3- xray production

8/3/2019 3- Xray Production

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X-ray Production

The following slides identify atomicstructure, the forces at work inside theatom, types of electromagnetic radiation

(including x-rays), x-ray characteristics,components of an x-ray machine and x-ray tube, how x-rays are formed and

ways to modify the x-ray beam.


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In navigating through the slides, you should clickon the left mouse button when you see themouse holding an x-ray tubehead or you aredone reading a slide. Hitting Enter or Page

Down will also work. To go back to the previousslide, hit backspace or page up.


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An atom is composed of electrons (with a negativecharge), protons (with a positive charge) andneutrons (no charge). The protons and neutronsare found in the nucleus of the atom and theelectrons rotate (orbit) around the nucleus. Thenumber of electrons equals the number of protonsin an atom so that the atom has no net charge(electrically neutral). Different materials (forexample, gold and lead) will have differentnumbers of protons/electrons in their atoms.However, all the atoms in a given material will havethe same number of electrons and protons. (Seediagram next slide)

Atomic Structure


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AtomThis atom has 7 protons and 7 neutrons in the nucleus.There are 7 electrons orbiting around the nucleus.

protons

neutrons

electrons


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The electrons are maintained in their orbitsaround the nucleus by two opposing forces.

The first of these, known as electrostatic force,is the attraction between the negative electronsand the positive protons. This attraction causesthe electrons to be pulled toward the protons in

the nucleus. In order to keep the electrons fromdropping into the nucleus, the other force,known as centrifugal force, pulls the electronsaway. The balance between these two forces

keeps the electrons in orbit.

(See next three slides)


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Electrostatic force is the attraction between thepositive protons and negative electrons. Electrons inthe orbit closest to the nucleus (the K-shell) will have

a greater electrostatic force than will electrons inorbits further from the nucleus. Another term oftenused is binding energy; this basically represents theamount of energy required to overcome the

electrostatic force to remove an electron from itsorbit. For our purposes, electrostatic force andbinding energy are the same. The higher the atomicnumber of an atom (more protons), the higher theelectrostatic force will be for all electrons in that

atom.


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Centrifugal force pulls the electronsaway from the nucleus


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The balance between electrostatic force andcentrifugal force keeps the electrons in orbit

around the nucleus

EF CF


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Electromagnetic Radiation

An x-ray is one type of electromagneticradiation. Electromagnetic radiationrepresents the movement of energy throughspace as a combination of electric and

magnetic fields. All types of electromagneticradiation, which also includes radiowaves, tvwaves, visible light, microwaves and gamma

rays, travel at the speed of light (186,000 milesper second). They travel through space inwave form.


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D

W

W

The waves of electromagnetic radiation have two

basic properties: wavelength and frequency. Thewavelength (W) is the distance from the crest of onewave 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 becomesshorter), the frequency will increase. The top waveabove has a shorter wavelength and a higherfrequency than the wave below it.

F = 3

F = 2


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radiowaves

tvwaves

visiblelight

x-rays gammarays

cosmicrays

Which of the above examples of electromagneticradiation has the shortest wavelength?

Which of the above has the lowest frequency?

Cosmic rays

Radio waves


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The energy of a wave of electromagneticradiation represents the ability to penetratean object. The higher the energy, the more

easily the wave will pass through the object.The shorter the wavelength, the greater theenergy will be and the higher the frequency,the greater the energy will be.

X-ray Energy


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A

B

CWhich of the above x-rays has the highest energy?

A: It has the shortest wavelength, highest frequency


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X-ray Characteristics

X-rays are high energy waves, with very shortwavelengths, and travel at the speed of light.

X-rays have no mass (weight) and no charge

(neutral). You cannot see x-rays; they are

invisible. X-rays travel in straight lines; they can not

curve around a corner.

An x-ray beam cannot be focused to a point; the

x-ray beam diverges (spreads out) as it travelstoward and through the patient. This is similarto a flashlight beam.


