INTERACTION OF XRAY WITH MATTER

SAHANA KAYASTHA

MSC MIT 1^ST YR

ROLL NO : 26

IOM, MMC

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Radiation: introduction�

• The term radiation applies to the emission and propagation of energy through space or a matter.
• Two categories of radiation importance in medical are Electromagnetic(EM) and particulate.

1. Electromagnetic radiation: Classified as

a. Non ionizing radiation(cannot ionize matter).

the types of radiation used in diagnostic ultrasonography and in magnetic resonance imaging, majority of the ultraviolet wavelengths, visible light, infrared, microwaves and radio waves.

b. Ionizing radiation (can ionize matter).

• Indirectly ionizing radiation (neutral particles): photons (x ray, gamma ray)

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Fig: The electro magnetic spectrum

2.Particulate radiation

• charged alpha particles, protons ,beta particles , positrons , energetic extranuclear electrons(directly ionizing charged particles) and uncharged particles, such as neutrons(indirectly ionizing).

Electromagnetic radiation

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• EM radiation has no mass, is unaffected by either electric or magnetic fields, and has a constant speed in a given medium.
• Lets talk about ionizing electromagnetic radiation that are used in diagnostic imaging.
• Gamma rays emitted by nuclei of radioactice atoms, are used to image the distribution of radiopharmaceuticals.
• Xrays ,produced outside the nuclei of atoms, are used in radiography,fluososcopy and computed tomography.

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Particulate Radiation

• Protons are found in the nuclei of all atoms. A proton has a positive charge and is identical to the nucleus of a hydrogen-1 atom.
• An atomic orbital electron has a negative electrical charge, equal in magnitude to that of a proton, and is approximately 1/1,800 the mass of a proton.
• Electrons emitted by the nuclei of radioactive atoms are referred to as beta particles.
• However, there are also positively charged electrons, referred to as beta-plus particles or positrons.
• Alpha particle consists of teo protons and two neutrons,ie identical to He nucleus
• Alpha particle are emitted by many high atomic number radioactive element, such as uranium, thorium, and radium.

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Particulate Radiation contd…�

• Neutron is an uncharged particle that has mass slightly greater than that of proton.
• Neutrons are released by nuclear fission and are used for radionuclide production

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X-rays

• X-rays have very short wavelengths, approximately 10^-8 to 10^-9 m.
• The higher the energy of an x-ray, the shorter its wavelength.
• Consequently, low energy x-rays tend to interact with whole atoms, which have diameters of approximately 10^-9 to 10^-10m; moderate energy x-rays generally interact with electrons, high energy x-rays generally interact with nuclei.
• X-ray interact at these various structural levels through four mechanisms ,first three of which play a role in diagnostic radiology and nuclear medicine.
• Coherent scattering
• Compton scattering
• Photoelectric absorption
• Pair production

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Importance of interaction�

• The selective interaction of x-ray photon with human body produces the image.
• Interaction of x-ray photon with receptor converts an x-ray image into one that can be viewed and recorded.
• Technical factor Kvp and MAs required to image tissue and if chosen appropriately may actually decrease the radiation dose to patient.
• Image visibility entirely depend upon the interaction of x-ray with matter.

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Some basic terminologies first

• Electron shell
• Binding energy
• Excitation
• ionization
• specific ionization
• Linear energy transfer
• Attenuation vs absorption
• Linear attenuation coeeficient
• Mass attenuation coeeficient
• Half value layer
• Beam hardening
• Differential absorption

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Electron shell

Binding energy

Binding energy in eV

Excitation

• If energy is delivered to an electron, it can be induced to rise to a higher orbit which is less tightly bound to the nucleus. This is termed excitation.
• The electron is raised to an excited state while a vacancy appears in a more stable bound state. This is an unnatural situation. If an electron falls back into the vacant slot in the lower orbit, then radiation must be released.

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Ionization�

Ionization of an outer shell electron

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• If the transferred energy exceeds the binding energy of the electron, ionization occurs,whereby the electron is ejected from the atom.
• The result of all these interactions is an ionization event which creates two charged particles: the liberated electron and the residual atom now positive since it is missing one electron and thus has one unpaired proton.

