Lect-1-Physics of x-ray and its interaction with matter PDF

Physics of x-ray and its interaction with matter By Prof.Dr.Aedah Z. AlKaisy Basic structure of an ATOM : Atomic structure An atom is made up of : NUCLEUS PROTON ( +ve charge ) NEUTRON ( neutral ) And Electron(-Ve charge in the outer shells) K shell : 2 electrons  L shell : 8 electrons  Each s...

Physics of x-ray and its interaction with matter By Prof.Dr.Aedah Z. AlKaisy Basic structure of an ATOM : Atomic structure An atom is made up of : NUCLEUS PROTON ( +ve charge ) NEUTRON ( neutral ) And Electron(-Ve charge in the outer shells) K shell : 2 electrons  L shell : 8 electrons  Each shell has a specific binding energy & The closer shell to the nucleus, the tighter bound to the nucleus. The electrons in the outermost shell are loosely bound to the nucleus & are hence called “free electrons”.  X-ray interaction with atom X-ray photons may interact either with orbital electrons or with the nucleus. In the diagnostic energy range, the interactions are always with orbital electrons. The molecular bonding energies, however are too small to influence the type and number of interactions . The most important factor is the atomic make up of a tissue and not its molecular structure.  Energy value of electronic shells is also determined by the atomic number of the atom.  K-shell electron are more tightly bound in elements of high atomic number. Pb : 88keV while Ca : 4keV.  Electrons in the K -shell are at a lower energy level than electrons in the L-shell. If we consider the outermost electrons as free ,than inner shell electrons are in energy debt. The energy debt is greatest when they are close to nucleus in an element with a high atomic number. Definition of x-ray X-rays are sometimes defined as having wavelengths between 10-10 and 10-12 m. A more robust definition of X-rays, however, is their mode of production. X-rays are produced through interactions in electron shells ■ Descriptione of x-ray emission? The x-ray radiation is emitted as: 1-bremsstrahlung x-ray radiation (about 80%) 2-and/or characteristic x-ray radiation. 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). X-Ray device Parts X-ray has three main components: 1-X-ray Tube 2-Operating Console 3-High Frequency Generator 4-Other Parts include Collimator ,Grid and X-ray Film THE X-RAY TUBE ■ X-ray tubes are designed and constructed to maximize x-ray production and to dissipate heat as rapidly as possible. ■ The x-ray tube is a relatively simple electrical device typically containing two principle elements: ■ A cathode and the anode represent(X-ray heart) X-ray heart ■ The heart of an X-ray machine is an electrode pair - a cathode and an anode - that sits inside a glass vacuum tube. ■ as the electrical current flows through the tube from cathode to anode, the electrons undergo an energy loss, which results in the generation of x-radiation. Anode ■ The anode is the component in which the x-radiation is produced. ■ It is a relatively large piece of metal that connects to the positive side of the electrical circuit. ■ The anode has two primary functions: ■ (1) to convert electronic energy into x-radiation, and (2) to dissipate the heat created in the process. The material for the anode is selected to enhance these functions. The anode is tilted at an angle of 12 to 17 degrees in order to maximise the contact area while focussing the resultant beam Glass envelope cathode anode electron beam filament rotor Focusing cup The anode is usually composed of tungsten or molybdenum as it must withstand very high temperatures (>3000 degrees oC) Correct warm up and stand by procedures are essential to maximise tube and filament life ■ The ideal situation would be if most of the electrons created x-ray photons rather than heat. The fraction of the total electronic energy that is converted into xradiation (efficiency) depends on two factors: ■ the atomic number (Z) of the anode material, and the energy of the electrons. Most x-ray tubes use tungsten, which has an atomic number of 74, as the anode material. In addition to a high atomic number, tungsten has several other characteristics that make it suited for this purpose. Tungsten for anode ■ 1-Tungsten is almost unique in its ability to maintain its strength at high temperatures. ■ 2- it has a high melting point and a relatively low rate of evaporation. ■ For many years, pure tungsten