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X-rays are differentially absorbed by the

materials they pass through. More densematerials (like an amalgam restoration) willabsorb more x-rays than less dense material (likeskin tissue). This characteristic allows us to see

images on an x-ray film. X-rays will cause certain materials to fluoresce

(give off light). We use this property withintensifying screens used in extraoral radiography.

X-rays can be harmful to living tissue. Because of

this, you must keep the number of films taken tothe minimum number needed to make a properdiagnosis.

X-ray Characteristics (continued)


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X-ray Equipment

X-ray equipment has three basic components:(1) the x-ray tubehead, which produces the x-rays, (2) support arms, which allow you tomove the tubehead around the patients head

and (3) the control panel, which allows you toalter the duration of the x-ray beam (exposuretime) and, on some x-ray machines, the

intensity (energy) of the x-ray beam.

1

3

2


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PID(cone)

X-ray

Tubehead

degrees

The x-ray tubehead is attached to the support arms

so that it can rotate up and down (vertically;measuredin degrees) and sideways (horizontally) to facilitateproper alignment of the x-ray beam. The PID (PositionIndicating Device) is attached to the x-ray tubehead

where the x-ray beam exits and it identifies thelocation of the x-ray beam. Some people refer to thePID as a cone; the PIDs on very old x-ray machinesused to be coneshaped.


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The control panel, like the one above left, allows youto change exposure time but nothing else. Somemachines, like the one above right, have controls for

changing the mA and kVp settings in addition toexposure time. The individual controls will bediscussed more later.

exposure time kVp control

mA control


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X-ray Tube

X-rays are produced in the x-ray tube, which is

located in the x-ray tubehead. X-rays aregenerated when electrons from the filament

cross the tube and interact with the target. Thetwo main components of the x-ray tube are thecathode and the anode.


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(tungsten)

CathodeFocusing

cup

Filament

The cathode is composed of a tungsten filamentwhich is centered in a focusing cup. Electrons areproduced by the filament (see next slide) and arefocused on the target of the anode where the x-rays

are produced. The focusing cup has a negativecharge, like the electrons, and this helps direct theelectrons to the target (focuses them; electrons

can be focused, x-rays cannot).

side view(cross-section)

front view(facing target)


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Thermionic Emission

x-sectionof

filament

hot

filament

When you depress the exposure button, electricityflows through the filament in the cathode, causing it toget hot. The hot filament then releases electrons whichsurround the filament (thermionic emission). The hotterthe filament gets, the greater the number of electrons

that are released. (Click to depress exposure button

and heat filament).

electrons


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Anode

Copper stem

Target

The anode in the x-ray tube is composed of atungsten target embedded in a copper stem. Whenelectrons from the filament enter the target andgenerate x-rays, a lot of heat is produced. The

copper helps to take some of the heat away fromthe target so that it doesnt get too hot.

side view front view

Target


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X-ray Tube Components

1

2

43

5

8

6

7

9

1. focusing cup 6. copper stem2. filament 7. leaded glass

3. electron stream 8. x-rays4. vacuum 9. beryllium window5. target

(for description, see next slide)


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1. Focusing cup: focuses electrons on target

2. Filament: releases electrons when heated3. Electron stream: electrons cross from filament totarget during length of exposure

4. Vacuum: no air or gases inside x-ray tube that mightinteract with electrons crossing tube

5. Target: x-rays produced when electrons strike target6. Copper stem: helps remove heat from target7. Leaded glass: Keeps x-rays from exiting tube in

wrong direction

8. X-rays produced in target are emitted in alldirections

9. Beryllium window: this non-leaded glass allowsx-rays to pass through. The PID would be

located directly in line with this window.

