Interaction of x-ray photons (and gamma ray photons) with matter

Interaction of x-ray photons with matter When a beam of x-ray photons passes through matter, its intensity (energy – or number of x-ray photons - flowing per second) is reduced: the beam has been attenuated, photon energy has been removed from the beam photon energy is attenuated by either being absorbed by the matter or scattered out of the beam Any photon energy not attenuated, is transmitted.

Electrical energy x-ray photon energy In the x-ray tube for diagnostic imaging or linear accelerator for radiotherapy treatment absorbed energy: radiation dose scattered x-ray energy transmitted x-ray energy Attenuation and transmission of x-ray photon energy in the patient converted into at the x-ray target

Absorption some or all of the x-ray photon energy may be absorbed when energy is transferred to matter The energy deposited per unit mass of matter is called absorbed dose Units: joule per kg = 1 gray (Gy) the energy deposited as absorbed dose causes ionisation and subsequent chemical changes that may result in biological effects

CT scan: differential absorption in tissues using kilovoltage x-ray photons RT treatment plan: absorbed dose distribution using megavoltage x-ray photons

Scatter Some x-ray photons are partially absorbed and their remaining energy scattered: deflected from their original path at high photon energies in megavoltage radiotherapy, scatter is in the forward direction and contributes to absorbed dose at low photon energies in diagnostic imaging, scatter occurs in all directions and may contribute to loss of image quality by increasing the overall density of the image receptor scattered x-ray photons contribute to the patient ’ s dose (by internal scatter) and exposure of staff

scatter Side scatter Forward scatter Back scatter x-ray source patient image receptor lead-glass screen air

Scattered radiation reaching image receptor blackening due to collimated x-ray photons in primary beam low density blackening around collimated beam due to scattered x-ray photons from collimators and object object

attenuation is exponential… Attenuation in matter is due to some x-ray photons being totally absorbed … … and some x-ray photons being partially absorbed and their remaining energy scattered X-ray photons that are not attenuated, are transmitted X-ray photons are attenuated differently in different materials depending on various factors: the individual x-ray photon ’ s energy, the material ’ s physical density, electron density, proton number and its thickness For a specific photon energy, an equal percentage (or fraction) of the energy in the beam is attenuated in equal thicknesses of material This is an exponential relationship: equal changes in one quantity give equal fractional changes in another

Exponential attenuation The linear attenuation coefficient (  ) gives the fractional reduction in intensity of a homogenous beam of x-ray photons per cm thickness of matter For example, the linear attenuation coefficient of soft tissue for 100 keV x-ray photons is approximately 0.2 (20%) (Bushong: table 29.1 in 6 th ed) 20% of photon energy is attenuated in each cm thickness of soft tissue; 80% is transmitted 1 cm 100%80%64%51%41%33%

Exponential attenuation The exponential relationship between the intensity of transmitted x-ray photons and the thickness of a specific material can be given as: WhereI t = transmitted intensity I o = original intensity e = exponential constant (2.718)  = linear attenuation coefficient of material x = thickness of material Provided the x-ray beam is i) homogenous ii) parallel I t = I o e -  x

Exponential attenuation of a beam of x-ray photons in equal thicknesses of aluminium is used to measure half value thickness (HVT) in an x-ray beam as part of routine quality assurance The linear attenuation coefficient is calculated for each individual voxel of tissue during a CT (computerised tomography) scan and mapped to a grey scale. Different tissues have slightly different coefficients and therefore map to a different grey to give the image Exponential attenuation in practice …

Experiment to measure HVT Ionisation chamber aluminium sheets x-ray source

CT scan of abdomen Each tissue has a slightly different linear attenuation coefficient, from which a CT number is calculated and mapped to a grey scale

Interaction Processes There are various interaction processes of x-ray (and gamma ray) photons, that may occur alongside each other: coherent (elastic) scatter (negligible) Compton (inelastic) scatter photoelectric absorption pair production

Relative importance of each interaction process in water

Mass attenuation coefficients in air Graham & Cloke (2003) p 299

Occurs when an x-ray photon interacts with a bound electron, in the inner shells of an atom Only occurs if the energy of the x-ray photon exceeds the binding energy of the shell The x-ray photon disappears by transferring all its energy to the bound electron This energy is used to overcome the binding energy of the electron, which then escapes the atom as a photoelectron carrying kinetic energy The photoelectron loses its KE via ionisation of surrounding atoms Photoelectric absorption

incident x-ray photon photoelectron c arrying KE photons of electromagnetic radiation z3E3z3E3 

The probability of photoelectric absorption  and is more likely to occur: in beams of low energy when the average x-ray photon energy is < 25 keV in dense matter with atoms of higher proton number eg bone, metal (shielding, filters), positive contrast media (barium sulphate, iodine) With a bound electron in an atom where the x-ray photon energy is just above the binding energy: this results in absorption edges: a large increase in photoelectric absorption of x-ray photons with an energy just above the binding energy of a specific shell – and an increase in x-ray photons being transmitted just below the binding energy z3E3z3E3

Photoelectric absorption: absorption edges Photoelectric absorption only occurs when the x-ray photon energy is equal to, or slightly greater than, the electron binding energy This results in bursts of absorption at the binding energy of each shell

Compton scatter Occurs alongside photoelectric absorption An x-ray photon interacts with a bound electron, whose binding energy is negligible in comparison with the x-ray photon energy Some of the x-ray photon energy is transferred to the electron, to overcome its binding energy. It escapes the atom as a Compton electron carrying kinetic energy The Compton electron loses its KE via ionisation of surrounding atoms The x-ray photon with reduced energy is deviated, or scattered, from its original path and will interact again until all its energy is lost and it disappears.

Compton scatter Compton electron carrying KE scattered x-ray photon with reduced energy incident x-ray photon angle of scatter  electron density E

The probability of Compton scatter and is more likely to occur In higher energy x-ray beams (average energy >25 keV) when electron binding energies in the attenuating material are negligible in comparison; as photon energy increases, the proportion of forward scatter increases. In diagnostic imaging, Compton scatter results in a loss of image quality. In megavoltage radiotherapy, it results in poor quality portal images but gives the main contribution to absorbed dose In materials containing atoms with a high electron density eg hydrogen. In diagnostic imaging, this results in more scatter in soft tissues…increasing the importance of collimation and shielding. In megavoltage radiotherapy, inhomogeneity correction is important in treatment planning  electron density E

Megavoltage imaging: example of DRR and portal image

Pair production A high energy x-ray photon (>1.02 MeV) interacts with a nucleus Its energy is converted into an electron and a positron, carrying any excess energy as kinetic energy The positron annihilates with an electron: the two particles are converted back into energy – two gamma ray photons of 511 keV Pair production only becomes important in very high energy beams of photons above 10 MeV

Pair production incident x-ray photon electron + KE positron + KE  -ray photon MeV  -ray photon MeV  E.z

Summary: Interaction of x-ray photons with matter When a beam of x-rays passes through matter, its intensity is reduced: attenuated attenuation is due to the interaction processes of photoelectric absorption, Compton scatter or pair production Attenuation increases with thickness and density of matter interaction process depends on x-ray photon energy, proton number and electron density of the matter

Energy transfer to matter Electrical energy  KE of electrons  x-ray production: characteristic/bremss  x-ray photon energy and heat energy  Interaction of x-ray photons Photoelectric absorption Compton scatter Pair production  Secondary electrons carrying KE  x-ray photon energy deposited Excitation and ionisation  Chemical/biological effects at x-ray target in matter