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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.
Dr. Ahmed Alsharef Farah 2
There are two types of X-ray spectrum:
1. Bremsstrahlung or continuous spectrum.2. Characteristic spectrum.
Dr. Ahmed Alsharef Farah 3
• 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.
Dr. Ahmed Alsharef Farah 4
Dr. Ahmed Alsharef Farah 5
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.
Dr. Ahmed Alsharef Farah 6
Characteristic X-rays of Tungsten and Their EffectiveEnergies (keV).
Dr. Ahmed Alsharef Farah 7
Characteristic x-ray emission spectrum for tungstencontains 15 different x-ray energies.
Dr. Ahmed Alsharef Farah 8
• 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.
Dr. Ahmed Alsharef Farah 9
• 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.
Dr. Ahmed Alsharef Farah 10
The characteristic x-ray emission spectrum isrepresented by a line at 69 keV.
Dr. Ahmed Alsharef Farah 11
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.
Dr. Ahmed Alsharef Farah 12
• A typical bremsstrahlung x-ray emissionspectrum is shown in Figure below.
Dr. Ahmed Alsharef Farah 13
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.
Dr. Ahmed Alsharef Farah 14
• 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.
Dr. Ahmed Alsharef Farah 15
• 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.
Dr. Ahmed Alsharef Farah 16
• 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.
Dr. Ahmed Alsharef Farah 17
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?
Dr. Ahmed Alsharef Farah 18
Answer
• The spectrum should look something likeFigure below.
Dr. Ahmed Alsharef Farah 19
• The curve intersects the energy axis at 0 and90 keV and has the general shape shown inFigure below.
Dr. Ahmed Alsharef Farah 20
• 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.
Dr. Ahmed Alsharef Farah 21
• 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
Dr. Ahmed Alsharef Farah 22
• 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.
Dr. Ahmed Alsharef Farah 23
• 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.
Dr. Ahmed Alsharef Farah 24
Factors that affect the size and relative position of x-ray emission spectrum.
Dr. Ahmed Alsharef Farah 25
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:
Dr. Ahmed Alsharef Farah 26
• 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.
Dr. Ahmed Alsharef Farah 27
Change in mA or mAs results in a proportionate changein the amplitude of the x-ray emission spectrum at allenergies.
Dr. Ahmed Alsharef Farah 28
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.
Dr. Ahmed Alsharef Farah 29
• 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.
Dr. Ahmed Alsharef Farah 30
• 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.
Dr. Ahmed Alsharef Farah 31
Effect of tube kilovoltage (kV) on X-ray spectra forthree tube potentials: A, 40 kV; B, 80 kV; and C, 120kV.
Dr. Ahmed Alsharef Farah 32
• 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%.
Dr. Ahmed Alsharef Farah 33
• 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.
Dr. Ahmed Alsharef Farah 34
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.
Dr. Ahmed Alsharef Farah 35
Adding filtration to an x-ray tube results in reduced x-ray intensity but increased effective energy.
Dr. Ahmed Alsharef Farah 36
The emission spectra represented here resulted from operationat the same mA and kVp but with different filtration.
Dr. Ahmed Alsharef Farah 37
• 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.
Dr. Ahmed Alsharef Farah 38
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.
Dr. Ahmed Alsharef Farah 39
• 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.
Dr. Ahmed Alsharef Farah 40
• 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.
Dr. Ahmed Alsharef Farah 41
Discrete emission spectrum shifts to the right with anincrease in the atomic number of the target material.
Dr. Ahmed Alsharef Farah 42
• 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.
Dr. Ahmed Alsharef Farah 43
• 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.
Dr. Ahmed Alsharef Farah 44
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.
Dr. Ahmed Alsharef Farah 45
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.
Dr. Ahmed Alsharef Farah 46
• Whatever the kV waveform, the maximumand minimum photon energies areunchanged.
Dr. Ahmed Alsharef Farah 47
Three-phase and high-frequency operations areconsiderably more efficient than single-phase operation.
Dr. Ahmed Alsharef Farah 48
• 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.
Dr. Ahmed Alsharef Farah 49
• 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.
Dr. Ahmed Alsharef Farah 50
Dr. Ahmed Alsharef Farah 51