narrow lead strips closely spaced on their edges and separated by spacers of low density material. Grids are used to reduce the amount of the scattered radiation towards the image receptor. Grids increases contrast and remove different type of un-sharpness during exposure.

There are two types of grids:

  1. Stationary grids: These are placed between cassette and patient and they do not move during exposure.


Types of stationary grids:

  • Linear grids
  • Crossed grids / cellular grids
  • Focused grids
  • Pseudo focus grids
  1. Moving grids: These grids are simply linear grids but they moves during the exposure. Therefore the shadow cast by the lead strips in stationary grid can be eliminated or removed by using these grids. These grids are always running at right angles with respect to the X-ray tube radiation. These grids are placed between the table and the cassette holder/bucky.

Types of moving grids

  • Single stork moving grids
  • Oscillating and reciproting grids

Grid ratio: It is the ratio of the height of lead strips or transparent passage to the width of the transparent passage.

Grid frequency: It is defined as the number of lead strips per inch or centimeter.

Scattered radiation: It is the type of radiation that occurred when the useful X-ray that occurred when the useful X-ray beam interacts with the object and deviate from its original path due to attenuation coefficient of the objects. The radiation which deviates from its original path

Attenuation: It is the reduction in the intensity of an X-ray beam as it transverse matter by either absorption or deflection of photon from the beam.

Two terms used to express the characteristics of X-ray beam are quality and quantity.

Quality refers to number of photons in the beam and quality refers to their energies. Intensity of a beam is product of number and energy of photon, so it depends on both quantity and quality.

Macro radiography: The technique of producing an image by direct magnification and enables fine anatomical details than the real and accurate diagnosis.

Micro radiography: The imaging technique used to examine very small objects or details, often with the use of high voltage, very small focal spot size and an ultrafine film emulsion. It is also called as fluoroscopy.

Collimation: The process by which X-ray beam are lined up to minimize divergence or convergence. Ideally, a collimated beam is a bundle of parallel rays perfectly lined up align an optical line of sight between transmitter and receiver.

Anode heel effect:

It refers to the lower field intensity towards the anode is comparison to the cathode due to lower X-ray emission from the target material at angles perpendicular to the electron beam.

Basic concept: The conversion of the electron beam into X-ray beam doesn’t simply occur at the surface of the target material but deep within it. Because the X-ray beam are produced deep in the target material they must Travers back out of it by they can proceed to the target field. More target materials needs to be transverse at emission angles that are perpendicular to the electron beam than at those more parallel to it. This increase in material leads to more desorption of the X-rays by the target material leads to more desorption of the X-ray by the target, resulting in fewer X-rays reaching the field at angles perpendicular to the electron beam, it also means that the X-ray emitted to angles closer to the incident beam and fewer are desorbed. The end result is that the field intensity towards the cathode is more than that towards the anode.

Factors:

  1. Anode angle: By increasing the anode angle the amount of target material is perpendicular to the anode is decrease resulting in less desorption of produced X-ray beam.
  2. Target to film distance: Increase in distance reduced heel effect by allowing more divergence of the beam which produces a more uniform image.
  3. Field size: The field will be more uniform at the center due to collimator absorbing the peripheral variations.
  4. Positioning: By aligning higher attenuation material towards the cathode and lower attenuating material towards the anode the resulting field is uniform.

Magnification: Magnified image can be produced by increasing the object to film distance (OFD) in which X-rays diverging from the point source will produce a directly magnified image.

Distortion: Misrepresentation of the true size and shape of an object is called as distortion. It results from unequal magnification of different parts of the same object.