x-ray emission spectrumkarary.edu.sd/site/assets/files/22114/04_x-ray_emission_spectrum-1.pdf• a...

X-ray Emission Spectrum

Dr. Ahmed Alsharef Farah

Dr. Ahmed Alsharef Farah 1

• X-ray photons produced by an X-ray tubeare heterogeneous in energy.

• The energy spectrum shows a continuousdistribution of energies for thebremsstrahlung photons superimposed bycharacteristic radiation of discreteenergies.


There are two types of X-ray spectrum:

1. Bremsstrahlung or continuous spectrum.2. Characteristic spectrum.


• A bremsstrahlung spectrum consists of X-ray photons of all energies up to maximumin a continuous fashion, which is alsoknown as white radiation, because of itssimilarity to white light.

• A characteristic spectrum consists of X-rayphotons of few energy, which is also calledas line spectrum.

• The position of the characteristic radiationdepends upon the atomic number of thetarget.


Characteristic X-ray Spectrum:

• The discrete energies of characteristic x-rays are characteristic of the differencesbetween electron binding energies in aparticular element.

• A characteristic x-ray from tungsten, forexample, can have 1 of 15 different energiesand no others.


Characteristic X-rays of Tungsten and Their EffectiveEnergies (keV).


Characteristic x-ray emission spectrum for tungstencontains 15 different x-ray energies.


• Such a plot is called the characteristic x-rayemission spectrum.

• Five vertical lines representing K x-rays andfour vertical lines representing L x-rays areincluded.

• The lower energy lines representcharacteristic emissions from the outerelectron shells.


• The relative intensity of the K x-rays isgreater than that of the lower energycharacteristic x-rays because of the natureof the interaction process.

• K x-rays are the only characteristic x-raysof tungsten with sufficient energy to be ofvalue in diagnostic radiology.

• Although there are five K x-rays, it iscustomary to represent them as one with asingle vertical line, at 69 keV.


The characteristic x-ray emission spectrum isrepresented by a line at 69 keV.


Bremsstrahlung X-ray Spectrum:

• If it were possible to measure the energycontained in each bremsstrahlung x-rayemitted from an x-ray tube, one would findthat these energies range from the peakelectron energy all the way down to zero.

• In other words, when an x-ray tube isoperated at 90 kVp, bremsstrahlung x-rayswith energies up to 90 keV are emitted.


• A typical bremsstrahlung x-ray emissionspectrum is shown in Figure below.


Question:• At what kVp was the x-ray imaging system

presented in Figure above operated?Answer:• Because the bremsstrahlung spectrum intersects the

energy axis at approximately 90 keV, the imagingsystem must have been operated at approximately 90kVp.


• The general shape of the bremsstrahlung x-ray spectrum is the same for all x-rayimaging systems.

• The maximum energy (in keV) of abremsstrahlung x-ray is numerically equalto the kVp of operation.


• The greatest number of x-rays is emittedwith energy approximately one third of themaximum energy.

• The number of x-rays emitted decreasesrapidly at very low energies.


• The energy of an x-ray is equal to theproduct of its frequency (f) and Planck’sconstant (h). X-ray energy is inverselyproportional to its wavelength. As x-raywavelength increases, x-ray energydecreases.

• The minimum wavelength of x-rayemission corresponds to the maximum x-ray energy, and the maximum x-rayenergy is numerically equal to the kVp.


Question

• What would be the expected emissionspectrum for an x-ray imaging system witha pure molybdenum (Mo) target (effectiveenergy of K x-ray = 19 keV) operated at 90kVp?


Answer

• The spectrum should look something likeFigure below.


• The curve intersects the energy axis at 0 and90 keV and has the general shape shown inFigure below.


• The bremsstrahlung spectrum is muchlower because the atomic number of Mo islow (Z = 42), and x-ray production is muchless efficient.

• A line extends above the curve at 19 keV torepresent the K-characteristic x-rays ofmolybdenum.


• The total number of x-rays emitted froman x-ray tube could be determined byadding together the number of x-raysemitted at each energy over the entirespectrum, a process called integration.

Factors affecting the x-ray emission spectrum


• Graphically, the total number of x-raysemitted is equivalent to the area under thecurve of the x-ray emission spectrum.

• The general shape of an emissionspectrum is always the same, but its relativeposition along the energy axis can change.

• The farther to the right a spectrum is, thehigher the effective energy or quality ofthe x-ray beam.


• The larger the area under the curve, thehigher is the x-ray intensity or quantity.

• A number of factors under the control ofradiographers influence the size and shapeof the x-ray emission spectrum andtherefore the quality and quantity of the x-ray beam.


Factors that affect the size and relative position of x-ray emission spectrum.


1. Effect of mA and mAs:

• If one changes the current from 200 to 400mA while all other conditions remainconstant, twice as many electrons will flowfrom the cathode to the anode, and the mAswill be doubled.

• This operating change will produce twice asmany x-rays at every energy.

Factors affecting the x-ray emissionspectrum:


• In other words the x-ray emissionspectrum will be changed in amplitude butnot in shape.