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Ionization and delta rays

• Sometimes, the ejected electrons posses sufficient energy to produce further ionizations called secondary ionization. These electrons are called delta rays

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SPECIFIC IONIZATION

• Average number of primary and secondary ion pairs produced per unit length of the charged particle’s path is called the specific ionization, expressed in ion pairs (IP)/mm.
• Specific ionization increases with the square of the electrical charge (Q) of the particle and decreases with the square of the incident particle velocity (v).
• A larger charge produces a greater coulombic field; as the particle loses kinetic energy, it slows down, allowing the coulombic field to interact at a given location for a longer period of time.
• The specific ionization of an alpha particle can be as high as approximately 7,000 IP/mm in air and about 10 million IP/mm in soft tissue and that of electron is much lower in the range of 5 to 10 IP/mm of aair.

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Linear Energy Transfer�

• The linear energy transfer (LET) is a measure of the average amount of energy deposited locally (near the incident particle track) in the absorber per unit path length.
• The LET is often expressed in units of keV or eV per micro meter.
• The LET of a charged particle is proportional to the square of the charge and inversely proportional to the particle’s kinetic energy.
• The LET of a particular type of radiation describes the local energy deposition density, which can have a substantial impact on the biologic consequences of radiation exposure.
• High LET radn(alpha particles,protons etc)deposit therir energy over much shorter range and are much more damaging to cells than low LET radn (beta particles and gamma ray, xrays

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Attenuation vs Absorption

• When xray photon enters a layer of matter such as human body, it is possible that three things can occur
• Transmission - It may penetrate through without any interaction
• Scattered in a different direction from that traveled by the incident photon
• Absorbed by the material such that no photon emerges
• Attenuation of the photon beam can be considered a combination of scattering and absorption.

i.e Attenuation = scattered + absorbed

• If the photons are scattered or absorbed, they are no longer travelling in the direction of the intended target.

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Source: IAEA, Interaction of radiation with matter - 3 X and Gamma Rays

� Linear Attenuation Coefficient�

• The fraction of photons removed from a monoenergetic beam of x-rays or gamma rays per unit thickness of material is called the linear attenuation coefficient (µ), typically expressed in units of inverse centimetres.
• The linear attenuation coefficient is the sum of the individual linear attenuation coefficients for each type of interaction.
• In the diagnostic energy range, the linear attenuation coefficient decreases with increasing energy except at absorption edges.
• Linear attenuation coeeficient for soft tissue ranges from approx. 0.35 to 0.16cm^-1.
• For a given thickness of material, the probability of interaction depends on the number of atoms the x-rays or gamma rays encounter per unit distance.
• The value of µ (E) is dependent on the phase state:

µwater vapor < µice < µwater

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MASS attenuation �coefficient

• The linear attenuation coefficient, normalized to unit density, is called the mass attenuation coefficient.
• the units of the mass attenuation coefficient are usually cm2/g.

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• The mass attenuation coefficient is independent of density.

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HALF VALUE LAYER�

• The half-value layer (HVL) is defined as the thickness of material required to reduce the intensity (e.g., air kerma rate) of an x-ray or gamma-ray beam to one half of its initial value.
• Units of HVL expressed in mm of Al for a Diagnostic x-ray beam.
• For monoenergetic photons under narrow-beam geometry conditions, the probability of attenuation remains the same for each additional HVL thickness placed in the beam.
• HVL is a function of (a)photon energy, (b) geometry, and (c) attenuating material.

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BEAM HARDENING

• The shift of the x-ray spectrum to higher effective energies as the beam transverses matter is called beam hardening.
• Low-energy (soft) x-rays will not penetrate the entire thickness of the body; thus, their removal reduces patient dose without affecting the diagnostic quality of the exam.
• X-ray machines remove most of this soft radiation with filters, thin plates of aluminum, copper, or other materials placed in the beam.
• This added filtration will result in an x-ray beam with a higher effective energy and thus a greater HVL.

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Xray interactions

Coherent scattering

• Reffered by variety of name including Thompson, Rayleigh, Classical and unmodified scattering.
• Name coherent is given to those interaction in which radiation undergoes change in direction without change in wavelength.
• When the energy of incoming x-ray photon is substantially less than the binding energy of strongly bound orbital electron, it may occur.
• JJ Thompson discovered classical scattering of x-ray with an electron.
• John Rayleigh discovered x-ray interaction and its momentarily absorption to the entire cloud of electron collectively around an atom.

