Physics#
Ionizing radiations#
Wilhem Röntgen a German physicist made the discovery in 1895 of X-rays during studies of vacuum tubes (Crookes). The first x-rays on film (the hand of his wife) date from this period.
Less than 20 years after their discovery, X-ray imaging was used medicinally in battle fields. Marie Curie, Nobel Prize winner in physics and chemistry, participates in the design of mobile radiology surgical units (see Petites Curies).
Some societal applications of X-rays appeared in the first half of the 20th century century, but radiobiological studies quickly put an end to these applications.
X-rays can be modeled twofold (it is the wave-particle duality):
as massless particles called photons characterised by their energy usually expressed in keV. The keV unit is used instead of Joules by reference to the mode of x-ray production which is based on electron acceleration: the kinetic energy of an electron (initially at rest) accelerated by a potential difference of 1V is 1eV. The speed of light in vaccum is \(3\times 10^8\) m\(\cdot\)s\(^{-1}\).
or else as electromagnetic radiation, characterized by its wavelength. The range of electromagnetic radiation is very wide.
The wavelength of x-rays is less than nm: atoms and their constituents are therefore resolved.
Photon
The photon energy \(E\) is related to the photon frequency \(\nu\):
When all photons of an x-ray beam have the same energy, the x-ray beam is called “monochromatic”, and named “polychromatic” otherwise. The momentum \(\mathbf{p}\) and wave vector \(\mathbf{k}\) are given by:
Mass particles exist at rest and their speed depends on their kinetic energy \(E_k\).
Mass particle
The energy of a mass particle is the sum of its rest energy energy \(E_0 = m_0 c^2\) and its kinetic energy \(E_k\):
where the Lorentz factor \(\gamma\) depends on the particle speed vector \(\mathbf{v}\) (it becomes \(+\infty\) if the particle speed is \(c\)):
or
The particle momentum \(\mathbf{p}\) is simply \(\mathbf{p} = \gamma m_0 \mathbf{v}\)
Attenuation law#
Photon-matter interactions are of two types: total absorption or scattering. In in the first case the energy of the photon is locally transferred to the atom. In the second case, a scattered photon is emitted after interaction with its own energy and direction. X-ray imaging is therefore based on attenuation, ie the proportion of photons that does not have any interaction with the matter, namely the radiation directly transmitted. This is the Beer-Lambert law.
Beer-Lambert attenation law: ratio of photons
The law of attenuation can be deduced from a thin beam model impinging on a plate of elementary thickness \(\text{d}l\). The drop in the number of directly transmitted photons is proportional to the number of incident photons \(N\) and the thickness of this plate.
Beer-Lambert attenation law: differential equation
which gives
\(\mu\) is therefore a linear attenuation coefficient which corresponds to a percentage of interaction per unit length.
Integrating this equation gives the exponential law of attenuation which presents a simple shape in the case of a homogeneous material and for a single radiation energy \(E\).
Beer-Lambert attenation law: Mono-E & mono-material
Directly transmitted photons (ie without interaction) in nr of photons or a monochromatic // beam in a homogeneous material:
In the case of materials with several homogeneous phases, the attenuation results from simple product of attenuations on the different constituent materials. In fact, for a locally non-divergent geometry, the order and positioning of the materials long radiation has no influence on the number of photons directly transmitted (unlike the proportion of scattered radiation arriving on the detector).
The Beer-Lambert law of attenuation generalizes to spectra large and heterogeneous materials in the form of a sum of exponentials. She can be expressed in number of photons, in energy, or in dose.
Generalized Beer-Lambert law for a polychromatic // beam and an heterogeneous material
The linear attenuation coefficient \(\mu\), which is the probabibility of interation per unit length, can be therefore interpreted as the sum over the different x-ray interactions with matter.
Linear attenuation coefficient \(\mu\)
The linear attenuation coefficient \(\mu\) is a sum of interaction probabilities per unit length (i.e. in cm\(^{-1}\))
The linear attenuation coefficient \(\mu\) varies linearly with the material density \(\rho\) and the cross section \(\sigma\), which represents a probability of interaction expressed as an area.