X-ray Tube Components (continued)


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Target

Beryllium Window

Focusing cup(filament located inside)

Photo of an X-ray Tube

Leaded glass


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X-ray Machine

Components

Control Panel X-rayTubehead

110, 220 line TimerExposure switch

mA selectorkVp selectorAutotransformer

Step-down transformerStep-up transformerX-ray TubeWires

Oil


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The x-ray machine is plugged into a 110-volt outlet

(most machines) or a 220-volt outlet (some extraoralmachines). The current flowing from these outlets is60-cycle alternating current. Each cycle iscomposed of a positive and negative phase. X-rays

are only produced during the positive phase; thetarget needs to be positive to attract the negativeelectrons from the filament. During the positiveportion of the cycle, the voltage starts out at zero

and climbs to the maximum voltage beforedropping back down to zero and entering thenegative phase. Each complete cycle lasts 1/60 of asecond; there are 60 cycles per second.

(See next slide)

X-ray Machine Voltage


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+ 110, 220

- 110, 220

positive

negative

target positive;electrons flow

target negative;no electron flow

target positive;electrons flow

0

voltage starts at zero and reachesa maximum of 110 or 220 beforegoing back to zero


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Direct Current (Constant Potential)

60-cycle Alternating Current

Many machines now convert the alternating currentinto a direct current (constant potential). Instead ofcycles going from zero to the maximum, both

positive and negative, the voltage stays at themaximum positive value, creating more effective x-ray production. This allows for shorter exposuretimes.


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Timer

The timer controls the length of the exposure. Theblack numbers above represent impulses. The rednumbers are seconds.


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Number of Impulses60 = seconds

1/60 sec.With alternating current, there are 60complete cycles each second; eachcycle represents an impulse and is 1/60

of a second. To change impulses intoseconds, divide the number of impulsesby 60. To convert seconds to impulses,multiply by 60.

Number of seconds X 60 = impulses

60 impulses/60 = 1 second

30 impulses/60 = 0.5 (1/2) second15 impulses/60 = 0.25 (1/4) second

0.75 (3/4) second X 60 = 45 impulses0.1 (1/10) second X 60 = 6 impulses


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There are two electrical circuits operating during anx-ray exposure. The first of these is the low-voltagecircuit that controls the heating of the filament.When the exposure button is depressed, this lowvoltage circuit operates for second or less to heatup the filament. There are no x-rays produced duringthis time. As you continue to depress the exposure

button, the high-voltage circuit is activated. Thiscircuit controls the flow of electrons across the x-raytube; during the positive portion of the alternatingcurrent cycle, the negative electrons are pulled

across the x-ray tube to the positive target. X-raysare produced until the exposure time ends. Thelength of time the high-voltage circuit is operatingrepresents the exposure time. (See next slide).


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X-ray Exposure

2. Activate low-voltage circuit to heat filament3. Activate high-voltage circuit to pull electrons across tube4. Electrons cross tube, strike target and produce x-rays1. Depress exposure button5. X-ray production stops when exposure time ends.Release exposure button


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Exposure Button

The timer determines the length of the exposure,not how long you hold down the exposurebutton; you cannot overexpose by holding theexposure button down for an extended period.However, you can underexpose by releasing theexposure button too soon; the exposureterminates as soon as you release the button.


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mA setting

milliAmpere (mA) selector

The mA (milliAmpere) setting determines theamount of current that will flow through thefilament in the cathode. This filament is verythin; it doesnt take much current (voltage) to

make it very hot. The higher the mA setting, thehigher the filament temperature and the greaterthe number of electrons that are produced.


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Step-Down Transformer

If the voltage flowing through the filament is toohigh, the filament will burn up. In order to reducethe voltage, the current flows through a step-down transformer before reaching the filament.

The voltage reaching the step-down transformeris determined by the mA setting. The step-downtransformer reduces the incoming voltage toabout 10 volts, which results in a current of 4-5

amps flowing through the filament.

Step Down Transformer


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Step-Down Transformer

Primary

Secondary

110 voltsor less

currentflow

10 volts

currentflow

The current enters the step-down transformer on theprimary (input) side and exits on the secondary (output)

side. The fewer turns in the coil on the secondary side, thelower the output voltage will be. The primary coil belowwould have 110 turns, the secondary coil would have 10.(Each loop of the coil is a turn; the number of turns in the

diagram below has been reduced for easier viewing).

kil V l k (kV ) l


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kiloVolt peak (kVp) control

kVp readout

kVp control knob

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 thevoltage, the faster the electrons will travel from thefilament to the target. The kVp control knob regulatesthe autotransformer (see next slide).

