2 It is recommended that after the completion of each section that you test your understanding by doing the quiz related to that section.
3 Some Background Concepts Slide Show 1:Some Background Concepts
4 This slide show will examine: 1. The formation of X-rays2. X-ray spectra
5 X-raysX-rays and gamma rays are both forms of ionising radiation. Both are forms of electromagnetic radiation But they differ in their source of origin.X-rays are produced through interactions in electron shells.Gamma rays are produced in the nucleus.
6 X-rays are sometimes defined as having wavelengths between and m. A more robust definition of X-rays, however, is their mode of production.X-rays are produced through interactions in electron shells
8 Anode-ve+veElectron beamXraycathodeTo produce x-rays projectile electrons are accelerated from the negative cathode to the positive anode.
9 When the electrons from the cathode are accelerated at high voltage to the anode: 99% of the energy is dissipated as heat(anode materials are selected to withstand the high temperatures they are able to withstand)1% is given off as x-rays.
10 The x-ray radiation is emitted as: bremsstrahlung x-ray radiation (about 80%)and/orcharacteristic x-ray radiation.
11 The energy of the x-rays (keV) is determined by the voltage applied (kVp). The amount of x-rays is determined by the current (mA).
13 X-ray spectra are composed of: 1. Continuous bremsstrahlung spectra2. In most cases, discrete spectra peaks known as characteristic x-rays.keVBremsstrahlung radiation makes up approximately 80% of the x-ray beam
15 Bremsstrahlung radiation X-rayProjectile electrons originating from the cathode filament impinge on atoms in the anode and will often pass close by the nucleus of these atoms.As the electrons pass through the target atom they slow down, with a loss in kinetic energy. This energy is emitted as x-rays. The process is known as bremsstrahlung or “braking energy”.
16 Bremsstrahlung xrays form a continuous energy spectra Bremsstrahlung xrays form a continuous energy spectra. The frequency distribution is continuous and shows that the Bremsstrahlung process produces more low energy that higher energy x-rays. The average energy is approximately 1/3 of the Emax.E max
17 23keV70KeVE maxThe Emax or the maximum energy of the x-rays measured as (keV) is equal to voltage applied to the Xray tube (kilovolt peak or kVp).For example:An applied voltage of 70 kVp produces an x-ray spectra with Emax of 70 KeV and average energy of about 23 keV.
19 Characteristic X raysE maxTo produce characteristic x-rays the projectile electrons must have sufficient energy to displace orbital electronsIf the projectile electron has sufficient energy, it may cause the ejection of an orbital electron (usually in the K shell) from an atom in the anode.An outer shell electron (usually from the L or M shells) fills the vacancy in the inner orbital and sheds energy as an x-ray of characteristic energy.
20 The most common transition is from L to K shell. Each shell transition has a characteristic energy and this energy is dependent on the atomic number of the atom.M-to-K transitions are less common and are of higher energy.For tungsten the characteristic Xray spectra are represented by peaks at 58 and 69 keV representing L-to-K and M-to-K shell transitions respectively.
21 Production of Characteristic X-rays K ShellL shellM shell
22 Note that the impinging electron from the cathode must have sufficient energy to displace the K shell electron of the anode.This energy is the excitation energy of the electron shells and the energy is characteristic for each and each elementFor a tungsten anode the electrons from the cathode must have at least 69.5 keV to dislodge a K shell electron.Consequently no lines will appear if the x-ray tube with a tungsten anode is operated at 20 kVp or 40 kVp40keV75keV
23 Note that characteristic X ray spectra are independent of voltage once the threshold values have been reached
24 A Schematic X ray Tube Glass envelope cathode anode electron beam filamentrotorfocal spotFocusing cupX-ray beamWindow
25 WindowrotoranodeFocusing cupfilamentcathodefocal spotGlass envelopeelectron beamX-ray beamThe filament is heated to boil off electrons which are then accelerated to the anodeThe filament is contained within the cathode which is cup shaped to focus the electrons onto the focus spot on the anodeTube currents of milliamperes are used whereas filament currents are in the range of 2-5 amperes
26 The anode is bevelled at an angle of 12 to 17 degrees in order to maximise the contact area while focussing the resultant beamGlass envelopecathodeanodeelectron beamfilamentrotorFocusing cupThe anode is usually composed of tungsten or molybdenum as it must withstand very high temperatures (>3000 degrees C)Correct warm up and stand by procedures are essential to maximise tube and filament life
27 End of Section 1Why not test your understanding using the quiz booklet
28 Factors affecting x-ray beam quality and quantity Slide Show 2:Factors affecting x-ray beam quality and quantity
29 This slide show will examine: The factors that affect the quantity of x-rays and their characteristics
30 The energy of the x-rays is determined by the voltage applied. The amount of x-rays is determined by the current.
31 Factors affecting x-ray beam quality and quantity Anode materialVoltage applied (kVp)Tube Current (mA)Filters used
32 1. Anode materialDifferent anode materials will produce different characteristic x-ray spectra and different amounts of bremsstrahlung radiation.
