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أستاذ المادة ايمن حميد عريبي التميمي       30/12/2018 05:44:21
lec 1 29-9-2014

د.أيمن حميد التميمي

introduction
an atom is composed of electrons (with a negative charge), protons (with a positive charge) and neutrons (no charge). the protons and neutrons are found in the nucleus of the atom and the electrons rotate (orbit) around the nucleus. the number of electrons equals the number of protons in an atom so that the atom has no net charge (electrically neutral). different materials (for example, gold and lead) will have different numbers of protons/electrons in their atoms. however, all the atoms in a given material will have the same number of electrons and protons.
the electrons are maintained in their orbits around the nucleus by two opposing forces. the first of these, known as electrostatic force, is the attraction between the negative electrons and the positive protons. this attraction causes the electrons to be pulled toward the protons in the nucleus. in order to keep the electrons from dropingping into the nucleus, the other force, known as centrifugal force, pulls the electrons away. the balance between these two forces keeps the electrons in orbit.
electrostatic force is the attraction between the positive protons and negative electrons. electrons in the orbit closest to the nucleus (the k-shell) will have a greater electrostatic force than will electrons in orbits further from the nucleus. another term often used is
binding energy this basically represents the amount of energy required to overcome the electrostatic force to remove an electron from its orbit. for our purposes, electrostatic force and binding energy are the same. the higher the atomic number of an atom (more protons), the higher the electrostatic force will be for all electrons in that atom.
centrifugal force pulls the electrons away from the nucleus

an x-ray is one type of electromagnetic radiation. electromagnetic radiation represents the movement of energy through space as a combination of electric and magnetic fields. all types of electromagnetic radiation, which also includes radiowaves, t ,v waves, visible light, microwaves and gamma rays, travel at the speed of light (186,000 miles per second).
the waves of electromagnetic radiation have two basic properties: wavelength and frequency.
the wavelength (w) is the distance from the crest of one wave to the crest of the next wave.
the frequency (f) is the number of waves in a given distance (d). if the distance between waves decreases (w becomes shorter), the frequency will increase.

the energy of a wave of electromagnetic radiation represents the ability to penetrate an object. the higher the energy, the more easily the wave will pass through the object. the shorter the wavelength, the greater the energy will be and the higher the frequency, the greater the energy will be.

1-x-rays are high energy waves, with very short wavelengths, and travel at the speed of light.

2-x-rays have no mass (weight) and no charge
(neutral). you cannot see x-rays they are
invisible.
3-x-rays travel in straight lines they can not
curve around a corner.
4- an x-ray beam cannot be focused to a point the
x-ray beam diverges (spreads out) as it travels
toward and through the patient. this is similar
to a flashlight beam.

5-x-rays are differentially absorbed by the
materials they pass through. more dense
materials (like an amalgam restoration) will
absorb more x-rays than less dense material (like
skin tissue). this characteristic allows us to see
images on an x-ray film.
6-x-rays will cause certain materials to fluoresce
(give off light). we use this property with
intensifying screens that used in extraoral radiography.
7-x-rays can be harmful to living tissue. because of
this, you must keep the number of films taken to
the minimum number needed to make a proper
diagnosis.

x-ray equipment has three basic components:
(1) the x-ray tubehead, which produces the x-rays,
(2) support arms, which allow you to move the tubehead around the patient’s head
(3) the control panel, which allows you to alter the duration of the x-ray beam (exposure time) and, on some x-ray machines, the intensity (energy) of the x-ray beam.

the x-ray tubehead is attached to the support arms so that it can rotate up and down (vertically measured in degrees) and sideways (horizontally) to facilitate proper alignment of the x-ray beam. the pid (position indicating device) is attached to the x-ray tubehead where the x-ray beam exits and it identifies the location of the x-ray beam. some people refer to the pid as a “cone” the pid’s on very old x-ray machines used to be cone shaped
the control panel, allows you to change exposure time but nothing else. some machines have controls for changing the ma and kvp settings in addition to exposure time. the individual controls will be discussed more later.

x ray production
x-rays are produced in the x-ray tube, which is located in the x-ray tubehead. x-rays are generated when electrons from the filament cross the tube and interact with the target. the two main components of the x-ray tube are the cathode and the anode
the cathode is composed of a tungsten filament which is centered in a focusing cup. electrons are produced by the filament and are focused on the target of the anode where the x-rays are produced. the focusing cup has a negative charge, like the electrons, and this helps direct the electrons to the target .
when you depress the exposure button, electricity flows through the filament in the cathode, causing it to get hot. the hot filament then releases electrons which surround the filament (thermionic emission). the hotter the filament gets, the greater the number of electrons that are release.
the anode in the x-ray tube is composed of a tungsten target embedded in a copper stem. when electrons from the filament enter the target and generate x-rays, a lot of heat is produced. the copper helps to take some of the heat away from the target so that it doesn’t get too hot.

