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Monte Carlo Simulation of Nuclear Medicine Imaging Systems
Published in Michael Ljungberg, Handbook of Nuclear Medicine and Molecular Imaging for Physicists, 2022
David Sarrut, Michael Ljungberg
Coherent scattering is an interaction between an incoming photon and an electron, in which the direction of the photon is changed but without energy loss. This type of interaction leads to photons that scatter mostly in the forward direction. The sampling technique for coherent scattering, which was also described by Persliden [13], is based on the Thompson cross-section multiplied by the atomic form factor [14], and the equation for this is shown in Chapter 3 (Eq. 3.20).
Nano-Sized CT Contrast Agents *
Published in Valerio Voliani, Nanomaterials and Neoplasms, 2021
Nohyun Lee, Seung Hong Choi, Taeghwan Hyeon
where I is the intensity of the transmitted X-ray, I0 is the intensity of the incident X-ray, µ is the mass attenuation coefficient of the object, and x is the thickness of the object. There are three types of interactions between X-rays and objects: coherent scattering, Compton scattering, and the photoelectric effect (Fig. 4.2a). In coherent scattering, also called Rayleigh scattering, Thompson scattering, or classical scattering, the energy of the incident X-rays is absorbed by an atom, and an X-ray photon of the same energy as the primary X-rays is emitted in a random direction. Because coherent scattering occurs mainly at low X-ray photon energies, its contribution is minor in most clinical situations.
Photon Interactions with Matter
Published in Eric Ford, Primer on Radiation Oncology Physics, 2020
In the coherent scattering process the incident photon interacts with an electron (Figure 5.1.2). The photon causes the electron to oscillate up and down (recall that photons can be thought of as electromagnetic waves and it is the oscillating electric field that produces a force on the electron). An accelerating charged particle always produces an electromagnetic wave. This is something we will consider in more detail in a later chapter. Therefore, as the electron oscillates, a second wave (or photon) is produced. This wave emerges in some other direction. The wavelength (or energy) of the emerging wave is equal to that of the incoming wave. That is, no energy is gained or lost in this interaction. There are two flavors of coherent scattering: Thompson scattering, in which the photon scatters from a free electron, and Rayleigh scattering, in which it scatters from an electron bound in an atom. It is the wavelength-dependent process of Rayleigh scattering from molecules in the atmosphere that makes the sky appear blue on the earth.
Estimation of energy absorption buildup factors of some human tissues at energies relevant to brachytherapy and external beam radiotherapy
Published in International Journal of Radiation Biology, 2019
The Monte Carlo simulation method was used to compare the obtained EABF values using MCNP6.1 (Goorley et al. 2012). MCNP6.1™ is a general-purpose, three-dimensional geometry, Monte Carlo radiation-transport code designed to track many particle types (Goorley et al. 2012). Thus, it is a general-purpose code for use in neutron, photon, and electron, etc. transport through different medium and in this work, it uses ENDF/B-VI.8 atomic data as the cross-section library. For calculation of the mfp of soft tissue and water, the mass attenuation coefficients were obtained using WinXCOM (Gerward et al. 2001, 2004). The mass energy absorption coefficients were obtained using data available at NIST (Hubbel and Seltzer 2004). For comparison, the soft tissue and the water were selected and their EABFs at 0.662 MeV (Cs-137 source), 1.173 and 1.25 MeV (Co-60 source) were obtained up to 20 mfp through MCNP simulation code assuming an isotropic photon source. The estimated error through the MCNP simulation was less than 1%. During the MCNP simulation, total and unscattered photon fluxes were calculated. In order to calculate the total photon flux, F2 tally was used while for unscattered photon flux, FT, FU, C and E cards were used besides F2 tally. FT and FU can count the number of interaction of photons with matter. C and E cards refer to angle of direction of photons and energy, respectively. Detailed explanation with more physics background (photoelectric effect, bound-electron Compton scattering, coherent scattering, bremsstrahlung photons, K and L X-rays, and pair production) can be found elsewhere (Rafiei and Tavakoli-Anbaran 2018).
Transient otoacoustic emissions and audiogram fine structure in the extended high-frequency region
Published in International Journal of Audiology, 2021
We consider here possible explanations for the findings of the current results within the cochlear mechanical theory outlined above. The relatively weaker ripple pattern at EHFs relative to conventional frequencies and the lack of any discernible peak in the reciprocal spectral periodicity distribution (Figure 3) may have several causes. The generation of audiogram ripple requires several different elements: inhomogeneities arising from longitudinal spatial variation in the wave-impedance along the basilar membrane, coherent scattering of the travelling wave from these inhomogeneities, active amplification of the travelling wave to give a “tall-and-broad” travelling wave envelope (Zweig and Shera 1995), and basal reflection from the stapes. Thus the weaker AFS ripple pattern seen in higher-frequency regions may arise from a lower cochlear amplifier gain, from less potent inhomogeneities, or potentially from a different interaction between the travelling wave and the inhomogeneities, perhaps due to differences in cochlear mechanical properties. Measurements of SFOAEs by Shera (2003; Figure 3) suggest that the travelling wave envelope becomes increasingly sharply tuned as the frequency is increased up to 7 kHz, which may be due to increasing cochlear amplifier gain. If this trend continues to the 8–16 kHz region, then we might expect greater cochlear amplifier gain, and hence a stronger AFS ripple pattern, leading to a greater amplitude in the reciprocal spectral periodicity distribution. There is, however, a complication: increasing the cochlear amplifier gain also leads to a narrower travelling wave envelope, which leads to reflections being less coherent (Zweig and Shera 1995). The consequence is that as the travelling wave envelope becomes narrower, any peak in the reciprocal spectral periodicity distribution is expected to become broader leading to less coherent scattering (Zweig and Shera 1995). A consequence of this is that the peak in the reciprocal spectral periodicity distribution may become less distinct as it comprises a broader range of spectral periods. A further possibility is that the interference pattern breaks down in the basal cochlear region as the peak in the travelling wave becomes located closer to the stapes, giving a restricted region in which the travelling wave can develop. However, simulations by Ku, Elliott, and Lineton (2008) using a cochlear model which incorporates the main elements of the cochlear mechanical theory, predicted cochlear standing waves (which would generate an AFS ripple pattern) in the basal, EHF region, thus suggesting that there is no inherent theoretical barrier to producing AFS ripple in the basal region.