33 2. Voltage (kVp)Note that increasing the applied voltage or kVp will increase the maximal energy, the average energy and the intensity of the x-rays. Characteristic x rays do not change with a change in kVp40keV75keV
34 3. Tube current (mA)Increasing the current (ie mA) will not change energy of the beam only the intensity (i.e. the amount) of x-rays. The quantity of x-rays is directly proportional to the tube current.100 mA200 mA75 keV
35 4. FiltrationOften filters of thin aluminium or other metals are used to filter out low energy x-rays.Filters will increase the average x-ray energy but decreases the intensity. The maximum energy and the characteristic x-rays remain unchangedAverage energy2mm filter4mm filter
36 End of Section 2Why not test your understanding using the quiz booklet
37 Interaction of X-ray Radiation with Matter Slide Show 3.Interaction of X-ray Radiation with Matter
38 This slide show will:1. Examine the energy transfer that accompanies interaction of radiation with matter2. Examine how x-rays interact with matter and the types of interaction
39 Interaction of Radiation with Matter (Transfer of Energy)If energy of particle or photon is absorbed - radiation will appear to have stoppedIf energy not completely deposited in the matter - remaining energy will pass through.If energy is absorbed – ionisation is more likelyIonization occurs when the energy transferred is sufficient to eject electron from the incident atom
40 The Process of Ionisation An Ion Pair is created
41 How X-rays Interact with Matter Slide Show 3:How X-rays Interact with Matter
42 This slide show will:Examine the 5 types of interactions between x-rays and matter
43 X-rays are classified as penetrative radiation Interaction of X-rays with MatterX-rays are classified as penetrative radiationThe penetration of x-rays (or conversely the amount of attenuation) is a function of: energy of the photonatomic number of irradiated matterthickness of irradiated materialdensity of irradiated material
44 Attenuation of X-raysX-rays are attenuated as they pass through matterThe degree of that any given material is able to attenuate x-rays is a function of its atomic number and its densityHalf Value ThicknessIt is conventional to refer to measure attenuation in terms of half value thicknessie the thickness of material required to reduce an x-ray to half its original intensity
45 Interaction of X-rays with matter There are five types of interactions:Coherent ScatteringPhotoelectric effectCompton scatteringPair productionPhotodisintegrationWith the exception of coherent scattering, all can result in ionisation of tissue
46 Interaction of X-rays with matter Four interactions resulting in ionisation:Photoelectric effectX-rays of low energyX-ray transfers energy to an electron which then ejectedCompton scatteringDominant in biological materialsX-ray is scattered at angle depending on amount of energy transferredPair productionX-rays > 1.02MeVPhotodisintegrationX-rays> 10MeV
47 Coherent scatteringAlso known as Classical or Thompson ScatteringChange in x-ray direction with no ionisationOccurs at energies <10 keV
48 Photoelectric effectX-ray transfers energy to an inner shell electron which then ejected.Filling the inner shell electron results in a characteristic x-ray.Characteristic x-rays from nitrogen, carbon and oxygen have very low energies.The final result is absorption of the x-ray (i.e. there is no exit radiation)X-rayPhotoelectron
49 Compton ScatteringThe incident x-ray is scattered by an outer shell electron which is also ejected (Compton electron)The X-ray is scattered at angle depending on amount of energy transferredThe energy of the incident x-ray is shared between the scattered x-ray and the Compton electronThe scattered X ray has lower energy and longer wavelengthCompton electron
50 The Medical Application of the Photoelectric Effect The photoelectric effect is responsible for most x-ray attenuation in tissuePhotoelectric attenuation increases with increasing atomic number.Bone absorbs 4x the x-ray than tissue at lower x-ray energiesPhotoelectric attenuation also decreases with increasing energy of the x-rayAbove 26 keV Compton Scattering becomes more dominant100%Compton50%Relative importancePhotoelectric50keV25keVX-ray Energy