components of the x-ray tube are

1. focusing cup 6. copper stem
3. electron stream 8. x-rays
4. vacuum 9. beryllium window
1.focusing cup: focuses electrons on target
2. filament: releases electrons when heated
3. electron stream: electrons cross from filament to
target during length of exposure
4. vacuum: no air or gases inside x-ray tube that might
interact with electrons crossing tube
5. target: x-rays produced when electrons strike target
6. copper stem: helps remove heat from target
7. leaded glass: keeps x-rays from exiting tube in
wrong direction
8. x-rays produced in target are emitted in all
directions
9. beryllium window: this non-leaded glass allows
x-rays to pass through. the pid would be
located directly in line with this window.

the x-ray machine is plugged into a 110-volt outlet (most machines) or a 220-volt outlet (some extraoral machines). the current flowing from these outlets is 60-cycle alternating current. each cycle is composed of a positive and negative phase. x-rays are only produced during the positive phase the target needs to be positive to attract the negative electrons from the filament. during the positive portion of the cycle, the voltage starts out at zero and climbs to the maximum voltage before dropingping back down to zero and entering the negative phase. each complete cycle lasts 1/60 of a second there are 60 cycles per second.

the timer controls the length of the exposure.
with alternating current, there are 60 complete cycles each second each cycle represents an impulse and is 1/60 of a second. to change impulses into seconds, divide the number of impulses by 60. to convert seconds to impulses, multiply by 60.
60 impulses/60 = 1 second
30 impulses/60 = 0.5 (1/2) second
15 impulses/60 = 0.25 (1/4) second

0.75 (3/4) second x 60 = 45 impulses
0.1 (1/10) second x 60 = 6 impulses
the ma (milliampere) setting determines the amount of current that will flow through the filament in the cathode. this filament is very thin it doesn’t take much current (voltage) to make it very hot. the higher the ma setting, the higher the filament temperature and the greater the number of electrons that are produced.
if the voltage flowing through the filament is too high, the filament will burn up. in order to reduce the voltage, the current flows through a step-down transformer before reaching the filament. the voltage reaching the step-down transformer is determined by the ma setting. the step-down transformer reduces the incoming voltage to about 10 volts, which results in a current of 4-5 amps flowing through the filament.
the kvp control regulates the voltage across the x-ray tube. (a kilovolt represents 1000 volts 70 kv equals 70,000 volts. a 70 kvp setting means the peak, or maximum voltage, is 70,000 volts). the higher the voltage, the faster the electrons will travel from the filament to the target. the kvp control knob regulates the autotransformer
the autotransformer determines how much voltage will go to the step-up transformer. basically, a transformer is a series of wire coils. in the autotransformer, the more turns of the coil that are selected (using the kvp control knob), the higher the voltage across the x-ray tube will be. this is similar to the function of a rheostat. the following slide shows how this works. the incoming line voltage will be 110 volts. the exiting voltage will be 65 volts if the kvp control is set at 65. the exiting voltage will be 80 volts if the kvp setting is 80.
the voltage coming from the autotransformer next passes through the step-up transformer, where it is dramatically increased. the ultimate voltage coming from the step-up transformer is roughly a thousand times more than the entering voltage. for example, if you set the kvp control knob to 65, 65 volts will exit the autotransformer. this 65 volts is increased to 65,000 volts by the step-up transformer. (the “k” in kvp stands for one thousand 65 kv is 65,000 volts). the side of the step-up transformer where the voltage enters (primary side) has far fewer turns in the coil than the exit (secondary)
there are two types of x-rays produced in the target of the x-ray tube. the majority are called
bremmstrahlung x-rays are produced when high-speed electrons from the filament are slowed down as they pass close to, or strike, the nuclei of the target atoms. the closer the electrons are to the nucleus, the more they will be slowed down. the higher the speed of the electrons crossing the target, the higher the average energy of the x-rays produced. the electrons may interact with several target atoms before losing all of their energy.
characteristic x-rays are produced when a high-speed electron from the filament collides with an electron in one of the orbits of a target atom the electron is knocked out of its orbit, creating a void (open space). this space is immediately filled by an electron from an outer orbit. when the electron dropings into the open space, energy is released in the form of a characteristic x-ray. the energy of the high-speed electron must be higher than the binding energy of the target electron with which it interacts in order to eject the target electron. both electrons leave the atom.
characteristic x-rays have energies “characteristic” of the target material. the energy will equal the difference between the binding energies of the target electrons involved. for example, if a k-shell electron is ejected and an l-shell electron dropings into the space, the energy of the x-ray will be equal to the difference in binding energies between the k- and l-shells. the binding energies are different for each type of material it is dependent on the number of protons in the nucleus (the atomic number)
an x-ray beam will have a wide range of x-ray energies this is called an x-ray spectrum. the average energy of the beam will be approximately 1/3 of the maximum energy. the maximum energy is determined by the kvp setting. if the kvp is 90, the maximum energy is 90 kev (90,000 electron volts) the average energy will be 30. as shown below, characteristic x-rays contribute a very small number of x-rays to the spectrum.
the x-ray spectrum results from three factors:
1- the varying distances between the high-speed electrons and the nucleus of the target atoms
2-multiple electron interactions. the high-speed electrons keep going until all energy is lost.
3- varying voltage. with an alternating current, the speed of the electrons will change as the voltage changes. the higher the voltage, the faster the electrons will travel. this is not a factor when the newer constant potential x-ray units are used.