was used as the anode material, but in recent years an alloy of tungsten and rhenium has been used as the target material but only for the surface of some anodes. ■ The anode body under the tungsten-rhenium surface on many tubes is manufactured from a material that is relatively light and has good heat storage capability. Two such materials are molybdenum and graphite. ■ They use molybdenum as an anode base material ■ Most x-ray tubes used for mammography have molybdenum-surface anodes. Does the density of the target material matter? ■ The target is commonly made from tungsten and other materials like cobalt, iron, or copper. Another important characteristic of the target material is its density. The material must be of high atomic mass for electron interaction. Remember that when the electron interacts with the target atoms the result is the generation of X-rays. Low density materials do not provide sufficient density for interaction. Design ■ Most anodes are shaped as beveled disks and attached to the shaft of an electric motor that rotates them at relatively high speeds during the x-ray production process. The purpose of anode rotation is to dissipate heat . Focal Spot ■ Not all of the anode is involved in x-ray production. The radiation is produced in a very small area on the surface of the anode known as the focal spot. The dimensions of the focal spot are determined by the dimensions of the electron beam arriving from the cathode. In most x-ray tubes, the focal spot is approximately rectangular. Benefit of focal spot ■ The dimensions of focal spots usually range from 0.1 mm to 2 mm. ■ X-ray tubes are designed to have specific focal spot sizes; ■ small focal spots produce less blurring and better visibility of detail, and ■ large focal spots have a greater heat-dissipating capacity. Focal spot size is one factor that must be considered when selecting an x-ray tube for a specific application. Tubes with small focal spots are used when high image visibility of detail is essential and the amount of radiation needed is relatively low because of small and thin body regions as in mammography. Most x-ray tubes have two focal spot sizes (small and large), which can be selected by the operator according to the imaging procedure. The Cathode ■ The cathode is the negative terminal of the tube assembly and includes the filament, which is a smallcoiled wire that is commonly made from tungsten. The filament provides the electrons for acceleration to the target (anode). Tungsten is metal with the desired properties for filaments. The filament is normally powered by an alternating current that is supplied to it by a separate transformer. Cathode function ■ The basic function of the cathode is to expel the electrons from the electrical circuit and focus them into a welldefined beam aimed at the anode. ■ The typical cathode consists of a small coil of wire (a filament) recessed within A cup-shaped region, as shown As you can see in the fig. ■ In many of the X-ray tubes, the current supplied to the filament ranges from a few hundred micro-amperes to several mille-amperes (mA). Filament current may be varied or fixed to maintain a constant tube current. Remember that the filament supplies the electrons. Adjustments in current to the filament varies the number of electrons that will boil off the filament. This in turn controls the number of X-rays that the tube is generating. Filament current controls the X-ray intensity. The cathode consists of: A spiral of heated low resistance (R) tungsten wire (filament) for electron emission. Wire is heated by filament current I=V/R ( V  10 V, I  3-6 A ) Ex.-Find the resistance of an x-ray wire, if you know that V=2mV, I=0.5 𝑚𝑎𝑚𝑝? So/ I=V/R. 0.5 = 2* 10-3/R R=….