Cross Section \(\sigma\)
The linear attenuation coefficient \(\mu\) is expressed in terms of the material cross section (\(\sigma\) in cm\(^2\))
The cross section (CS) \(\sigma\) is usually not 3D-isotropic and is expressed as a differential cross section (DCS) per solid angle
where the elementary solid angle \(\mathrm{d}\Omega\) is
angle \(\theta\) is taken with respect to the photon direction
angle \(\phi\) is the polarisation angle
The DCS for unpolarized photons is rotational symmetric and can be expressed in function of angle \(\theta\)
The cross-section \(\sigma\) does not depend on the material density \(\rho\). The mass attenuation coefficient \({\mu}/{\rho}\), which is expressed in \(\text{cm}^2\cdot\text{g}^{-1}\), is therefore independent of the density \(\rho\)
The Bragg additivity rule makes it possible to relate the mass attenuation coefficient \({\mu}/{\rho}\) to calculate that of a mixture.
Bragg’s additivity rule
Example: computation of the linear attenuation of salted water (SW) at 10 g/l
\[ \mu_{SW} = \rho_{SW} \left( \omega_{\text{NaCl}} \times \left.\frac{\mu}{\rho}\right|_{\text{NaCl}} + \omega_{\text{H2O}} \times \left.\frac{\mu}{\rho}\right|_{\text{H2O}}\right) \]Salted water density: no increase of volume (hyp)
\[ \rho_{\text{SW}} = \frac{1000 \text{[g]} + 10 \text{[g]}}{1000 \text{[cm}^3\text{]}} = 1.01 \text{[g}\cdot\text{cm}^{-3}\text{]} \]Mass fractions
\[ \omega_{\text{NaCl}} = 10 \text{[g]} / 1010 \text{[g]} = 0.99 \% \]\[ \omega_{\text{H2O}} = 1. - \omega_{\text{NaCl}} = 99.01 \% \]Mass attenuation coefficents
\[ \left.\frac{\mu}{\rho}\right|_{\text{NaCl}} = \frac{A_{\text{Na}}}{A_{\text{Na}}+A_{\text{Cl}}} \left.\frac{\mu}{\rho}\right|_{\text{Na}} + \frac{A_{\text{Cl}}}{A_{\text{Na}}+A_{\text{Cl}}} \left.\frac{\mu}{\rho}\right|_{\text{Cl}} \]\[ \left.\frac{\mu}{\rho}\right|_{\text{H2O}} = \frac{2 A_{\text{H}}}{2 A_{\text{H}}+A_{\text{O}}} \left.\frac{\mu}{\rho}\right|_{\text{H}} + \frac{A_{\text{O}}}{2 A_{\text{H}}+A_{\text{O}}} \left.\frac{\mu}{\rho}\right|_{\text{O}} \]
Interactions#
Rayleigh scattering#
Rayleigh scattering, coherent and elastic, is an interaction with the atom as a whole, without ionization or energy transfer.
Characteristics:
never the dominant photon interaction
total cross section \({\sigma_R}\left/{\rho}\right. \propto Z\left/E^2\right.\)
small scattering angle
no excitation/ionization
atom recoil-energy negligible \(E_{\mathrm{scatter}} = E\)
The scattered photon has a differential (angular) cross section, denoted DCS, maximum in the incident direction, ie the angle of scattering \(\theta \approx 0\).
The probability of backscattering, ie \(\theta > \pi/2\), is not zero but remains low, even at low energy.
The interest of this scattering is its sensitivity to atomic arrangement. For materials different but having the same attenuation, Rayleigh broadcast DCS can be very different. The independent atom approximation (IAA) is not good enough for modelling scattering compared to a more complex interference function model (IFM) to take into account extra-atomic interference (see Fig. 30 where the Rayleigh DCS is plotted in long-dashed lines).
It is the idea of so-called dark field imagery that allows puts access to these DCS by Talbot-Lau interferometry. The information thus extracted are complementary to those of simple attenuation.
Regular atomic arrangements generate diffraction patterns in directions related to Bragg’s law according to the directions of the planes of atoms. This gives rise to small (SAXS) or wide angle (WAXS) diffraction imaging techniques.