x-ray production is a very inefficient process. only 1% of the interactions between the high-speed electrons and the target atoms result in x-rays. 99 % of the interactions result in heat production. the excess heat is controlled by the high melting point of the tungsten target, the conductive properties of the copper sleeve and the cooling from the oil surrounding the x-ray tube.
the energy of the x-ray beam and the number of x-rays are primarily regulated by the kvp control, the ma setting and the exposure time. one, two or all three of these exposure factors may need to be adjusted, depending on the size of the patient’s head, the likelihood of patient movement due to tremors or the inability to hold still, etc.. if the exposure factors are not set properly for the current patient, the resultant film may be too light or too dark
the kvp primarily controls the energy or penetrating quality of the x-ray beam. the higher the kvp, the higher the maximum energy and the higher the average energy of the beam. a higher kvp allows the x-ray beam to pass through more dense tissue in a larger individual, resulting in a more acceptable radiographic image. in addition to increasing penetrating ability, a higher kvp will also result in the production of more x-rays. because of this, an increase in kvp will allow for a decrease in exposure time, which may be helpful in children or in adults with uncontrolled head movement.
the ma setting determines the heating of the filament. the hotter the filament, the more electrons that are emitted the more electrons crossing the x-ray tube, the greater the number of x-rays that result. there is no change in the average energy or maximum energy of the x-ray beam. doubling the ma setting results in twice as many x-rays.

an increase in exposure time will result in an increase in the number of x-rays. doubling the exposure time doubles the number of x-rays produced. exposure time has no effect on the average or maximum energy of the x-ray beam. (click to change exposure time from 5 impulses to 10 impulses).
mas = milliamperes (ma) x seconds (s)
mai = milliamperes (ma) x impulses (i)
all x-ray machines have an ma setting (may be fixed or variable) and an exposure time setting (always variable) for each radiograph taken. the product of the ma setting times the exposure time equals mas or mai, depending on whether the exposure time is in seconds or impulses. as long as the mas remains constant for a given patient size, the x-ray output will remain the same. for example, if the ma setting is 5 and the exposure time is 30 impulses, the mai would be 150 (5 times 30). if we change the ma setting to 10 and decrease the exposure time to 15, the mai is still 150 (10 times 15). there will be no change in the number of x-rays. if an x-ray machine has variable ma settings, increasing the ma will allow for a decrease in exposure time this will be advantageous in most cases.
filtration
low-energy x-rays do not contribute to the formation of an x-ray image all they do is expose the body to radiation. therefore, we need to get rid of them. the process of removing these low-energy x-rays from the x-ray beam is known as filtration. filtration increases the average energy (quality) of the x-ray beam.there are two components to x-ray filtration. the first of these, called inherent filtration, results from the materials present in the x-ray machine that the x-rays have to pass through. these include the beryllium window of the x-ray tube, the oil in the tubehead and the barrier material that keeps the oil from leaking out of the tubehead. this removes very weak x-rays.
the second component is the addition of aluminum disks placed in the path of the x-ray beam (added filtration). these disks remove the x-rays that had enough energy to get through the inherent filtration but are still not energetic enough to contribute to image formation.
disks of varying thicknesses, when combined with the inherent filtration, produce the total filtration for the x-ray machine. federal regulations require that an x-ray machine capable of operating at 70 kvp or higher must have total filtration of 2.5 mm aluminum equivalent. (the inherent filtration is “equivalent” to a certain thickness of aluminum). x-ray machines operating below 70 kvp need to have a total filtration of 1.5 mm aluminum equivalent.
collimation is used to restrict the area of the head that the x-rays will contact. we want to cover the entire film with the x-ray beam, but don’t want to overexpose the patient. also, when x-rays from the tubehead interact with the tissues of the face, scatter radiation is produced .
this scatter radiation creates additional exposure of the patient and also decreases the quality of the x-ray image. (scatter will be discussed in greater detail in the section on biological effects of x-rays).
collimation
the collimator, located in the end of the pid where it attaches to the tubehead, is a lead disk with a hole in the middle (basically a lead washer). the size of the hole determines the ultimate size of the x-ray beam. the shape of the hole will determine the shape of the x-ray beam.
the shape of the opening in the collimator determines the shape of the x-ray beam. the size of the opening determines the size of the beam at the end of the pid. pid’s come in varying lengths longer pid’s have a smaller opening in the collimator.
the x-ray beam continues to spread out as you get further from the x-ray source (target). more surface is exposed on the exit side of the patient than the entrance side. by collimating the beam, less overall surface is exposed and as a result, less scatter radiation is produced. both of these things reduce patient exposure. 2.75 inches (7 cm) is the maximum diameter of a circular beam or the maximum length of the long side of a rectangular beam at the end of the pid.
if you switch from a 7 cm round pid to a 6 cm round pid, the patient receives 25% less radiation because the area covered by the beam is reduced by 25%.

rectangular collimation results in the patient receiving 55 % less radiation when compared to what they would receive with a 7 cm round pid.

with my best wishes

المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .