X-ray scatter imaging in medicine: Model and experimental validation.

摘要

The images produced in all radiological exams today are formed by capturing the undeviated (primary) x-ray photons. The scattered field, however, can offer different and often more useful information. I have devised a simple but useful model that considers the imaging via scattered x-rays of a target object against a background material of the same dimensions when both are situated within a water phantom. The model predicts that there exist specific medical regimes where x-ray scatter imaging could offer higher contrast ( C) and signal-to-noise ratio (SNR) than primary imaging. For example, the imaging of blood vessels versus white brain matter in a 15-cm-thick water phantom with the forward-scattered photons (2° to 12°) results in a maximum SNR, over all energies, greater than primary for essentially all vessel sizes ≤23 mm. Using an 80 kV beam would require dose increases of about 47% (scatter) and 205% (primary) relative to using a monoenergetic beam.; The model was validated experimentally with plastics placed at the centre of a 15-cm-diameter spherical water phantom. For example, to image a 2-cm-thick poly-methyl methacrylate/polycarbonate combination using an 80 kV beam with the primary photons we obtain Cexpt = 0.01 +/- 0.02, Cpred = 0.008 +/- 0.002, SNRexpt / Kairc = 0.86 +/- 1.6 (mJ/kg)-1/2 and SNR pred/ Kairc = 0.51 +/- 0.14 (mJ/kg)-1/2, where Kairc = air collision kerma [subscripts "expt" = experiment and "pred" = prediction]. The values obtained with the theta = 4° scattered field were Cexpt = 0.26 +/- 0.06, Cpred = 0.19 +/- 0.01, SNR,expt / Kairc = 3.8 +/- 0.8 (mJ/kg)-1/2 and SNRpred / Kairc = 3.2 +/- 0.3 (mJ/kg)-1/2. The significant variation of C and SNR with different form factor data emphasizes the need for more accurate x-ray diffraction measurements. This work confirms the usefulness of scattered x-rays and provides a model that can be used as a tool for designing and optimizing an x-ray scatter imaging system.