Bragg’s law
Teams recently proposed a diffraction contrasy tomography (DCT) combining attenuation and diffraction on the same detector. Diffraction spots are associated in pairs at \(\pi\) angular distance during the rotation of the source, which makes it possible to find the local orientation of the atomic structure (see Fig. 35).
Compton scattering#
Compton scattering, inelastic and incoherent, is the dominant interaction for most share of materials in a wide range of energy. The photon transfers a part of its energy to the atom which expels a peripheral electron (therefore weakly bound).
Characteristics:
high probability for medium energy range
total cross section \(\sigma_C\left/\rho\right. \approx \) cstt then \(\propto 1\left/E\right.\)
independent of \(Z\)
atom is ionized
loosely bound electron
The angular differential cross section (DCS) is much more isotropic than for the Rayleigh scattering: at low energy, the most likely direction for the scattered photon Compton is even backwards, ie \(\theta \approx \pi\).
The laws of conservation of energy and momentum allow to model the energy of the scattered Compton \(E_2\) photon as a function of the scattering angle \(\theta\). This law is decreasing monotonically with \(\theta\), and the energy is maximum and close to the energy incident for small angles.
Scattered energy
Proof:
Energy-momentum relations (with \(E_0 = m_e c^2\) and \(p = |\boldsymbol{p}|\)): \(E_i^2 = (p_ic)^2\) with \(i\in\{1,2\}\) and \(E_e^2 = E_0^2 + (p_ec)^2\)
Conservation of the momentum (vector): \(\boldsymbol{p_1} = \boldsymbol{p_2} + \boldsymbol{p_e}\)
Conservation of the energy: \(E_1 + E_0 = E_2 + \sqrt{E_0 + (p_ec)^2}\)
This relationship assumes that the expelled Compton electron is at rest, in practice there is Doppler broadening. The atom is therefore ionized as a result of this interaction, this is what gave the terminology of “ionizing”.
Compton electron kinetic energy
Compton backscattering has been the subject of scanning imaging protocol development. whole body, especially for border control at airports. Source collimated in a brush sweeps and turns around the person, the detector is the same side as the source.
Photoelectric absorption#
The photoelectric effect is a total absorption interaction, the photon transfers any its energy to the atom. A strongly bound electron is expelled from the atom. This electron called photoelectron leaves with a kinetic energy equal to that of the photon incident minus its initial binding energy.
Characteristics:
high probability for low energy
total cross section \(\tau\left/\rho\right. \propto Z^n\left/E^3\right.\) with \(n\in[3\;4]\)
shell discontinuitites
ionized atom
tightly bound electron
\(KE_{\mathrm{Photo-e}^{-}} = E - E_{\mathrm{binding}}\)
At this point the atom is ionized on an internal shell, a cascade of re-arrangement of the electronic procession takes place and the associated energy releases result from a competition between two phenomena exclusive: the expulsion of another but peripheral electron called the Auger electron, or well the emission of a so-called fluorescence photon. The energies of fluorescent radiation are characteristic of the atom (eg use to measure lead in paintings). See the NIST database for the list of transition energies per element and the XDB table for the compiled list of emission lines. The photoelectric absorption depends also on the local molecular structure: this phenomenon is used in the x-ray absorption spectroscopy (XAS).
The fluorescence efficiency is very close to 100% for materials with large atomic number. On the other hand, for materials with low atomic number, there will be no fluorescence radiation but emission of Auger electrons.
Application to 3D fluorescence imaging of the different constituents of a cell.
Pair production#
The pair production is a total absorption interaction, the photon transfers any its energy to the atom. A pair of electron and its antiparticle, a positron, is created in the vicinity of the atom. The remaning energy, ie the energy of the incident photon minus twice the rest mass of the electron (ie 1.022 MeV), is transfered as kinetic energy to teh electron and positron.
Characteristics:
dominant for high energy
total cross section proportional to atomic number
exists only for incident photon energies \(E>2m_ec^2=1.022\) MeV$
\(KE_{e^-} + KE_{e^+} = E - 2 m_{e}c^2\)
When the positron comes at rest, it is annihilated with a neighboring electron and two \(511\) keV are emitted in opposite directions. This electron-positron annihilation is the principle of PET medical imaging.