51 Pair Production Occurs with high energy x-ray (> 1.02MeV) 0.51MeV positron0.51MeV electron
52 Photonuclear Disintegration Only occurs with very high energy x-ray (> 10 MeV)Nuclear fragment
53 Linear Energy Transfer (LET) Rate of energy transfer - ionisations per mm or keV/mmHigh LET radiation is not penetrative (ie energy is deposited in a small distance)Low LET radiation is penetrative (ie much less chance energy is deposited in a small distance)X-rays are lower LET radiation
54 Linear Energy Transfer (LET) Rate of energy transfer - ionisations per mm or keV/mmAlphaNot penetrativeHigher LET than beta20Beta1X-raysMore penetrativeSlightly lower LET than beta1
55 End of Section 4Why not test your understanding using the quiz booklet
56 Slide Show 4:The Effect of Radiation on Living Organisms
57 This slide show will:1. Examine the effect of ionising radiation on living organisms2. Describe units of exposure and dose
58 Effect of Radiation on Living Organisms Molecular Effect of Ionising RadiationDisruption of bonds - reduced molecular weightAlteration of the tertiary and quaternary structureCross-linking
59 Molecular Effect of Ionising Radiation Direct EffectRadiolysis of DNAPrimary feature of high LET radiationIndirect EffectFree radicals by radiolysis of water.2H H2O+ + H20-H2O+ OH. + H+Hydroxyl radicals react with other molecules (such as DNA)damaging them.
60 Effect of Radiation on Cells Relationship to dose rate (cell survival curve)At lower doses cells are able to repair damage without cell death(shoulder region)Higher doses cell death occurs directly proportional to doseHigher LET there is no shoulder region (i.e. cell repair mechanism overwhelmed by radiationDoseCellSurvival
61 Effect of Radiation on Tissues and Cells Tissue Type (Law of Bergonne and Tribondeau)1. Rapidly dividing tissue is more radiosensitive2. Rapidly growing cells are more radiosensitive3. Younger and more immature cells are more radiosensitive4. Mature cells are less radiosensitive(cf. tissue weighting factor)NB: Dividing cells are more sensitive in G2 and G1 parts of the cell cycleOrgan toxicityEye lens is particularly sensitive
62 Effect of Radiation on Humans Stochastic effectsThreshold after which there is an all or nothing effecte.g. Cancer or genetic effectsDeterministic EffectsVary with Dosee.g. lens opacification, blood changesTotal body irradiationHighly unlikely that an individual would survive a total exposure of more than 3 Gray without intensive medical treatmentPartial body irradiationCataracts are formed if eyes are exposed to more than 2 GrayHair loss occurs at exposures over 3 Gray
64 Units of Radiation Exposure and Dose Exposure (Roentgens)Absorbed dose (Gray)Dose Equivalence (Sievert)Relative biological effectiveness of different types of ionising radiationThe Effective Dose Rate (Sievert)
65 Exposure Unit is Roentgen Amount of x-rays that will cause 1 gram of air to absorb 86.9 ergsUseful for gamma and x-rays only
66 Absorbed dose SI Unit is Gray (Gy); old unit is rad Dose absorbed by the irradiated material accompanied by 1 joule (100 ergs) of energy.The quantity of energy absorbed per gram per Roentgen is dependent on the materialTherefore the absorbed dose is a useful measure and is applicable to any type or energy of ionising radiation
67 Dose Equivalence Unit is Sievert (Sv); old unit is rem Dose is multiplied by a radiation weighting factor (WR) similar to LETDose Equivalence = D x WRRadiation Weighting factors of emissions are approximately:Alpha particles = 20Protons, neutrons = 10Beta particles = 1Gamma rays and x-rays = 1
68 The Sievert takes into account the Biological Effectiveness of the radiation It can be thought of the absorbed dose of any radiation that produces the same biological effect as 1 Gray of therapeutic x-raysFor example:If 2.5 Sieverts of radiation are required for a given biological effect – then this could be delivered by 2.5 Gray of therapeutic x-rays or 0.25 Gray of neutronsExplanation:neutrons are 10 x more effective at producing the same biological effect (ie: have a Quality Factor of 10) and hence 1/10 effective dose of neutrons is required for the same biological effect.