• Increasing the mA does not affect theshape of the spectrum but increases theoutput of both bremsstrahlung andcharacteristic radiation proportionately.

• The area under the x-ray emissionspectrum varies in proportion to changes inmA or mAs, as does the x-ray quantity.


Change in mA or mAs results in a proportionate changein the amplitude of the x-ray emission spectrum at allenergies.


2. Effect of kVp:

• As the kVp is raised, the area under thecurve increases to an area approximatingthe square of the factor by which kVp wasincreased.

• Accordingly, the x-ray quantity increaseswith the square of this factor.


• When kVp is increased, the relativedistribution of emitted x-ray energy shiftsto the right to a higher average x-ray energy.

• The maximum energy of x-ray emissionalways remains numerically equal to thekVp.


• Increasing the kV shifts the spectrumupwards and to the right.

• It increases the maximum and effectiveenergies and the total number of x-rayphotons.

• Below a certain kV (70 kV for a tungstentarget), the characteristic K-radiation isnot produced.


Effect of tube kilovoltage (kV) on X-ray spectra forthree tube potentials: A, 40 kV; B, 80 kV; and C, 120kV.


• A 15% increase in kVp does not double thex-ray intensity but is equivalent to doublingthe mAs to the image receptor.

• To double the output intensity by increasingkVp, one would have to raise the kVp by asmuch as 40%.


• Radiographically, only a 15% increase inkVp is necessary because with increasedkVp, the penetrability of the x-ray beam isincreased.

• Therefore, less radiation is absorbed by thepatient, leaving a proportionately greaternumber of x-rays to expose the imagereceptor.


3. Effect of Added Filtration:

• Adding filtration to the useful x-ray beamreduces x-ray beam intensity whileincreasing the average energy.

• Added filtration more effectively absorbslow-energy x-rays than high-energy x-rays;therefore, the bremsstrahlung x-rayemission spectrum is reduced further on theleft than on the right.


Adding filtration to an x-ray tube results in reduced x-ray intensity but increased effective energy.


The emission spectra represented here resulted from operationat the same mA and kVp but with different filtration.


• Adding filtration is sometimes calledhardening the x-ray beam because of therelative increase in average energy.

• The characteristic spectrum is notaffected, nor is the maximum energy of x-ray emission.


4. Effect of Target Material:

• The atomic number of the target affectsboth the number (quantity) and theeffective energy (quality) of x-rays.

• As the atomic number of the target materialincreases, the efficiency of the production ofbremsstrahlung radiation increases, andhigh-energy x-rays increase in number to agreater extent than low-energy x-rays.


• The change in the bremsstrahlung x-rayspectrum is not nearly as pronounced as thechange in the characteristic spectrum.

• After an increase in the atomic number ofthe target material, the characteristicspectrum is shifted to the right, representingthe higher energy characteristic radiation.This phenomenon is a direct result of thehigher electron binding energies associatedwith increasing atomic number.


• Changing the target to one of loweratomic number reduces the output ofbremsstrahlung but does not otherwiseaffect its spectrum, unless the filtration isalso changed.

• The photon energy of the characteristiclines will also be less.

• Elements of low atomic number alsoproduce low-energy characteristic x-rays.


Discrete emission spectrum shifts to the right with anincrease in the atomic number of the target material.


• Tungsten is the primary component of x-raytube targets, but some specialty x-ray tubesuse gold as target material.

• The atomic number for tungsten is 74 andgold is 79.


• Molybdenum (Z = 42) and rhodium (Z =45) are target elements used formammography.

• In many dedicated mammography imagingsystems, these elements are incorporatedseparately into the target.

• The x-ray quantity from suchmammography target material is lowowing to the inefficiency of x-rayproduction.


5. Effect of Voltage Wave form:

• There are five voltage waveforms:I. Half-wave–rectified.II. Full-wave–rectified.III. Three-phase/six-pulse.IV. Three-phase/ 12-pulse.V. High-frequency waveforms.


Waveforms of high-voltage generators:(a) singlephase, half wave–rectified; (b) single-phase,full wave–rectified; (c) three-phase, six-pulse; and (d)high-frequency generator.


• Whatever the kV waveform, the maximumand minimum photon energies areunchanged.


Three-phase and high-frequency operations areconsiderably more efficient than single-phase operation.


• The number of x-rays emitted is low at lowervoltages and increases at higher voltages.

• The quantity of x-rays is much greater atpeak voltages than at lower voltages.

• Consequently, voltage waveforms of three-phase or high-frequency operation result inconsiderably more intense x-ray emissionthan those of single-phase operation.


• The relationship between x-ray quantityand type of high-voltage generator providesthe basis for another rule of thumb used byradiologic technologists.

• If a radiographic technique calls for 72kVp on single-phase equipment, then onthree-phase equipment, approximately 64kVp — a 12% reduction — will producesimilar results.


x-ray emission spectrumkarary.edu.sd/site/assets/files/22114/04_x-ray_emission_spectrum-1.pdf• a...

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