Nuclear interactions#
Photonuclear interaction (aka nuclear photoelectric effect) is the absorption of gamma-ray photons by atomic nuclei and the accompanying ejection of protons p, neutrons n, or heavier particles from the nuclei. The photonuclear cross-section has the appearance of a high energy resonance (eg around 14MeV for tungsten or 23MeV for carbon), with a distribution shape similar to a Lorentz function. The peak energy of the distribution of this giant dipole resonance (gdr) is directly related to the atomic mass,
and the full-width half maximum of the gdr distribution is small, around 7MeV. Nuclear fluorescence resonance (NRF) may also occur, it is a \((\gamma,\gamma')\) nuclear reaction. NRF is the process by which an excited nuclear state emits \(\gamma\)-rays of specific energies to de-excite to its ground state. NRF spectroscopy has been used in NDT to prevent illicit drug smuggling across borders and seaports (see Nature DOI).
Electron-matter interactions#
X-ray imaging, which aims at detecting directly transmitted radiation only (ie without interaction), can be parasitized by the presence of many types of secondary radiation (Rayleigh and Compton scattering, Fluorescence…) as Fig. 50 shows. Electrons (and possibly positrons if the energy is greater than 1.02 MeV) are also by-products of interactions.
Fig. 51 shows the traces (in red) of electrons of 500 keV impacting from the left a W plate (100 microns thick and in vacuum). We see that the electrons have trajectories very disturbed and travel less than 100 microns. On the other hand, on the hundred electron launched, a few generated photons (in green).
Electron-matter interactions are shared between collision (ionizations or excitation) and radiative processes. Collisions are more likely for electrons with low kinetic energies and might ionized atoms, therefore Fluorescence radiation may result from collisions. Radiative processes are more likely for electrons with very high kinetic energies and generate Bremsstrahlung x-ray photons.
if \(b\approx a \Rightarrow\) hard collisions leading to excitation and ionisation, thus possibly emission of Auger electron or Fluorescence radiation)
if \(b \ll a \Rightarrow\) Coulomb-Force interactions with the external nuclear field, during which the electron is deflected in this process and gives a significant fraction of its kinetic energy to a photon (called Bresmstrahlung). From Attix DOI
Electron stopping power: energy loss per unit length
radiative stopping power \(S_{\text{radiative}} \propto E_0^{-2} \Rightarrow\) negligible for heavy charged particles
secondary x-ray radiation: Fluorescence + Bremsstrahlung (see section X-ray production)
The radiative efficiency is very low (less than % as shown by Fig. 56) in the range energy used in imaging. However, it is mainly Bremsstrahlung and less those of fluorescence (from ionizations by electrons) which are used in x-ray generators to produce the radiation. The Heavy metal anodes increase radiative efficiency.
Bremsstrahlung radiation yield involved in the radiative fraction \(g\) (average radiative yield for all \(e^-\) produced) for defining the mass energy-absorption coefficient \(\mu_{en}\) (see section Linear energy-absorption coefficient)
Radioprotection \(\Rightarrow\) low \(Z\) shields electrons
For a given kinetic energy of the electron, the maximum traversed range is defined by the CSDA (continuous slowing-down approwimation) range. The path of electrons, normalized in density, depends on the energy of the electrons but very bit of the material. The electrons expelled during photon-matter interactions are absorbed locally (this is the notion of dose): the path of 100 keV electrons in water is less than 200 microns.
Summary#
Attenuation coefficient#
Mass attenuation coefficient curves for carbon (Fig. 60) and tungsten (Fig. 61). The discontinuities for the photoelectric effect (PhE) are visible for tungsten because the electron binding energies of the atom are greater (70 keV for the K layer of tungsten while it is only 0.3 keV for carbon).
Note the relative constant of the coefficient of mass Compton scattering attenuation (CS) between the two materials, while that of the photoelectric effect is much more dominant for tungsten. The creation of electron-positron pair (PC) is a total absorption interaction that occurs at very high energy.
The iso-probability curve between the interactions by photoelectric effect and by scattering Compton is shown in Fig. 62. Materials with a low atomic number will essentially interact with X-rays by Compton scattering, while heavy metals mainly by photoelectric effect. These domains are usefull for the choice of energy (to minimize secondary radiations) or material (for radioprotection}
In radioprotection, two quantities based on the linear attenuation coefficient are usually used:
The half-attenuation layer or HVL (couche de demie-atténuation ou CDA) represents the thickness necessary to attenuate by a factor of 2 the number of photons transmitted without interaction.