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Coherent scattering

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• Is an interaction between low energy x-rays(energies below approx. 10kev) and atoms.
• The incident x-ray interacts with the target atom, causing it to become excited.
• The target atom immediately releases this excess energy as a scattered x-ray with wavelength equal to that of the incident x-ray(ƛ = ƛ’) and therefore equal energy. Only there is slight change in direction.
• There is no energy transfer and therefore no ionization.

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• absorption of radiation
• vibration of the atom
• emmision of radiation
• atoms return to undisturbed state

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Energry of scattered photon = Energy of incident x-ray photon

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• Only a small percentage of radiation undergoes coherent scattering(generally less than 5%).
• Some coherent scattering however, occurs throughout the diagnostic range.
• For eg; at 70kvp, a small percent of x-rays(3%) undergo coherent scattering contributing to film fog or image noise(the general graying of an image that reduces image contrast) but is of little importance in diagnostic radiology.

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Compton scattering

• Also k/a inelastic, nonclassic, modified and incoherent scattering.
• Discovered by American physicist, Arthur Compton.
• Interactions occur between incident x-rays and outer shell electrons.
• Electron is then ejected from atom, thereby ionizing the atom. this ejected electron is called a Compton electron or recoil electron.
• Also the x-ray continues in a different direction with less energy i.e scattered, so called the Compton scatter.

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Energy distribution�

• The energy of Compton scattered x-ray is equal to the difference between the energy of the incident x-ray and the energy of the ejected electron.
• And again the energy of ejected electron is equal to its binding energy plus the kinetic energy with which it leaves the atom.
• Mathematically,

Ei = Es+(Eb + EKE)

Ei = energy of the incident x-ray

Es = energy of the scattered x-ray

Eb = electron binding energy

EKE = kinetic energy of ejected electron

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• During Compton scattering, most of the energy is divided between the scattered x-ray and the Compton electron .usually , the scattered x-ray retains most of the energy.

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• Both the scattered x-ray and the Compton electron may have sufficient energy to undergo additional ionizing interactions before they lose all their energy.
• Ultimately , the scattered x-ray is absorbed photoelectrically.

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Factor determining the energy of photon

Features of Compton Scattering

Relative probability of interaction

Fig.: The probability that x-ray will interact through Compton scattering

is about the same for atoms of soft tissue and those of bone.

This probability decreases with increasing x-ray energy

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• Compton scattering in tissue can occur with all x-rays having some considerable importance in x-ray imaging:
• 97% of scatter x-rays originate from Compton interaction.
• Compton scattering = scatter radiation = the source of most of the occupational radiation exposure that radiographers receive(mostly in fluoroscopy)
• Provides no useful information on the radiograph. rather producing uniform optical density on the screen- film radiograph and uniform intensity on the digital image receptor that results in reduced image contrast.
• Cannot be totally eliminated .so, we must accept them and tolerate an image of diminished quality.

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Photoelectric effect

• The photoelectric effect can occur when an incident photon has energy equal to or greater than the binding energy of electron in atom
• Photon can ionize the atom by ejecting an electron from the inner shell
• Photon gives all its energy to the atom.
• Here the x-ray is not scattered but is totally absorbed.
• The ejected electron called secondary electron or photoelectron escapes with kinetic energy equal to the difference between the incident x-ray and binding energy of the electron.
• Mathematically ;

Ei = Eb + EKE where, Ei = energy of incident x-ray

Eb = the electron binding energy

EKE = the kinetic energy of the electron

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• Vacant site fulfilled by electron jumping inward from another shell farther away from nucleus, accompanied by emission of characteristics x radiation in the form of secondary photon whose energy is equal to difference between the binding energies of two shells involved.

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Energy transfer in photoelectric effect

• Energy transfer is two step process.

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• The 1st is, photon transfers all its energy to an electron located in one of the atomic shell and the electron is ejected from the atom known as photoelectron and begins to pass through surrounding matter penetrates tissues but never out of patients body and never reach IR
• Finally the photoelectron deposits the energy in the surrounding.
• Summarily , photon energy is divided in two portions: a portion of the energy is used to overcome electrons binding energy and to remove it from atom. the remaining energy is transferred to electron as KE and is deposited near interaction site.