A f


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Autotransformer

The autotransformer determines how muchvoltage will go to the step-up transformer.Basically, a transformer is a series of wire coils.In the autotransformer, the more turns of the coilthat are selected (using the kVp control knob),the higher the voltage across the x-ray tube willbe. This is similar to the function of a rheostat.The following slide shows how this works. Theincoming line voltage will be 110 volts. Theexiting voltage will be 65 volts if the kVp controlis set at 65. The exiting voltage will be 80 volts ifthe kVp setting is 80.

Autotransformer: the initial setting is 65; 65 voltsAutotransformer: if the setting is changed to 80


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110 V

65 volts

curre

ntflow

Autotransformer: the initial setting is 65; 65 voltsleave the autotransformer.

80 volts

to step-up transformer

kVpselector

Autotransformer: if the setting is changed to 80,80 volts leave the autotransformer.


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The voltage coming from the autotransformer nextpasses through the step-up transformer, where it is

dramatically increased. The ultimate voltage comingfrom the step-up transformer is roughly a thousandtimes more than the entering voltage. For example, if

you set the kVp control knob to 65, 65 volts will exitthe autotransformer. This 65 volts is increased to65,000 volts by the step-up transformer. (The k inkVp stands for one thousand; 65 kV is 65,000 volts).

The side of the step-up transformer where the voltageenters (primary side) has far fewer turns in the coilthan the exit (secondary) side.

Step-Up Transformer

Step Up Transformer


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Step-Up Transformer

Primary

Secondary

65-90 volts

currentflow

65,000 to90,000 volts

currentflow

The current enters the step-down transformer on theprimary (input) side and exits on the secondary (output)

side. The more turns in the coil on the secondary side, thehigher the output voltage will be. The secondary coil in thestep-up transformer has 1000 times as many turns as theprimary coil. (Again, the number of turns has been reduced

for easier viewing).

Th l ti hi f th i hi t


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65,000 to90,000 volts

kVp

filament

110 volts

10 volts

The relationship of the various x-ray machine componentsare shown in the diagram below. They form the high-voltageand low-voltage circuits. For a more detailed review of thecomponents, see next slide.

Th l h f h i l d i h h iWhen the exposure button is depressed the current canWhile keeping the exposure button depressed, theThe x-rays pass through the filter and collimator


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exposurebutton

oil

filter

The x-ray machine is plugged into the electricaloutlet (110 volts usually).

The length of the exposure is selected with the timer.When the exposure button is depressed, the current canflow into the x-ray tubehead. This activates the low-voltage circuit which heats the filament; this lasts for second (Click to depress exposure button).

While keeping the exposure button depressed, thehigh-voltage circuit is activated to pull theelectrons from the filament to the target, producingx-rays. (Click to produce x-rays).

filament

The x rays pass through the filter and collimatorbefore exiting through the PID. (Click for next slide)

Th t b h d i fill d ith il hi h d th


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The tubehead is filled with oil which surrounds thetransformers, x-ray tube and electrical wires. The primaryfunction of the oil is to insulate the electrical

components. It also helps to cool the anode and, as wewill discuss later, it helps in filtration of the x-ray beam.The barrier material prevents the oil from leaking out ofthe tubehead but still allows most x-rays to pass through.

oil barriermaterial

Step-upTrans

Step-downTrans


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X-ray Production

There are two types of x-rays produced in thetarget of the x-ray tube. The majority arecalled Bremmstrahlung radiation and the

others are called Characteristic radiation.


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Bremmstrahlung x-rays are produced when high-speed electrons from the filament are slowed

down as they pass close to, or strike, the nuclei ofthe target atoms. The closer the electrons are tothe 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-raysproduced. The electrons may interact with severaltarget atoms before losing all of their energy.