Ω Thermionic emission ■ Electrons that flow through electrical circuits cannot generally escape from the conductor material and move into free space. They can if they are given sufficient energy in a process known as thermionic emission, thermal energy (or heat) is used to expel the electrons from the cathode. The filament of the cathode is heated in the same way as a light bulb filament by passing a current through it. This heating current is not the same as the current flowing through the x-ray tube (the MA) that produces the x-radiation. During tube operation, the cathode is heated to a glowing temperature, and the heat energy expels some of the electrons from the cathode. Envelope ■ The anode and cathode are contained in an airtight enclosure, or envelope. The majority of x-ray tubes have glass envelopes, although tubes for some applications have metal and ceramic envelopes. Functions of the envelope The primary functions of the envelope are :  to provide support and electrical insulation for the anode and cathode assemblies and  to maintain a vacuum in the tube. The presence of gases in the x-ray tube would allow electricity to flow through the tube freely, rather than only in the electron beam. This would interfere with x-ray production and possibly damage the circuit. Housing ■ The x-ray tube housing provides several functions in addition to enclosing and supporting the other components. Its functions as a shield and absorbs radiation, except for the radiation that passes through the window as the useful x-ray beam. Its relatively large exterior surface dissipates most of the heat created within the tube. The space between the housing and insert is filled with oil, which provides electrical insulation and transfers heat from the insert to the housing surface. THE X-RAY CIRCUIT ■ The energy used by the x-ray tube to produce x-radiation is supplied by an electrical circuit .The circuit connects the tube to the source of electrical energy, that in the x-ray room is often referred to as the generator, the generator receives the electrical ■ energy from the electrical power system and converts it into the appropriate form (DC, direct current) to apply to the x-ray tube. Operating console ■ The operating console allows the radiologic technologist to control the xray tube current and voltage so that the useful x-ray beam is of proper quantity and quality. •Radiation quantity refers to the number of x-rays or the intensity of the x-ray beam. ■ Radiation quantity is usually expressed in mille roentgens (mR)or milliroentgens/milliampere-second (mR/mAs). Collimator and Grid ■ Collimator is a device used to minimize the field of view, avoid unnecessary exposure using lead plates. Lead shutter are used to restrict the beam. The collimator is attached to the X-ray below the glass window where the useful beams is emitted. ■ Grid is similar to a collimator except they have different positions. Grid is placed right after the patient. It is made up of lead strips, which is used to eliminated scattered light. These strips only allow rays at 90o X-Ray Film ■ The film is placed after the bulky. It turns black when Xrays interact with it and stays white where the X-rays are absorbed. This causes an image to be formed that is in black, grays and white. Medical applications of X-ray ■ The uses of x rays in the fields of medicine and dentistry have been extremely important. Examples might include the observation of the broken bones and torn ligaments of football players, the detection of breast cancer in women, or the discovery of cavities and impacted wisdom teeth. ■ Since x rays can be produced with energies sufficient to ionize the atoms making up human tissue, it can be used to kill these cells. X-ray production Electrons are released by thermionic emission, the electron current is determined by the temperature which depends on the wire current. The electron current is approximately 5 to 10 times less than the wire current. a focusing cup with a negative bias voltage applied to focus the electron distribution. The free electron collides with the tungsten atom, knocking an electron How can they produce X-ray ? Anode -ve +ve Electron beam Xray cathode To produce x-rays projectile electrons are accelerated from the negative cathode to the positive anode. ■ The X-rays pass through a tube window (with low Xray absorption) perpendicular to the electron beam. ■ Usually the low energy component of the X-ray spectrum does not provide any information because it is completely absorbed in the body tissue of the patient. It does however contribute significantly to the absorbed dose of the patient which excess the acceptable dose limit. ■ These lower energies are therefore filtered out by aluminum or copper absorbers of various thickness X-ray spectra are composed of: 1. Continuous bremsstrahlung spectra Bremsstrahlung radiation makes up approximately 80% of the x-ray beam keV 2. In most cases, discrete spectra peaks known as characteristic x-rays. Continuous Spectrum ■ X-rays are produced whenever matter is