X-ray forward-scatter imaging: Experimental validation of model
Medical Physics , Volume 28 - Issue 2 p. 210- 219
In our research program we have investigated, through modeling and related numerical calculations, the potential use of scattered photons for medical x-ray imaging. In this work, we present an experimental validation of the primary and of the forward-scatter x-ray imaging models. Incident polyenergetic photon beams generated from a conventional rotating anode x-ray tube were used. To compare quantitatively the results between primary and forward-scatter imaging, an ionization chamber was used to record the incident air collision kerma, Kc/air. Plots of contrast (C) and the signal-to-noise ratio (SNR) as a function of the imaging task are presented. We have chosen to make measurements with plastics [polymethyl methacrylate (PMMA), polycarbonate, polystyrene, polyethylene, and nylon] placed at the center of a 15 cm diam spherical water phantom. Good agreement between experiment (expt) and prediction (pred) was obtained for many imaging tasks. For example, to image a 2 cm thick PMMA/polycarbonate combination using an 80 kV beam with the primary photons we obtain Cexpt=0.01 ± 0.02, Cpred=0.008 ± 0.002, SNRexpt/√Kc air=0.86 ± 1.6 (mJ/kg)-1/2 and SNRpred/√Kc air=0.51 ± 0.14 (mJ/kg)-1/2. The values obtained by using the θ=4° scattered field were Cexpt=0.26 ± 0.06, Cpred=0.19 ± 0.01, SNRexpt/√Kc air=3.8 ± 0.8 (mJ/kg)-1/2, and SNRpred/√Kc air=3.2 ± 0.3 (mJ/kg)-1/2. We have, however, shown that using form factor data from different authors can have a significant effect on the predicted values of C and SNR. The use of our semianalytic expressions for the numbers of transmitted and scattered photons combined with our experimental measurements allowed us to quantify the amount of water contamination in our measurements. Some preliminary results in air with biological materials (liver, muscle, water) are also presented. We are confident that our model can be used as a tool for designing and optimizing an x-ray scatter imaging system.