Bremsstrahlung Radiation

(Also known as braking radiation or general

radiation)


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Bremsstrahlung X-ray Production

+High-speedelectron fromfilament enterstungsten atom

Electron sloweddown by positivecharge ofnucelus; energy

released in formof x-ray

Electron continues on indifferent direction to interactwith other atoms until all of its

energy is lost

Bremsstrahlung X-ray Production


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Bremsstrahlung X-ray ProductionMaximum energy

High-speed electronfrom filament enterstungsten atom andstrikes target, losingall its energy anddisappearing

The x-ray produced has energyequal to the energy of thehigh-speed electron; this is themaximum energy possible

+


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Characteristic Radiation

Characteristic x-rays are produced when a high-speed electron from the filament collides with anelectron in one of the orbits of a target atom; theelectron is knocked out of its orbit, creating a

void (open space). This space is immediatelyfilled by an electron from an outer orbit. When theelectron drops into the open space, energy isreleased in the form of a characteristic x-ray. The

energy of the high-speed electron must be higherthan the binding energy of the target electronwith which it interacts in order to eject the targetelectron. Both electrons leave the atom.

Characteristic Radiation (continued)


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Characteristic x-rays have energies characteristic ofthe target material. The energy will equal the difference

between the binding energies of the target electronsinvolved. For example, if a K-shell electron is ejectedand an L-shell electron drops into the space, the energyof the x-ray will be equal to the difference in binding

energies between the K- and L-shells. The bindingenergies are different for each type of material; it isdependent on the number of protons in the nucleus(the atomic number).

Characteristic Radiation (continued)

K-shell

M-shell

L-shell


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Characteristic X-ray Production

LK

MHigh-speed electronwith at least 70 keVof energy (must bemore than the

binding energy of k-shell Tungsten atom)strikes electron inthe K shell, knockingit out of its orbit

Ejected electronleaves atom

Recoil electron(with very littleenergy) exitsatom

vacancy

X-ray with 59keV of energyproduced. 70(bindingenergy of K-

shell electron)minus 11(bindingenergy of L-shell electron)= 59.

Electron in L-shelldrops down to fillvacancy in K-shell

X-ray Spectrum


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X-ray Spectrum

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 maximumenergy. The maximum energy is determined by the kVpsetting. 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 verysmall number of x-rays to the spectrum.

X-ray energy (keV)

characteristicx-rays

(59 & 67 keV)

#ofx-rays

Bremmstrahlungx-rays

X ray Spectrum (continued)


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X-ray Spectrum (continued)

The x-ray spectrum results from threefactors:

(1) the varying distances between thehigh-speed electrons and thenucleus of the target atoms

(2) multiple electron interactions. The

high-speed electrons keep goinguntil all energy is lost.

(3) varying voltage. With an alternatingcurrent, the speed of the electrons

will change as the voltage changes.The higher the voltage, the fasterthe electrons will travel. This is nota factor when the newer constantpotential x-ray units are used.


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X-ray production is a very inefficient process. Only1% of the interactions between the high-speedelectrons and the target atoms result in x-rays. 99

% of the interactions result in heat production. Theexcess heat is controlled by the high melting pointof the tungsten target, the conductive properties ofthe copper sleeve and the cooling from the oil

surrounding the x-ray tube.

heat


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X-ray Beam Modifiers

The following slides identify the various waysof changing the energy of the x-ray beam and

the number of x-rays produced during an x-ray exposure.

Exposure Factors


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Exposure Factors

The energy of the x-ray beam and the numberof x-rays are primarily regulated by the kVpcontrol, the mA setting and the exposure time.

One, two or all three of these exposure factors

may need to be adjusted, depending on thesize of the patients head, the likelihood ofpatient movement due to tremors or the

inability to hold still, etc.. If the exposure

factors are not set properly for the currentpatient, the resultant film may be too light ortoo dark (see next slide).


