DIAGNOSTIC RADIOLOGY. Lec.3

Transcription

1 DIAGNOSTIC RADIOLOGY Lec.3 -

2 x-ray beam shape and position: Generation of the x- ray When the filament in the cathode is electrically heated. Cloud of electron is formed in the vacuum. When the high voltage circuit of the x- ray tube is activated, the electron are repelled from the cathode and attached to the anode. The speed with which electron travel across the gap between the cathode and anode depends on the potential difference between the two electrodes. The potential is controlled on the x- ray machine panel by a kilovolt selector. Only ( 0.2 %) of the kinetic energy will converted into x ray radiation at 65 K V P and the remaining is lost as heat. The heat must be transmitted to out side of the tube other wise the tube will be severely damage. The heat is carried away and absorbing by the insulating oil in the tube head.

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4 X ray radiation : High energy radiation produced by the collision of electrons with metal target in an x-ray tube. X ray beam : x ray that created focal spot of anode and travel in all direction. Useful x ray : X-ray photon which travel from anode in abeam with direction of 90 0 to the electron stream between cathode and anode this beam called useful X-ray beam it emerges through the window of x ray tube. Central ray : In the beam x ray photons that traveling in very center of the cone of radiation called central ray and this is commonly used to fix and locate the position of x ray beam. Inverse square law : The intensity of x ray inversely proportion with square of distance measured from the source of radiation to the point of measuring the radiation intensity. I= intensity. D= distance. Intensity: The number of photon per unit area. I1/I2 over (D2)2 /(D2)2

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6 X- ray machine consist of : 1. The component parts. 2. The x ray tube. 3. The x ray generating apparatus. 1. The component parts : Consist of A. The control panel.1. On- off switch, indicator light. 2. Exposure button, indicator light. 3. Control devices time,kilovolt age,mill amperage selectors. B. The extension arm : allows for the movement and positioning of the tube head. C. Tube head : Is tightly sealed,and the component parts of the tube head are : 1. Metal housing surrounds the x ray tube head and transformers,it filled with oil, it protect the tube and grounds the high voltage component. 2. Insulating oil it prevents over heating absorbing the heat created by the production of x ray. 3. Transformer is advice that used to either increase or decrease the voltage in the electrical circuit. 4.Aluminum filter disk or sheets of 0.5 mm placed in the path of the x- ray beam, they filter out the non penetrating, long wave length x- ray. 5. Lead collimator or lead plate with central hole, it restricts the size of the x- ray beam.

7 2. X- ray tube: Is the heart of the x ray generating system. The x ray tube is glass vacuum tube from all of the air has been removed because if there is air there will be an interaction with air or gas molecules, and this interaction will prevent the electron from reaching the anode.x ray tube consist of : A. Leaded glass housing : Is leaded glass vacuum tube that prevents x ray from escaping in all direction. B. Cathode ve : consist of tungsten wire filament in cup shaped holder made of molybdenum. The purpose of the cathode is to supply the electrons necessary to generate x ray. C. Anode + ve : consist of thin tungsten plate embedded in solid copper rod. The purpose of the anode is to convert electrons into x ray photons.

8 Rectification : The x ray beam used in dentistry's not continuo stream of radiation, but comes from the tube as pulses, the frequency of which depends on the number of cycle per second of alternative current A C. Rectification is the process of converting alternative current A C to direct current D C.

9 Half value layer : Is a method used to monitor the penetrating quality of the beam, half value layer is determine by placing filtering material such as Aluminum filter in front of the beam, so the thickness of this filter need to reduce the number of x ray photons passing through it to one half, is present to half value layer for this beam of radiation. The higher half value layer. the more is the penetration ability of the x ray beam. in oral diagnosis, the half value layer of the beam is approximately 2 mm of aluminum.

10 X ray spectrum : x ray beam consist of many photons of different wave length. Short wave length high frequency more energy. Long wave length low frequency less energy. The wave length of the x ray are so short that they are measured in Angstrom unite. x ray wave length used in diagnostic radiation range ( o.1- o.5 ) Angstrom.one Angstrom equal to one over cm.

11 x ray are absorbed by any form of matter ( solid, liquid and gasses ). When photons reach on atom four things can happen : 1. Photon can pass through the atom without any change occurring to either the atom or the photon.

12 2. It can be deviated from its direction by the atom with out any change in the atom, but the photon after deviation become a photon of scattered radiation.

13 3. photon can strike an electron of atom and be completely absorbed, and this called photo-electronic effect.the electron of the atom is accelerated out of its orbit. Photo electron either colliding with electron of other atom and gives its kinetic energy, or interaction with nuclei of the atoms and producing electro magnetic radiation of long wave length.

14 4.When x ray photon hit an electron of atom and gives up only part of its energy, called Compton effect, the result is an electron that travel of high speed and photon that change its direction, so photon become with low energy and long wave length.

15 Collimation Control the size and shape of x-ray beam, it made from lead with hole in its center used to control the size and shape of x- ray beam. the hole is either round or rectangular in shape Rectangular collimation reduce patient exposure more than round. Collimation can be achieved by one of two methods : 1. Using diaphragm (round or rectangular shape). 2- Using metal cylinder, cones and rectangular tubes. Diaphragm:-consist of metal plate or disk made from lead A hole is present in the center of disk allow the beam to pass throw it only the shape of x-ray beam determine by the shape of the hole of diaphragm such diaphragm is place over the opining in the head of x-ray machine.

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17 Filtration : x ray used must be able to penetrate hard and soft tissues bones,teeth, the longer wave length x ray are not useful in diagnostic radiology thus removal of these long waves photons from the beam by passing the beam through filter made from aluminum, this either built into the x ray machine by manufacture or added as an extra filter. The effect of Filtration on x ray beam is of most of long wave photons so the resulting x ray beam will consist of mainly x ray photons of short wave length with high energy photons and high penetrating power absorption that s why they named hard x ray beam. we have 2 type of filter : 1. Inherent or built in the machine ex glass of the x ray tube,oil surrounding the tube,any plastic pointed cone attached to the head of the x ray machine. 2.Added filter is the sum of all extra filters.

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