INTERACTION OF X-RAYS WITH MATTER

Properties of X rays: Travel in straight line, Undergo reflection: follow laws of reflection, Undergo refraction: follow laws of refraction, Ionize the gas/ medium through which they pass,

INTERACTIONS Three fates -Transmitted, (If we consider X ray beam it is attenuated by matter) -Absorbed, -Reflected: Scattered: adds noise to system.

INTERACTIONS 1.COHERENT SCATTERING 2.PHOTOELECTRIC EFFECT (Characteristic radiation) 3.COMPTON SCATTERING (General radiation) 4.PAIR PRODUCTION 5.PHOTODISINTEGRATION

COHERENT SCATTERING Significance : 5% of all interactions. Radiation undergoes change in direction without a change in wavelength. 2 Types 1.Thomson Scattering-involves single electron of the atom 2.Rayleigh Scattering- involves all electrons of the atom Significance : Not so important in practical radiology. Produces scattered radiation, Contributes in formation to film fog.

Absorption of radiation Excitation of atom Emission of radiation RAYLEIGH SCATTERING Absorption of radiation Excitation of atom Emission of radiation Energy absorbed is same as energy emitted as wavelengths of both radiations is same. AND SO Only interaction which doesn’t cause ionization of atom involved. No energy is transferred to the atom.

PHOTOELECTRIC EFFECT 3 End products CHARACTERISTIC RADIATION POSITIVE ION PHOTOELECTRON: Also known as negative ion. Inner orbit electron escapes parent atom as photoelectron. Almost all energy of photon is given to electron. Get absorbed immediately - less penetrating power.

CHARACTERISTIC RADIATION One electron from outer orbit jumps into K- shell / inner orbit of atom (that has high potential energy.) Energy is given up by the electron before entering inner orbit which is particular / characteristic for that atom / element - characteristic radiation. In an X-ray tube a high speed electron ejects the bound electron while In a photoelectric reaction an X-ray photon ejects the bound electron.

X-ray photon strikes k-shell electron K-shell electron is ejected Outer shell electron fills K-shell void An atom deficient in one electron and so is positive ion.

For Tungston: k shell energy 70 keV, l shell 12 keV, and so characteristic radiation: 58 keV.

Significance: Most imp when low energy radiation is incidenting on high atomic wt element. When photoelectric effect predominates attenuation is more/ mechanism of k edge filters. PROBABILTY OF OCCURANCE OF PHOTOELECTRIC EFFECT: It depends upon three things. Incident photon must have sufficient energy to overcome binding energy of electron of inner high energy orbit. Energy : When photon energy and electron binding energy are nearly same. Example-Iodine K-shell B. Energy- 33.9 KeV Probability of photoelectric effect decreases with increase in energy of radiation. Photoelectric effect proportional to 1/(energy)3. Tighter an electron is bound in it’s orbit, more likely it to be involved. Inner shell electrons are more tightly bound. Tighter an electron is bound in it’s orbit, more likely it to be involved. Electrons are tightly bound in atoms with high atomic no. Photoelectric effect proportional to (atomic no.)³

APPLICATIONS Advantage: Radiographic image of excellent quality. - doesn’t produce scatter radiation - enhances tissue contrast. Disadvantage: Patient exposure is more.

COMPTON SCATTERING Most important reaction in diagnostic radiology High energy photon strikes outer shell electron. Outer shell electron ejected. Photon gets deflected from its path Atom is left with electron deficit state

Diff from photoelectric effect : Photon survives and is not completely absorbed by electron, Outer orbit electron is escaping not inner high energy electron, So no external electron to take its place No charact radiation

COMPTON SCATTERING COMPONENTS OF REACTION Scattered Photon Recoil Electron Positive Ion Part of energy of incident photon goes to recoil electron as kinetic energy, rest retained by photon. Retained energy of photon depends upon: -Initial energy -Angle of deflection of recoil electron.

0 degree deflection is like coherent scattering

Change in wavelength of scattered photon , Δλ =0.024 (1-cos ɵ). In diagnostic radiology, most of energy is retained by incident photon. Significance / Disadv: (Photon survives and deflection by narrow angle) Reduced image quality by forming film fog. Produces major safety hazard scatter radiation. Difficult to remove from x-ray beam. -Can’t be removed by filters(too energitic) - Can’t be removed by grids(small deflection) Probability of occurrence : Depends upon total no. of electron in an absorber. Initial energy of photon.

PAIR PRODUCTION High energy photon strikes with nucleus and produces 2 particles -Positron and Electron. Do not occur in diagnostic radiology range which uses photons of 10-150 keV. THIS Interaction doesn’t occur if photon energy less than 1.02 MeV.

PHOTODISINTEGRATION Part of nucleus is ejected by high energy photon. Not useful in diagnostic radiology which uses 10 to 150 keV. Doesn’t take place with photons <7MeV energy. Ejected portion may be proton, neutron, an alpha particle.

