X - RAY MACHINE: AN INTRODUCTION

X-rays founded by physicist Wilhelm Conrad roentgen(1895).He called it as the new kind of rays or x-ray .by using this ray he taken photograph of his wife and showing the her bones ,wedding rings. Afterwards it’s used to visualize the internal anatomy of humans. Nowadays x-rays are used to diagnostic tool in medical profession and used in simple chest radiography to a digital subtraction angiography, CT (computer tomography) depends on use of x-rays. Depends on anatomical structures of body have different densities for the x-rays. When x-ray from source penetrate to the body, the internal body absorbs varying amount of radiation. The radiation coming from body has spatial intensity variation that’s image of the internal structure of body. We are using photographic film for the visualization of x-ray intensity distribution.2 properties of x-rays which are used for diagnostics are

  • It has a capability to penetrate matter coupled with different absorption in various material.
  • It can produce luminescence and effect of photographic emulsions. The x-ray picture is called radiography that’s shadow picture produced by x-ray emanating from a source.
  • NATURE OF X-RAYS

    x-rays are electromagnetic radiation located at low wavelength in electromagnetic spectrum. For diagnostic purpose, using wavelength of order of 10^-10m and speed of 3*10^10 cm/s which is unaffected by electric and magnetic fields. According to quantum theory ,interaction of matter involves energy exchange and their relationship between the wavelength and the photon


    E=hv=h(c/wavelength)

    h-Planck’s constant =6.32*10^-34js , c- velocity of propagation of photons (3*10^10 cm/s), v-velocity of radiation .

    To characterize x-rays by the voltage ,which is measure of energy of the radiation.

    Properties of x-ray

    It has short wavelength and extremely high energy so which can penetrate through material which can absorb & reflect visible light. X-ray can produce secondary radiation which is composed of scattered radiation. It can produce ionization in gases and influence the electrical properties of liquids and solids, which can also produce fluorescence in certain material to help them emit light.

    Units of x-ray

    Radiological units adopted Rontgen(R) as a measure of the quality of of x-ray radiation. one R can produce 2.08*10^9 ion pairs per cubic cm of air at standard temperature and pressure. Other units are millirontgen and microrontgen.The units are based on the ionization produced by the rays and which is not effects like the blackening of photographic films. The biological effects of x-rays are due to energy that’s very closely correlated with absorbed dose(D).unit of absorbed dose is rad.(one rad=1.0 *10^-2j/kg in irradiated material).The relationship between roentgen and absorbed dose is D=f R,where f=.86rad/R for air and 1 rad/R for soft tissue.


    PRODUCTION OF X-RAYS

    X-rays produced by electrons collide by high speed with matter and are thus suddenly stopped. The energy produced by electrons are in the form of highly penetrating electromagnetic waves(x-rays) having different wavelength. x-rays are produced in a constructed glass tubes which comprises of 1) source for production of electrons 2) energy source to accelerate an electrons 3)free electrons pathways 4) a mean of electron beam and last one a device to stop the electrons. 2 main types of x-ray are stationary anode tubes and rotating anode tubes.

    Stationary anode tube

    Electrons are generated from the filament of tube which is vacuum diode. The electron stream is focused on a target on a anode by suitable shaped of cathode cup. The kinetic energy of electrons is converting to x-rays .here , electrons emitted by hot filament become current carriers across tube.so we have adjust filament temperature for tube current and primary voltage for tube voltage. Some of the tubes function as a triode with bias voltage, it can used to control the size and shape of the focal spot by focusing electron beam. The cathode block contains filament made up of nickel or stain less steel. Where filament is closely wound helix of tungsten wire o.2 mm thick and 1.0-1.5 diameter. the target is comprised a small tablet of tungsten about 15 cm wide .20mm long, and 3mm thick soldered into block of copper. Tungsten is used because of high melting point and high atomic number (24).copper is best thermal conductor which can carry heat away from target .The heat flow from anode to outside of the tube. For additionally electrons have high voltage and shielded.the tube will emit electrons in all directions and protection provided.we apply metal container for completely surrounding the tube.thats shield. We are using this tubes in small capacity x-ray machines.

    Rotating tubes

    For more penetrating x-rays we need higher tube voltages and current. In rotating tubes, heat is generated in anode and heat capacity is function of the focal spot area. Here tubes are based on removal of the target from electron beam before it reaches high temperature under electron bombardment and rapid replacement by another cooling target. The anode is an alloy of tungsten and it rotates at a speed of 3000-3600 or 9000-10000.Due to largest area of the anode ,increasing the heat loading capacity and it allows high power levels to be used which produces more x-rays. We are using lubricants like lead, gold, graphite or silver .For diagnostic applications, usually employ high mill amperes and lower exposure time.

    X-ray Machines

    Block diagram of basic x-ray machine sub system.
    , there are two parts of the circuit.one of them to produce high voltage that’s applied to the tube’s anode and consist of high voltage transformer followed by rectification, current is follows HT pathway that’s measured by mA meter. A kV selector switch facilitates change in voltage between exposures. The exposure switch controls the timer. The second part controlling heating of x-ray tube filament, which is heated by 6-12v of ac supply at current of 3-5 amperes. The filament temperature determines the tube current and attached a mA selector. The basic design of x-ray generators has not changed for last 50 years. Voltages in the range of 30-200kV are required for the production of x-rays for diagnostic purposes, it is generated by high voltage transformer. Modern x-ray machines use of high frequency generators for producing high voltages. High tension cables-Insulated cables are connected to generators by view of high voltages to x-ray tubes. A typical capacitance of cables is 130230pF/m. collimators and grids are used to increase image contrast and to reduce dose to patient thats the x-ray beam must be limited to area of interest.collimator is placed between x-ray tube and patient whereas grids are inserted between patient and cassette. A timer is used to initiate and terminate the x-ray exposure,it controls the voltage to primary of high voltage transformer.All modern x-ray machines using digital timers.

    VISUALIZATION OF X-RAYS

    X-rays cannot detected or visualized by human eye so we use indirect methods to produce image of x-rays that have passed through the body of patient. x-ray films-x-ray which having shorter wavelength and it will react with photographic emulsions. X-ray films are insensitive to x-rays so we use intensifying screens consist of fluorescent material bounded to plastic base. Here, film is sandwiched between 2 screens and held in cassette. Fluorescent screens-In fluoroscopy, x-rays are converted to visual image of fluorescent screen. Screen is consist of plastic base coated with thin layer of fluorescent material, zinc cadmium sulfide that’s bonded to lead-glass plate. Zinc cadmium sulfide produce light (550nm) that can visible to eye in green part of the spectrum. X-ray image intensifier television system-It consist of large evacuated glass tube with input screen diameter (15-32cm).The input screen is converted the x-ray image to light image. Where light image produce is transmitted through the glass of the tube to photo –cathode which convert the light image to equivalent electron image. Modern x-ray image intensifiers use of a thin layer of cesium iodide.