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Vacancy fulfillment

• Since the interaction creates vacancy in one of the electron shell, typically k or l.
• An electron moves down to fill in.
• The drop in energy of filling electron often produces a characteristics x-ray photon which is the characteristics of each element.
• These characteristics x-rays consists of secondary radiation and behave in the same manner as scatter radiation.
• This effect yields three end product:
• Characteristic radiation
• Negative ion(the photon electron)
• A positive electron (an atom deficient one electron)

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Features of Photoelectric Effect

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Fig: relative probability that a given x-ray will undergo a photoelectric interaction

is inversely proportional to 3^rd power of x-ray energy and directly proportional to

3^rd power of atomic number of absorber

Significance of Photoelectric absorption

• The radiographic contrast is the result of penetration and photoelectric absorption
• It enhances contrast between adjacent tissues when contrast media is introduced into patient’s body
• All the radiation dose to the patient and other individuals exposed is the result of photoelectric absorption

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Pair production

• Does not occur in diagnostic radiology
• High energy x-ray photon may escape interaction with electrons and come close enough to be influenced by the strong nuclear field.
• This causes the photon to disappear,
• Its energy is converted into matter in the form of two particles.
• One is electron and the other is positron , a matter with mass as an electron but with positive charge.
• Cannot take place with photon energies less than 1.02MeV.
• Produces two electron with mass of one equal to 0.51MeV.

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• Fig: pair production

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Process of pair production

• Incoming x-ray photon strongly interacts with nucleus.
• Energy of photon is transformed into two new particles: a negatron(an ordinary electron) and a positron(positively charged electron)
• They have same mass and magnitude but different sign.
• The electron loses its kinetic energy by excitation and ionizing atoms and is captured by an atom in need of electron.
• Positron acts destructively with nearby electron.
• during this interaction, positron and electron annihilate with each other, a conversion of mass into energy in accordance with Einsteins’s theory of relativity as E = mc2

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• Tis energy that appears from annihilation of electron and positron is carried off by two 0.511 MeV photon moving in opposite direction.
• Annihilation radiation is used in Positron Emission Tomography(PET)

Photodisintegration

• Occurs with energy more than 10 MeV.
• The high energy photons can escape interaction with electrons and the nuclear field and be absorbed directly by the nucleus.
• This will raised the nucleus to an excited state and instantly emits a nucleon or other nuclear fragment such as neutron, proton, an alpha particle or a cluster of particles.

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Process of photodisintegration

• A high energy photon collides with the nucleus of an atom, which directly absorbs the photon energy.
• This energy excess creates an instability that in resukt emits neutron by the nucleus.
• Other types of emissions, a proton or proton neutron combination or even alpha particles are possible if sufficient energy is absorbed by the nucleus.

Differential absorption

• Out of five way an x-ray interaction with tissue ,only two are important to radiology by differential absorption.
• 1. Compton scattering
• 2. Photoelectric effect
• Differential absorption determines the degree of contrast of X-ray image.
• X-ray image result from the difference between x-ray absorbed photoelectrically in the patient and those transmitted to the image receptor . This difference is differential absorption.

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Characteristics of differential absorption

Summary

• Two interaction of xradiation: photoelectric absorption and Compton scattering are important in diagnostic radiology
• The photoelectric effect is the basis of radiographing imaging whereas the Compton effect is its bane.
• Within the energy range of diagnostic radiology 23 to 150 kvp which also includes mammography, when kvp is decreased the number of photoelectric interaction increases but the Compton interaction decreases however dose to patient increases
• When kvp is increased patient receives a lower dose but the image quality is compromised.

• Compton effect is responsible for vast majoritt of scatter radiation.
• If coherent scattering accounts fotr the 5% of the interaction, Compton scattering for 20% and the photoelectric effect for 75% and the total 100%.

Comparison of Photoelectric effect and Compton effect

• The probability of PE effect & Compton effect is equal for soft tissue at 20 keV and the effects are equal for bone at 40 keV

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REFERENCEs

• The Essential Physics of Medical Imaging ,JERROLD T. BUSHBERG, PhD.
• Radiologic Science for Technologists, Stewart Carlyle Bushong.
• IAEA (International Atomic Energy Agency), Lecture 2 - Basic Nuclear Physics 2 - Ionization and X-rays
• IAEA, Lecture 3 - Rad. Interaction with Matter 3 - X rays, Photons
• Christensen’s physics of radiography
• Various websites.

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thank you