HVL
Half-Value Layer (HVL), expressed in cm:
Proof:
\[ N_{\mathrm{DT}} = \frac{N_0}{2} = N_0\;\exp\left(-\mu T_{\mathrm{HVL}}\right) \quad\Leftrightarrow\quad T_{\text{HVL}} = \frac{\ln 2}{\mu} \]
The mean free path or MFP is the averaged distance traversed by a photon without interaction to the next interaction.
MFP
Mean Free Path (MFP), expressed in cm :
Proof:
\[\begin{split} T_{\mathrm{MFP}} & = \left<x_1\right> \\ & = \int_0^\infty \mu x e^{-\mu x} \text{d}x \\ & = \left[-xe^{-\mu x}\right]_0^\infty + \int_0^\infty e^{-\mu x} \text{d}x \\ & = 0 + \left[-\frac{1}{\mu}e^{-\mu x}\right]_0^\infty \\ & = \frac{1}{\mu} \end{split}\]
Propagation#
The divergence of the beam is directly conditioned by the solid angle between a surface and the source of radiation. This relation can be approximated by an inverse relation- is proportional to the square of the distance from the radiation source, considering that the surface is small and perpendicular to the direction of propagation.
Diverging beam (no attenuation): Distance law
Solid angles should be used to be more precise if the x-ray source emission is expressed per solid angle (eg in \(4\pi\) for the whole space). To compute the number of photons passing through a given surface, we need to determine its solid angle, ie the equivalent spherical surface \(\text{d}a\) on a given sphere of radius \(R\).
Solid angle
The global X-ray propagation formula is therefore the product between the term of divergence and attenuation.
Propagation law: directly transmitted photons
NB : to account for secondary radiations, a build-up factor \(>1\) is sometimes used, especially in radioprotection to get a better estimate of all the radiations that pass through.
Wave#
Wave phenomena are really present for X-rays, but the differences in indices being very small, it is difficult to observe them with standard x-ray sources. But for a coherent x-ray beam, diffraction patterns are much more likely to be visible.
A link between the Beer-Lambert attenuation law and the propagation of the electromagnetic wave can be established under some assumptions. Energy directly transmitted \(N_ {DT} E\) is therefore the squared modulus of the wave, and the integral along the radius of the attenuation coefficient linear \(\mu\) becomes the integral along the radius of the refractive index \(n\), of which the imaginary part \(\beta\) is linked to \(\mu\). The exponential law is found so similar on the phase term with \(\delta\).
with
refraction index \(n(r) = 1 - \delta(r) + i \beta(r)\)
\(\delta(r) \propto \rho(r) \lambda^2\)
\(\beta(r) = \lambda \mu(r) (4 \pi)^{-1}\)
In energy we get
\(\Rightarrow\) Beer-Lambert law where \(I_0^2 = N_0 E\)
As an example, Fig. 67 to Fig. 71 illustrate Fresnel interference by a fiber 150 microns in diameter with a detector placed at 56.7 cm from the fiber. We can clearly see the phase contrast on the acquired image. The simulation reproduces this phenomenon well when the impulse response of the detector or PSF is taken into account. These interferences have a very small spatial extent (a few microns).
The interest in phase imaging is the gain in sensitivity. The phase of the wave is up to 1000 times more sensitive to variations in density than its amplitude (see Fig. 72).
Monte Carlo simulation#
A few examples are given below of the use of Monte Carlo simulations. In high energy physics experiments:
To optimize the setup of a medica device:
For treatment planning in radiotherapy:
For DNA damage simulations to get the energy deposit, the number of single and double strand breaks:
In NDT to visualize the different secondary radiation contributions:
For further details
\(\mu/\rho\) database \(\Longrightarrow\) See XCOM from the National Institute of Standards and Technology (NIST)
Interactive plots of \(\mu\), the DCS \(\Longrightarrow\) Jupyter Notebook.
Monte Carlo simulations software \(\Longrightarrow\) See for example the Geant4 website of CERN