irradiated with a beam of high-energy charged particles ■ In an x-ray tube, the interactions are between the electrons and the target. Since energy must be conserved, the energy loss from the interaction results in the release of x-ray photons ■ The energy (wavelength) will be equal to the energy loss E  12.398  ■ This process generates a broad band of continuous radiation (bremsstrahlung or white radiation) 2-Bremsstrahlung Radiation Bremsstrahlung x-rays occur when electrons are (de)accelerated in the Coulomb field of a nucleus Bremsstrahlung radiation X-ray Projectile 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”. The probability of bremsstrahlung goes as Z2, hence high Z targets are more effective than low Z The energy of the x-rays varies from zero to the maximum kinetic energy of the electron (x-ray tube kVp) The frequency distribution is continuous and shows that the Bremsstrahlung process produces more low energy than higher energy x-rays. The average energy is approximately 1/3 of the Emax. E max Name and explain the Characteristic of X-ray Radiation Generating Characteristic Radiation ■ The photoelectric effect is responsible for generation of characteristic x-rays. ■ An incoming high-energy photoelectron dislodges a k-shell electron in the target, leaving a vacancy in the shell ■ An outer shell electron then “jumps” to fill the vacancy ■ A characteristic x-ray (equivalent to the energy change in the “jump”) is generated L-shell to K-shell jump produces a K x-ray M-shell to K-shell jump produces a K x-ray Characteristic X rays E max To produce characteristic x-rays the projectile electrons must have sufficient energy to displace orbital electrons If 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. The most common transition is from L to K shell. ■ 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. ■ 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. Note that characteristic X ray spectra are independent of voltage once the threshold values have been reached. Dual characteristics of X-rays  X-rays belong to a group of radiation called electromagnetic radiation  Electromagnetic radiation has dual characteristic, comprises of both Wave Particle  Wave concept: Propagated through space in the form of waves. Waves of all types have associated wavelength and frequency  Relationship : c=λν. c=velocity of light λ=wavelength. ν=frequency The wavelength of diagnostic X-rays is very short around 0.1 to 1A. Wave concept explains why it can be reflected. Particle concept used to describe interaction between radiation &matter The amount of energy carried by each photon is given by E=hν E=photon’s energy = Planck's constant * ν=frequency c= νλ or ν=c/λ so substituting c/λforνwe get E= hc/λh=4.13x 10-18keV/sec c=3 x108m/sec E=12.4 E=Energy in Kev.λ=wavelength in A0 Ex. ■ Find the wave length and the frequency of X-ray photon jumps from E2 (18ev) to E1(8eV) ? So/ METHODS OF INTERACTIONS Photons : absorbed / scattered. Attenuation: Reduction of intensity. Difference in attenuation gives the radiographic image. Absorbed: completely removed from the x- ray beam & cease to exist. Scattered: Random course. No useful information. No image only darkness. Adds noise to the system. Film quality affected “film fog”. ■ About 1% of the x- rays that strike a patient's body emerge from the body to produce the final image. The radiographic image is formed on a radiographic plate that is similar to the film of a camera. ■ Remaining 99% of the x-rays ---Scattered Absorbed. BASIC INTERACTIONS BETWEEN X-RAYS AND MATTER There are12mechanism,outofwhich five basic ways in which an x-ray photon may interact with matter. These are :Broadly classified on the basis ofA: PHOTON B:PHOTON. SCATTERING DISAPPEARANCE PHOTOELECTRIC COHERENTSCATTERING COMPTONSCATTERING PAIRPRODUCTION PHOTODISINTEGRATION 1. COHERENT SCATTERING What happens in coherent scattering ? Low energy radiation encounters electrons Electrons are set into vibration Vibrating electron, emits radiation s t a t e Atom returns to its undisturbed state Fig : Rayleigh scattering 1. COHERENT SCATTERING No ionization --- why??? because, no energy transfer. Only change of direction. Only effect is to change direction of incident photon. Less than 5%. Not important in diagnostic radiology. Produces scattered radiation but of negligible quantity. 