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Exposure factors too high

(too dark)

Correct exposure factors

Exposure factors too low(too light)

kVp (kilovolt peak)


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kVp (kilovolt peak)

The kVp primarily controls the energy or

penetrating quality of the x-ray beam. The higherthe kVp, the higher the maximum energy and thehigher the average energy of the beam. A higherkVp allows the x-ray beam to pass through more

dense tissue in a larger individual, resulting in amore acceptable radiographic image. In additionto increasing penetrating ability, a higher kVp willalso result in the production of more x-rays.

Because of this, an increase in kVp will allow for adecrease in exposure time, which may be helpfulin children or in adults with uncontrolled headmovement.

kVp (kiloVolt peak)


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kVp (kiloVolt peak)

X-ray Energy (keV)

NumberofX-rays

70 90

90 kVp

70 kVp

In switching from 70 kVp to 90 kVp, the average

energy increases (dotted lines below), the maximumenergy increases (from 70 keV to 90 keV) and thenumber of x-rays increases. (Click to change from70 kVp to 90 kVp).

mA (milliampere)


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mA (milliampere)

The mA setting determines the heating of the filament.

The hotter the filament, the more electrons that areemitted; the more electrons crossing the x-ray tube, thegreater the number of x-rays that result. There is nochange in the average energy or maximum energy of thex-ray beam. Doubling the mA setting results in twice asmany x-rays. (Click to change from 5 mA to 10 mA).

NumberofX-rays

X-ray Energy

10 mA (twice as many x-rays)

5 mA

maximum energy

average energy

(no change)

(no change)

Exposure Time


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NumberofX-rays

X-ray Energy

10 impulses(twice as many x-rays)

5 impulses

maximum energy

average energy

Exposure Time

An increase in exposure time will result in anincrease in the number of x-rays. Doubling theexposure time doubles the number of x-raysproduced. Exposure time has no effect on theaverage or maximum energy of the x-ray beam.(Click to change exposure time from 5 impulsesto 10 impulses).

(no change)

(no change)

mAs or mAi


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mAs or mAi

mAs = milliamperes (mA) x seconds (s)mAi = milliamperes (mA) x impulses (i)

All x-ray machines have an mA setting (may be fixed orvariable) and an exposure time setting (always variable) foreach radiograph taken. The product of the mA setting timesthe exposure time equals mAs or mAi, depending on whetherthe exposure time is in seconds or impulses. As long as themAs remains constant for a given patient size, the x-rayoutput will remain the same. For example, if the mA setting is5 and the exposure time is 30 impulses, the mAi would be 150

(5 times 30). If we change the mA setting to 10 and decreasethe 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-raymachine has variable mA settings, increasing the mA willallow for a decrease in exposure time; this will be

advantageous in most cases.

In the following situations, would you expect the x-rayfil t b (A) d (B) tl d


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1. Recommended kVp, mA, exposure time (e.t.)

2. Increase mA; no change in kVp, e.t.

3. Decrease e.t.; no change in kVp, mA

4. Increase kVp; no change in mA, e.t.

5. Double mA, halve e.t.; no change in kVp

A CB

B

A

C

A

B

overexposed correct exposure underexposed

film to be (A), overexposed, (B) correctly exposed or(C) underexposed? (No change in patient size).

Filtration


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Filtration

Low-energy x-rays do not contribute to the formation

of an x-ray image; all they do is expose the body toradiation. Therefore, we need to get rid of them. The

process of removing these low-energy x-rays fromthe x-ray beam is known as filtration. Filtrationincreases the average energy (quality) of the x-ray

beam.There are two components to x-ray filtration. The firstof these, called inherent filtration, results from thematerials present in the x-ray machine that the x-rays

have to pass through. These include the berylliumwindow of the x-ray tube, the oil in the tubehead andthe barrier material that keeps the oil from leakingout of the tubehead. This removes very weak x-rays.

Filtration (continued)


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Filtration (continued)

The second component is the addition of aluminumdisks placed in the path of the x-ray beam (addedfiltration). These disks remove the x-rays that hadenough energy to get through the inherent filtration butare still not energetic enough to contribute to imageformation.