69 The Effective Dose Unit = Sievert (Sv) Takes into account how different parts of the body react to ionising radiationEffective Dose Rate = D x Q x wTwT is a tissue weighting factor for organs and tissuese.g. wT gonads = 0.2 while wT Skin is 0.01
71 Dose ICRP Prescribed Limits per annum Members of public 1 mSv per annum above background5 mSv to eye20 mSv to handsRadiation workers20 mSv per annum above background150 mSv to eye500 mSv to handsPregnant women must receive no more than 2mSv per annum
72 Note that:Exposure limits are set for all ‘members of the public’ including pregnant women, babies etc.University staff members and students should consider themselves members of the public for the purposes of setting exposure limits
73 Background Dose in NZ Is approx 1.8 mSv per annum Background depends on activity (e.g. number of medical x-rays received in a year)Note- that airline crew on international flights are the most occupationally exposed group in NZ mSv per annum received as a result of increased cosmic radiation received at higher altitudes
74 Measurement of dose Dosimeters Measurement of dose can only really be obtained with dosimeters. These range from film badges, to thermiluminescent detectors to hand-held monitors.Hand held dosimeters are available in the UniversityGeiger Muller metersGeiger Muller meters only measure ionisation events impinging on the tube. These meters are useful for detecting point leakage from x-ray apparatus.Any readings obtained should be used with care as the high x-ray intensity can give alarming results Always verify any reading with a dosimeter
75 End of Section 4Why not test your understanding using the quiz booklet
77 X-ray Analytical Equipment While most X-ray generated in analytical x-ray units are of low energy and the x-ray beam is very narrow, their intensity is very high. If an operator was able to put their hand in the path of the xray beam they would sustain an x-ray burn.In order to ensure safe operation of such machines the manufacturers have installed interlocks and designed the operation of the machine so that it is extremely difficult for the operator to come in contact with the x-ray beam.
78 X-ray Diffraction Equipment X-ray Tubes employ special anode (Mo, Cu, Fe, CO and Cr) to prouce the correct type of characteristic radiationSpecial windows are employed – Berrylium, mica or low absorption glass to minimise loss of low energy radiationVoltages are characteristically (30-50 kV) low but amperages employed are high (15-20 mA)Incidents with X-ray diffraction equipment are rare, but those that have occurred are serious
79 The cornerstone of safe operation of these units is to ensure: 1. Only trained operators who are aware of the safe operation use these machines.2. The safe operation of the interlocks are checked regularly3. X-ray leakage and scatter are monitored on a regular basis4. The equipment is secured against unauthorised and untrained use
80 Safety with X-ray Apparatus Four Principles of Radiation Safety:1. Minimise Exposure Time2. Maximise Distance from Source3. Use Correct Shielding4. Follow Manufacturers Instructions5. Follow the ALARA principle -keep dose A Low As Reasonably Achievable
81 1. TimeEnsure all personnel minimise time spent close to machine2. DistanceExposure falls at the square of distance from sourceTherefore use distance to your advantage
82 3. ShieldingMost analytical X-ray machines have adequate shielding providedIt is important to periodically verify that shielding is functioning properlyNever tamper with or alter shielding
83 4. Follow Manufacturers Instructions Only authorised operators of X-ray apparatus who are familiar with safe operation of machine use the apparatus.Ensure there are procedures to prevent unauthorised use of or access to X-ray equipment (ie only authorised key-holders can use apparatus)Never over-ride safety interlocksDocument all repairs
84 Legal ObligationsAll x-ray apparatus must have a current licence-holder in charge of the apparatus. The National Radiation Laboratory (NRL) is the New Zealand statutory body which regulates use of radioisotopes and irradiating apparatus.All x-ray apparatus must be operated in accordance with the current NRL Safe Code of Practice (NRL C17 - X-ray Analytical Equipment)All x-ray equipment must be secured against unauthorised useNRL must be notified of any disposal of x-ray equipment
85 Requirements of NRL C17 - Safe Code of Practice for X-ray Analytical Equipment There must be a current licence holder in chargeAll users are authorised by licence holderAll users are properly trained (training is documented)Access to the machine is restricted by locking access to room and restricting access to switchProminent warning signs on doors and equipmentLog of authorised users and log of use and repairs
86 Code of Practice readily available to all users Interlocks are tested periodicallyRepairs are undertaken by authorised personsEquipment is periodically monitored for x-ray scatter and leakageEmergency procedures are in place to ensure machine is powered down in an emergency
87 End of Section 5Why not test your understanding using the quiz booklet