2. PHOTOELECTRIC EFFECT What happens in Photoelectric effect ? An incident PHOTON encounters a K shell electron and ejects it from the orbit The photon disappears, giving up ( nearly) all its energy to the electron The electron ( now free of its energy debt) flies off into space as a photoelectron carrying the excess energy as kinetic energy. The K shell electron void filled immediately by another electron and hence the excess energy is released as CHARACTERISTIC RADIATION. The atom is ionized. PHOTOELECTRIC EFFECT Percentage of photoelectric reactions Radiation energy (Kev) 20 Water 65 Compact bone 89 Sodium iodide 94 60 7 31 95 100 2 9 88 CHARACTERISTIC of RADIATION ■ Characteristic of radiation generated by the photoelectric effect is exactly the same .The only difference in the modality used to eject the inner shell electron. ■ In x ray tube a high speed electron ejects the bound electron, while In photoelectric effect an X ray photon does the trick. In both cases ■ the atom is left with an excess of energy = the binding energy of an ejected electron ■ Usually referred to as Secondary Radiation to differentiate It from scatter radiation...... End result is same for both, “A Photon that is deflected from its original path” How does this happen ? ■ After the electron has been ejected, the atom is left with avoid in the K shell & an excess of energy equivalent to the binding energy. ■ This state of the atom is highly unstable & to achieve a low energy stable state ( as all physical systems seek the lowest possible energy state ) an electron immediately drops in to fill the void. ■ As the electron drops into the K shell, it gives up its excess energy in the form of an x-ray photon. The amount of energy released is characteristic of each element & hence the radiation produced is called Characteristic radiation. 2. PHOTOELECTRIC EFFECT ■ Thus the Photoelectric effect yields three end products ■ Characteristic radiation ■ A -ve ion ( photoelectron ) ■ A+ve ion (atom deficient in one electron ) 2. PHOTOELECTRIC EFFECT ■ Probability of occurrence : ■ The incident photon energy>binding energy of the electron. ■ Photon energy similar to electron binding energy ■ Photoelectric effect 1 (energy)3 ■ The probability of a reaction increases sharply as the atomic no. increases ■ Photoelectric effect. (atomic no.)3  Low atomic number : interaction mostly at the K shell.  High atomic number : interaction mostly at L and M shell.  In summary, Photoelectric reactions are most likely to occur with low energy photons and elements with high atomic numbers provided the photons have sufficient energy to overcome the forces binding the electrons in their cells. K-shell electron binding energies of elements important in diagnostic radiology Atom Calcium Iodine Barium Tungsten Atomic number 20 53 56 74 K-shell 4.04 33.2 37.4 69.5 Lead 82 88.0 2. PHOTOELECTRIC EFFECT : Applications in diagnostic radiology ■ Advantages : Disadvantage: ■ Excellent radiographic images • Maximum radiation :No scatter radiation. exposure. ■ Enhances natural tissue contrast. • All the energy is absorbed Depends on 3rdpowerof the atomic no., so it magnifies the by the patient whereas in difference in tissues composed of other reactions only part of different elements, such as bone & soft tissue the incident photon’s energy ■ Lower energy photons : total is absorbed. absorption. Dominant up to 500keV. 3.COMPTON EFFECT The Compton effect occurs when the incident x-ray photon with relatively high energy ejects an electron from an atom and a x-ray photon of lower energy is scattered from the atom. The reaction produces an ion pair A +ve atom A–ve electron ( recoil electron ) Almost all the scatter radiation that we encountered In diagnostic radiology comes from Compton Scattering 3.COMPTON EFFECT Kinetic energy of recoil electron ■ Energy of photon distributed Retained by the deflected photon Two factors determine the amount of energy the photon transmits :  The initial energy of the photon.  