Disks of varying thicknesses, when combined with theinherent filtration, produce the total filtration for the x-ray machine. Federal regulations require that an x-raymachine capable of operating at 70 kVp or higher must

have total filtration of 2.5 mm aluminum equivalent. (Theinherent filtration is equivalent to a certain thicknessof aluminum). X-ray machines operating below 70 kVpneed to have a total filtration of 1.5 mm aluminumequivalent.

Filtration


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Inherent

beryllium windowof x-ray tube

Added

Aluminum filter (s)

TotalOil/Metal barrier

filter

PID

collimator

barriermaterial

berylliumwindow

oil


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filter

PID

The filter is usuallylocated in the end ofthe PID which attaches

to the tubehead.

Collimation


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primary x-ray

scattered x-ray

Collimation is used to restrict the area of the head that thex-rays will contact. We want to cover the entire film with

the x-ray beam, but dont want to overexpose the patient.Also, when x-rays from the tubehead interact with thetissues of the face, scatter radiation is produced (seebelow). This scatter radiation creates additional exposureof the patient and also decreases the quality of the x-ray

image. (Scatter will be discussed in greater detail in thesection on biological effects of x-rays).

Collimation


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Collimation

The collimator, located in the end of the PID where

it attaches to the tubehead, is a lead disk with ahole in the middle (basically a lead washer). Thesize of the hole determines the ultimate size of thex-ray beam. The shape of the hole will determinethe shape of the x-ray beam.

You are looking up through thePID at the collimator (red arrows),

which is a circular lead washerwith a circular cutout in themiddle. This will produce a roundx-ray beam. The light gray area inthe center is the aluminum filter.

Collimation


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The shape of the openingin the collimatordetermines the shape of

the x-ray beam. The sizeof the openingdetermines the size of thebeam at the end of thePID. PIDs come in

varying lengths; longerPIDs have a smaller

opening in the collimator.

round

rectangular

Collimation

The x-ray beam continues Collimation


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The x-ray beam continuesto spread out as you getfurther from the x-raysource (target). More

surface is exposed on theexit side of the patientthan the entrance side. Bycollimating the beam, lessoverall surface is exposed

and as a result, lessscatter radiation isproduced. Both of thesethings reduce patientexposure. 2.75 inches (7

cm) is the maximumdiameter of a circularbeam or the maximumlength of the long side of arectangular beam at theend of the PID.

collimatedbeam

collimator

target(x-ray source)

Collimation

7 cm Collimation


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If you switch from a 7 cm

round PID to a 6 cmround PID, the patientreceives 25% lessradiation because thearea covered by the

beam is reduced by 25%.

Rectangular collimation(dotted line at left)results in the patient

receiving 55 % lessradiation when comparedto what they wouldreceive with a 7 cmround PID.

6 cm round

film(4.5 cm long)

entrance

entrance

exit

exit

6 cm

7 cm

area covered at skin surface (6 cm round PID)

area covered as beam exits (6 cm round PID)

area covered at skin surface (7 cm round PID)

area covered as beam exits (7 cm round PID)

Collimation

The quality, or average energy, of the x-ray beam is


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Quality Quantity

(primarily)kVp

mA

Time

Filtration

no change

no change

Collimation does not change the energy or number ofx-rays in the x-ray beam that reach the film; it just

limits the size and shape of the beam.

q y, g gy, yincreased with an increase in kVp or an increase infiltration. The quantity, or number of x-rays, is

increased with an increase in kVp, mA setting andkVp setting.

Thi l d th ti X


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This concludes the section on X-rayProduction.

Additional self-study modules are availableat: http://dent.osu.edu/radiology/resources.php

If you have any questions, you may e-mailme at: [email protected]

Robert M. Jaynes, DDS, MS

Director, Radiology GroupCollege of DentistryOhio State University
http://dent.osu.edu/radiology/resources.phphttp://dent.osu.edu/radiology/resources.php

3- xray production

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