Incorporating corrections for the head-holder and compensation filter when calculating skin dose during fluoroscopically guided interventions

Abstract

The skin dose tracking system (DTS) that we developed provides a color-coded illustration of the cumulative skin dose distribution on a 3D graphic of the patient during fluoroscopic procedures for immediate feedback to the interventionist. To improve the accuracy of dose calculation, we now have incorporated two additional important corrections (1) for the holder used to immobilize the head in neuro-interventions and (2) for the built-in compensation filters used for beam equalization. Both devices have been modeled in the DTS software so that beam intensity corrections can be made. The head-holder is modeled as two concentric hemi-cylindrical surfaces such that the path length between those surfaces can be determined for rays to individual points on the skin surface. The head-holder on the imaging system we used was measured to attenuate the primary x-rays by 10 to 20% for normal incidence, and up to 40% at non-normal incidence. In addition, three compensation filters of different shape are built into the collimator apparatus and were measured to have attenuation factors ranging from 58% to 99%, depending on kVp and beam filtration. These filters can translate and rotate in the beam and their motion is tracked by the DTS using the digital signal from the imaging system. When it is determined that a ray to a given point on the skin passes through the compensation filter, the appropriate attenuation correction is applied. These corrections have been successfully incorporated in the DTS software to provide a more accurate determination of skin dose.

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

“…Skin dose calculation while using compensation filters has been reported previously by us, 10 and there was a discrepancy between the Gafchromic film measurement and DTS calculation in the attenuated periphery, since the backscatter dose component was not accounted for in the calculation. Figures 4(a) illustrates the DTS color dose distribution calculated using the backscatter PSF n convolution technique for the PMMA graphic with the two compensation filter outlines, and figure 4(b) shows the corresponding line-dose profile comparisons between the Gafchromic film and DTS.…”

“…ROI attenuators used for dose reduction and compensation filters used for intensity equalization reduce the entrance skin dose and modulate the spatial backscatter scatter distribution in the FOV. 6–10 Backscatter spreads from the point of primary interaction and the spatial distribution of backscatter from the patient varies in relation to the location of the ROI and compensation attenuators in beam. 11 We propose a method for the estimation and automatic mapping of cumulative skin dose for spatially shaped x-ray fields through the convolution of a backscatter to primary point-spread function (PSF n ) with the primary dose distribution.…”

Estimation of patient skin dose in fluoroscopy: summary of a joint report by AAPM TG357 and EFOMP

“…Hence an increased accuracy may be provided for skin dose estimation and data can be displayed in real-time to the operator (e.g., intra-event exposure parameter variations, collimation and influence of wedge compensation filters). 64 Regarding exposure or dose distribution mapping, there are three main types of solutions available:…”

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

“…Furthermore, when the primary beam is not uniform, a single backscatter factor will not be able to describe the distribution of backscatter dose. For example, beam shaping devices like region-of-interest (ROI) attenuators 5–8 and compensation filters 9 modify the primary beam distribution and concomitantly the backscatter distribution.…”