Its angle of deflection. ■ 1.Initial energy :-Higher the energy more difficult to deflect. High energy : Travel straight retaining most of the Low energy : Most scatter back at angle of 180o ■ 2.Angle of deflection :-Greater the angle, lesser the energy transmitted. With a direct hit, maximum energy is transferred to the recoil electron. The photon retains some energy & deflects back along its original path at an angle of 180o. 3.COMPTON EFFECT ■ Probability of occurrence : ■ It depends on :■ Total number of electrons: It further depends on density and number of electrons per gram of the absorber. All elements contain approx. the same no. of electrons per gram, regardless of their atomic no. Therefore the no. of Compton reactions is independent of the atomic no. of the absorber. ■ Energy of the radiation :The no. of reactions gradually diminishes as photon energy increases, so that a high energy photon is more likely to pass through the body than a low energy photon. Two subsequent points should also be noted: ■ Firstly, the photoelectron can cause ionizations along its track. ■ Secondly, X-ray emission can occur when the vacancy left by the photoelectron is filled by an electron from an outer shell of the atom. Disadvantages of Compton reaction : ■ Scatter radiation :Almost all the scatter radiation that we encounter in diagnostic radiology comes from Compton scattering. In the diagnostic energy range, the photon retains most of its original energy. This creates a serious problem, because photons that are scattered at narrow angles have an excellent chance of reaching an x-ray film &producing fog. ■ Exceedingly difficult to remove– ►cannot be removed by filters because they are too energetic. ►cannot be removed by grids because of narrow angles of deflection. 4. PAIR PRODUCTION ■ No importance in diagnostic radiology. ■ What happens in Pair production ? ■ A high energy photon interacts with the nucleus of an atom. The photon disappears &its energy is converted into matter in the form of two particles ■ An electron Apositron(particle with same mass as electron, but with +ve ■ charge.) ■ Mass of one electron is 0.51 MeV. ■ 2 electron masses are produced. ■ So the interaction cannot take place with photon energy less than 1.02MeV. ■ Positron annihilation.What happens to thePositron ? ■ Slowly moving Positron combines with a free electron to produce two photons of radiation. 2 mass units are converted, giving a total energy of 1.022MeV. To conserve momentum, two photons each with 0.511 MeV energy are ejected in opposite direction. 5. PHOTODISINTEGRATION A photon with extremely high energy ( 7-15 MeV), interacts directly with the nucleus of an atom. May eject a neutron, proton or on rare occasions even an alpha particle. No diagnostic importance. We rarely use radiation>150 Kev in diagnostic radiology. ■ What happens in Photodisintegration ? ■ A high energy photon encounters the nucleus of an atom. ■ Part of the nucleus which may be a neutron, a proton, an alpha particle or a cluster of particles, is ejected. RELATIVE FREQUENCY OF BASIC INTERACTIONS  Coherent scattering :About 5% . Minor role throughout the diagnostic energy range.  Compton scattering : Dominant interaction in water. Water is used to represent tissues with low atomic nos. such as air, fat and muscle.  Photoelectric reaction :usually seen in the contrast Agents because of their high atomic numbers.  Bone is intermediate between water &the contrast agents. At low energies, Photoelectric reactions are more common, while at high energies, Compton scattering is dominant. Scatter Radiation ■ Definition ■ A type of secondary radiation composed of photons of lower energy than the photons that produced them and which travel in a different direction. ■ The term scatter radiation is synonymous with secondary radiation in the context of x-rays Scatter radiation&Contrast - overview ■ Radiographic images are maps of radiation attenuation. Bone attenuate the most, air in lungs the least. ■ Good radiograph : maximum contrast difference between different tissues. X-Ray beam enters body. Large number of interactions producing scatter radiation. Image contrast reduced depending on scatter radiation content reaching film. Factors affecting scatter radiation  Field Size  Most important factor in the production of scatter radiation. ■ A small x ray field usually called Narrow beam irradiates less tissue and generates fewer scattered photons. Contrast Improvement by Reducing X-Ray Effects of scatter radiation Reduction of contrast: Scattered photons Carry no useful information Contribute to film blackness(film fog) Increased patient dose Increased risk to personnel Prevention of scatter radiation ■ Different techniques are used to keep the scatter radiation from reaching the films. ■ By using ray filters and X ray beam restrictors Grids