Direct measurement and analytical modeling of scatter in portal imaging

Abstract

In this study a direct measurement of scatter in portal imaging for various air gaps and scatterer thicknesses at a beam energy of 6 MV is presented. The experimental data are compared with results from a Monte Carlo (MC) scatter model. In the regime where the air gap is larger than 9.3 cm the MC and the experiment agree. Based on this MC model an analytical model is developed, which takes all important interaction processes into account. It comprises a rigorous treatment of first order scattering and an estimation of photons scattered more than once within the phantom. This estimation is based on the assumption that higher order scattering can be considered as isotropically distributed around a certain scatter origin located in the midplane of the phantom. It is found that relative deviations between the MC model and the analytical model are of 2% to 3% in regions where scattering is very large.

Bone mineral density in cone beam computed tomography: Only a few shades of gray

“…However, it may have significant implications in the gray values of the structures, with small volume FOVs associated with reduced values of density [18] . The decrease of gray values in the smallest FOV may be explained by the reduction of the diameter of the X-ray beam so as to irradiate only the region of interest [45][46][47][48] . This X-ray beam limitation may lead to the decrease of the amount of low-energy photons and to the increase in the capacity of penetration of X-rays [49] , resulting in a relative reduction of the value of attenuation of X-rays and gray values [43] .…”

Depth distribution of multiple order X-ray scatter

“…For the energy range where Compton scatter dominates, the behavior of the first order scatter can be studied by using the Klein-Nishina expression for Compton scattering cross-section (Spies et al, 2000). Here, we prove the conclusion that for the first order scatter of a parallel and monoenergetic photon beam in a slab of homogeneous scattering material, the ZLAST distribution always approximately exponentially increases with depth only if the attenuation coefficient of the material for the primary photons is less than that for the scattered photons.…”

“…In order to understand the nature and characteristics of scatter and to attempt to correct for it, the extensive studies by, for example Johns and Yaffe (1982), Boone (1988), Ljungberg et al (1990), Spies et al (2000), Jonsson and Larsson (2001), Meinel et al (2003), have been done and some important properties of scatter have been reported in literature. The strength and distribution of scatter have been shown to depend on incident photon energy, attenuation properties of the object, X-ray field size, air gap between the scattering object and the detector, and so on.…”

On Radiotherapy Dose Verification With a Flat-Panel Imager

“…More rigorous treatments of scatter from a patient or a phantom to the EPID, especially for smaller air gaps, are described in the literature. 46,74,75,[112][113][114]119 …”