Monte Carlo investigation of backscatter point spread function for x-ray imaging examinations

Abstract

X-ray imaging examinations, especially complex interventions, may result in relatively high doses to the patient’s skin inducing skin injuries. A method was developed to determine the skin-dose distribution for non-uniform x-ray beams by convolving the backscatter point-spread-function (PSF) with the primary-dose distribution to generate the backscatter distribution that, when added to the primary dose, gives the total-dose distribution. This technique was incorporated in the dose-tracking system (DTS), which provides a real-time color-coded 3D-mapping of skin dose during fluoroscopic procedures. The aim of this work is to investigate the variation of the backscatter PSF with different parameters. A backscatter PSF of a 1-mm x-ray beam was generated by EGSnrc Monte-Carlo code for different x-ray beam energies, different soft-tissue thickness above bone, different bone thickness and different entrance-beam angles, as well as for different locations on the SK-150 anthropomorphic head phantom. The results show a reduction of the peak scatter to primary dose ratio of 48% when X-ray beam voltage is increased from 40 keV to 120 keV. The backscatter dose was reduced when bone was beneath the soft tissue layer and this reduction increased with thinner soft tissue and thicker bone layers. The backscatter factor increased about 21% as the angle of incidence of the beam with the entrance surface decreased from 90° (perpendicular) to 30°. The backscatter PSF differed for different locations on the SK-150 phantom by up to 15%. The results of this study can be used to improve the accuracy of dose calculation when using PSF convolution in the DTS.

Calculation of forward scatter dose distribution at the skin entrance from the patient table for fluoroscopically guided interventions using a pencil beam convolution kernel

“…6, 7, 8 In that case, the exposure was delivered cross-table to avoid the forward scatter dose contribution from the patient table, and hence included only the primary dose and backscatter dose distribution. For the current study, the skin dose distribution is determined for a posterior-anterior vertical x-ray beam incident on a 20 cm thick PMMA block placed over the patient table using the DTS and XR-QA2 Gafchromic film.…”

Calculation of the entrance skin dose distribution for fluoroscopically guided interventions using a pencil beam backscatter model

“…Furthermore, studies with EGSnrc Monte Carlo on a water phantom have shown a dependence of backscatter PSF on the incident beam angle. 29 In addition, the PSFn may change with location in the field for a nonhomogenous object with internal anatomic structures that may modify the backscatter; 29,30 this is seen in the difference that we determined between the PSFn for the PMMA and SK-150 head phantoms. In this study, the PSFn calculated for the normal incidence pencil beam is convolved across the entire entrance field over the phantom surface with the assumption that the primary x-rays have nearly normal incidence.…”

“…The details of the Monte Carlo code and validation of the spectrum are given elsewhere. 29 A phase space file containing the spatial and energy information of the beam was used as input to DOSXYZnrc to obtain the dose absorbed per photon in the phantom model. To obtain the PSFn, EGSnrc is used to generate a 1-mm square pencil beam (80 kVp, 3.1-mm Al HVL) consisting of 4 × 10 9 photons incident on a phantom, and the total primary plus backscatter entrance dose distribution is determined for 1 m × 1 m × 0.5-mm-thick voxels on the surface with 0.02% statistical uncertainty for the central voxel.…”

“…The backscatter PSF will be dependent on various factors, such as beam energy, primary beam entrance angle, and location on the surface of a nonuniform phantom/patient. 29 The detailed evaluation of the dependence of backscatter PSF on these parameters is beyond the scope of this work. Benmakhlouf et al 4 have reported the backscatter factor variation with energy within the diagnostic range to be from 5% to 20%, depending on the field size.…”

Skin dose mapping for non-uniform x-ray fields using a backscatter point spread function

“…12,26 The voxel size of the PMMA EGS nrc model was maintained at 1 × 1 × 1 mm since the design was simulated. The beam is enclosed by a single voxel and the dose intensity estimated per voxel is the average across its entire volume.…”