Dep on atomic no 3 energy of radiation Imp practically, source of scattered radiation and is well effective at soft tissue / water density 5 % only does not change much with other parameters

BASIC TERMINOLOGY Quality of X-ray Beam-Refers to energy of photons. Quantity of X-Ray Beam-No. of photons Intensity of X-Ray Beam-Product of no. and energy of photons. Monochromatic Radiation : Spectrum of photons of same wavelength. Polychromatic Radiation : Spectrum of photons of different wavelength

ATTENUATION Definition: Reduction in intensity of X-ray beam as it traverses matter by either absorption or deflection of photons from the beam. It depends upon quantity as well as quality of radiation. In case of MONOCHRMATIC RADIATION: Number of photons remaining in the beam decreases by the same percentage with each increment of absorber . This is called as Exponential Attenuation.

Monochromatic rays

ATTENUATION Exponential equation for attenuation is, N = N0e_µx N = no. of transmitted photons, N0 = no. of incident photons e = base of natural log, µ= linear attenuation coefficient X = absorber thickness.

ATTENUATION COEFFICIENT It is measure of quantity of radiation attenuated by absorber thickness. 1.Linear Attenuation Coefficient 2.Mass Attenuation Coefficient

Linear Attenuation Coefficient(µ) It is quantitative measurement of attenuation per centimeter of absorber. Most important in diagnostic radiology. More practical and useful. Unit : per cm Linear Attenuation Coefficient , µ = µ coherent + µ Phptoelectric Effect + µ compton We can predict total amount of attenuation depending upon percentage of each type of interaction.

Linear Attenuation Coefficient(µ) Specific for energy of beam and type of absorber. Does not depend on thickness of absorber. Energy of beam increases-linear attenuation coefficient decreases. Half Value Layer thickness(HVL): It is absorber thickness required to reduce the intensity of the original beam by one half. So unit is cm. HVL = 0.693/µ

Mass Attenuation Coefficient It is used to quantitate attenuation of materials per mass of absorber. It is independent of physical state of material. Eg . Same for 1 gm of ice, water, water vapor. It is obtained by dividing linear attenuation coefficient by the density(ρ) MAC = µ/ρ Unit – per g/cm2 or cm2/g

FACTORS AFFECTING ATTENUATION Radiation Energy / kVp of radiation. Density Of Matter Atomic Number Electrons per cm3 RELATION BETWEEN DENSITY, ATOMIC NO. In general elements with high atomic no. are denser than elements with low atomic no. Elements with low atomic no. have more electrons per gram but not necessarily electrons per cm3 than those with high atomic no.

Effect of Energy and Atomic Number As radiation energy increases percentage of photoelectric reaction decreases and compton reaction becomes predominant reaction. As atomic no increases photoelectric reaction becomes predominant reaction. Attenuation is always greater when photoelectric effect predominates. Low energy of radiation At low kVp. Increased at wt Low at wt

Effect of Energy and Atomic Number : As energy increases attenuation decreases. In case of high atomic no. attenuation may actually increase with increase in energy. K edge : It is binding energy of K-shell electron. with radiation energy below K edge fairly large percentage of photons are transmitted, while above K edge transmisssion drops. Effect Of Density On Attenuation Increase in density increases attenuation. Relationship is linear. Difference in tissue densities is primary reasons why we see an x-ray image.

Effect Of Electrons Per cm3 Compton reaction depends up no. electrons in a given thickness i.e. electrons per cubic centimeter. Electrons/g x density = electrons/cm3 Bone has fewer e/g than water, bone still attenuates more radiation, because it has more e/cm3.

Polychromatic Radiation Contains spectrum of photons with different energies / different kVp x rays. Mean energy of polychromatic radiation is between one third and one half of its peak energy. Transmitted photons undergo change in both quality and quantity. Attenuation is not exponential. Larger percentage of low energy photons are attenuated by first few cms of absorber, so quality of remaining beam increases.

Increased mean energy to 57 from 40 at begining 2 8 4 Imagine a diagram for monochromatic radiation: 1000-500-250-125 (50 % or constant fraction of beam is lost by each unit block), 40 kVp beam persists till end.

APPLICATIONS of attenuation : X-ray image – only transmitted photons reach film. - Because of differential attenuation between different tissues variable number of photons are transmitted through various tissues. Differential attenuation may be due to predominance of photoelectric interactions or compton scattering . Compton predominance – at low energy radiation and low density, no. of electrons per cm3. Photoelectric predominance – at higher atomic no.

FACTORS AFFECTING SCATTER RADIATION: Kilovoltage : Scatter radiation is maximum with high kVp techniques. Less important as we can’t compromise kVp. Only variable we can control. Field Size : Most important . Scatter radiation increases with increase in field size, then gradually tapers off until finally it reaches plateau or saturation point. Part Thickness : Reaches saturation with increase in part thickness. Some authors (Yochum says use grid to decrease scatter radiation if skeletal part thickness is > 10 cm.) Collimation, filter decrease scatter radiation. Air gap technique, grid decrease scatter reaching film.